ive been researching lately about fabrication of polycarbonate & acrylic to attempt a one of a kind enclosure with a very large sheet of 15/16" lexan, so far this is what ive found from searching the web for various literature on the subject,........
Both acrylic and polycarbonate
are rigid plastic products and are available in clear plastic sheet, rod and tube form which are transparent and almost glass like in appearance. This is where the similarities end, however, as these plastic products perform different roles.
Acrylic or Perspex
is the plastic sheet generally used for plastic fabrications, boat windows, display cabinets, raffle barrels, brochure holders, etc.
Acrylic has 8 times the impact strength of glass, has a high degree of transparency and can be fabricated, by bending, cutting and moulding, to almost any shape.
There are many colours of acrylic on the market, making it a fabulous plastic product to use in signage and displays. Also the good weatherability of acrylic is useful for sign writers.
Acrylic will melt at temperatures of 100°C. This is the plastic to use for your everyday plastic fabrication requirements.
Polycarbonate or lexan is the plastic sheet used when safety
and security plastics are required. It is renowned for its high impact strength; it has 250 times the impact strength of glass and 30 times that of acrylic.
Polycarbonate is virtually unbreakable, light weight, durable, is UV stabilised (has good weather resistance) and is thermo formable to almost any shape. Its major uses are safety machine guards, riot shields, safety glazing, windows in race cars, telephone booth panels, etc. This is a strong plastic product, but it will scratch more than acrylic.
There is a mar resistant polycarbonate available, which has the same basic properties as regular polycarbonate, but it has a reduction in abrasion and scratching. Polycarbonate will melt at temperatures of 149°C.
So, to sum up, acrylic is used for non safety items and all types of displays. Polycarbonate is used for industrial strength applications where safety is a priority and where high temperatures are used. Naturally acrylic is cheaper than polycarbonate and regular polycarbonate is cheaper than the mar resistant grade.
Acrylic, [COLOR=yellow ! important][COLOR=yellow ! important]Polycarbonate[/COLOR][/COLOR]
, Plexiglas, Lexan; they are all interchangeable terms in most peoples vocabulary. Getting the terminology correct could save your project from becoming a disaster.
Of all four materials listed above there are really only two plastics involved, polycarbonate and acrylic. Lexan is General Electric's trade name for Polycarbonate and acrylics trade name is Plexiglas.
Acrylic = Plexiglas
Polycarbonate = Lexan
Why does all this matter? In two words I would answer "Impact Resistance." If you’re planning any sort of project involving one of these two transparent materials you will want to know the one major drawback acrylic has to offer. Impact resistance is the key. If you want your project to hold up for any length of time under normal use, polycarbonate is the material of choice. Acrylic has a very low impact resistance and is prone to stress fracturing. Polycarbonate on the other hand has impact resistance 30x that of acrylic. Polycarbonate is also a harder material than acrylic making it less prone to scratching.
Acrylic / Plexiglas Extruded
Impact Strength 0.4 ft lbs / in
Hardness Rockwell M: 93
Polycarbonate / Lexan
Impact Strength 12 ft lbs / in
Hardness Rockwell R: 118
These numbers are for the most common types of each material. Higher grades of each material are available but can cost quite a bit more. Even with high grade acrylic and polycarbonate the trend continues. Polycarbonate beats out acrylic in all the areas that make a difference for our uses.
What can acrylic be used for? Window mods are probably ok as long as you’re not playing football with your case. ITX cases, mATX might be pushing it depending on design. For standard ATX acrylic is a very bad choice. Even if you don’t kick your case, micro fracturing can occur around high stress areas. Polycarbonate is more expensive than but the costs far outweigh the disadvantages of acrylic.
Here is a price comparison of what I paid for each material.
Home Depot 24x48 sheet of .25” thick acrylic $30
McMastercarr 24x48 sheet of .25” thick polycarbonate $45
UPS ground for 2 sheets was only $11.75!
Where can you purchase these materials?
Home depot usually stocks acrylic but polycarbonate can be scarce
is an excellent choice for either. They even stock bullet resistant polycarbonate sheets!
I hope this is of some use in your future projects
Polycarbonate and acrylic plastics have achieved very wide usage in the design and manufacture of luminaire lenses.
Their advantages over glass are many, including impact resistance, weight and viscosity of flow characteristics that allow them to be injection molded into optical designs of extremely high precision. Yet, for all their similarities, there are significant differences in performance in the areas of optical stability and impact resistance. These differences make the occasional claims touting one or the other as the superior lens material difficult to evaluate. In fact, comparative analysis shows that neither material meets all design requirements better than the other. Combinations of factors may make acrylic the choice for one application; change those needs slightly and polycarbonate is the best selection.
Time and again, specifiers and owners of a project have been disappointed in the performance and longevity of luminaires that are being overdriven or misapplied. Purchasing a fixture that uses lamps too hot for the lens material will lead to premature failure. For instance, a 10" square by 7" deep fixture with a polycarbonate lens and a given optical configuration can easily support a 70 HPS source and provide long life and performance, A 100 HPS lamp in the same fixture may well cause yellowing of the lens and loss of performance and aesthetic characteristics. A larger fixture must be used if the higher source is the design choice.
This paper will describe the materials and then explore the two critical performance areas of impact resistance and optical stability with emphasis on thermal characteristics and effects. Further, primary application criteria will be established. The goal is to enable the specifier, through evaluation of his specific project, to select the most appropriate lens material.
Both acrylic and polycarbonate are thermoplastics. This means the material can be re-melted from solid state for use in injection or casting process. Acrylic is derived from methyl methacrylate. This plastic has excellent optical qualities, good chemical resistance and thermal and electrical properties and has moderate strength. This strength can be enhanced by use of an additive in the formative chemical reaction, yielding high impact acrylic. The additive, however, reduces clarity, weatherability and flexural modulus.(1)
Polycarbonate results from the linking of dihydric or polyhedric phenols through carbonate groups. The material has very high impact resistance, is easily processed by all thermoplastic methods and has high temperature performance.(2) Additives will make it resistant to ultraviolet radiation which can cause long term degradation of the plastic.
From these descriptions alone, guidelines for use begin to take shape. The following specific analyses will make these guidelines clear.
Two frequently used methods to evaluate strength of these materials are the notched Izod test and falling dart impact. Results are measured in footpounds. The notched Izod test evaluates shear stress while the falling dart measures resistance to direct, penetrative impact. Comparison of polycarbonate and two types of acrylic shows:
High Impact Acrylic(4)
In refractor design, the falling dart test is the most appropriate because concern is focused on the likelihood of vandalism or the impact of an object on the lens.
Factors come into play that can radically affect each material's resistance to impact. Temperature change can reduce material strength. High impact acrylic shows straight line correlation between temperature and strength. The 8.0 ft/lb value at 60°F in the dart test decreases to 4 ft/lbs at 9°F. Conversely, its impacts strength will increase beyond 8 ft/lbs as temperatures exceed 60°F. (5) Polycarbonate also exhibits some loss of strength in low temperatures but over the same 0°-60°F range, it loses only 15% as opposed to acrylic's 50% loss.(6) Polycarbonate demonstrates exceptional low temperature tolerance.
Another factor altering impact resistance is lens design itself. With respect to high impact acrylic, W.C. Burkhardt notes that there is ". . . difference in the impact strength of lighting parts depending on whether the prismatic or non-prismatic side was impacted. Unfortunately, impacting the smooth side usually resulted in reduced impact strength."(7) The variations in impact strength appear to be functions of part thickness, size and shape of the optical element and even the molding process used to create the optical element. Burkhardt further notes "…whenever prisms or other light obscuring or controlling elements are to be used', if possible they are best designed on the outside of a lens from an impact strength stand-point."(8)
This is an application drawback. In terms of dirt depreciation and maintenance, it is far preferable to keep the optical elements on the inside of the lens and present a flat, smooth surface to the exterior. Dirt will not adhere readily to such a surface and it is easily cleaned.
Polycarbonate exhibits some stressing characteristics that are typical of curved surfaces. Sharp corners and radii are high stress points, concentrating loads in very localized areas. Lenses designed with softer corners and larger radii would show correspondingly less stress. However, the larger radius solution leads to some small loss of light control capability.
When it comes to selecting a lens material for strength, the, consider these factors: basic strength, ambient temperature and design. Will the luminaire be located in an area where damage through vandalism is likely or would the only lens contact be occasional and light? Will it be exposed to temperatures with wide shifts in range? Will maintenance be a problem? Is the environment dusty or dirty? In the following chart boxes with an "X" indicate an appropriate design choice.
X Optical Performance:
The second major attribute of these plastics is their light transmitting abilities. Both acrylic and polycarbonate are excellent choices for lens material as they have high transmissivity ratings and show very little hazing.
High Impact Acrylic
Of the three, standard acrylic performs best as an optical medium. However, because of strength considerations, polycarbonate and high impact acrylic are the most common choices for outdoor use. The critical difference between these two in the long run is the fashion in which each reacts to the environmental stresses of ultraviolet radiation and heat.
In lighting applications UV is a well known stressing agent of plastics - all transparent plastics will yellow under UV - but it is in many ways the most controllable. Polycarbonate without a UV inhibiting additive will show strong yellowing upon exposure to natural and artificial sources of ultraviolet (such as sunlight and HID lamps). High impact acrylic also yellows, though not to the same degree, and standard acrylic shows little UV induced yellowing. The use of a UV inhibitor in polycarbonate formulation reduces yellowing significantly.
Aside from the nature of the plastic itself, lamp selection can affect the yellowing process. Figure 1 shows typical energy emissions of the three major HID lamps.
Note that the mercury vapor and metal halide lamps both emit ultraviolet radiation while the high pressure sodium does not. Naturally, selection of a given lamp type would in itself indicate the degree to which UV would affect the lens. However, between LTV stabilization in polycarbonate, the general acrylic characteristics and judicious lamp selection, LTV is a minor problem. A more pressing concern for both materials is heat.
Refer again to Figure 1. Note that all lamps emit a relatively large portion of their total output in convective/conductive energy. This is heat; it has a remarkably adverse effect on acrylic and polycarbonate.
Figures 2, 3 and 4 clearly illustrate the close correlation between heat and yellowing in thermoplastics exposed to HID sources. (Note that the time scale for the acrylic test differs from the polycarbonate scale.)
(Charts 2 and 3 are black and white reproductions of colored photos demonstrating relative Yellowness Index over time These charts are available on request.)
Aside from lamp, other factors that influence the rate of heat induced yellowing include ambient temperature, size of luminaire enclosure, distance from lens to lamp center as well as the distance from lens to reflector and the shape of the reflector itself. The charts therefore represent specific and controlled tests and should be considered a guide to possible material responses in the most general sense.
Does yellowing present more than an aesthetically displeasing effect? Strangely, the data are split. Standard falling ball impact tests on polycarbonate indicate that there is no loss of material strength. Furthermore, an ASTM Yellowness Index Rating of 25 for polycarbonate results in a loss in transmissivity of only 5%.
On the other hand, yellowing is a sign of degradation of the plastic molecule. Heat and ultraviolet act to break the molecules. This surrenders the intrinsic strength of the material as the molecular structure no longer consists of long intertwined chains but fractured segments. This may be reflected in reduced strength of parts with formed surfaces as these surfaces tend to localize stresses.
From a specification point of view, one must be concerned with the temperature of the lens. Polycarbonate has a viable working temperature of around 90°C (approximately 195'F). Working temperature is that maximum allowable temperature for a material that will not result in a loss of physical characteristics. For acrylic, ". . . the designer concerned with retention of of optical properties would certainly want to design for plastic temperatures in the 150°-160° F range."(14)
Unless the lens/housing enclosure is large, these temperature limitations mean the use of lamps with high amounts of conductive/convective radiation is not recommended. This includes the mercury vapor and metal halide lamps. High pressure sodium is a good choice as its higher efficiency makes the use of a lower wattage lamp with less emitted heat feasible. At any rate, in specifying relatively small fixtures with plastic lenses, it is advisable to request a tested internal lens temperature. This can allow the specifier to easily determine if the source/wattage combination he desires is compatible with long lens life.
Unfortunately, there are other causes of yellowing that are extremely difficult to anticipate. Stresses in the manufacturing process can cause premature yellowing even if the other environmental stresses are within the limitations described. Some of these fabrication stresses include the amount of regrind or reprocessed material in the lens, excessive molded in stresses (a function of the lens design itself), processing temperature and improperly or insufficiently dried resin. These factors are not perceivable in a finished lens and the manufacturer must be constantly aware of the possibilities of molding stress and must test samples for quality.
Another phenomenon of the thermoplastic lens is haze. The percentage amount of haze (see Optical Performance chart) indicates how much light is scattered outside a small conical angle by the material. The larger the amount of haze, the more diffuse the source will appear to be. In some applications, this is desirable but if acrylic or polycarbonate is selected because of transmissivity or clarity, haze is a definite shortcoming.
Little has been tested as to the causes in haze generation. There appears to be a link between the causes of yellowing and an increased amount of haze but the percentage of light improperly diffused rarely increases by more than a few percentage points.
Selection of a lens with an emphasis on retention of optical characteristics requires an analysis based on heat. This is where the tested temperature comes in handy. If a high UV emitting source is preferred, also keep in mind that these lamps are quite hot.
As pointed out earlier, the superiority of a given plastic lens material depends not on intrinsic attributes but is determined by the application itself. An overall comparison shows that polycarbonate is more applicable and fits more situations. Acrylic is a good performer when chances of surface impact are small and when continued aesthetic appearance is important with the use of high UV sources. The single most destructive element, though, is heat. In specifying any fixture with a thermoplastic lens this must be considered carefully.
Acrylic sheet can be thermoformed using several types of equipment such as vacuum, pressure, or stretching equipment and a variety of heating methods including coiled nichrome wire, metal (cal) rod, hot air ovens, ceramic elements, and quartz tube (nichrome filament and tungsten filament). These heat sources derive average life spans ranging from 1, 500 to 20,000 hours at varied efficiency levels.
The following technical information is offered as a resource for thermoforming equipment and procedures. Users should undertake sufficient verification and testing to determine the suitability for their own particular purpose.
Forced circulating-air ovens heat uniformly at consistent temperatures and are commercially available. These ovens consist of an outer and inner shell separated by a space containing fiber or rockwool insulation. Inside, the ovens have thermostatically
controlled heaters. Baffles and electric fans can also be used to ensure even heat distribution. Because of its relatively low molecular weight, acrylic sheet cannot be hung from a single edge. A clamping frame should be used for support and to facilitate sheet transfer to the forming station. Before heating with an air oven, be sure that you can reliably control temperature thermostatically within 10(F ( 5(C) between 340-350( F; air velocities across the sheet range between 200-1000 feet per minute (1-5 m/s); temperatures throughout the oven are uniform; and the clamping frame exerts constant uniform pressure on all sides as the sheet becomes soft.
Using horizontal infrared heaters (ceramic elements for instance) instead of hot air ovens are faster and less labor intensive. Horizontal units are also more flexible because either the heater or the tooling can be moved. Working clearances can often be improved by moving the heaters, and systems can be designed to handle custom made blanks.
When designing a horizontal heating system, independently controlled zones can be set up to improve control and flexibility.
Thermoforming machines are ideal when large volumes of the same shape are being produced. However, tooling costs are usually high.
Multiple shapes can be formed at the same time from a single sheet. Post machining will be needed to separate and finish the parts.
Vacuum and Pressure-Forming Equipment
Many types and sizes of forming equipment are commercially available. Trade publications dealing with plastics list many equipment manufacturers.
How to Make a Vacuum Chamber
An airtight vacuum chamber can be fabricated from welded steel plates. Steel should not be used for the forming plate since it may "chill" the hot sheet. A hardboard forming plate about 1/2 inch in thickness is recommended.
The detachable forming plate can be sealed to a flange on the top of the vacuum box using a gasket. The sheet blank is clamped in place over the forming plate using a clamping or holding ring and several toggle clamps.
The shape of the formed part's base will be determined by the shape of the cutout in the forming plate. Regulating the vacuum between the vacuum chamber and a vacuum storage tank will control the height or depth of the part. Required equipment: a high speed vacuum pump (10 cfm minimum), a vacuum storage tank, a 1" gate valve, a vent valve for releasing vacuum after the part is formed, and a vacuum gauge.
The required equipment for free blowing includes a plywood board with an air hose attached to its underside and a forming plate for controlling the piece's contour at its base. To evenly distribute incoming air, baffle the air intake with foam, felt, or cardboard. Cover the plywood board with flannel or polyurethane foam to prevent mark-off. The forming plate or ring should be made from approximately 1 1/2" thick hardboard. quick-acting toggle clamps may be used to attach the heated sheet and the ring to the plywood base.
Plug-and-Ring Forming Equipment
Rings or plates can be made from hardboard, plywood, or metal; plugs are usually made from hardwood; and the equipment should be coated with flocked rubber sheet to minimize mark-off.
For producing large volumes of parts, durable aluminum rings and plugs can be used.
Be sure to follow manufacturers' safety recommendations for equipment and material as various acrylic sheet products react differently when heated to forming temperatures.
Because of the orientation imparted during manufacture, acrylic sheet shrinks slightly when heated to thermoforming temperatures. Manufacturing direction can be determined from the sheet label or print on the masking. The lines of print are perpendicular to the direction of manufacture.
Original dimensions won't change in fabrication operations not requiring heat. However, sheet heated to thermoforming temperature changes dimensionally by about 3 percent maximum shrinkage in the manufacturing direction and approximately 0.5 percent maximum width increase (transverse direction).
Measure the shrinkage in a preliminary test, if acrylic sheet isn't held in a retaining frame. Then, determine the size of material required to compensate for shrinkage before cutting any blanks.
Predrying acrylic sheet is rarely necessary. Keep the sheet wrapped until used. To prevent blistering, dry high-water-content sheet in a forced-circulation drying or vacuum oven before heating. Drying time depends on water content and material thickness. 24 hours at 176(F (80(C) dries most sheets.
To reduce the length of the forming cycle, pre-dry the sheet in a spare oven and transfer it directly into the forming oven at 176(F (80(C) after the drying period.
To avoid blistering or distortion, heat the sheet to the low end of the forming temperature range using convection, conduction, or radiation heating.
The standard procedure for a vacuum forming machine is to clamp the cold sheet in a frame and heat it by infrared radiation. Stresses may arise due to the sheet becoming hot while the clamped edges stay cold. Tearing, edge distortion, and asymmetrical shapes may also occur. To avoid these problems, heat the clamping frame to 140-180(F (60-82(C). Clamp the sheet and continue heating.
When using high-intensity quartz tube heating panels, a short soak cycle should be included after the heating cycle and before forming. This results in more even heat distribution across the sheet's thickness.
The forming range for acrylic sheet is 290-320(F (142-160(C). Even temperature distribution throughout the sheet's thickness is recommended. Before forming, the sheet's temperature must be higher than the desired temperature to allow for cooling that will occur prior to the start of forming.
Specification of adequate oven temperature makes for easy adjustment and control. In cases like infrared heating, where temperature specification is impossible, find the approximate temperature by using a pyrometer calibrated for plastics. Thermometer papers that show the sheet's surface temperature by a color change can also be used.
Temperature requirements depend on forming conditions - the degree of shaping (stretching) and the forming rate. To prevent pimples, blisters, shading changes, and other damage, avoid higher-than-necessary temperatures. A template or mold can mark overheated material.
Heating time depends on material thickness and heating method. Conditions during heating like air velocity in the oven, panel-to-heater distance, etc. also affect heating time. Final product and surroundings are other factors.
These variables are too numerous to predict mathematically. Minimum heating times should be determined by running test cycles. With some experience, cycles that result in evenly and thoroughly heated sheets can be developed.
The maximum forming rate of a sheet is limited to the speed at which it will stretch without exceeding its strength and fracturing. The minimum forming rate must be fast enough to prevent the sheet from cooling appreciably.
A highly pigmented sheet should be formed slower than a colorless or transparent material.
Excessive fast forming rates will impart high stresses and cause low craze resistance. To minimize stresses, use moderate forming rates and ensure a uniform temperature distribution over the surface of the sheet and across its thickness.
Higher forming temperatures are needed to achieve greater "draws" or increased definition. For a "slow" forming operation it may be necessary to continue with infrared heating while the part is being formed.
Here, acrylic sheet is heated and bent over a positive (male) or into a negative (female) mold. Female molds are best because they compensate for sheet shrinkage during cooling and sheet "memory." To deter mark-off, cover the molds with rubberized flocking or billiard table felt. For the same reason, set mold temperature high and forming temperature low.
Cover the surface so the cooling rate is the same on both sides of the mold (thick cloth or felt blankets make good covers). If the mold and blanks are larger than the finished part, trim off clamping frame marks.
Form compound contoured parts like those used for bus rear windows and roof glazing. After heating to the formed temperature, gently position and clamp the sheet over a positive mold with the shape of the finished part's external contours.
Free Blowing and Vacuum Molding
Many configurations can be free blown using a shaped clamping ring or vacuum-drawn using a mold box. Items of high optical quality can be produced by these methods because the surface of the material never touches the mold walls. Thus, no mark-off or local cooling occurs.
A variety of spherical surface shapes can be molded depending on the geometry of the clamping frame when using these techniques.
The pressure or vacuum is varied to get the desired height or depth. The height is demarcated with a jig or soft material designed to avoid marking. Pressure or vacuum can also be controlled automatically with optical light sensors
. Although this method is expensive, its lack of contact creates an advantage in optical-critical production.
A simple suction or blowing mold consists of a base plate with a clamping frame. A sufficient amount of mechanical or hydraulic toggle clamps must be provided to maintain frame rigidity and to withstand the forming pressure. Using screw clamps has the disadvantage of being time consuming and may allow the sheet to cool excessively before it is formed. Beaded clamping frames will seal better than flat frames.
For free blowing, use up to 75 psi of air pressure. Provide large air connections so that large parts can be shaped quickly. Be sure that incoming compressed air does not hit the hot panels directly and cause local cooling. Installing baffles or screens in front of the inlet opening can deflect the incoming air.
Resting the sheet blank on a cold base plate may cause undesired cooling while installing the clamping frame. To reduce this, heat the plate or cover it with thick, non-linting cloth.
Vacuum forming requires the same fundamental conditions as pressure forming except that less clamping force is needed because suction seals the sheet to the vacuum box virtually automatically and the pressure difference is limited to 15 psi or less. If possible, arrange the suction ducts in a ring around the edge of the vacuum box to prevent airflow from cooling only one side of the part. For processing large parts, fit a reservoir (or vacuum tank) in front of the vacuum pump for quick evacuation of large volumes of air.
Vacuum with Plug Assist in Negative Mold
Essentially the same considerations apply here, as with blowing, except that the smaller available pressure difference - one atmosphere - limits this technique's application to simple moldings without strongly undercut portions.
Vacuum to a Positive Mold with Mechanical Pre-stretching
By comparison with combined processes using negative molds, sucking onto a positive mold has the advantage that the mold becomes the pre-stretching plug. Also, marks may appear on only one surface.
Where vacuum provides insufficient force, compressed air may be used. In either case, the mold should be heated and suction or venting holes must be provided at its extreme points.
Plug and Ring Forming
Use this method for trays, sign faces, lighting fixture diffusers, or any part not deep-drawn for which mark-off at the inside corners is acceptable. The mold includes a forming plate, clamping plate, and a male plug.
The molded part's outside contour conforms to the forming plate, but its opening is larger. To provide a sheet's thickness clearance between the male and female mold parts, the slightly tapered male plug is smaller than the forming plate's inside dimension.
The mold can be set up in an air cylinder press or in an arbor or drill press for small parts. Position the heated sheet on the forming plate and hold it using the clamping plate (toggle clamps or C-clamps can be used to hold the forming and clamping plates together). The male plug is forced through the clamping and forming rings to a predetermined depth.
Vacuum Snap-Back Forming
Free blowing and vacuum forming produce natural "surface tension" shapes or bubbles which a forming plate can control only at the base. Vacuum snap-back forming employs a male plug attached to an air cylinder's ram. The plug is lowered into the vacuum-formed bubble. This method combines plug-and-ring and vacuum forming. Drawing vacuum on a hot sheet forms a bubble. A male plug is positioned inside the bubble. Gradually, vacuum pressure is released and the hot sheet, because of its "elastic memory," snaps back and clings to the male plug.
The plug, frequently of hardwood, should include a 2( to 3( taper (draft), easing removal after cooling and contraction as well as vent holes to avoid trapping air at the bottom surface. This method enables production of irregular shapes, with the forming plate's cutout and the male plug's shape controlling the contour.
After forming, cool the parts to below 140-160 F (60-70 C). Don't just cool the surface - the interior must also cool. Provide uniform cooling on all sides to prevent stress. Completely cover slow cooling thick-walled parts with felt or blankets to block drafts.
There's no rule of thumb for predicting sheet interior's cooling time. Factors include material thickness, ambient air temperature, and airflow to the part. Experience is the best teacher.
As it cools, the sheet shrinks to reverse heat-caused expansion. Allow it to move freely to prevent stress. Shrinkage on the mold can also cause stress, it is best to remove the part as soon as it achieves dimensional stability.
Uneven heating temperatures
Clamping frame not hot
Be sure all heaters are functioning. Eliminate drafts. Baffle heat on all sides.
Preheat clamping fram. Preheat entire sheet.
Heating too rapidly
Lower temperature. Increase distance between heaters and sheet.
Be sure all heaters are functioning. Use screening to balance heat.
Predry. Preheat, Keep masking on sheet until formed. Use older material first.,
Improper mold surface
Dirt on sheet
Use proper mold covering (foam, felt, flocking)
Clean with deionized air.
Excessive forming temperature differential
Preheat clamping frame. Use slip clame frame (low/high).
Heat frames to proper temperature before inserting sheet. Add supplemental heat to corners.
Cracking in corners
Heat sheet evenly. Preheat frames or use heated frames. Add supplemental heat to corners.
For more information contact: CYRO Industries, P.O. Box 5055, Rockaway, NJ 07866; Tel: 800-531-6384; Fax: 973-442-6117; or Web site: Object moved
# # # #
NOTE - Acrylic sheet is a combustible thermoplastic. Precautions should be taken to protect this material from flames and high heat sources. If not extinguished, acrylic sheet will burn rapidly to completion. The products of combustion, if sufficient air is present, are carbon dioxide and water. However, in many fires sufficient air will not be available and toxic carbon monoxide will be formed, as it will when other common combustible materials are burned. Good judgement is urged in the use of acrylic sheet. It is recommended that building codes be followed carefully to assure proper use.
From high speed trains to street furniture, and from
snowmobiles to motorway signs, Lexan® sheet
products are designed and manufactured in a
diverse range of shapes and sizes. One of the most
economical methods of producing these parts is
Thermoforming Lexan® polycarbonate sheet is an
established process that offers the designer the
freedom to develop complex shapes and forms
with cost/performance characteristics that have
significant advantages over more traditional methods
of production. Low cost tooling, large part production
and reduced lead times all contribute to the
advantages of producing sheet products in this
way. With the introduction of Lexan® Exell® D and
Lexan® Margard FMR, thermoformed applications
can now be produced in both added value surface
treated products, providing the engineer with
enhanced design opportunities.
Whilst the thermoforming process is basically very
simple, the number of different processing,
production, design and finishing steps are quite
varied. To assist the designer and the converter in
the selection of a suitable production method, the
following section outlines some of the techniques
used in the design and manufacture of thermoformed
products in Lexan® polycarbonate sheet.
Table 1.0: Formability
Material Vacuum Drape Twin sheet Pressure Hot/Cold Cold Flat Curved
forming* forming forming* forming* line bending** curving lamination lamination
Lexan® 9030 x x x x x x x x
Margard® MR5E x
Margard® MRA3 x
Margard® HLG5 x x
Margard® HLGA3 x x
Margard® FMR x x x
Margard® FLG5 x x x x
Exell®D x x x x x x
Exell® D ST x x x x x x
Sign grades x x x x x x
CTG x x
FR grades x x x x x x
Gepax® x x x x x x
Ultem® x x x x x
* Transparent sheet in contact with mould surface can cause haze and an optically distorted surface.
** The mar resistant or U.V. protected surfaces can be damaged around the bent area during the bending process.
The majority of thermoplastic resins, including
sheet products, are hygroscopic, which means that
they absorb moisture. Moisture builds up in the
polymer sheet during manufacture, transportation
and storage. In the ‘as extruded’ condition it
presents no problem. However, during forming,
excess moisture can cause bubbling and other
surface appearance problems as well as a reduction
in property performance.
Whilst the amount of water absorbed is not
significant compared to other hygroscopic
materials, it is essential that it is removed prior to
forming. A hot-air circulating oven at 125°C ± 3°C
is recommended. To avoid warpage, drying
temperatures should not exceed 125°C and the air
volume in the oven should be changed six times
per hour to allow for the removal of water vapour.
After removing the protective masking, the sheets
should be hung vertically in the drying oven and
pre-dried according to the recommendations
outlined in Table 1.1. Alternatively, the sheets can be
placed in racks with a seperation of approximateley
1.0 to 2.5 cm between the sheets. Following predrying
the sheet should be processed within a few
hours. The time limit depends upon the wall
thickness and local environmental conditions.
Table 1.1: Recommended Drying Times
Sheet Thickness (mm) Drying Time (hrs)
1.2 Thermoforming Techniques
Lexan® polycarbonate sheet products are easily
thermoformed and a wide variety of applications
can be produced using the process. The basic steps
involve the heating, shaping and cooling of a
thermoplastic sheet product. There are a number
of different forming techniques, some of which only
require heating to allow the sheet to conform to
a simple positive or negative mould as in drape
forming. Others, such as vacuum and pressure
forming require that, after heating, the sheet is
made to conform to a mould by applying pressure
or a vacuum.
Whilst each process is slightly different, as illustrated
in Figures 1.1-1.4, the basic steps are very similar.
The sheet is firstly clamped along all edges inside
a clamping frame. A heat source is moved over the
sheet raising its temperature until it is elastic.
The heat source is removed and the mould table
raised. The air in the space between the sheet and
the mould is evacuated and the sheet is drawn
towards the mould and takes its form. Pressure can
also be applied to the positive side of the mould to
reproduce detailed mould features. The sheet
is cooled, the mould moved downwards, and the
product taken out of the machine. The clamping
edges are removed from the product and,
if necessary, additional machining is carried out
to finish the product.
As a manufacturing process the technique offers
significant advantages and is widely used for its
simplicity and low production costs.
However, in order to preserve its protective coatings,
Lexan® Margard® cannot be thermoformed.
The major benefits of thermoforming are listed
• Small to large part production
• Short lead times
• Small to medium size series
Fig.1.1: Positive Forming Fig.1.2: Negative Forming
Figure 1.3 Bubble Forming Figure 1.4 Plug-Assisted Forming
Heater (Double Sided)
1.3 Heating and Cooling
uniform heating of
sheet is the critical factor in the production of good
quality thermoformed parts. Sandwich type heaters
are recommended as they provide slow even heat
on both sides of the sheet. These may be of the
ceramic or quartz infra-red type. Proportional timers,
together with a controlled heating rate are
recommended, and due care and attention should
be paid to the influence of power variations and air
draughts. Slow heating rates will balance out hot
spots and allow the sheet edges to reach the
required forming temperature. Pre-heating of the
clamping frame to 120°C-130°C is recommended.
Since Lexan® polycarbonate sheet cools rapidly,
it is essential that final control and heating is carried
out on the forming machine itself. Normal sheet
temperatures are in the range of 170°C-225°C for
mechanical and vacuum forming.
Optimum forming conditions depend upon part
design, draw ratio, sheet thickness and the forming
technique employed. However, the following basic
rules still apply:
• Forming at low temperatures gives the best hot
strength, minimum spot thinning, and generally
shorter cycle time.
• Forming at high temperatures gives the lowest
internal stress levels but it increases mould
shrinkage and material thickness may not be
A compromise between the two will usually
produce parts with acceptable properties within
a satisfactory cycle time.
Cooling times are
a number of factors.
These include ambient, forming and mould
temperatures, mould material, cooling system,
part thickness and design geometry. However,
since Lexan® polycarbonate has a relatively high
heat distortion temperature, formed parts can be
removed from the mould at around 125°C. Forced
cooling air or water cooling is not recommended.
Fig. 1.5: Sandwich Heating
1.4 Drape Forming
Drape forming is the simplest of all the
thermoforming techniques. Using either a male or a
female mould, the sheet is heated and allowed to
conform to the shape of the mould under its own
weight or with slight mechanical pressure.
The process involves placing the sheet (without the
masking) and mould in a hot-air circulating oven.
The temperature is raised to the point where the
sheet sags (between 140°C-155°C) and conforms
to the shape of the mould. Both items can then
be removed from the oven and allowed to cool.
Figures 1.6 and 1.7 illustrate the basic steps.
Exceeding the glass transition point of Lexan
materials will result in a decreased optical quality.
Pre-drying is not necessary due to the low
The drape forming process can be a combination of
different methods. These include:
• Shaping under its own weight at a temperature
• Shaping under its own weight with
a slight mechanical pressure. (Temp. 155°C)
• Cold curving into a jig and placing in an oven
at temperatures between 140°C-155°C.
• Cold curving the sheet over a mould, exposure
to a temperature of 150°C and application of
vacuum to obtain a 3D shape.
Cold curving guide-lines must be strictly followed,
to avoid surface cracking of coated products.
Always allow for slow and unforced cooling. When
shaping is carried out under the sheet’s own weight,
use oversized sheets in order to avoid material
shrinkage problems. Alternatively, the sheets can be
placed in the oven with the mould directly outside.
Once the sheet has reached the required
temperature, it should be quickly removed and
allowed to drape itself over the mould. The transition
between the oven and the mould should be handled
very fast since the Lexan® sheet sets-up rapidly once
it has been removed from the oven.
Typical applications include visors and automotive
safety glazing where the Lexan® sheet products
easily meet the demanding quality requirements.
In these types of application the mould needs to
be made from a high gloss material such as steel,
aluminium, or even glass in order to achieve the
necessary optical quality.
Fig. 1.6 / 1.7: Typical Drape Forming Set-up
1.5 Pressure Forming
Pressure forming is basically the same as vacuum
forming. However, during the final forming stage,
compressed air is applied to the positive side of the
mould to force the sheet to conform more closely
to the mould. The result is a component with sharp
features and detailed geometry.
The basic steps are illustrated in Figure 1.8, showing
the pressure chamber mounted above the mould.
Textured surfaces and small radii are typical of the
fine detail which can be achieved with this process.
Fig.1.8: Pressure Forming
Air Pressure Applied
1.6 Twin Sheet Forming
Twin sheet forming is a development of vacuum
forming technology whereby two sheets are formed
at the same time producing an application with
a hollow sealed section. The basic steps in the
process are outlined in Figures 1.9a-d.
Accurate temperature control is an essential
element when using this technique since only one
side of the sheet is heated. The ability to control
heating in individual areas of the sheet is vital.
Photocells also need to be installed to control
sagging and hot-air is often used to keep the
two sheets from touching each other.
As a highly competitive process for producing
hollow sectioned parts, it is particularly suited to
the production of large applications. These typically
include luggage boxes, air ducts, roof domes and
The connection joint between the two parts is
obtained by a combination of melting of the two
materials and the exposed pressure of the moulds.
No additional glue or other adhesive is therefore
necessary. This method can be used to produce
parts consisting of two different materials, colours
and gauges. Fully automatic controlled equipment
is manufactured by Geiss-Germany and Shelley-U.K.
Fig. 1.9a-d: Twin Sheet Forming
with air pressure
1.7 Product Design
The major factors that affect thermoforming product
design fall into four main categories: function,
economics, aesthetics and manufacturing. The first
three of these are largely dependent upon the actual
product. However, within the manufacturing area,
certain limitations are imposed by the nature of the
process. To assist the designer and the producer in
the design process, the main factors affecting
manufacturing are as follows.
the degree of sheet
stretching which, in turn, is a function of the draw
ratio. The draw ratio is the relationship between the
surface area of the thermoformed product (S) and
the available sheet surface inside the clamping
frame (s). (See Figure 1.10.a-b)
Draw ratio (QS) = S / s
= LW + 2LH + 2WH
A similar relationship also exists between the sheet
thickness and the average product thickness.
QT = T / T'
The above recommendations assume an even
thickness distribution throughout the part, with
a more or less symmetrical part geometry. If the
component is long and slender, stretching may be
unidirectional causing excessive thinning in certain
areas. In these cases it is recommended that the
depth of drawing be limited to a value equal to the
smallest width of the product. For vacuum formed
products, a draw ratio of 3:1 is commonly accepted
as a maximum.
In all cases, whether
positive or negative
forming, all geometry
changes should be accompanied by a generous radii.
The basic criterion is that all radii should be at least
equal to the wall thickness. General guidelines are
illustrated in Figure 1.12.
sheet products, in
common with all
thermoplastic materials, shrink upon cooling. It is
therefore essential that all surfaces should be given
adequate draft angles to ensure easy release of the
part from the mould.
For positive moulds a minimum of 2° to 3° is
recommended. However, 5° to 7° is preferable when
part geometry allows. For negative forming 0.5° to
1° is a recommended minimum. If, however, the
mould is textured, a minimum of 2° to 3° is advised.
To avoid stress build-up and difficult removal
through post mould shrinkage (0.8-1%), removal
of the Lexan® part should take place at a part
temperature of 120°C.
Fig. 1.10a-b: Blank Size Determination
Heated area + 2 x 15 mm for clamping
Fig.1.11: Blank size required for forming
1.7 Product Design
Undercuts are possible
with vacuum forming.
However, this often
makes the moulds more complex and the processing
more critical. Undercuts are more common in
negative forming and the simplest method is to use
a loose, removable part in the mould.
A typical example is that of a rim around the
circumference of the part as shown in Figure 1.13.
The loose part can be a ring in two or more pieces
that is removed from the part once it is formed.
This method of producing an undercut is labour
intensive and will obviously increase the cycle time.
For large series, moving parts can be installed into
the mould activated by both pneumatic and hydraulic
Fig. 1.12: Minimum Radii Recommendations Fig. 1.13: Typical Undercut Design
T - Initial sheet thickness
Tmin - Minimum formed thickness
Rp - Radius on positive mould
Rn - Radius on negative mould
Rs - Radius at or near area of high stress
1.8 Moulds and Mould Design
Moulds used for forming Lexan® polycarbonate
sheet products are relatively inexpensive and can
be made from a variety of different materials.
Depending upon the number of production parts
required and their quality, moulds can be made
from wood, plaster of Paris, epoxy resins, metalfilled
polyester or metals. Since they only need to
withstand atmospheric pressure there is little wear
and the flow of the plastic against the mould
surface is minimal.
For prototypes and small series production, wood
can be used. Whilst it has significant advantages in
terms of availability and ease of processing, it does
have disadvantages. Wooden moulds are not
dimensionally stable, particularly at high forming
temperatures, and often with large mouldings the
release pressure can damage the mould surface.
For medium to large production runs, cold curing
epoxies or acrylics, or mould materials filled with
aluminium are recommended. In these cases it may
be necessary to provide cooling channels in the
mould to conduct away the heat build-up. It is
essential for part consistency that the mould
temperature is kept constant during forming.
To allow for post
0.8%-1% should be added to all dimensions.
The evacuation of air
from the mould needs
to achieved as quickly
as possible. However, the vacuum holes should not
be so large as to leave witness marks on the
product after forming.
To avoid marks on the moulding, 0.5-0.75 mm
diameter holes are recommended. The holes can
be recessed on the underside of the mould to
improve evacuation, as illustrated in Figure 1.15a
Figure 1.15b illustrates the spacer and slot design.
Fig.1.14: Typical Vacuum Forming Mould
Fig.1.15a: Vacuum Hole Recommendations
Fig.1.15b: Slot Design Recommendations
Diameter holes 0.5-0.75 mm
back drilling recommended
ø 0.5-0.75 mm
1.9 Domes and Pyramids
Domes are probably the simplest applications made
by the thermoforming process. The technique
involves clamping the edges of the sheet and, after
heating, applying gentle pressure to the underside.
The sheet then stretches like an elastic membrane
to form the dome. With accurate pressure control,
the shape is maintained until the sheet has cooled.
The basic steps in the process are illustrated in
Taking the process one step further, pyramids can
be produced, as shown in Figure 1.17. A simple
wooden skeleton acts as a mould and, after
applying the pressure, the mould is raised and the
sheet allowed to cool on the mould. Contact with
the mould is limited to the edges of the pyramid
and optical quality parts can easily be produced.
Recommended processing temperatures are
Fig. 1.16: Free-blown Domes
Fig.1.17: Typical Pyramid Formation
1.10 Hot Line Bending
Hot line bending is a process involving the
application of heat to the bending zone to enable
thicker sheets and more acute angles to be formed.
The sheet is heated locally along the line of the
bend using a radiation heater, typically an electrical
resistance heater. Depending upon the set-up, the
sheet can be heated from one or two sides. In the
case of single side heating, the sheet needs to be
turned several times to achieve optimum heating.
The protective masking can be left on the sheet
during the hot line bending process.
When the sheet has reached a temperature
155°C-165°C, the heaters are switched off and the
sheet bent to the required angle. For close
tolerances and/or high volume production, the use
of a bending machine equipped with temperature
controlled heaters on both sides is recommended.
A typical set-up is illustrated in Figure 1.18.
Since the process involves localised heating, the
expansion characteristics of the sheet are not
entirely predictable. For sheet widths of up to 1m,
the bend line is usually straight. For sheet widths
greater than 1m however, the line of the bend is
often concave with the outer edges lifting, as shown
in Figure 1.19.
Simple jigs can be constructed to allow the sheet
to cool in position which reduces the degree of
distortion. In all cases it is recommended that
prototype parts are produced to determine the
feasibility of the bending operation.
Fig.1.18:Typical Set-up for Hot Line Bending Fig.1.19: Concave Edge Effect on Wide Sheets
frame with cooling
1.11 Cold Curving
This technique involve placing the sheet under
stress. However, provided certain precautions are
taken, the performance characteristics of the sheet
are not substantially changed.
This technique simply
involves installing a
curved sheet, thereby
placing a slight bending stress across the sheet.
The stress levels in the curve are a function of sheet
thickness and radii, and, provided they do not
exceed a recommended maximum, the stress will
have no influence upon the property performance.
The basic criteria for the minimum radii is 100 times
sheet thickness for uncoated Lexan® sheet products,
175 times sheet thickness for Lexan® Exell® D and 300
times the sheet thickness for Lexan® Margard® FMR.
Table 1.2a outlines the recommended radii for a
range of sheet thicknesses. This technique is not
recommended for Lexan® Margard® MR5E. As
combinations of high stress and unfavourable
chemical conditions can lead to environmental
stress-cracking in the contact areas, it is essential
that all materials are checked for chemical
compatibility prior to installation.
Curving Lexan® CTG or uncoated Lexan® sheet, prior
to the drape forming process, may be done at radii
100 times to the sheet thickness.
Table 1.2a: Minimum Cold Curving Radii
Sheet Lexan® Exell® D Uncoated Lexan®
Thickness Min. Radius Products Min. Rad.
(mm) (mm) (mm)
1.0 - 100
1.5 - 150
2.0 350 200
3.0 525 300
4.0 700 400
5.0 875 500
6.0 1050 600
8.0 1400 800
Table 1.2b: Minimum Cold Curving Radii
Thickness Margard® FMR5E/FLG5*
Cold line bending is
possible, since Lexan
sheet products are very
ductile, even at low temperatures. However, the
process does involve some degree of permanent
plastic deformation and the results are dependent
upon sheet thickness, tooling and the angle of
strain bending. A typical cold line bending
operation is illustrated in Figures 1.20 and 1.21.
Recommendations for Cold Line Bending
• Use sharp tool edges.
• Allow sufficient time for sheet relaxation after
bending (± 1-2 days).
• Do not reduce bending angle during installation
or force the sheet into the desired position.
• Bending operation should be performed quickly
for optimum results.
• Textured sheets should only be bent so that the
textured surface is in compression.
• Due to stress relaxation immediately following
bending, overbending is usually required to
achieve the desired angle.
• Coloured sheets can show tint variations along
the bend following bending.
Smooth and notch-free edges (rounded and/or 45°
tapered edges) of the Lexan® sheet are necessary to
avoid-side cracking during bending. In order to limit
the critical elastic strain, cold line bending is usually
restricted to an angle of 90° or higher, for sheet
thicknesses up to 6 mm.
Thicker Lexan® sheet of 8, 9.5 and 12 mm, can be
cold line bent up to an angle of 135°.
Following bending, residual stresses will remain in
the sheet and will reduce the impact strength of the
material in the area along the bend. This technique
should therefore be limited to less demanding
The mar resistant coating of Lexan® Margard® MR5E
and FMR sheet and the U.V. protected surface of
Lexan® Exell® D sheet may be damaged around the
bent area during the bending process. For more
information on any of the forming techniques
please contact your local GE Plastics Structured
Technical Service Centre.
Fig.1.20 and 1.21: Cold Line Bending
Cold Line Bending
* One side coated FMR5E
2.0 Fabricating Techniques
Fabrication can be defined as the construction,
manufacture or assembly of a number of related
component parts. For Lexan® polycarbonate sheet
products, that could involve the construction
of window panels, the manufacture of a large
motorway sign or the assembly of a safety shield
around a piece of machinery. In one way or another
each of these applications requires fabrication.
The following section discusses the techniques
and processes used to fabricate finished products
from Lexan® polycarbonate sheet and provides
recommendations and advice on how to achieve
the best results.
2.1 Cutting and Sawing
Lexan® polycarbonate sheet products can be cut
and sawn easily and accurately using standard
workshop equipment. Circular saws, band saws,
jig saws and common hacksaws can all be used
successfully. However, certain important guide-lines
should be followed. General guide-lines are listed
below with specific recommendations outlined in
each cutting section.
• The sheet must always be securely clamped
to avoid undesirable vibration and rough cut
• All tools should be set for cutting plastics with
fine toothed panel blades.
• The protective masking should be left on the
sheet to prevent scratching and other surface
• When finishing the edges of all Lexan sheet
products should be clean and free of notches.
• If possible swarf and dust build-up should be
blown away with a compressed air supply.
This type of cutting
operation is the most
common and, whilst
cutting speeds and feeds are not so critical as with
other thermoplastics, it is important to follow the
• Tungsten carbide tipped saw blades are
preferred with alternative teeth bevelled at 45°
on both sides to improve cutting and reduce
• Always use a low feed to get a clean cut.
• Always start cutting with the blade at full speed.
• For single sheets less than 3 mm thick, bandsaws
or jig saws are preferred to circular saws.
These can be of the
conventional vertical type
or the specially developed
horizontal type suitable for plastic sheet materials.
In both cases it is vital that the sheet is adequately
supported and clamped during the cutting operation.
The saw guides should be as close to the sheet as
possible to reduce blade twist and off-line cutting.
The most important
consideration with this
type of cutting is
support and clamping,
particularly with the use of a jig saw. Blades having
a tooth spacing of 2-2.5 mm are ideal with the
emphasis upon low cutting feeds.
Circular Saw Bandsaw
Clearance Angle 20°-30° 20°-30°
Rake Angle 5-15° 0-5°
Rotation Speed 1800-2400 m/min 600-1000 m/min
Tooth Spacing 9-15 mm 1.5-4 mm
Table 2.1: Cutting and Sawing Recommendations
Fig. 2.2: Typical Tungsten Carbide Tipped Circular Saw
suitable for Lexan® sheet products
A tungsten carbide tipped
saw blade suitable for cutting
Details of a typical saw:
Diameter 400 mm
Tooth spacing 12 mm
Gullet depth 11 mm
Shaft speed 4000 rev/min
Alternate teeth bevelled
at 45° on both sides
angle of hook
3° positive to
Circular Saws 3° negative
Standard high speed steel twist drills or drills with
an angular wedged bit can be used for drilling
Lexan® sheet products. Carbide-tipped drills can also
be used since they retain their sharp cutting edge.
The most important factor to consider when drilling
Lexan® sheet products is the heat generated during
the actual process. In order to produce a clean, wellfinished
hole that is stress-free, the heat generated
must be kept to an absolute minimum. By following
a few basic guide-lines, clean, stress-free holes can
easily be produced.
• The drill hole must be cleared frequently to
prevent swarf build-up and excessive frictional
• The drill must be raised from the hole frequently
and cooled with compressed air.
• The sheet or product must be adequately
clamped and supported to reduce vibration and
ensure a correctly sized hole.
• Holes should not be drilled closer to the edge of
the sheet than 1-1.5 times the diameter of the
• All holes must be larger than the bolt, screw or
fixing to allow for thermal expansion and
• For long production runs the use of carbidetipped
twist drills is recommended.
Drilling feeds and speeds are outlined in Table 2.2
with the various drill configurations in Figures
2.3 to 2.6.
Table 2.2: Drilling Recommendations
Hole Diameter Speed (rev/min) Feed (mm/min)
3 1750 125
6 1500 100
9 1000 75
12 650 50
18 350 25
Recommended drill angles:
Clearance Angle α 15°
Rake Angle λ 0°-5°
Included Tip Angle ϕ 120°-160°
Helix Angle β 30°
Fig. 2.3 and 2.4: Typical Drill Configuration
parallel to surface to
Fig. 2.6: Drill suitable for thin sheet
Fig. 2.5: Drill suitable for large holes
wrong right way
Lexan® sheet products can be machined using
conventional milling machines fitted with standard
high speed knife cutting tools.
Once again the importance of suitable clamping
cannot be over-emphasised. Mechanical jigs and
fixtures, or vacuum chucks provide a suitable
Table 2.3 outlines appropriate cutting speeds and
feeds with a typical cutting tool illustrated in Figure
2.7. Forced-air cooling enables higher cutting rates.
However, care should be taken not to over-heat the
material. The use of cutting fluids to lubricate or
cool the sheet is not recommended.
Computerised trimming is a fully automatic milling
process. It is extremely accurate and operates
horizontally as well as vertically. The use of a
vacuum-operated jig avoids vibration of the part
ensuring a smooth cut. Standard high speed, twosided
cutting routers with tungsten carbide tips are
recommended, with a cutting speed of
approximately 250 m/min at 25.000/30.000 RPM at a
sheet thickness of 4 mm.
Table 2.3: Milling Recommendations
Clearance Angle 5°-10°
Rake Angle 0°-10°
Cutting Speed 100-500 m/min
Cutting Feed 0.1-0.5 mm/rev.
Fig. 2.7: Typical Milling Cutter
2.4 Mechanical Fastening Devices
With a few exceptions, all mechanical assembly
techniques involve some form of additional
fastening device. The choice of device is often
dependent upon the nature of the fastening
required. Whilst rivets tend to be permanent,
screws and nuts can be made detachable and some
of the spring clips types can be either permanent or
There are many different types of mechanical
fastening system which can be used successfully to
assemble plastic sheet components.
Within the limitations of this publication only a small
number can be discussed.
For simplicity they are divided into three groups:
• Screws, nuts and bolts
• Spring clips and other fastening devices
Two important factors need to be considered with
all these fastening systems. Firstly, allowance needs
to be made for thermal expansion and contraction.
All holes, slots and cut-outs must be machined
over-size to allow for the dimensional changes as
a result of temperature changes. Secondly, the
distribution of tightening torque should be equal.
With the aid of compatible rubber washers and
large screw and rivet heads, the tightening torque
should be spread over as wide an area as possible
and should not be excessive.
Material m/m°C x 10-5
Lexan sheet 6.7
Glass 0.7 - 0.9
Aluminium 21. - 2.3
Steel 1.2 - 1.5
2.4.1 Screws, Nuts and Bolts
The majority of these
screws are made from
steel, but other metals
and alloys are used for specialised applications.
Several examples of this type of fastening system
are shown on this page. Figures 2.9 and 2.10
illustrate sheet fastening devices known as ‘blind
screw’ and ‘blind nut’ anchors.
Self-tapping screws are
widely used within the
Basically they produce their own thread as they
are driven into a hole and may be considered
whenever an assembly is likely to be dismantled
and re-assembled. Whilst the majority of these
screws are designed for plastic mouldings, with the
aid of spring clips and washers they can be adapted
for sheet applications. Figures 2.11 to 2.14 show
some typical fastening systems.
If the application calls
for a screwed assembly,
it is vitally important
that the following recommendations are considered.
• Do not use countersunk head screws as the
‘wedging’ action of the countersunk head causes
excessive hoop stress on the sheet. This can
lead to part failure.
• Be sure that all oil, grease and other coatings
are removed from the screws before assembly.
Certain oils and greases can cause
environmental stress cracking.
Fig. 2.9 and 2.10: Blind Nut and Blind Screw Anchor
Fig. 2.11-2.14: Other Typical Fastening Systems
The stems washer is free-spinning
and preassembled to the screw.
A controlled spring load is applied
to the plastic.
diecast or plastic of
Available in a wide variety
of thread forms.
Available with a variety
of point styles.
Available in a variety of
Threads inside washer
provide resistance to
stripping in thin sheet
The standoff of the stems
washer carries clamp load.
Metal washer with
2.4.2 Riveting Systems
Whilst riveting is a popular and effective assembly
technique, certain guide-lines should always be
followed when considering this type of assembly
method. Riveting can induce both radial and
compressive stresses in the plastic sheet and
precautions should be taken to distribute these
forces over as wide an area as possible.
In a plastic-to-plastic assembly a metal back-up
washer with laminated rubber is recommended to
reduce the compressive stresses. If the diameter of
the rivet with a rubber washer is slightly bigger than
the hole diameter, then the hoop stresses will be
transmitted to the washer rather than the plastic
sheet. For plastic-to-metal joints, the head of the
rivet with a rubber washer should be against the
plastic, and the hole in the sheet should be large
enough to allow for thermal movement. Holesize is
1.5 x expanded rivet diameter.
Rivet diameters should be as large as possible and
spacing should be between 5-10 times their diameter.
GE Plastics Structured Products recommends the
use of aluminium, brass and copper rivets.
There are several different types of riveting system,
however, the most popular is the ‘pop-rivet’. This
type of rivet provides the means to assemble two
components together with access restricted to one
side only. Figures 2.15 and 2.16 illustrate typical
Fig. 2.16: Typical Pop-Rivet Assembly
Metal back-up washer
with laminated rubber
rubber washer Oversized hole
2.5 Miscellaneous Fabricating Techniques
Many different techniques are used to cut and
fabricate Lexan® polycarbonate sheet products.
These techniques include:
• Laser Cutting
• Water Jet Cutting
Whilst these techniques are used, they are not
recommended since they either induce unnecessary
stress on the finished part or result in a poor surface
With both shearing and punching the process
involves a shearing action with a guillotine or
a punch which tends to leave a roughly cut surface.
This surface often contains micro-cracks which may
lead to premature failure.
Tapping is possible in Lexan® polycarbonate. However,
the process is usually restricted to moulded parts.
Self-tapping screws or machine screws require a
minimum depth to achieve the necessary holding
power and sheet products do not have the
Laser cutting of GE sheet products is not
recommended due to the following disadvantages:
• Rough cutting edges
• Carbon deposition on cutting edges
• Stress level increase in thick sheets
With water jet cutting, the following considerations
should be taken into account:
• No stress at any sheet thickness
• Cutting edge requires further finishing
• Limited cutting speed
• Expensive equipment
For more information regarding fabricating
techniques or any assembly process please contact
your nearest GE Structured Products Technical
3.0 Finishing, Decorating and Cleaning
As the final steps in an assembly process, finishing
and decorating often takes up the most time and
require the most significant input. The operations
are usually performed on the surface appearance of
the assembly. Attention to detail is essential if the
components are to perform their required function.
In this particular area, it is also vital that appropriate
consideration is given to the chemicals coming into
contact with the Lexan® polycarbonate sheet product.
Whether the chemical is a paint system, an adhesive
system or a cleaning agent, it is essential that it
is compatible with the Lexan® sheet product.
GE Structured Products has a comprehensive
database listing compatible systems, and advice
and support is always available from all
GE Structured Products Technical Service Centres.
3.1 Chemical Resistance
The chemical resistance of a thermoplastic is
dependent upon five major factors:
1. Stress level in the application
3. Exposure time
4. Chemical concentration
5. Type of chemical involved
Lexan® polycarbonate resin has a good chemical
resistance, at room temperature, to a variety of
dilute organic and inorganic acids. Water, vegetable
oils, solutions of neutral salts, aliphatic hydrocarbons
and alcohols are also included in this category.
When a thermoplastic is attacked by a chemical
it usually takes one of three forms. In the first case
the chemical is absorbed into the plastic,
and plasticisation and/or crystallisation occurs.
The visible signs of this type of attack are
swelling or surface whitening. Lexan® polycarbonate
is affected in this way by partial solvents such as
low molecular weight aldehydes and ethers,
ketones, esters, aromatic hydrocarbons and
In addition, chemical attack ranging from partial to
complete destruction of Lexan® polycarbonate
occurs in contact with alkalines, alkali salts, amines
and high ozone concentrations.
The third type of attack is often the most difficult
to predict since environmental conditions dictate
whether or not the plastic will be affected.
Combinations of certain environments, coupled
with stress and/or strain upon the material, cause
stress cracking or crazing of the polycarbonate.
Crazing can be induced at moderate to high stress
levels by low molecular weight hydrocarbons.
Products such as acetone and xylene may cause
stress cracking even at very low stress levels and
should therefore be avoided.
Taking into account the complexity of chemical
compatibility, all chemicals which come into contact
with polycarbonate should be tested. For sheet
products the most common contact is with sealants,
gaskets and the various cleaning media.
Chemical compatibility testing, Figure 3.1, is an
on-going process at GE Structured Products and
many standard products have already been tested.
A complete list of recommended cleaners, gaskets
and sealants is available upon request. However, a
shortened list of some of the more common
compounds is outlined in the respective sections in
In case of doubt about any aspect of chemical
compatibility of Lexan® polycarbonate sheet always
consult your nearest
GE Structured Products Technical Service Centre for
The ‘mar’ resistant
coating of Lexan®
an additional benefit
in terms of chemical resistance. The proprietary
coating is resistant to a range of chemical agents
that under normal circumstances are detrimental
to Lexan® polycarbonate.
Table 3.1 outlines the results of a series of tests
carried out on coated and uncoated
Lexan® polycarbonate sheet.
The tests also included an evaluation of impact
resistance and in each case the application of
the chemical showed no significant effect upon
the impact resistance of the Lexan® Margard®.
The tests were conducted on 3 mm samples
with an exposure time of 5 minutes, at room
temperature and stress-free.
Fig. 3.1: Lexan® sheet chemical compatibility summary
Chemical class Effects
Acids (Mineral) No effect under most conditions of
concentration and temperature.
Alcohols Generally compatible.
Alkalis Acceptable at low concentration and
temperature. Higher concentrations and
temperatures result in etching and attack
as evidenced by decomposition.
Aliphatic Hydrocarbons Generally compatible.
Amines Surface crystallisation and chemical attack.
Aromatic Hydrocarbons Solvents and severe stress-cracking agents.
Detergents and Cleaners Mild soap solutions are compatible.
Strongly alkaline ammonia materials should
Esters Cause severe crystallisation. Partial solvents.
Fruit Juices and Soft Drinks Compatible at low stress levels.
Some concentrates not recommended.
Gasoline Not compatible at elevated temperatures and
Greases and Oils Pure petroleum types generally compatible.
Many additives used with them are not,
thus materials containing additives should
Halogenated Hydrocarbons Solvents and severe stress-cracking agents.
Ketones Cause severe crystallisation and stresscracking.
Silicone Oils and Greases Generally compatible up to 80°C.
Table 3.1: Lexan® Margard® Chemical Resistance Tests
W = surface whitening
S = surface dissolution
Chemicals Uncoated PC Lexan® Margard® MR5E
Toluene W/S ok
Acetone S ok
Methylethylketone S ok
Dichloromethane W/S ok
Sulphuric acid (95-97%) ok ok
Hydrochloric acid (32%) ok ok
Ammonia (25%) ok ok
Thinner (Sikkens 1-2-3) W/S ok
Super Gasoline (Esso) W/S ok
Diesel Fuel (Esso) ok ok
Fuel C ok ok
Hairspray ok ok
Chemical Resistance of
Lexan® Margard® MR5E
Be it simple or complex, decorative or functional,
hand-controlled or automatic, painting Lexan® sheet
products offers the designer the freedom to create
a dramatic effect in a sign or a simple colour code
Provided certain basic recommendations are
followed, most techniques used to apply paint
to wood, metal, building materials and other
plastics can be used for Lexan® sheet products.
The important factor is once again one of compatibility.
Only approved paint systems should be used. Some
paint and thinner systems are not compatible with
Lexan® sheet products and can cause stress
cracking and a reduction in impact performance.
Paint systems for Lexan sheet should be flexible.
Combinations of flexible primers and hard top coats
could also work. Any paint system should be
flexible at subzero temperatures.
In view of adhesion problems, painting is not
recommended for decorating the coated sides
of either Lexan® Margard® MR5E or
Lexan® Margard® FMR.
• Clean the sheet and remove static with a damp
chamois cloth or ionised air treatment.
• Avoid too high a delivery rate and too heavy
a wet coat thickness.
• Allow adequate drying before applying spray
mask to painted areas.
• Do not expose painted faces to a low
temperature and high humidity environment
• Use dry air in all compressed air lines.
Drain water taps frequently.
• Paint solvents should be evaporated from the
paint surface as quickly as possible by
providing appropriate air circulation.
• Follow recommended machining and trimming
practices for finishing post-decorated faces.
Table 3.2: Painting Systems for Uncoated Lexan® sheet
Supplier Paints Thinner Comments
AKZO Coatings Autocryl - 2K Acrylic
01-69004 06-302007 Primer/2K/PUR
Class 45 Top coat/2K/PUR
Diegel PA 21 24896 1K Flex. acrylic
Schaepman C1 F57 VOA 462 Acrylic
C1 W28 Water Acrylic/water based
C4 P212 VOA421/H4P4 2K Acrylic
Herberts R 47633 - 2K Primer
41605 11098 Basecoat BMW metallic
R 4790 - 2K Clearcoat
R 4780 - 2K One layer system
Becker TH 130 NT19 2K Top coat
DJ-331-5176 ET-134 1K Primer (flexible)
TC 132 - 2K Clear coat
HSH Interplan 1000 1K Water-based
Morton L446 U987 1K Acrylic System
NB. For information regarding application techniques and property values please contact the relevant paint supplier.
3.2.2 Screen Printing
Silk-screen printing is a well-established process
that offers a wide variety of options for a decorative
finish. However, in most cases the printing must be
accomplished prior to installation, since the process
is basically a horizontal one and is generally
restricted to small-to-medium part sizes.
The process involves forcing viscous inks through
a very fine, thin screen that is treated in such a way
as to allow the ink only through to the patterned
area. Special inks are required that are formulated
so that they will pass through the mesh, while being
sufficiently viscous to prevent run-out.
This type of finishing operation is often used in the
sign industry and a wide variety of screen printing
inks and thinners are available.
Once again the importance of chemical compatibility
cannot be over-emphasised and only paints and
thinners recommended for use with Lexan® sheet
products should be used.
This process is not suitable for decorating
Lexan® Margard® MR5E, Lexan® Margard® FMR5E
and Lexan® Margard® MRA3.
The one side coated Margard® grades such as
Lexan® Margard® HLG5, Lexan® Margard® FLG5 and
Lexan® Margard® HLGA3 can be screenprinted at the
Sericol Seritec TH Polyplast PY
Gibbon Inks & Coating Ltd. Malercryl
öll Jet 200/Thermo-Jet /
Marabu Marastar SR/Maraplast D
Table 3.3: Silk Screen Inks for Uncoated Lexan® sheet
• Use only approved paints and thinners.
• Do not mix different paints and inks.
• Do not substitute spray thinners for screen
• Do not add solvents such as toluene, xylene,
cellulose acetate, methylethylketones or other
related chemicals to the inks.
• Use water-moistened chamois or soft cloths to
avoid abrasion or scratching during cleaning
prior to printing.
• Use the correct colour of paint to achieve
• Provide good air circulation and ventilation
3.2.3 Anti-static Treatment
As common with all insulating materials, Lexan®
polycarbonate sheet tends to build up a static
charge. It is often necessary to clean and discharge
the surface prior to painting or screen printing.
Wiping the sheet with a damp chamois or applying
de-ionised air to the surface is often all that is
required. Another effective method in minimising
static charge build-up is control of the relative
humidity: the higher the relative humidity, the lower
the static charge build-up will be. Relative humidity
preferably should always be above 60%.
3.3 Adhesives and Sealants
The use of adhesives to bond dissimilar materials is
now universal. Over the past twenty years polymer
technologists have developed adhesives with a
wide range of properties and application profiles.
Adhesion technology has become a branch of the
plastics industry in its own right, offering a
technique that is one of the most efficient, effective
and economical methods of joining plastic
components to themselves and to other materials.
However, it is a technology that often causes the
most problems. Whilst some adhesives/sealants
form a flexible bond, others form a rigid bond.
Some are capable of filling gaps, whilst others
are for close contact. Some can withstand high
temperatures, whilst others cannot. The choice
of adhesive types is vast, as are the applications
areas. It is vitally important, therefore, to select the
adhesive carefully, ensuring its compatibility with
the materials being used and the working
The importance of chemical compatibility was
discussed in Section 3.1 and adhesive selection and
testing is an ongoing process at GE Structured Products.
A comprehensive data-base of suitable adhesives is
available and in all cases it is strongly recommended
that all adhesives are checked for compatibility
Table 3.5 presents an overview of some of the initial
criteria used to select an adhesive and Table 3.6
provides a list of compatible adhesives indicating
generic types, trade names and application areas.
Figures 3.2 and 3.3 outline some typical joint
configurations and can be a guide in determining
the right joint geometry for an application.
3.3 Adhesives and Sealants
Table 3.6: Adhesive Selection Chart for Lexan® polycarbonate sheet products
* These products are compatible with coated Lexan® sheet products, Lexan® Exell® D, Lexan® Margard®. Other silicone sealants may contain AMINO or BENZAMID curing agents which are not compatible
with Lexan® sheet and may cause stress corrosion. Consult the manufacturer before using other silicone sealants.
The listed adhesives, adhesive tapes and sealants have only been tested under normal atmospheric conditions to determine their compatibility and adhesion performance with Lexan® sheet products.
The actual choice of adhesive will depend upon the design of the joint, the circumstances under which the joint will be used and the prevailing environmental conditions. In all cases the adhesive type
should be fully tested under exact conditions to determine complete compatibility and performance.
Adhesive Product Joins Lexan® 1/2 Part Supplier Comments
Type Name sheet to: System
Epoxy Scotch Weld Metals, Plastics, 2 part 3M Company Fast curing, epoxy
DP 110 Rubbers with high shear strength
Epoxy Scotch Weld Plastics 2 part 3M Company Epoxy
DP 190 with high shear strength
Polyurethane Bison PUR Plastics, Metals, 2 part Perfecta
Polyurethane Plio-grip 6000 Plastics, Metals, 2 part Good Year Flexible, very short
Wood pot life (10min.)
Hot Melt Jet Melt 3736 Plastics, Wood 1 part 3M Company Good heat resistance.
Jet Melt 3764 Plastics, Wood Oil and water resistant
Hot Melt Macromelt XS6335 Plastics, Metal, 1 part Henkel Clear
Silicone *Silpruf® SCS2000 Lexan® uncoated 1 part GE Bayer Silicones Excellent adhesion,
Lexan® Exell® D, UV and weather
Lexan® Margard® MR5E + FMR resistant, flexible.
Silicone *SEA 210 Plastics, Glass, 2 part GE Bayer Silicones Fast Cure
Silicone Multi Sil Lexan® uncoated 1 part GE Bayer Silicones Excellent adhesion,
Lexan® Exell® D, UV and weather
Lexan® Margard® MR5E + FMR resistant, flexible.
Tapes Scotchtape Plastics, Glass, - 3M Company Double sided
VHB Range Metals Pressure sensitive
Tapes Fas Tape Metals/ Plastic - Fasson Double coated
Tapes PS-18 - - Velcro Hook and loop tape
Tapes SR 321 - - Multifoil PE Foam, 2 sides
SW 321 PE Foam, 2 sides
Tapes 5669 - - Sellotape PE Foam, 2 sides
Table 3.5: Adhesive Groups and Property Profile
Impact Moisture Number of Temperature Gap Filling
Behaviour Behaviour Components Limits (°C)
Epoxy Bad Very Good 1 or 2 200 + +
Polyurethane Very Good Good 1 or 2 140 +
Hot Melt Good Good 1 60 +/-
Silicone Excellent Very good 1 or 2 250 +
3.3 Adhesives and Sealants
The double butt lap joint provides maximum
uniform stress distribution in the load bearing area.
The joggle lap joint allows a more uniform stress
distribution than a single tapered lap joint.
A tapered single lap joint is more efficient than a
single lap joint, allowing for bending of the joint
edge under stress.
A double lap joint allows for greater rigidity than a
single lap joint.
A simple lap joint could create cleavage and peel
stress under loading, particularly in bonding thin
A round lap joint can be used to add rigidity and
strength to an assembly and minimise the
deflection of flat sheets.
Double scarf lap joints have better resistance to
bending forces than double butt joints.
Rounded tongue and groove joints are self-aligning
and can act as an overflow reservoir for adhesives.
Landed scarf tongue and groove joints function as
control stops for adhesive line thickness.
Recessed tongue and groove joints improve
cleavage resistance of straight butt end joints.
Straight butt end joints are not usually
recommended for most types of applications.
Fig. 3.3: Joint Design Configurations
1. Rounded Tongue
and Groove Joint
2. Landed Scarf Tongue
3. Tongue and Groove Joint
4. Butt Joint
Fig. 3.2: Joint Design Configurations
1. Double Butt Lap Joint
2. Joggle Lap Joint
3. Tapered Single Lap Joint
4. Double Lap Joint
5. Simple Lap Joint
6. Round Lap Joint
7. Double Scarf Lap Joint
3.4 Cleaning Recommendations
Periodic cleaning of all Lexan® polycarbonate sheet
products can be accomplished easily and without
the need for specialised cleaning agents. However,
as is the case with all thermoplastic materials,
certain chemicals can cause structural as well
surface damage and precautions need to be taken
to avoid any aggressive cleaning agents.
The basic cleaning agent for all Lexan® polycarbonate
products is a solution of lukewarm water with mild
soap or household detergent, using a soft cloth or
sponge to loosen any dirt and grime.
All surfaces are then rinsed with cold water and
dried with a soft cloth to prevent water spotting.
However, in some cases this may not be sufficient
and certain solvent cleaners may be needed to
remove stubborn stains, graffiti etc. In these cases
the following list of cleaning agents are approved
for use at room temperature:
• Methyl alcohol
• Ethyl alcohol
• Butyl alcohol
• Isopropyl alcohol
• White spirit
• Petroleum ether (BP 65°)
Should it be necessary
prior to forming to
clean Lexan® sheet,
it is recommended that the dust is blown off with
an ionising air gun or the sheet is wiped with a soft
cloth dipped in water or a mixture of isopropanol
Recommendations for Cleaning
The unique surface of Lexan® Margard® sheet
provides superior protection against chemical
attack. Even graffiti such as spray paint are easily
and quickly removed. Although Lexan® Margard®
has a mar resistant coating, the use of abrasive
cleaners and/or sharp cleaning instruments that
may damage or scratch the coating should be
The recommended cleaning procedure for the
removal of graffiti etc. is as follows:
• Paints, marking pens, inks, lipstick, etc, use
graffiti remover. (See Table 3.7).
• Labels, stickers, use kerosene or white spirit.
• Final wash with warm soap solution, followed
by rinse with clean water.
Points to remember
• Don’t use abrasive or highly alkaline cleaners.
• Never scrape the sheet with squeegees, razor
blades or other sharp instruments.
• Don’t clean Lexan® sheet products in the hot
sun or at elevated temperatures as this can
lead to staining.
lots to look through, eh?