Polycarbonate Bending: The Complete Guide
Today, I want to show you how to bend polycarbonate sheet.
For newbies, it can be a difficult process – but trust me, I will make it easier for you.
In fact, once you have the right tools (all which you will learn here), you can do-it-yourself.
So, in this guide, I will teach you the basic steps, tools and precautions you must follow when bending polycarbonate sheet.
By the end of this guide, you will definitely be an expert in polycarbonate sheet bending process.
Here’s what you should do:
Polycarbonate Sheet Basics
Polycarbonates are carbon polymers with organic groups attached in a long, continuous chain.
They are thermoplastic, i.e. at specific temperatures, they soften and can be molded into various shapes.
Polycarbonates can withstand large impact forces and generally do not shatter.
They are naturally transparent, and amorphous in form.
During the production process, additives can be added to alter its properties.
The additives may reduce the transparency of the polycarbonate, increase its fire resistance ability, or make its surface less susceptible to scratching.
They can also be transformed into various forms, the most common being as sheets, rods or tubes.
This article shall focus on polycarbonate sheets, and the various techniques and equipment that are used to mold them into the desired final product.
Polycarbonate Sheet Bending Machine and Equipment
Since there are different sheet bending techniques, it follows that the machine and equipment to be used are different.
Let’s explore some of these techniques and equipment.
Main Techniques of Bending Polycarbonate Sheet
Some of the most common technique include:
1. Cold Line Bending
Even without heating, a polycarbonate sheet can be bent.
However, there are a couple of factors to be considered, and recommendations to follow for the best results.
These factors include sheet thickness, angle of bending and the tooling.
It’s thus recommended that:
- The operator uses tools with sharp edges
- The operator allows plenty of time after bending, say one or two days.
- The operator doesn’t force the sheet into the desired final form or reduce the bending angle during installation.
- Overbending may be required, to overcome the effect of springback, the attempt by the bent polycarbonate to revert to its original position.
- The operator attempts cold line bending on a sample piece before committing a larger piece.
Once satisfied with the test run on the sample piece, cut the sheet to its pre-bending size.
Then give the edges a smooth finish, to eliminate the possibility of a crack forming from the bend line.
The sheet is then bent rapidly, with the protective film still in place in most cases.
Now, to ensure that the bend maintains the desired angle after springback, the sheet should be bent 20-40o larger than the desired angle.
NOTE:Cold line bends usually do not exceed 90o, as this may exceed its elastic limit.
This technique isn’t recommended for polycarbonate variants that are hard coated, or even UV-protected.
It is because this type of bending will likely weaken the efficacy of such additives along the bend line.
Polycarbonate sheet cold line bending
Similarly, since residual stress will remain in the polycarbonate sheet, this process should be limited to producing items that will be used in low impact circumstances.
2. Cold Curving
As could be inferred from its name, cold curving is the process of bending the whole polycarbonate sheet to create a dome or an arch.
Curved polycarbonate sheet on swimming pool enclosure
In the formation of such a shape, the significance of the cold forming radius comes into play.
This gives the minimum radius that should be attained if the final form is to remain in place.
It is given by multiplying the thickness of the sheet by 100, i.e.
Minimum cold forming radius = sheet thickness x 100
This radius is applied in all cold bending procedures
In the image above, the values are all in inches, but the principle often holds even if the units are in mm.
That said, it is important to appreciate that this is generally for plain polycarbonate sheets as variants with additives may have different radii.
In effect, up to a limit, the harder the variant of polycarbonate, the greater its minimum cold forming radius has to be.
So, while in the plain polycarbonate the thickness is multiplied by 100; there are types of polycarbonate sheet in which the minimum radius is 300 times the sheet’s thickness.
3. Break Bending Polycarbonate
Brake bending is a technique that uses a device known as a press brake to alter the sheet into the desired final form.
The press brake has a long history of use in shaping sheet metal.
And, the relative flexibility of polycarbonate sheets, which can be bent to a limit without breaking, has allowed the transfer of this technology in the processing of polycarbonates.
In principle, brake bending occurs when a sheet of polycarbonate is held between two pieces of metal, called a punch/ram.
It is basically a mobile, and a die, which is often immobile, and the mobile section moves the sheet to force it into the desired form.
There are different types of press brakes in the market, divide broadly into manual press brakes, CNC press brakes and hydraulic press brake.
· Manual Press Brake
As can be inferred from the name, a manual press brake operates by the application of physical force by the operator.
The operator uses levers to move the equivalent of the punch, to cause the sheet to bend.
This type of brake requires the operator to manually adjust all the critical parameters of the brake, such as desired bending size, and angle.
The operator is also the one to clamp the sheet in place, before attempting to bend the sheet.
Manual brakes range from small portable brakes barely exceeding an arm’s length, to industrial variants weighing hundreds of kilograms.
Consequently, there are manual press brakes that are purely mechanical, while there are others that require some electricity to operate.
· Hydraulic Press Brake
This is an electric press in which hydraulic force is used to move a ram, positioned above the polycarbonate.
In the hydraulic press brake, there is a top die with a predesigned groove.
To bend polycarbonate, the press moves downwards at a predetermined force to result in the bending of the sheet.
Hydraulic press brakes can be further classified into several subgroups depending on the direction of its ram and die.
Or, the coordination of the hydraulic system resulting in sub-types such as hybrid press and torsion synchronization brakes.
Press break bending graph
· CNC Press Brakes
These are brakes where the degree of bending is controlled by a computer numerical controlled system.
They ensure a high degree of accuracy, as the punch and die system can be moved on several axes beyond up, down and sideways.
CNC brakes are easily programmed via a screen attached to the system.
Through these screens the operator can run simulations to see the final product in some models.
As the system monitors input and output simultaneously, it is possible to fine tune the process in real time.
Schematics of brake press
The above image shows the schematics of a press brake.
Hydraulic, and CNC press brakes generally have this outlay, with some components, such as a CNC controller, accounting for the difference in classification.
In brake bending, it is recommended that one performs the operation very fast.
Of course, this is to compensate for springback – you should overbend the sheet.
It is not advised to bend flame-retardant variants of polycarbonate sheets as this may compromise their quality.
4. Hot Line Bending
Taking advantage of the thermoplastic nature of polycarbonates, hot line bending involves:
- Softening a length of the sheet using a narrow, heated strip, such as a hot wire or an electric heater.
- The polycarbonate sheet can be either be heated on one side or on both sides, depending on factors such as its thickness
- If the polycarbonate sheet is thicker than 3mm, it is recommended that double-sided heating is used
Furthermore, if the sheet is thicker than 6mm, it is advisable to remove the protective film protecting the polycarbonate sheet.
Doing so at least along the bend line, prevents it from melting onto the polycarbonate sheet.
Usually, at between 155oC and 165oC, the heated region becomes pliable enough to be bent to the desired angle.
A double-sided heating setup for hot line bending
A consequence of the localized heating used in hot line bending is that the sheet may expand and distort as it cools.
Consequently, it is always advised that one test the efficacy of the hot line bending set up with a small sample of polycarbonate sheet before committing the larger sheet.
With this sample so transformed, one can also check whether the sheet’s integrity has been compromised by the bend.
This is done by hitting the bend line with a hammer; if it breaks, then the set temperature was too low.
Therefore, you should adjust it upwards for the next sample, until one finds the sheet maintains its integrity despite such impacts.
One way of reducing stress-induced cracking on the bend line is by the process of annealing.
It is the process of heating the sheet to a specific temperature and then letting it cool down slowly at a predetermined rate.
This alters its properties, such as hardness, making it more flexible.
It is also possible that, when using metal contact heaters, these may stick to the surface and cause unwanted scratches on the polycarbonate sheet.
In any case, one shouldn’t allow the wire to contact the polycarbonate sheet.
Furthermore, when applied to polycarbonate sheets larger than 1 meter, hot line bending may result in the sheet curving into an unplanned concave design.
It is normally due to the outer edges lifting off.
It is thus advisable to create a simple jig that will allow the sheet to cool in place with a minimal chance of this distortion.
Polycarbonate sheet end curving upwards
Caution should be taken to avoid the risk of the sheet’s end curving up as seen in this diagram
To avoid such distortions, longer sheets need to be preheated (the whole sheet); 200oF (93.3oC).
It works for some commercial versions.
However, it is important review the product guide to find the ideal preheating temperature for the polycarbonate sheet in your possession.
As the thickness of the sheet increases, it may be prudent to fit the heating element within a V-groove.
This is because polycarbonate tends to absorb moisture and this setup allows moisture to escape without creating bubbles within the sheet.
While at the same time allowing the formation of a sharp angle.
5. Bending Polycarbonate Sheet with a Heat Gun
Principally, using a heat gun to bend a polycarbonate sheet is the same as hot line bending.
Only that the operator is now forced to move the heat gun constantly along the bend line and flip over the polycarbonate sheet to ensure both sides get heated.
You can see this in the video below:
In this case then, one simply needs a vice/clamp to hold the sheet in place and a heat gun.
The problem with using a heat gun however is that, because the process is so manual, the possibility of uneven heating is very likely.
In turn, this means bubbles are likely to form along the bend line, and this will affect the durability of the polycarbonate at the bend site.
6. Forming a Bend by Thermoforming Polycarbonate
Thermoforming uses heat and a combination of pressure or molds to transform a sheet of polycarbonate into a desired final form.
While the other techniques are mostly limited to making linear bends, with thermoforming, complex 3D designs are possible.
It is because the whole sheet is open to alteration within a controlled environment.
- Vacuum forming
- Pressure forming
- Plug-assisted forming
- Twin sheet forming
- Drape forming.
As aforementioned, polycarbonate tends to absorb moisture, moisture which may bubble out during the thermoforming process.
It is thus imperative that, prior to thermoforming, the polycarbonate sheet must be pre-dried.
To commence pre-drying, remove the protective layer on polycarbonate sheets and hang the polycarbonate sheets vertically or place within a rack an air-circulating oven.
The sheets should be 2.5 cm apart, to allow air to move in between; without this critical spacing, the sheets will not dry.
The oven should be set to a temperature of at least 120oC and shouldn’t exceed 125oC, as beyond those temperatures, the sheets may become warped.
Commercially available polycarbonate sheets do come with a guide that includes pre-drying durations.
This duration is hugely dependent upon the thickness of the sheet.
And, the thicker the sheet, the longer its pre-drying time is.
For instance, a Lexan™ sheet 1 mm thick will require a pre-drying time of 2 hours, while a sheet 6 mm thick will require 12 hours.
If for some reason one doesn’t have such a guide, the alternative means to pre-drying requires one to take about three sample pieces of polycarbonate sheet, and dry them in the oven.
After about 2 hours, take out a piece, heat it to its forming temperature, and see if bubbles appear.
If they do, the pieces still need more pre-drying time.
Repeat until the sample no longer forms bubbles.
Either way, it is important to change the circulating air in the oven about six times every hour to ensure the removal of water vapor.
Because the sheet will begin picking up moisture shortly after it is removed from the drying oven, it is advisable to process it immediately.
ii. Recommendations for Forming Process
When the sheet is heated to its forming temperature, it will sag; it is thus advised to ensure enough space between the mold and the clamp to factor this in.
Ascertain that the vacuum tank installed is sufficient for the task, capable of sustaining at least 20” Hg pressure for the duration of the forming process.
Double-sided heaters (aka sandwich heaters) are better suited for this process, as they are more likely to evenly heat the sheet.
Preheat the clamping frame and mold to avoid warping and other defects
For continuous production, aluminium molds are the best, especially if cored with temperature control lines to keep its temperature in check.
Molds made of wood, or epoxy may be used if the production is limited.
An extremely-polished mold surface may cause the polycarbonate to stick and introduce air pockets.
It is better to have a blemish free mold with a slight matte finish.
Make sure the molds in use have factored in the natural shrinkage that will occur as the polycarbonate cools.
iii. Drape Forming
This forming process is for products that require a gradual curve, like windscreens.
Also called oven forming, in this technique, the sheet is heated to its forming temperature in an oven and then placed over the mold.
It is then held in place until it cools down.
You should have thermal gloves when removing the sheet from the oven.
iv. Vacuum forming
In this process, the polycarbonate sheet is heated to its forming temperature and then placed upon a mold.
It is then forced to take the form of the mold by the creation of a vacuum.
The process is straightforward: in the vacuum forming machine, the sheet is heated until it becomes pliable while being held in place by the clamping frame.
At this point, the pliable polycarbonate sheet is lowered onto the mold.
It has tiny vacuum holes, to facilitate the vacuum pump, which sucks out the air between the mold and the sheet.
After it’s been thermoformed, the end product must cool down to retain its shape.
In some machines, fans and a mist spray are engaged to hasten the cooling process.
Afterwards, the formed polycarbonate can be trimmed as desired.
This process can be used to make shop signs, yoghurt cups, boat hulls, refrigerator liners and many other components.
Molds used can be either male or female i.e.
The decision to use either model is informed by whether the aesthetics of the exterior surface are more crucial (if so, the female mold is used).
Or, whether it’s the product’s interior (in which case the male mold is used).
v. Plug-assisted Thermoforming
This is a variant of the vacuum forming process in which a plug is used to force the sheet into a more consistent thickness.
You can compare it to the single step vacuum process.
This process is also called mechanical thermoforming.
The plug is designed to fit with the mold in use, like a lock and a key fit each other.
In this procedure, after heating the polycarbonate sheet to its forming temperature, the sheet is lowered to the mold, followed by the plug.
It sandwiches the sheet between the mold and the plug.
A vacuum is created to enhance the sheet’s conformity to the mold.
Plug assisted molding
vi. Pressure thermoforming
Compressed air (set up to a reading of 100 psi) is used to force the polycarbonate sheet into conforming to the shape of its mold.
In pressure thermoforming, a vacuum force is also applied.
Because it applies two forces working in tandem on the polycarbonate sheets, its products are more detailed than products from vacuum forming.
The image below illustrates the process.
vii. Twin Sheet Thermoforming
In this process, two polycarbonate sheets are clamped together.
Using air pressure and the vacuum effect, cast to form one fused product from two different molds.
The starting set up is as follows:
Twin sheet thermoforming
The molds do not have to be of the same shape, neither do the polycarbonates have to be of the same properties (color, thickness etc.).
That said, it is advisable that the difference in thickness between the sheets does not exceed 0.0625 inches.
This would expose the thinner sheet to a longer greater temperatures while attempting to reach the forming temperature.
When the sheets have reached the forming temperature, and the molds have been latched in place, the vacuum pump is activated
Simultaneously drawing the upper and lower sheet into its mold.
To accelerate this process, pressure is introduced in the gap between the sheets, by means of an air injecting needle pumping hot air.
The two sheets are fused together on either end by the combined application of pressure by the two molds and heat.
Twin sheet thermoforming
The Use of hot air during twin sheet forming results in the trapping of this air as the product is sealed on both ends.
If this trapped air cools down, the product could collapse due to the resulting vacuum; molds with vent holes for cool air are thus ideal for this process.
A great advantage of twin sheet formed products is the hollow center can be filled with reinforcing material, such as foam, to further enhance is properties.
Factors Affecting Quality of Polycarbonate Bending
There are quite a number of factors that affect the polycarbonate bending process. They include:
- Polycarbonate sheet bending radius
- Magnitude of stress when bending polycarbonate sheet
- Polycarbonate sheet thickness
- Polycarbonate sheet bending temperature
Applications of Polycarbonate Bending
The flexibility of polycarbonates means that it can be molded into countless designs.
Implying, there are uncountable devices you can make by bending polycarbonate using any of the above processes.
In transport, polycarbonates have been used to create car roofs, cockpit canopy on jets, car lights, and other components.
It’s also used in making eyewear, visors, riot shields, toys, roofing materials among many other items.
FAQ on Polycarbonate Bending
Now, before I warp up this guide, here are some of the most common questions people ask me every day.
1) How do I cut the polycarbonate?
A circular saw is recommended for cutting the polycarbonate.
2) How are polycarbonates cleaned?
Polycarbonate are cleaned using a mild soap and warm water.
Use a soft cloth and then rinse with warm water.
Use isopropyl alcohol dabbed on a soft cloth to remove grease and paint, and then wash with mild soap and rinse with warm water.
3) What is a multi-wall polycarbonate?
This is a sheet of polycarbonate that comprises several layers, with pockets of air in between these layers.
4) What is Lexan?
Lexan is the trademark used by General Electric for their polycarbonate products.
There are many other trade names for polycarbonate sheets, such as Makrolon (Bayer).
5) Can polycarbonates be recycled?
Yes, they are recyclable, thou for some applications, always go for virgin polycarbonate sheet.
6) Can polycarbonates be painted?
Certain paints are incompatible with polycarbonates as their component chemicals may degrade the product.
It is thus advisable to consult the product guide for advice on which paint to use.
Polycarbonate sheets are versatile and durable material with one major weakness; they are not resistant to scratching.
However, since there are additives to remedy this, and impart other qualities upon the sheet.
It is clear that polycarbonates are the materials to use in lieu of other clear compounds, where strength is desired.
The best part is, you can bend polycarbonate to make virtually any product of any shape.
For more information on polycarbonate bending, feel free to contact us.