Temperatures for thermoforming Polycarbonate

When Polycarbonate is cooled below 150 C / 302 F, it transitions from a flexible structure to a rigid structure that locks into what ever shape it is in; this temperature is known as the glass transition temperature. Conversely, when Polycarbonate it heated above its glass transition temperature it becomes flexible and can be bent into various shapes. This property is used in the process of thermoforming.

Thermoforming can be carried out at any temperature above the glass transition temperature and below the melt temperature of 267 C / 512 F, although in practice the Polycarbonate becomes more flexible the higher the temperature and it is not necessary to approach the melt temperature. The Polycarbonate actually becomes difficult to use much above a temperature of 215 C / 450 F.

There are three broad categories of forming – Cold forming, Low temperature thermoforming and high temperature thermoforming.

Cold forming.

Cold forming uses a frame to hold the Polycarbonate sheet in the desired shape. The sheet is then heated to between 302 F and 340 F for several hours until the entire sheet (interior and not just the surface) rises above the glass transition temperature. The sheet is then cooled below the glass transition temperature to set the shape. Cold forming is a simple process, but can only be used for relatively simple shapes (often two dimensional) without tight radius bends.

Low temperature thermoforming.

Low temperature thermoforming is carried out between 350 F and 370 F. This process is often used for simple shapes where the Polycarbonate sheet drapes over a mold or into a mold. While it is possible to achieve relatively simple 3D shapes with low temperature thermoforming, complex shapes with lots of detail are not possible. One advantage of low temperature thermoforming is that pre-drying of the sheets is not necessary.

High temperature thermoforming.

High temperature thermoforming is carried out between 370 F and 420 F. Complex shapes, sharp details and deep draws are all possible with high temperature thermo-forming. Many thermoforming processes use vacuum to achieve some of the complex shapes. One of the disadvantages of high temperature thermoforming is that all moisture must be removed from the sheet by drying the sheet prior to thermoforming. If this drying is not done, the higher temperatures will cause moisture evaporation bubbles to appear in the sheet during thermoforming.

Drying needs to be carried out above the boiling point of water and it is recommended that the sheet is heated to 120 C / 250 F to dry the material. The drying time is dependent upon the sheet thickness. For 0.118” thick sheet about 10 hours of drying is recommended, for 0.236” sheet, this can increase to closer to 30 hours. After drying the sheet should be used within a reasonably short time frame to prevent the sheet re-absorbing moisture from the air.

Hard coatings.

One thing to remember with thermoforming Polycarbonate sheet is that raising the temperature above the glass transition temperature will make the sheet flexible; any hard coating on the sheet will probably not be flexible and will crack during the thermoforming process. When purchasing Polycarbonate sheet for thermoforming it is important to use only hard coatings designed for thermoforming. These coatings are slightly more expensive than standard hard coats, but are considerably cheaper than the alternative of post coating any thermoformed parts.