Thermoforming: Pre-Drying

a. General Guidelines:

Almost all types of PALSUN sheets are suitable for the various thermoforming (TF) procedures. However, due to a native small moisture content absorbed after manufacture, they demand a thorough pre-drying treatment prior to most the remoforming techniques. During this process, the sheet’s temperature will be raised to over 160 °C (320 °F). Avoiding this preliminary treatment may result in moisture blisters, marring the appearance of the finished product, and/or reduce its properties.
The recommended procedure entails using a recirculating air oven, operating at 120-125°C (250-260°F). the duration depends on sheet thickness. Higher thicknesses require longer periods in the drying oven.

Table 17: Typical Pre-Drying Time
 in Oven for Various Thicknesses
(time for thicker sheets arrived at by interpolation).

The sheets are put into an oven, with the protective film removed from both sides, and arranged 20-30mm 3/4”-1”) apart, to enable free air circulation. They can be stacked horizontally (on stays or suspended) or vertically- just so they will not be distorted or twisted.
The pre-drying process should be performed as close as possible to the actual forming. Fully dried sheets taken out of the oven and cooled down to roomtemperature may be workable within 1 to 10 hours (depending on relative humidity and temperature in the workshop).
Longer delay may necessitate repeated pre-drying session.
A good idea is (if possible) leaving the pre-dried sheets in the switched-off oven until the actual thermoforming process. This way saves energy and time on the thermoforming apparatus.

b. Guidelines to the Heating Process
Good quality thermoformed products can only be achieved through a careful and controlled heating process. All parts of the treated sheet should reach even, uniform temperature, achieved by a slow, controlled heating rate, avoiding sudden changes in air circulation and temperature. Such events may result in hot spots and possible distortions.
the sheet’s edges must maintain the same forming temperature as the whole sheet.

Pre-heating of the clamping frame from120? to130?C is recommended.

Temperature Regulation: Continuous regulation of the sheet’s temperature must be maintained inside The thermoforming device itself. PALSUN sheet (of any type) tends to cool quite quickly, and may need a regulating system for adding or dispersing of excess heat on the spot. The sheet’s temperature at the thermoforming zone (or the whole sheet) should always be kept above the “Glass Transition Temperature” (approx.150 °C) during the forming process.
Forcing the sheet to form at a lower temperature may induce detrimental internal stresses, reducing the sheet’s impact resistance and increasing its chemical sensitivity. Internal stresses are invisible and can be detected only by polarized light.

Annealing may solve the problem, but it is a complicated process, which is inefficient or impossible in most cases.

Protective Polyethylene (PE) masking in Thermoforming: Special PE masking is available for TF and should be ordered for sheets intended for TF. This masking may be left on the sheet using most thermoforming methods, and peeled off at the last moment. In case of regular masking, it should be removed prior to the thermal treatment, otherwise it may fuse into the sheet’s face.