Annealing Acrylic Thermoform Plastics
Annealing is the process of relieving stresses in molded or formed plastics by heating to a predetermined temperature, maintaining this temperature for a set period of time, and slowly cooling the parts.Sometimes, formed parts are placed in jigs to prevent distortion as internal stresses are relieved during annealing.
To anneal acrylic sheet, heat it to 180°F (80°C), just below the deflection temperature, and cool slowly. Heat one hour per millimeter of thickness – for thin sheet, at least two hours total.
Cooling times are generally shorter than heating times – see the chart below. For sheet thickness above 8mm, cooling time in hours should equal thickness in millimeters divided by four. Cool slowly to avoid thermal stresses – for Thermoform Plastics the thicker the part, the slower the cooling rate.
Wait until oven temperature falls below 140°F (60°C) before removing items.Removing a part too soon can offset annealing’s positive effects.
While annealing acrylic sheet parts, support them to avoid stress. For example, a part’s raised center section will need independent support – it can’t be supported from the ends.Lack of support may inhibit relaxation or cause warpage. Be sure parts are clean and dry before annealing. Remove paper masking to avoid baking it onto the material. Remove any spray masking, protective tape, or similar material. Plastic masking may remain in place.
If the only fabrication you have done is surface machining and you do not need to anneal cemented joints, heating time can be reduced. This reflects the fact that machining forms stresses only at and slightly below the surface – the entire sheet thickness needn’t be annealed. Heat at least two hours; cool the same amount of time. If holes have been drilled entirely through the sheet, position the part so heated air flows through the holes.
If you are annealing following cementing,, including Thermoform Plastics, allow the part to sit at least six hours to avoid bubble formation resulting from rapid solvent evaporation in the joint.
SHEET THICKNESS VS. HEATING/COOLING TIME
|Thickness||Heating Time||Cooling Time||Cooling Rate|
Category: Thermoform Plastics