Blow­Back Forming

One of the main advantages of blow­back forming is in the reproduction of complex drawn shapes. Also, deeply drawn parts made by the blow~back forming method show less reduction in Chemcast® GP thickness than when vacuum assist Plug and ring forming is used. This method consists of clamping a heated Chemcast® GP sheet between a pressure box and an oversize clamping ring. A male form shaped to the inside contour of the part is then pressed into the sheet to the required depth and locked in this position thus stretching the sheet. Compressed air at 50 to 100 psi is then admitted to the pressure box, forcing the heated Chemcast® GP sheet back against the male form. The parts are held in this position by air pressure until the Chemcast® GP cools and becomes rigid. The forming cycle must be fast enough to insure that the Chemcast® GP sheet is still well above the minimum forming temperature as it is blown back against the male form.

Care must be used in design to prevent the Chemcast® GP sheet from folding at the corners when it is blown back against the male mold. One way to overcome this tendency is to add dummy blocks to the mold to equalize stretching in all directions.

Reverse Blow Forming

The main advantage of reverse blow forming is good control of wall thickness.

This method consists of clamping to a pressure box a heated blank which is twice as large as the projected area of the part. The box must be strong and air­tight. A Chemcast® GP bubble is blown in the sheet clamped to the box by admitting air to the box. The male mold is then forced down into the bubble. Contact with the mold tends to prevent further thinning due to friction and cooling. As the male mold descends the bubble will wrap around it due to the air pressure maintained in the box. The pressure should be adjusted by a relief valve so that excess pressures are not built up as the male mold and formed Chemcast® GP displace the volume. The male mold must be vented at undercuts or any areas which trap air between the mold and the formed part. The vented air also can be used to blow the formed part off the mold.

Ridge Forming

Ridge forms are open or skeletal rather than solid forms and they may be used to good advantage in many forming operations. They are generally easier and less costly to construct than solid forms.

Ridge forms may be used with nearly all methods of forming including press forming, vacuum or pressure forming, snap back or reverse blow forming and vacuum Platen forming. Ridge forms contact the heated Chemcast® GP only along ridges necessary to determine the size and shape of the formed part. Consequently, mark­off is minimized.

Because hot Chemcast® GP tends to resume its original flat shape, the area between ridges of the form or between ridges and clamping ring are stretched taut. Areas in a plane enclosed by ridges tend to form flat planes in the formed part. In other shapes with ridges which do not fall in a Plane, the intervening areas tend to be concave.

The principles of ridge forming can be extended to the construction of both male and female forms (Figure 37) so that reverse curves, flanges and flutes can be formed with a minimum of distortion.

Several shapes can be formed in a single piece by dividing a vacuum box with partitions and using separate control valves for each compartment. The Chemcast® GP itself forms a seal when it is drawn against the ridges or partitions. The pressure differentials which may be used are limited by the tension in the Chemcast® GP sheet as it is stretched across the opening. A tighter seal can be made if ridges in the male form press the Chemcast® GP against corresponding grooves or ridges in the female form.

Male and Female Forming

Male and female forming may be used to form Chemcast® GP by surface molding and embossing the material between matched male and female dies. Both surfaces of the Chemcast® GP sheet formed by this method are in continuous contact with the forms and will reproduce the mold surfaces if high enough pressures are used.

Matched male and female dies usually cost more than tooling used for other forming methods. These dies should be metal to withstand the high pressures which may be developed. If used hot, metal dies will prolong cooling. They should, therefore, be cored to permit heating and cooling.

Forming Acrylic Sheet

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