EQUIPMENT AND DESIGN Selection of a suitable nonporous, flexible m o l d is the first concern in developing hydrostatic pressing tech niques. I n t h e past, r u b b e r — i n the form of tubes, balloons, bags, or specially shaped molds—has b e e n used. T h e chief disadvantage of r u b b e r containers is t h a t the desired sizes, shapes, a n d wall thicknesses are not readily available. L a r g e producers of d i p p e d or e x t r u d e d r u b b e r shapes are r e l u c t a n t to scale their facilities for short r u n p r o d u c t i o n of special items. A possible solution to this p r o b l e m rests with small j o b shops which are willing to devote special a t t e n t i o n to requests for r u b b e r shapes of n o n s t a n d a r d dimensions. A l t h o u g h this a r r a n g e m e n t t e n d s to m a k e t h e r u b b e r con tainers relatively expensive, the extra cost c a n usually be justified by the i m p o r t a n c e a n d value of the end p r o d u c t . O t h e r disadvantages of r u b b e r containers are nonuniformity of wall thickness, r a p i d r e t u r n a b l e stretch, a n d the t e n d e n c y to fold or crease d u r i n g compaction. These shortcomings are considered second a r y in t h a t they c a n be circum vented by holding r u b b e r m o l d p r o ducers to rigid specifications a n d by special mold filling practices. Thin plastic materials have emerged recently as a substitute for r u b b e r in hydrostatic c o m p a c t i o n molds. A d v a n t a g e s for plastic con tainers are cited as uniform thick ness, low cost, availability in a wide r a n g e of sizes a n d thicknesses, slow r e t u r n a b l e stretch, high stretch-tobreak factor, a n d the thermoplastic feature t h a t permits sealing to de sired shapes (
