The two most widely used methods of extrusion blow mold constructions are by machining or casting. The choice of method is dependent on factors such as, number of cavities, mold material, intricacy of design and accuracy of cavity sizes. Generally, if many cavities are involved, and the material is one that lends itself to casting, and cavity size tolerances are not critical, then casting produces the lowest cost mold. Some designs, due to the intricacy of detail, are easier and therefore less costly to produce by casting rather than machining and vice versa.
The mold material to be used is probably the most important factor in selecting the fabrication process. Zinc, which is readily cast, can produce excellent results.by gravity casting over a metal master made from mild steel. The detail is sharp and the accuracy of cavity sizes is as good as any casting technique providing times and temperatures during casting are closely controlled. If a ceramic master is used then both detail and accuracy are sacrificed.
Aluminum is difficult to cast and only certain alloys are castable. Generally alloys that are high in silicon are best suited casting, while a high strength alloy such as 7075 cannot be cast and be machined. The best detail and accuracy in an aluminum casting can obtained by pressure casting and die casting wherein a hardened metal master usually made from an H-13 steel is used. Neither of these casting methods has been popular because of the ease with which a higher strength alloy can be machined with the inherent accuracy of cavity sizes in this cutting process. Gravity casting of aluminum over ceramic has been limited primarily to large mold cavities of the 5 gal. Variety and up. Even in these castings the aluminum foundry will core out a portion of the casting to permit as nearly uniform thickness of wall section as possible in an attempt to keep porosity at a minimum.
Beryllium copper, like aluminum, can best be pressure cast to prevent porosity in molds of varying cross sections. A hardened metal master of H-13 steel is usually used, but under repeated heating and cooling the H-13 becomes annealed. Subsequent re-hardening of the master can cause distortion and a change in size which affects the size and accuracy of the ultimate cavity. Machined beryllium copper has produced the highest quality blow mold.
Regardless of whether a mold is machined or cast, shrinkage factors must be considered. The machined mold cavity must allow for shrinkage of the plastic used plus any other shrinkage which will take place in the subsequent post-molding operations on the blow molded part. For a cast mold, the above shrinkages must be considered together with an allowance for the amount the metal will shrink during casting. Once all shrinkage factors have been predicted, the pattern is displaced to check its size before metal is either cast or cut. This check verifies the mathematical computations used to determine the pattern size initially and prevents possible costly errors in the actual mold construction.