The blow mold has undergone many changes in its design evolution, but its basic requirement of good heat-transfer properties and resistance to wear remains of maximum importance. These two factors must be given major consideration in the design of blow molds.

Although many metallic and non-metallic materials have been used with varying degrees of success, this presentation will consider only metallic materials as the best production material for blow molds. The most commonly used materials are aluminum, beryllium copper and zinc,and their applicable properties are listed in Table 1 and rated for those characteristics of greatest interest to a manufacturer and user of blow molds. A discussion of their characteristics follows to clarify the reasons for the arbitrary ratings of excellent, good, fair and poor.

Table 1

Beryllium Copper

Beryllium copper is chosen as a mold material where the prime consideration are hardness, high wear resistance, high thermal conductivity, resistance to corrosion, and ease of reparability. Copper, when alloyed with a Very small percentage of beryllium to gain hardness and castability, loses some thermal conductivity and machinability. As noted in Table 11.1 thermal conductivity can vary from 0.15 to 0.61 cal/sq cm/sec/°C. Typically, beryllium copper having 2% beryllium, .5% cobalt, and the balance copper will show a conductivity of .15 in the cast state. Heat treating this same material and increasing its hardness from Rockwell B55-70 to C37-42 can improve its conductivity to .31. By reducing the beryllium content to .6% and increasing the cobalt content to 2,60%,the conductivity of cast material can range from a low of .25 to a high of .61 when undergoing a change in hardness from Rockwell B30-45 to B90-100.

A compromise formulation containing approximately 2.0% beryllium is usually chosen when casting blow molds of beryllium copper. If mold inserts or cavities are to be machined directly from bar stock, the percentage beryllium usually used is approximately 1.65.

With material cost in 1986 ranging between $6.70 per lb. for casting ingot to $8.00 per lb. for high density stock, beryllium copper is the most expensive mold material presently used for blow molds. Oftentimes, to keep costs at a minimum, beryllium copper is used only in those areas of a mold where its most desirable properties are required. Usually these are in the neck and base areas where the plastics material is squeezed between the mold halves. Even if cost is of no object, the material weight can sometimes eliminate use of beryllium copper for mold construction because of mold machine limitations or because of in-plant handling considerations.

Its ductility, as in the case of aluminum, permits minor repairing by peen-ing. Inert gas welding is usually employed for major repairs. While some annealing takes place as a result of welding, the serviceability as a blow mold material is not jeopardized.

While the tendency of beryllium copper to work harden and its toughness make it difficult to machine, adherence to proper cutting angles, coolants, etc. can facilitate this problem. Booklets describing best machining practices are available from the manufacturers of beryllium copper.


Leave a Reply

Your email address will not be published. Required fields are marked *