Tool steels have historically been over-used for injection mold cores and cavities, simply because they've been both cheap and familiar. But soaring price tags on many tool steels, coupled with the push for more productive molding operations, is triggering an investigation of alternative mold construction materials. While not necessarily cheaper than steel on a price basis, these alternatives have thermal engineering or maintenance cost advantages that make them more attractive than ever. Copper alloys are one choice. They can speed the removal of heat from critical sections of the tool, potentially improving cycle times 20 to 50 percent and even improving the dimensional stability of the molded parts themselves. Look for more stainless steels, too. Stainless has always been the material of choice when molding acid-emitting resins, such as PVC. But now use of stainless is spreading, particularly for expensive tools, because corrosion-related maintenance efforts and costs are reduced through the life of the tool. Finally, important advances are also under development with aluminum mold materials, particularly in strength, surface hardness, corrosion resistance, and uniformity after heat treating (go to the Design News Materials Forum at http://rbi.ims.ca/4921-511 for more information). Here's a look at three recent mold materials that can serve as steel stand-ins:
RoyAlloy is a new stainless mold-base steel, developed by Edro Specialty Steels, to overcome shortcomings associated with prehardened 420F (1.2085) stainless holder steels: a lack of toughness, unpredictable stability, inconsistent machinability and poor weldability. Edro says thermal treatments give the material improved dimensional stability after machining without the need for stress relieving.
The photo shows a MoldStar 90 beryllium-free copper alloy core for a 32-cavity mold running a polypropylene part used in a packaging application. "Because of the conductivity of the alloy, a water channel is cut only in the base of the core, in place of an intricate, small, water system normally needed had the material been tool steel," comments Cliff Moberg, president of Performance Alloys.
MoldMAX V is a new copper-nickel-silicon-chrome alloy that has a hardness of 30 HRC and thermal conductivity approximately four to five times greater than P20 tool steel. The new grade is the second in a series of non-beryllium-copper alloys introduced by Brush Wellman. Moldmax XL is a copper-nickel-tin alloy with similar properties.