Molds are often the limiting factor in developing the most cost-effective plastic parts. Take a look at new developments in mold materials to make sure you’re getting the most possible from a tool. In addition, the push for high productivity, coupled with soaring price tags on tool steels, is triggering an investigation of more efficient materials of construction. Another thought: better thermal engineering of a mold can improve dimensional stability of molded parts.
Tool steels have been over-used for cores and cavities, simply because they’re cheaper and because of inertia. Integration of copper alloys into the design of a mold can speed removal of heat from critical sections. Cycle times can be improved 20 to 50 percent with aggressive design ideas for core-side inserts coupled with a robust injection molding machine.
The big advances in mold materials are coming from better understanding of hardness, grain structure, and the effect of metal processing equipment, from machining to welding. In designing copper alloys for molds, for example, consider carefully whether the material is wrought or cast. Wrought parts have a more uniform grain structure and size because they are hot worked. Cast products have better ductility. Each has its rightful place in the tool. The highest hardness for copper alloys may reduce ductility and effectiveness.
Big changes are also under way in stainless steels as well as aluminum. Stainless was always the material of choice when molding acid-emitting resins, such as polyvinyl chloride. But now use of stainless is spreading, particularly for expensive tools because maintenance efforts and costs are reduced through the life of the tool. One reason: water-cooling channels remain corrosion free for maximum cooling efficiency. Stainless molds and components also remain corrosion-free during storage.
Important advances are also under development with aluminum mold materials, particularly in strength and surface hardness. There are also new aluminum alloys that have better uniformity after heat treating. Another benefit: improved mechanical properties while maintaining high thermal conductivity. R&D efforts are also aimed at reducing susceptibility to corrosion of some high-strength aluminum alloys. Manufacturers also promise that design engineers will be able to dimension these aluminum materials more like steel, particularly with regard to placement of water lines.
Here’s a look a three new tooling materials:
Additional cooling capacity can simplify mold construction dramatically. Consider the recent use of 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. www.performancealloys.net
RoyAlloy is a new stainless mold base steel developed by Edro Specialty Steelsthat is said to overcome shortcomings associated with prehardened 420F (1.2085) type stainless holder steels: a lack of toughness, unpredictable stability, inconsistent machinability and poor weldability. Edro claims that special thermal treatment of the material provides improved dimensional stability after machining without the need for stress relieving. RoyAlloy is a 400 series martensitic stainless steel designed specifically for improved plastics and rubber mold tooling. It’s available from stock in flat sizes up to 12 inches thick and rounds up to 12 inches in diameter. www.edro.com
MoldMAX V is a new copper-nickel-silicon-chrome alloy that has a hardness of 30 HRC and thermal conductivity approximately 4-5 times greater than P20 tool steel. The new grade is the second in a series of non beryllium copper alloys introduced by Brush Wellman, Cleveland, OH. Moldmax XL is a copper-nickel-tin alloy with similar properties. Benefits of the new series include reduce cycle times through improved heat removal as well as better dimensionally stability of the molded part. Another advantage is availability of the material in large block sizes. The photos show forged rings made of an earlier (beryllium containing) grade of MoldMAX. For more information, go to www.moldmax.com.