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Mold Material/Surface Treatment 38 MoldMaking Technology May 2015 By Robert Lammon and Harry Raimondi A luminum tooling has a variety of natural characteristics that can benefit the injection molding process. For example, aluminum's faster cutting and overall manufacturing times allow for greater throughput efficiencies, its high thermal conductivity enables it to be processed more easily, and it is very strong, to name a few of its advan- tages. While these characteristics help to yield faster cycle times and reduced waste, hard-coating an aluminum mold with nickel boron or electroless nickel can further improve its durability and its ability to produce plastic parts with Class A surface finishes when high volumes are required. The cost of hard-coating an aluminum mold depends on the type of hard coating and the size of the mold. It typically adds just a fraction to the overall cost of the tooling, similar to the cost of a texture or grain application. Hard-coating also adds only a minimal amount of depth to the tooling. For example, electroless nickel plating is an auto- catalytic process that plates evenly once the part is submerged in the tank, without using an electrical current. The plating thickness can be held in tight tolerance with a recommended 0.0002 to 0.0003 inch per side. Typically, an application of as much as 0.0005 inch after part sampling validation is used to account for any potential mold adjustments, such as a tight part tolerance. The primary benefits of hard-coating are protection, surface finish, lubricity, durability and return on investment, which are further explained here. Protection Hard-coating protects the mold integrity for the life of the tool by adding additional hardness and corrosion resistance. Hard-coating provides a number of benefits that make aluminum tooling an attractive alternative to steel. EXPANDING Aluminum's Potential Aluminum has a low hardness level, close to 18 HRC, and applying any nickel-based coating will bring that hardness level to 45 HRC or higher. Nickel coatings also provide a layer of corrosion resistance to further protect the molding surface. Chrome coatings are often considered for added cor- rosion resistance, but they do not adhere uniformly and leave microscopic areas uncoated in which corrosion can begin. Electroless nickel seals and protects the surface better. When molding abrasive resins such as glass-filled nylons, a hard coating with a hardness level that is higher than that of the mold's base material (aluminum) will protect the mold's surface from excessive wear. The abrasive resin will wear away the harder coating on the surface of the mold before it reaches the base material. Unprotected, the aluminum would erode, damaging mold details, removing texture and rounding sharp edges. Nickel coating can be applied as thick as 0.0006 inch, and can be easily stripped and reapplied without any dam- age to the aluminum base material. The whole part is simply placed into a stripping solution and removed when the coating has been dissolved. When part revisions or mold repair requiring either weld- ing or re-cutting of the aluminum are needed, the coating must first be stripped and then reapplied after the welding or Images courtesy of Phoenix Proto Technologies and Bales Mold Service. This automotive instrument panel light guide was made from an acrylic resin in a one-cavity 7000 series aluminum mold coated with electroless nickel for a Class A surface fnish. FIGURE 1