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moldmakingtechnology.com 39 recutting is completed. Stripping will return the part to just the base material. After any changes or repairs are completed, the coating is reapplied to bring the mold back to optimum working condition. Welding over the plating is not recom- mended because the plating's "slip" or lubricity may prevent the weld from properly adhering. Surface Finish Aluminum is a soft and porous base material. This presents a challenge when the mold must be polished to a Class A surface finish, which is required, for example, for producing optical components such as clear lenses, prisms and light pipes. Any mold imperfections will reproduce in these optical components. Hard-coating can help the moldmaker achieve a Class A surface finish in aluminum, if it is done properly. For example, an electroless nickel surface coating can enable the aluminum base material to be polished with superior results, but if the hard coating is not applied to a uniform depth, it will result in a poor finish. An experienced polisher can polish the plating, but the ability to do so without the plating wearing off or without breaking through the plating takes a long time to master. Texture and grains on a mold surface can also be protected with a hard-coat process. The coating is harder than the base material, so it can withstand an abrasive molding material, like the glass-filled nylon mentioned earlier. A minimal amount of coating (0.0002 to 0.0003 inch) will cover the texture without filling it in and protect the peaks and valleys from wear, pro- viding more consistent results between the first and the last parts run (see Figure 1). Lubricity Nickel boron hard-coating is often used on aluminum molds that have complex geometries or minimal draft allowances that can interfere with part release. Such geometries include deep ribs or thin-wall conditions that assist in part processing and ejection from the mold surfaces. These types of geome- tries can be difficult to fill with resin, and the pressure exerted against the mold by the resin can make it difficult for the part to be released from the mold. Nickel coatings are applied using an electroless process in which the whole part is submerged in the plating bath. Nickel boron is co-deposited (the electroless nickel and boron are in the same bath and applied at the same time) for a uniform thickness, providing high lubricity and part release from the mold. Electroless nickel has a coefficient of friction of 0.45 or less, while the nickel boron coating has a coefficient of 0.05 or less. The lower the coefficient of friction, the less friction between mold and part, and the easier the release of the part (see Figure 2). Durability High part volumes such as in the hundreds of thousands for parts molded from abrasive resins, and in the millions for parts made from non-abrasive resins, take a toll on mold durability, and hard-coating can protect the mold surface. It's usually nec- essary to apply the hard coating after part approval but before production startup in order to help keep shutoffs and any wear areas on the mold clean and sharp. Even a short run, especially with a abrasive material, can alter tool dimensions. Plating the tool early helps avoid the need for buildup and repair later. Again, nickel coatings raise the hardness level of aluminum to 45-54 HRC, which results in increased wearability of alu- minum molds. Coatings also can serve as great wear indica- tors. Once it is applied, the actual buildup of the hard coating can be measured, documented and monitored as part of the preventative maintenance performed after production runs. When the base material wears through the nickel coating, it is time to strip and recoat the mold (see Figure 3). This motor fan was made from a talc-flled polypropylene in a one-cavity 7000 series aluminum mold coated with nickel boron for lubricity. This automotive bottle cap was made from a polypropylene resin in a two-cavity aluminum production mold coated with nickel boron for durability. FIGURE 2 FIGURE 3