Moldmaking Tips: Polishing and plating

Steven Bales, president of Bales Mold Service Inc. in Downers Grove, IL, offers moldmakers these tips for prepping molds before having them polished, plated, or welded. In business since 1978, Bales knows the tricks of the trade that can help produce a higher quality mold.

Polishing: Get a Head Start
Before you send a tool out for polishing, determine if it makes sense to have smaller tool parts polished to spec while other parts are completed. There's almost always a point where components of a mold are set aside during manufacturing. If they're at a point where polishing can be done, why not get a jump on the schedule and have it taken care of?

Second, know what finish is best for the end product (Figure 1). Your vendor can advise you as to what finish suits the application. A well-marked blueprint or piece part is helpful, as is the steel type. Keep in mind that the labor required to produce an A-1 diamond finish with flat lapping optical quality can be double that required to obtain an A-2 finish. Why pay more if you don't have to?

Third, when the tool (or component) is almost complete, have your machinist slow down the feed of the mill, lathe, or EDM to create a smoother surface. This can speed up turnaround. Also, it's important to clean all parting lines, shutoffs, runners, and vents before diamond polishing if the tool is to be plated.

Figure 1. On the right are the five previous SPI finishes used in moldmaking. Over the years, these finishes have been refined into more specific categories, as described in the second column (A-1, A-2, etc.). This chart shows how the new SPI finish numbers compare with and correspond to the predecessors.

Successful Plating and Coating
What are some of the basics? Use vapor honing or degreasing to clean the mold before sending it out for plating and coating. Also, if any welding or plating repairs have been done, tell your polishing and plating professional. As with polishing, choose the proper coating or plating process. For example, injection molders should opt for a codeposit of nickel and polytetrafluoroethylene (PTFE) in lieu of using a mold release spray. Sprays tend to cake up the mold and cause extra maintenance. Using nickel/PTFE not only serves as a permanent release agent without the buildup, it also increases resin flow, decreases cycle time, and offers excellent corrosion protection.

The choice between chrome and electroless nickel is important. Keep in mind that hard chrome offers great abrasion resistance, but is not effective for corrosion protection. Electroless nickel is the best bet for fighting corrosion, and is an especially good choice for PVC or other corrosive materials. Additionally, no matter how complex, all surfaces plate uniformly with electroless nickel. Chrome, however, does not easily plate inside corners, ribs, and bosses without a conforming anode. The construction of a conforming anode is necessary for full detail coverage.