Hard Chrome Plating on Plastics Tooling: Now More Than Ever
For years it was only used to salvage tools, now it can be used for more…
For many years, hard chrome plating on plastics tooling was only a means for salvaging mold components of intricate shape and usually used for increasing shut-off dimension (more or less a shimming approach by the mold maker).
Then, as the onslaught of high-performance tooling demanded tighter and tighter tolerances, such words as scoring and galling became more reason to take advantage of hard chrome's intrinsic properties: anti-galling and lowest coefficient of friction of all other engineering metals. Finally, with the advent of highly corrosive and highly erosive materials coupled with both optical and aesthetic quality requirements, the need for properties that only chrome could offer has made it the talk of not only the mold maker but the mold designer as well.
The success of hard chrome in plastics tooling and industrial applications can probably be attributed to its unique combination of properties, which are not possessed by any other commercially available material. The most important of these being hardness.
The hardness alone would not be sufficient to secure widespread use in plastics tooling applications, because a number of other hard materials and other hardening processes are available. It is the combination of great hardness with extremely good corrosion resistance (equal or superior under most conditions to that of such noble metals as gold or platinum) and very low coefficient of friction that give such remarkable results in plastics tooling applications. To this should be added the ease of stripping and replating for repeated salvage when the plate wears beyond permissible limits.
The benefits of hard chrome deposits are not obtained unless the coating is deposited on a sufficiently hard basis metal and to a sufficient thickness. Even a relatively heavy deposit of hard chrome may be crushed or indented on a soft metal such as copper. The best possible adhesion is also important, particularly where the surface may be subjected to severe stress and shock. Both conditions being prevalent in the molding cycle.
The low coefficient of friction and desirable surface properties of chromium are realized for the most part only on relatively smooth surfaces. The deposits can be ground and lapped to size. The deposits are easily ground but are sensitive to heat and should be ground with very light cuts, soft wheels and plenty of coolant.
In most cases, bright deposits are obtained on highly polished molding surfaces with no further mechanical treatment required. By carefully controlling the plating operation, it is possible to plate to size within very close limits.
Overall, the mold designer, mold maker and molder have at their disposal a unique combination of physical properties, which for the most part offer some level solution for the most common of plastics tooling problems. How does one, then, determine when, where and how much?
Application Before the Problems
The mold designer is the one individual who must initially realize his creation. Although the most common concerns have a variety of “rule-of-thumb” solutions that are justified, the word “plating” seems to be somewhat blackballed in the plastics tooling vocabulary.
However, 0.0002 inch of hard chrome on a mirror-finished core (especially a large core) can save hundreds, if not thousands, of dollars in production just by preventing oxidation from forming on a sweating mold. It won’t appreciably affect any dimensions. If maintenance forgets to oil it down before storage, it won’t rust. With today’s plating methods, adhesion quality (from qualified sources) is approaching perfection. Now why doesn’t the mold designer know this? Because, to the average mold designer hard chrome means someone made a mistake (salvage).