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Electroless vs. electrolytic plating: Which is right for your mold?

TAGS: Materials

Editor’s note: Steven Bales is president of Bales Mold Service Inc., a supplier of engineered coatings and finishes.

When a customer is hot for a job—and that’s most customers these days—it’s common to be completely focused on mold construction, or if you’re a molder, on getting it up and running as quickly as possible. Potential problems are usually an afterthought, which is not the best way to keep a customer coming back.

There are some easy ways to avoid this unpleasant experience and look good at the same time. It all begins with asking three simple but key questions well before that due date arrives:

  • What type of material is the mold being constructed from? (Prehardened, tool steel, stainless, or beryllium?
  • What type of material will be molded? (Thermoplastic, rubber, wax, phenolic, or something else?)
  • What are you trying to achieve? (Corrosion protection, abrasion protection, lubricity, or some combination of these?)
Toolmakers typically have answers to the first two questions, and perhaps also the third, from the beginning. But it’s interesting to note that more times than not, they forget to look beyond the blueprint and consider more directly what plating process will give the customer the optimal production outcomes over time.

Similarly, molders may assume that the toolmaker did his homework and gave the customer what is required. Both might send it to the local plating vendor and say, “Plate it with chrome.” The tool gets plated with chrome, but the mold delivery is running late and the squeeze is put on the plating vendor to get the job done swiftly and correctly. The vendor sends a sizable bill for the service, and a month later the customer is screaming because parts are sticking and the mold is showing small or microscopic corrosion.

It sounds extreme, but it happens time and again. You can use hindsight and say, “Well, the mold obviously needed nickel plating and not chrome,” but how much easier and cost-effective would it have been to have done some homework about electroless vs. electrolytic coatings and when and why to use them?

Understanding the Differences

There are some distinct differences between electroless and electrolytic plating materials. Electrolytic deposits require d-c voltage and a conforming anode to be constructed in order to manipulate the plating process and direct it to the cathode (mold). Electrolytic deposits are not uniform and plate somewhat heavily on outside corners that are high-current-density areas, while plating more lightly in low-current regions of the mold. This is especially true in tight detail spots such as ribs or bosses. However, the low-current-density areas of the mold that are closest to the conforming anode will get more plating, resulting in a more even deposit overall. Hard chrome is an electrolytic process, as is diamond chrome. Copper, cobalt, nickel, tin, and cadmium are also examples of electrolytic plating.

Consider the importance of the anode further. Detailed anode construction enables more even coverage where you need it. Without it, plating would be heavy where you don’t need it, such as on parting lines or outside corners, and too light where you do need it, like on cavity details and inside corners.

Electroless deposits do not require an anode or cathode because they are autocatalytic and require no d-c current. The plating solution begins to plate the mold immediately upon contact, and it plates uniformly, even in highly detailed areas. Electroless nickel, nickel-cobalt alloys, nickel-PTFE, nickel-boron, and other nickel-based composites are examples of electroless plating materials.

Because the electrolytic plating process requires that a conforming anode be constructed, more time and skilled craftsmanship are involved, and that results in a sizable vendor invoice. Failure to realize this (when chrome is the proper plating process) can also lead to late delivery of the mold. Electroless plating processes cost less, but only in terms of time and materials. Whether they are, in fact, more economical, depends on the desired results, so you must ask the three questions listed earlier—and then some.

For example, if you have a mold with complex details and are molding thermoset materials with high glass or mineral content, chances are electroless nickel, at 50 Rockwell C hardness, will be ineffective. However, if you find you have only 10% glass or mineral content, nickel may work well, especially if your main concern is release properties and corrosion protection. On the other hand, a mold running resin with more than 20% abrasive materials should be hard chromed at 72 Rockwell C, or diamond chromed at more than 85 Rockwell C. You could use electroless nickel, but you will encounter more frequent maintenance and replating. Here’s why.

Electroless nickel deposits between 45 and 55 Rockwell C, depending on the level of phosphorous in the bath, offer significantly less abrasion resistance than chrome but excellent corrosion resistance. (The higher the phosphorous, the higher the corrosion protection and the lower the hardness.) In fact, in salt spray chambers where corrosion protection is commonly measured and tested by industry analysts, a .001-inch deposit of electroless nickel will last approximately 1000 hours before red rust develops, as compared to only 100 hours for the same thickness of chrome deposit.

It should be stressed that electroless nickel plates absolutely everything it touches. Masking may be required, adding to the cost of the process. We recommend allowing for coating thickness during the design and construction of the tool because effective, complete coverage of a mold’s surface requires at least a .0003-inch thickness of electroless nickel. Anything less leaves microscopic areas where the base material is vulnerable to gaseous emissions from the molding materials. A .0005-inch deposit is most desirable when molding corrosive material but we find that, in many cases, customers only allow for deposits of between .0001 and .0002 inch.

The best bet is to plan for the electroless nickel coating during mold construction. Consult with your plating vendor if necessary. Whatever you do, realize that plating can eat up some time allowances made for tool assembly if the process has not been properly planned for in advance.

Analyze what parameters you’re working within a bit further. How complex is the mold detail? If you want good release, corrosion protection, and abrasion resistance, which one is most important? Are there budget constraints? Is the tool a prototype or production tool? A good plating vendor will ask these questions up front and know the appropriate steps to take, but it helps if you have the answers ready ahead of time.

Realize also that preparation of a mold for plating varies according to the customer’s requirements and the plating material chosen. Steel must always be prepped by an experienced plating professional using variations of alkaline and acid baths. Stainless steel, ferrous steel, aluminum, and beryllium each require different pretreatment processes, so it’s imperative that your vendor knows the base materials he or she is working with. Literally 95% of plating failures can be traced back to improper pretreatment of the tool.

Understanding the differences between electroless and electrolytic plating processes can make a big difference in mold performance. Many key issues have been outlined here that should shed light on the dynamics of nickel- and chrome-based products. Plating science is continually evolving and more coatings are being developed every day, each with its own benefits and new challenges. Making a study of the latest technology will keep you up to date and your customers happy.

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