How to preserve mold performance

A better understanding of what the latest mold coatings can do can make your customer relations invulnerable. —Steven J. Bales, edited by Carl Kirkland

This chromium-matrix diamond composite coating provides a high dispersion of nanometer-sized, spherical diamond particles.

If there is one common thread binding good relations between moldmakers and molders, it’s the need to continuously preserve, and even enhance or improve, a mold’s performance. This thread is what helps create a valuable partnership to maintain customer loyalty and perpetuate a mutually profitable alliance.

One effective way to solidify that thread is for moldmakers to more clearly understand the importance of using the correct mold coating. It’s no longer just a question of using nickel or chrome. There are other smart options available.

First, rest assured that there is still a place—and there probably always will be a place—for hard chrome and electroless nickel in the plastics tooling industry. However, with the growing field of new molding materials being used today, it naturally follows that new coatings have either become available or soon will be available that are designed to help take mold performance to a new level, as shown in the chart on p. 36.

Some of these newer molding materials and compounds can be brutal on a mold. In addition to glass and mineral fillers, we now regularly deal with rice hulls, wood fibers, metal powders, flame retardants, and other additives, to name just a few. Add to these the frequent occurrence of outgassing and moisture acidity, and your very expensive tools are in for a beating.

Hard chrome vs. diamond chrome

The first step in choosing the best coating for your mold is to assess your needs. What kind of material is being molded? What kind of tool steel is being used? How intricate are the parts being molded? How many cycles do you expect the mold to perform between scheduled maintenance? What are the tolerances expected?

Hard chrome may be the answer if you are molding very intricate parts using glass-filled materials. Its main advantage is that it offers a tough 72 Rockwell C hardness and is applied at a very low temperature, providing excellent protection from abrasion, plus moderate corrosion protection. Hard chrome also gives you the ability to achieve any SPI finish.

Hard chrome can be costly, though, especially if the parts you’re molding are very intricate and detailed, because it requires the construction of an anode to apply it. The more intricate the part, the longer and more involved the anode construction. This also means a higher price.

An alternative could be a newer nickel-cobalt alloy, a unique alternative that does not require an anode to apply it, thereby rendering it a cost-effective alternative to hard chrome. Also, because it has electroless properties, it plates more uniformly. And the cobalt gives the mold good abrasion resistance at 62 Rockwell C hardness.

So what is the best choice? It depends on the percentage of glass in the molding material you’re running. You have to decide which matters most: abrasion or corrosion protection.

Another new alternative

For very high-wear conditions that even hard chrome and nickel-cobalt can’t effectively protect, consider an even newer coating product called diamond-chrome. With a Rockwell C hardness of 85-plus, this chromium-matrix diamond composite coating provides a high dispersion of nanometer-sized, spherical diamond particles (see photo, above). These diamond particles give this coating its impressive ability to offer superior abrasion protection.
A noted benefit is that diamond-chrome does not compromise the integrity of the tool as do similar products on the market, because it is applied at a very low temperature—about 130°F. Also, diamond-chrome will plate all commonly used tool steels, even when they are heat-treated, nitrided, or prehardened.
It’s easily strippable by using reverse electrolysis in a caustic solution and it can be deposited in any controlled thickness, from 0.000020-0.001 inch. Add to that its coefficient of friction (COF), which measures 0.15 against steel, and you can see it offers great release properties.

Options for excellent release

If good release is your main objective, there is a coating available that has proven to be highly successful: nickel-PTFE, a coating that was originally developed 20 years ago for NASA.
With a COF as low as 0.10, it can greatly improve part release and even enhance resin flow by as much as 4-8% over other coating options. This certainly is due to the PTFE, but applying PTFE alone is not the answer because it will wear down. It has no hardness. But when a dispersion of 25% PTFE by volume in an electroless nickel codeposit is created, you benefit from a 45 Rockwell C hardness for corrosion and wear protection, as well as the good release properties.

If reduced cycle times cause your customer to sing your praises, consider applying nickel-PTFE on zero-draft cores, deep ribs, and textured surfaces. It also can solve the problem of sticky polymers.

The next generation

For superior release properties and better protection against wear, corrosion, and heat, an electroless nickel-phosphorus matrix coating containing boron nitride particles is a good choice. Called nickel-boron nitride, its COF measures 0.05 and its hardness rating is higher than nickel-PTFE at 54 Rockwell C.

This new technology makes it ideal for the unscrewing cores in closure molds, where reduced cycle times are essential. An added benefit is that the hardness rating can be increased to 67 RC if the tool is heat treated at 500°F.

Nickel-boron nitride can be applied to any substrate at only 185°F and also can be easily stripped without compromising the base material. Although it is about 15% more expensive than nickel-PTFE, it outperforms nickel-PTFE at up to 1250°F, which far surpasses the 550°F maximum limit for all PTFE-based coatings.

Like nickel-cobalt, nickel-boron nitride requires no anode for its application. Rather, it is an autocatalytic process. It saves time and money, and it will not compromise the thermal conductivity of the tool.

The right coatings can help you solidify that common thread between you and your customer by providing the best possible tooling and, subsequently, the best possible molded parts. a

Author Steve Bales (sbales@balesmold.com) is president of Downers Grove, IL-based Bales Mold Service Inc. (www.balesmold.com), a provider of finishes and engineered coatings for molds, dies, tooling, and components.