IMM 8/99

We are looking for a good steel plating method for molds running soft PVC for transparent medical disposables. At present, we are using a D-2 grade from Bohler and Stavax from Uddeholm with hard chrome plating for our inserts. But hard chrome does not last longer than 100,000 shots. With the Stavax, due to lower hardness, we get flash after 100,000 shots We have tried titanium nitriding, too It works well on core inserts, but due to process limitations, it is not usable on inside cavity surfaces. Can anyone advise?

We have made many multicavity molds for the medical industry running soft PVC. We have experienced excellent results with our molds running millions of cycles, using Stavax with a
high-quality nickel plating on the molding surfaces. PVC releases hydrochloric gas during molding. This strips chromium on the surface as well as in the steel.
-M. Hoffmann, Caco Pacific Corp., Covina, CA, (626) 331-3361.

Hard chrome deposits in thicknesses less than .0005 inch are basically porous and cracked. The chloride gases produced during PVC molding will degrade the chrome deposit and base material. Having plated both chrome and electroless nickel on molds for many years, I see this problem quite often. Electroless nickel at .0005-inch deposit is nonporous and crack-free, providing no path to the base metal. Also, chloride gases have no effect on the deposit. Electroless nickel has the same corrosion protection, regardless of the base material.
-S. Bales, Bales Mold Service Inc., Downers Grove, IL, (800) 215-6653.

We have been specifying Dicronite dry film lubricant for tooling parts. This may be an alternative to chrome plating. Dicronite is produced to military specification DOD-L-85645 and is economical, nondistortive, friction-free, very clean, and is molecularly bonded to the material being processed.
- S. White, Wabash MPI, Wabash, IN, (219) 563-1184.

Titanium nitride coating using the PACVD process allows coatings to uniformly cover cavity inserts. This is a gas process performed at 900F. D-2 steels from the mentioned suppliers or DC-53 from International Mold Steel have a secondary hardening curve around I000F. PACVD coatings can be applied 50 to 100 deg F below the tempering temperature without loss of core hardness. As a result, coatings can be applied to the cavity and retain the original polished finish and heat treat properties. The stainless steel grades of 420 will lose some core hardness during the process. Ultra Chem, 15/5, or 17/4 will retain the original core hardness of 32 to 36 Rockwell C. NAK 55 and 80, PX5, or P-20 will retain core hardness after the PACVD process as well.
-R. Monahan, Ionic Technologies Inc., Greenville, SC, (864) 288-9111.

With 45 years of experience in metal finishing, I can relate to your problem. The answer is not in finding the right steel plating method, but in finding a higher grade of tool steel for your application. The D-2 tool steel you are using now will give you an approximate maximum hardness of 64 to 66 Rockwell C. A harder, denser tool steel such as T-15 or M-42, with a good microfinish, and a potential hardness of 70 Rockwell C or higher, together with one of General Magnaplate's synergistic coatings, may be the solution. Coating the inside of an insert is generally not a problem. A synergistic coating would improve the life of your cavity inserts. There are several possible choices, and one could be customized for use in your application.

The reason chrome is giving you limited life is because you are molding soft PVC. When PVC is heated, there is always some chlorine gas combining with a little humidity that forms hydrochloric acid. You cannot avoid getting this reaction with chrome, which limits the life of this coating. In fact, many people use hydrochloric acid to strip chrome plating.
-W. Alma, General Magnaplate Corp., Linden, NJ, (908) 862-6200.

The performance of any surface treatment depends greatly upon the ability of the tooling material to support the surface layer. Differentials in thermal expansion between the coating and the substrate, excessive flexure of thin details, and/or plastic deformation of mold surfaces (e.g., on shutoffs, interlocks, parting lines, and so on) will cause undue stress on the coating/tooling material interface and may result in delamination or spalling. Once the coating delaminates, it may become wear particles as it flows across mold surfaces, thus accelerating normal wear.

The solution to the problem at hand depends, first of all, on whether the plating is actually being worn away or if it is in fact spalling. Secondly, a solution depends on the primary reason for applying the plating in the first place-for example, corrosion resistance, wear resistance, lubricity, filling properties, and so on. In the majority of molding applications, platings are specified to improve wear resistance and/or corrosion resistance. Assuming that this latter scenario is the case, we have experienced excellent results with inserts manufactured from our powder metallurgically produced materials (Vanadis 4, 6, 10, and Elmax) in both highly abrasive and corrosive molding applications. The high hardness to toughness ratios for these grades and excellent dimensional stability make them an outstanding substrate for most coating processes. Additionally, in many cases, the high levels of wear resistance-and in the case of Elmax, good corrosion resistance-can even eliminate the requirement for a surface treatment. To illustrate this, Elmax is commonly utilized as a mated slide material with Stavax ESR (AISI-420) in cleanroom operations where coatings are not an option, as they present a possible source of contamination.
-P. Roche, Uddeholm, Rolling Meadows, IL, (800) 552-5233.

RPC has developed a substrate impregnation process (SIP) to penetrate up to 4µm into mold substrates, such as 6000 and 7000 series aircraft aluminum, S-7, H-13, A-2, P-20 and so forth. SIP maintains an operating temperature of 70C in mold components, ensuring no change in metallography in the bulk substrate. Both surface imperfections and highly polished decorative surfaces are maintained through this conformal process. Corrosion-resistant metals can be impregnated into mold component substrates to combat extreme environments and high-wear applications. We will gladly answer any questions.
-V. Sciortino, Rapid Prototyping Corp., Longmont, CO. (970) 535-0550.

DiamondBlack Technologies offers a coating that will prevent wear and corrosion on your cavity surfaces. The coating is harder than chrome at 90 to 93 Rockwell C. It is applied at temperatures less than 250F and will not have an adverse effect on the steel. We have customers that are heavy PVC molders that swear their tools stay in perfect condition, even after one million shots, by using this coating. You don't even have to make dimensional changes to allow for the coating as it's only .00008 inch thick per surface.
-M. Hill, DiamondBlack Technologies, Conover, NC, (800) 368-9968, ext. 3132.

Our company offers ionitriding, a diffusion-based surface hardening process for molds. It creates wear-resistance of .010- to .020-inch depth and hardness of 62 to 65 Rockwell C and over. Call for details.
-G. Idriceanu, Sun Steel Treating Inc., South Lyon, MI, (877) 471-0844.