Mold polishing 101: A veteran's instructions

By: 
July 17, 1999

Walter Dinkel immigrated from Germany to Chicago in 1952. He started polishing metal and molds in 1953, and he’s been at it ever since. An expert on mold steel quality, one of the most basic tasks Dinkel has perfected is that of quickly polishing a mold. And the thing of it, he says, is that it’s not as hard as most moldmakers think.

“Mold polishing is a peculiar trade,” he explains. “It’s a mystical thing for many toolmakers. They think it’s a trade secret, but I like to share my technique. My competitors think I’m nuts. But it’s no secret.” Aside from the obvious goal of creating a shiny, mirrorlike finish, Dinkel also uses polishing to assess the quality of the tool steel with which he’s working. He says moldmakers can be overwhelmed by the complexity of polishing instructions often provided by steel producers and steel suppliers. But, with the right tools and just 10 to 20 minutes, he says any competent moldmaker can achieve an excellent finish and learn a lot about the steel he’s polishing.

Following are his instructions to achieve an SPE-SPI #2 finish on any tool steel used to construct cavities or cores for injection molds.

1. On a ground (flat) surface, use a flat stone about 1-by-1-by-.25 inch with a center detent to permit room for a profiler drive. The stone should be in the K hardness range of either aluminum oxide or silicon carbide. The profiler should be adjusted for a 2- to 3-mm stroke. The area to be flat stoned should be worked until all grinding marks have been completely eliminated. The final grit stone mark count should be 600.

2. Remove all abrasive stoning grit from the surface. Then, using a number 15 or number 9 µm diamond compound, brush out all 600 grit stone marks with a Tampico fiber rotary brush.

3. Follow the brushing with a number 9 µm diamond compound on a hard felt buff until all brush marks have been removed. After this buffing, clean the polished area.

4. For the final finish, use a soft felt (F-10) buff with a number 3 µm diamond compound. Use a naked fluorescent light to assess surface quality and detail.

The key to this relatively simple polishing process is the diamond compound. Dinkel says it should have a minimum of 1 carat (200 mg)/g concentration. “Using a weak diamond compound can make a good piece of tool steel look inferior,” he says. He also says never to use an ultrasonic device if the work to be done must have a truly flat finished surface.

On unmarked steel, this polishing technique, performed on a small surface area, can help a moldmaker distinguish between 420 stainless steel, H-13, and free-machined steel.

Many ingots, side-by-side, look alike, Dinkel says. Polishing can reveal the true character of a steel. He says that 420 stainless, because of its 12 to 14 percent chrome content, tends to pit if polished with too much rotary action or too much heat. For most injection molding applications, Dinkel’s experience is that H-13 steel produces, in general, the best polished surface.


Contact information
W.D. Mold Finishing Inc.
Schiller Park, IL
Walter Dinkel
Phone: (847) 678-8449
Fax: (847) 678-8450

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