The Designer's Edge: Parting Line Bearing Surface

Randy shares his view on the impact of adequate parting line bearing surface on venting.


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Coining that occurs from a lack of parting line bearing surface.

A lack of venting or closed vents causes maintenance and part quality issues. Today I’ll break down how parting line bearing surface impacts venting. Closed-off vents occur every day as toolmakers maintain thousands of molds, and in most cases this relates directly to a lack of bearing surface. Parting line bearing surface helps keep vents open, but it is often overlooked as a solution. The more bearing you have, the better.

Parting line relief used to be a standard process because of all the hand work required to spot a tool., but with today’s technology and being able to cut very close to true net, parting line relief is not a necessity.  Adequate parting line bearing surface reduces coining of vents, parting line burrs along cavity edges and stress on shut-off details that can break. I have a tool that has actually been coined in 0.025 over the years to where the vent relief channels where actually closed.

It would be beneficial to industry if it would focus on tonnage to square inches in relation to bearing surface. It boils down to the press tonnage and actual square inches of bearing surface on the parting line. I say “actual” because in many cases intended bearing surface is ground away by the toolmaker in the spotting process. You cannot just calculate the designed bearing surface when calculating tonnage per square inch. If you can reduce your tonnage per square inches by half, your coining will be cut in half.

I prefer zero parting line relief. For mold bases that have cavity inserts, parting line bearing pads should be used where the mold base outside the cavity is typically relieved. Moving from a 2 inch x 2 inch (4 square inches) bearing pad to a 4 inch x 4 inch (16 square inches) increases your bearing surface by four times. Although this may seem like basic math and logic, it is often not obvious.


Close up of coining.

Next month, I will move onto the venting of mold components.