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Tooling Presses On

Many plastic parts are made with the process of injection molding. This involves creating two large blocks of steel that have the mold shape, jamming them together, shoving melted plastic in at 50-200 tons of pressure, letting it cool briefly, pulling the blocks apart, and pulling the piece of plastic out. This process is then repeated many many times.

Designing the parts for injection molding is challenging. The wall thicknesses must be even, how the plastic flows must be accounted for, and texture must be added to hide defects in the process. All parts must have draft as well, which is the angle of the part so that it can slide out of the mold easily. You have to take care to avoid undercuts and side actions, sink and part lock.

Then the part is translated into a mold. The mold designer creates a mold that has two pieces, sprues that carry the plastic from the gate to the cavity, and sometimes cooling lines. They add ejector pins as well, to push the part out of the mold. The material choice is important, too, as different materials have different levels of shrinkage, and so the mold must be sized accordingly.

As of this writing, the mold making is a week ahead of schedule, but we’ve run into a bit of a problem. When the part is made, the mold opens up, and it is supposed to release from one side and stick to the other, and then the ejector pins push it out of the other side. What’s currently happening is that it is sticking to the wrong side. There are no ejector pins on that side, so it just stays there and needs to be pulled out manually, resulting in warpage of the part. It also can’t be automated. What should happen is the part needs to release cleanly from one side and stick to the other side consistently. Then an automated arm drops in and a vacuum catches the parts. Then the ejector pins release the part from the other side of the mold and the arm carries the part away.

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The top and bottom are in the same mold, but when the mold separates, the two halves don’t stay on the same side of the mold. This is a problem.