Re: Circles in Plastic Castings


Dennis Storzek
 




---In STMFC@..., <chris_hillman@...> wrote :


Thanks Schuyler, ---- So THAT'S what they're from. Strange thing is many other hoppers & other cars show no signs of these markings, at least not in clearly visible areas. Either the other cars were ejected differently, or the Proto 2000 (and others) were ejected too soon when the plastic was still too soft ???
 
Paul Hillman
================

Where the ejector pin prints end up is dictated by how the mold designer orients the part in the mold. There have been many molds built over the years, and each designer has to weigh all his options, then make what he feels are the best choices.

Aside from where the ejector pins will go, the other consideration is where the plastic will be injected into the part, the "sprue" and "gate". Unless this is a really complicated mold, the "E pins" will always be on the side opposite the gate.

If you want the pin prints to be on the underside of the slope sheets, that means the gate needs to be in the center of the interior of the part, which either puts it on the cross ridge, or the centersill over the center hopper on a triple. In addition, it puts the gate deep into the part, which means a long sprue. Since sprues need to be tapered, the longer it is, the bigger the scar it leaves on the part. Heated sprue bushings can allow a "pin point" gate, but accommodating the heated bushing in the narrow confines of the core that makes the car interior becomes problematic.

The other way to design things is to orient the part so the gate is on the underside, between the hoppers. This puts the e-pins on the slope sheets, but since the slope sheets are the traditional location for car weights, they  can be covered by separate slope sheet parts.

Indented pin prints are not from ejecting the part too hot, but is rather intentional to make little seats so the part stays on the pin ends and comes out of the mold straight, without dragging a detailed surface on part of the mold. It also helps insure that the pin prints don't stand above the surrounding surface and form shallow bumps that get in the way of other parts. Ejector pins in the typical mold for an HO scale body are somewhere about 6" or 7" long, and change length relative to the rest of the mold as the mold temperature changes, so a perfect match is seldom possible.

Dennis Storzek


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