Fried eggs, frying pans and blowing air in the wrong direction

Mold releases are a black art in my book, mainly because they have to do the impossible: Let the part fly out of the mold, yet leave no residue on the part, or buildup in the mold cavities. This concept is so old and works so well in the IM industry that I wondered about the cooking spray I use on my frying pan at breakfast. In a sense, it’s a mold release, and while it lets my fried egg slide onto my plate, it does leave a residue, and a significant one at that.

So why use releases? I’ve often wondered why so many molds need them, yet the practice, in short-run molding especially, of spraying release into the mold every few shots is still with us.

One mold I worked on involved a small TPO gasket that refused to stay with the cores, preferring to hide on the cavity side of the parting line where it could cause maximum trouble, mainly because the machine optics didn’t see a potential crash. The press cycled happily, and the inevitable disaster followed. With no time to work out a real solution, which would always involve pulling the mold, the operator was issued a half a dozen cans of release and instructed to shoot into the cavities every third or fourth shot. It worked, but only because the part was non-critical, didn’t need finishing and stayed profitable, despite the extra steps.

Was there a better way? Ejection was always suspect. I must confess that I’ve always felt that many mold designers skimp on the ejection side. Why not add a pin or two? Cost, of course, but in many cases where a faster-cycling mold hung up on parts cocked in the mold during ejection, the common solution I’ve seen often is an ancillary air blast, or … spray releases. Both often require longer open times and add cost that could be avoided in the first place by adding pins.

One trick rarely used in the smaller cold-runner molds is to pressurize the venting. This is one that I refused to believe when an old moldmaker explained it to me, but the more you think about it, the better it looks when the job can’t justify serious reengineering.

The “old trick” I’m referring to involved routing compressed air through the ejector pin bushings or vents to give an added push as the mold opens. “Isn’t that blowing air the wrong way?” I asked naively, suggesting that such a system would require extensive valving, with controls that would sequence the air blast just as the mold opened, while allowing venting to ambient as the cavities packed. As it turns out, however, moderate pressures don’t inhibit the outrush of vent air, but can add a little extra push at mold open, while operating continously.

This still seems counter-intuitive to me, but I’ve seen it work with as little as a drilled hole and a quick-release connector feeding regulated shop air. It’s ugly, but considered as an “internal air blast” it can work brilliantly if there isn’t enough clearance, open time, or a long enough production run to justify a serious fix.

At a production rate in my kitchen of one twenty-fourth of a part per hour, however, the economics still suggest a shot of Pam.