Lexan 63 Glove box door

[Larry L.]

At a flee market a member of our chaper bought for two dollars a really nice Lexan golve box door. When he brought to our chapter meeting. We all looked it over and decided it was a female mold of the real glove box door. You could see the ejection pins from the mold. Anybody know how the orig. doors were made. We guessed that the part was a master for a plaster mold, that was used to make the orig. molds. It a perfect mold, correct in every detail but exactly opposite of a real door.
We need help on this one.
Thanks
Larry

[Chuck S.]

Re: Lexan 63 Glove box door

Molds for thermoplastic injection molding consist of both the male and female halves. For a part the size of a glove box door, the full mold assembly would be a layered steel volume of 3-4 cubic feet (assumes only one cavity needed). You may have found just one of the cavity plates; if you can see the ejector pin openings, then it's probably the backside surface. If it's not hardened steel, then you may be looking at a prototype used to make one of the cavity half plates.

The male and female halves of the mold are aligned by large "guide pins" that permit the two halves to be opened for ejection of the part, and then closed again with precision. The line on the part made by separation of the mold halves is called the "parting line". The glove box door details would be "machined" (EDM) and polished into each half; one half cavity surface for the front of the glove box door, and one half cavity surface for the inside glove box door surface.

The injection cycle starts when the molding machine closes the mold. The injection screw begins to turn, melting thermoplastic pellets from a feed hopper. After a few seconds of screw turning to melt the plastic, the hydraulic cylinder in the molding machine forces the screw forward in the barrel to inject molten plastic into the mold's channel and gate system. On a large part like a glove box door, several "gates" or small diameter openings will have to be distributed around the part to provide uniform plastic filling from one side to the other without cold joint lines.

Some defects in plastic parts are due to poor mold design, and can not be removed by processing. Some defects are due to poor part design. Other defects are from improper processing (cycle timing)...it all has to be right to get a good part. You may have observed plastic parts with a depression on a flat surface...that is called "sink", and it generally occurs opposite large masses of plastic like a reinforcing gusset. Since the plastic shrinks as it cools, the mold has to be slightly "over-filled" to avoid "sink", and the pressure held on the injection screw a few seconds until the "gates" solidify. This prevents the molten plastic from escaping the cavity.

The molds have cooling water circulating through them to quickly cool the part to a solid piece of plastic. After the cooling period, the mold opens and the molding machine hydraulically actuates the ejector pins to kick the part out, and the cycle starts over again...cycle time will be less than one minute.

Lexan is the GE brand name for polycarbonate, an extremely strong, tough, resilient thermoplastic. I believe the plastic headlight covers on the C6 and other cars is polycarbonate...a lick from a concrete chunk big as your two fists will bounce off at 60mph with hardly a scratch. I doubt the door is made of polycarbonate because I don't think it had been invented in 63, but I could be wrong. If it IS polycarbonate, then the General was out on the leading edge. Polycarbonate is also expensive, and the extra cost would have required an application with justification.

[Jack H.]

I agree with Chuck...

A polycarbonate material and injection molding would have been 'out of line' for the intended application (simple interior glove box cover). In those days, Zora was fighting/scratching for every penny in his tooling budget!

It's more likely a different clear plastic material would have selected for the application...one that could be easily fabricated with simple vacuum-form technology.