Plastic Injection Molding - Measuring Mold Filling

Thanks a lot for the valuable information.

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My father was making molds for plastic from steel. First rule was that mold has to be as symmetrical as possible, with feed going to center of plate and then branching out to fill molds, and each mold had outlet for air in form of 1mm long 1/2mm diameter pipe that become conical and 1/2cm in diameter. It was made between plates so half was on one plate and half on other. He did it all mostly by hand because tools were scarce and there were no computers nor CNC machines. Based on experience, he later made round holes where product engraving should be and inset round brass pieces with engravings, or fixed brass backside form at least, because it did not corrode, acted >>greasy<< with plastic so it was easy to open mold and get part out, and it was softer to engrave than steel. Also, seriall engraving by pantograph from one bigger model was possible and in case something went wrong in process of engraving (like broken bit for engraving making damage), only one inset was lost, not whole mold. Brass also conduct temperature better so there were less problems with cooling of plastic too fast, and faster cooling when cooling was required.....

Since air outlets were so miniature and formed balls or short sticks on the other side, it was simple to break them away by hand leaving allmost no trace, and they could have been made so that they were engraved on mechanism that push product out of mold, cutting those tiny strands of plastic away in process. There were variants where act of pushing product out was tearing this tiny sticks off product and they remained in mold, to be expelled by force of air comming out of mold cavities when new dose of plastic was injected. Since this pieces were even smaller than originall grains of plastic, they were fed back into heating chamber without problem. Same technique was used to remove plastic from inlet channels........

I hope this would help, even if today there is whole science formed about molding and there are programs to simulate this to find optimal solutions in each case.....

THANKS!!Henrik14..

Pressure sensors in the cavity can be enormously useful in monitoring the process.

Check out http://www.rjginc.com/

David Hunt, PE

davidhunt@outdrs.net

I am a new comer to plastic injection so please be patient! Do the steel molds show wear in certain areas and not in the other? If so what is done to "balance" the wear. Besides nitriding are there other surface treatments of the molds to reduce wear in critical areas? www. technovations.ws technovationsintl@gmail.com

Injection of plastic do not wear steel. But, if mold get damaged by accident, it usually has to be replaced as whole. Plastic would more likely glue itself to steel mold then to Brass insets, and those are not so prone to micro crumbling because of rapid and frequent changes in temperature of mold. Steel is also more prone to material fatigue so cracks can develop due to normal usage. I have seen molds cracked in half because of this.......

Sorry that it is just >>second hand<< expertise that is also 20+ years old........