Selcting the right injection molder

[Andy Fritz]

Hi, I realize theis might not be the optimal form for injection molding, but I am sure there must be some one know more about this than I do, as I am a complete novice.

I am going to buy a used 80's machine and the size doesn't seem to affect the price much. I would use it to produce parts ranging in weight from 0.5 - 7 oz.

These are my questions: Is the mold size directed by the platen size of the machine? If I am running a small part that would normally be ok in a mold of 2*3*3 In. Would I then be forced to use a mold of say 8*6*10 just because this happens to be the size of the machines platens and minimum mold thickness? It would make sense if the injection pressure can not be adjusted, but is this usually the case?

When shot weight is specified, am I right to assume the practical shot weight is only around 65% of the stated shot weight?

Are injection molders typically longlived? I have an old Bridgeport mill from the early 80's and it just keeps running like new. Are molders more sensitive?

At ebay just about any machine is selling from 2 - 10K USD. Are these machines usually ready for the scrap yard or could they be ok for low volume operation?

Andy

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[dpryor]

Andy,

You have a lot of questions! I am not a molder but a mold designer so I can answer a few of your questions. As far as sizing the mold to the press, it is a combination of factors. If your mold is too small, then you run the risk of crushing the parting line due to clamp pressure or peening the mold into the platens. Your platen size just limits the size of the mold. It is important to size your part shot size to the press. That way you dont leave material residing in the barrel too long.