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Aluminum cavity inserts in a steel mold base?

Aluminum cavity inserts in a steel mold base?

Aluminum cavity inserts in a steel mold base?

We are going to make some aluminum cavity inserts for 3 different molded parts that will be interchangeable in a single cavity mold base. I am most comfortable with a steel mold base but am concerned about thermal expansion. The inserts will be about 10" square. The material we will be running these is acetal with a mold temperature of around 125F -160F.
Anyone have any experience with this? My fear is that the inserts will expand and crack the steel mold plates. Am I worried about nothing? Is there a better practice?


RE: Aluminum cavity inserts in a steel mold base?

I am mostly experienced with a molder who made his molds from aluminum plate, laminated with high strength bolts, wrenched by an impact wrench for part ejection, and shot mostly in nylon and acetal. He won awards for some of the parts, and always met our expectations wrt size and finish.
I had one thick Delrin part that he managed to mold with shrink marks, i.e., cavities, on the _inside_, and nice flat surfaces on the outside. Much to my regret, he has not been available for consultation since his death; I'm sure he's none too pleased about it either.

I tried insert molding, once, using steel inserts of ~2" dia in steel mold bases, with another molder. Our intent was to allow production of up to six different parts of similar size in one multicavity mold base, which was sort of the basis for using that molder, based on assurances that turned out to not be promises, much less guarantees. We found he could make good parts with one insert at a time, but he could never get even two good parts from one shot, and the inserts didn't really interchange from one position to another either.

If your mold base has clearance around the inserts, or thick walls, I wouldn't worry about it too much, >>>provided that I had personally made a site visit and witnessed that molder swapping aluminum inserts in steel bases as proposed and getting good parts<<<. If the large inserts are your idea, just buy the individual mold bases; they are cheap.

Mike Halloran
Pembroke Pines, FL, USA

RE: Aluminum cavity inserts in a steel mold base?

Mike suggestion is good-individual mold bases are cheap.

If you still want to use aluminium cavity inserts then my question is why are you concerned about the mold plates cracking due to thermal expansion?

This suggests to me the inserts will be in pockets of the mold plates.

If designed correctly, pockets are not needed. Just bolt the insert directly to the the face of the mould plate.

Paul Kuklych

RE: Aluminum cavity inserts in a steel mold base?

Thanks for the responses Mike and Paul,
My experience is with steel cavities in standard steel mold bases. My customer is looking for cheaper cavities and this is a low volume project with 3 pretty big parts (9" diameter) thus my thought was to make aluminum cavities that would be interchangeable in pockets in a steel mold base. This mold base could also be used for different parts in the future. Thus my concern about the difference in expansion between the steel and the aluminum.
While the cost of aluminum mold bases may be cheap, for this size and the need for 3 would make it more economical to go with steel.
Paul, could you expand on your thought of bolting the cavities directly to the mold plate?
Since the pockets basically line up the inserts, how do you accurately locate the inserts to bolt them on?
Do you eliminate the A and B plates and use aluminum instead?

Thanks again for your thoughts.

Dell M4800
SW 2017

RE: Aluminum cavity inserts in a steel mold base?

We have many moulds with steel bolsters and alloy inserts set tightly in pockets. Never a problem with expansion. The only problem you will have with many cavities to fit a single bolster is that you may eventually run out of space for ejector pins in the bolster rear face and the ejector plate.

You might also find that alloy will cool the acetal too quick - giving a poor surface finish.


Politicians like to panic, they need activity. It is their substitute for achievement.

RE: Aluminum cavity inserts in a steel mold base?

The cavity block is located to the mould plate using 2 dowel pins. Its very simple to make.

If you look at the 4 cavity mould video in the link below, all 4 cavities & cores are directly bolted to the plates with dowel pins present on the cavity side.


I avoid designing moulds with pockets because usually they serve no purpose. The cavity & core inserts align themselves using some form of taper locking between them. Machining a pocket is just extra work. If you have a pocket then have a good reason for it.

In any case if you already have a steel base with pockets then it will be cheaper for you to just use that.

Paul Kuklych

RE: Aluminum cavity inserts in a steel mold base?

Thanks again Paul and Pud.
I do not have a mold base yet and I think I will go with bolting the cavities on the plates and eliminating the pockets. As you say Paul, there is no reason for pockets if the cavities can have some kind of alignment feature. I love gaining a different perspective instead of just what I have always seen, thanks for changing mine.

Dell M4800
SW 2017

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