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sizzer33 (Mechanical) (OP)
4 May 11 4:54

I would like to find out what is the most economical way to manufacture PMMA cubes with dimension of 25X25X25(mm). It will need light texture on the surface. Material to be used is Plexiglass PMMA 8N. I've only experience in injection molding and see that the part is not really feasible for this process. Any advice is appreciated.

patprimmer (Publican)
4 May 11 7:24
Cast it from PMMA casting resins.

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patprimmer (Publican)
4 May 11 7:25
Alternativly machine it from 25mm thick sheet.

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MacGyverS2000 (Electrical)
4 May 11 9:27
I would machine it from bar/sheet stock (hmmm, I'm starting to sound like a broken record).  Someone else has already done the molding part, all you need to do is cut, possibly some media blasting to get your texture.

Dan - Owner

sizzer33 (Mechanical) (OP)
5 May 11 2:11
Thanks for your suggestions. There will possibly be other shapes like half circle and cylinder shape. I'm actually wondering how some of these jewellers make the acrylic jewels because i'm thinking we would need suppliers like that.

So from your suggestions given, machining from PMMA stock would be a feasible way to do it for production phase. We're looking at 500,000pcs per month type of volume.

I saw in YouTube they do like plastic caps with compression molding and they were really fast. Can this be done with compression molding too?
patprimmer (Publican)
5 May 11 4:04
At 500,000 per month I would injection mould with a lot of cavities in the mould.

I would estimate at least 2 minute cycle time, probably more like 3 or 4 min. Acrylic tap handles typically took about 90 sec I think.

That's about 12,000 a month per cavity 24/6 production.

That equals 45 cavities allowing for a few not working.

With a 24 cavity mould it would require about 10 tonne min clap and a 500 gram shot.

It would require at least 2 of these to keep up with production.

Other possibility is cell cast. Cycle time is a lot longer, but tooling is cheap so a few hundred cavities is feasible. Materials like acrylic sheet are traditionally cell cast from a pre-condensate or partially polymerized syrup.

Parts will weigh about 20 grams each or 50 per kg.

500,000 parts is 10 tonnes per month. Raw materials suppliers will be very happy to give you all sorts of advice as to how to make them either way if they believe you are for real and have a realistic project.


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sizzer33 (Mechanical) (OP)
5 May 11 5:30
Thanks Patprimmer. I guess we'd have to weigh in the pros and cons of injection moulding process & casting to see which are cheap and more efficient. Possibly will need to look for some casting suppliers.

From your experience in cell casting, could you advise typically how many cavity moulds you can build and whats the cycle time like? I would assume in production the whole casting process would be automated from putting in the liquid PMMA to ejecting the parts?   
patprimmer (Publican)
5 May 11 8:52
Probably it is not all that automated. Control of exotherm is the big issue. You heat it tocure it, but it is exothermic. To much of a start and it boils the monomer and makes bubbles. To little and it does not fully cure.

They cast acrylic sheet between flat sheets of glass clamped together with spacers of the required sheet thickness in between.

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