I knew a guy who made a machine like that. I don't think the injection cylinder had any seals at all. It was just a long heated vertical cylinder with a funnel at the top, and a really big air cylinder above that drove a close- fitting ram into it from above the funnel to force an injection.
He could make very good parts by precisely choreographing the cycle, all done by hand.
- Insert mold
- Measure and pour granules
- Wait for granules to melt
- Inject
- Withdraw ram, release mold clamps
- Disassemble the mold with an impact wrench
- Extract the part
- Reassemble the mold with an impact wrench
- repeat
His cycle time was usually around a minute. He'd tell you it was longer while negotiating prices, but you could hear the air cylinder in the background and time it yourself.
His molds were 'laminated', i.e. made from pieces of aluminum plate, keyed and bolted together, with no draft.
I think there are a few other guys who do things similarly. If you just need parts to use or sell, it would be cheaper to find one of those guys and work with him. If you're intending to go into the business, you'll need to accrue a lot of odd skills to get started.
Like Mike says, a simple ram can work. A hydraulic ram gives a lot more control than an air cylinder though, and I think is a lot safer with regard to hot plastic potentially squirting out if the injection stroke is fired without the nozzle having a good seal with the mould.
Regards
Pat
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