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Old 09-28-2018, 11:39 AM
Eric Trulson Eric Trulson is offline
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Ceramic blow-molds?

So I've got a question for the hivemind. Does anybody have experience with attempting to use ceramic blow-molds? (Successfully or not. Even knowing about previous attempts and failures would be helpful)

To the best of my knowledge, the standard blow-mold materials for glass tend to be hardwoods, graphite, and metals (mostly cast-iron). One of the common downsides with all of those materials is the need to start from a solid billet and remove material to form the mold cavity, which tends to push individual mold price fairly high once you account for material cost and CNC machine time.

I'm compelled to try and figure out a good workflow for 3D printing a PLA/foam positive of the form to be blown, casting around it with some type of ceramic to form the actual mold, then burning out the PLA/foam positive to leave the cavity.

Doesn't need to survive for huge production runs, but a single-use mold doesn't solve the problems I'm going after. Ideally, this would be suited for doing small-volume production runs, say around 10-30 pieces before the mold fails. This would be for turn molds only.

The two biggest issues I see are lubrication/cooling (keeping the mold surface cool enough to stop hot glass from sticking to it), and avoiding cracking or thermal shock in the ceramic (minor surface cracking might be fine, but as soon as things shift enough to expose a jagged edge to the glass, that mold is toast). My initial instinct would be to use a fine-grained high-alumina castable with a decently thick wall (1"+), and use a combination of wax and graphite powder for the lubrication.

Any stories of past attempts at this, or thoughts for better approaches?
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Old 09-28-2018, 03:24 PM
Jordan Kube Jordan Kube is offline
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In my travels down the PLA/foam core with ceramic overlay I ran into problems with the clay shrinking and cracking around the form. You would probably want a castable type product with low setting shrinkage.
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Old 09-28-2018, 03:28 PM
Jordan Kube Jordan Kube is offline
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Might be best to go from foam or PLA to plaster, then to clay.
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Old 09-28-2018, 05:42 PM
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Why not plaster?
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Old 09-28-2018, 08:21 PM
Rosanna Gusler Rosanna Gusler is offline
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Research quartz inversion temp when thinking about using ceramic.
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Old 09-29-2018, 11:25 PM
Steven O'Day Steven O'Day is offline
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If it is a blow mold you will need to be able to remove the glass piece so you should be able to get the PLA out without burning it out. Like Jordan said ceramic would not work because it shrinks as it dries and will crack around the mold.

I would try plaster, make sure the PLA is sprayed with mold release or vaseline.

Metal casters use a material referred to as ceramic shell. It is much tougher but requires a setup to keep it in suspension then it is fired.

Jewelers us a plaster/refractory mix that is more heat resistant than pottery but is more expensive.
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Old 09-30-2018, 12:28 PM
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Ancient blow molds are thought to have been made out of terracotta. If your mold shrinks and/or cracks then the clay body has not been properly fired.

Some blowing with terracotta molds.
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Old 09-30-2018, 01:12 PM
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Those "bench" rails are wild!
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Old 10-01-2018, 01:53 AM
Peter Bowles Peter Bowles is offline
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I've made one piece, two piece, turn and still molds using all sorts of refractory castables. Often cast into a plywood box to give structure and to allow hinges, handles, latch clamps and wheels to be easily mounted. I lined them all with cork powder and pva. Works fine, no need to overthink this stuff.
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Old 10-01-2018, 02:45 PM
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If you make a refractory blow mold, as Peter suggests, you could burn out the PLA without worrying about release agents, etc. I would still seal the PLA first with some shellac or similar product.
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Old 10-01-2018, 03:33 PM
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Old 10-01-2018, 06:39 PM
Peter Bowles Peter Bowles is offline
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Learning to blow nicely into molds is a whole skill set in itself and can challenge your normal ways of setting up bubbles, gathers and timing greatly. Give some real thought to how you intend to finish the work (hot or cold) when you are designing the mold.
One of the secrets of blowing into molds is to go in with the glass as cool as you can get away with - not as hot as you can.
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