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Vacumforming canopys?????

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I am using a dental vacforming machine (plenty on sale on ebay and the like, sourced from China)

The problem with these, and I suppose with the Mattel one, is the lack of fine temperature control. When using PETG (one commercial brand is Vivak, but there are others) you need to control the temperature to avoid bubbles forming in the plastic (bubbles come from the moisture trapped IN the plastic and vapourising). On my machine, this is done by scrutinising the plastic sheet and adjusting the height, i.e. distance from the heating resistance. Not always successfully in fairness.

Solutions for this : pre-dehydrate the PETG sheets by letting them one or two hours in the oven at 50° C, or heat the plastic at 150° C by plunging it in hot cooking oil in the machine for French fries, assuming you have one with fine temperature control (but a cooking thermometer can help here). Any vacuum source, like the Mattel machine, will work then. A plywood box with holes plugged to the domestic vacuum cleaner will also work ;) 



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This threads reminds me of the issues Zacto had getting canopies for his A-7 correction kit.  He made it sound like vacuum forming canopies was some kind of lost black art.  It was his reasoning for discontinuing the kit.  

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It's not a black art but it takes some setting up to get consistent results, here are a few tips, btw I've been doing this since 1987.

1. Your master part needs to be strong and very well finished and shiny, a trick to get this made easier is to vacuum form over your master and then vac over that again, I assure you it works, and you get a crystal clear result.

2. have your master raised off the deck by a slab that recesses in slightly, that way the plastic will tuck under slightly rather than web out which leaves you with no edge to cut to.

3. A lot of machines have timers and if you are doing mulitples it allows you to get the correct heat setting before pulling your mould up. The plastic usually drops down as its melting and then tightens up, once you see a little bit of smoke a few seconds afterwards this is the time to pull up the mould.

4. Allow the mould to fully rise into the plastic before you put the vacuum on.

5. Some plastics need to dry out before use to remove any hint of moisture before starting.

6. If you ever get a forming stuck to a mould a quick blast from an airline between the mould and the edge of the plastic will pop it off easily.

7. Don't be too stingy with the size of the sheet you are going to use the more you use the less likely 'webbing' will occur.


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