Vacuforming: What Is the Max Size

Joseph Humpston

Hey There Stan Winston School of Character arts! I'm now in the process of constructing a vacuforming machine. Before I start trying to cast random stuff, I just wanted to know how big a casts I can make. I know that the limiting factor on the x-axis (length) and the y-axis (Width) is the length and width of your machine's vacuum table and frame, but I don't really how big casts can be on the z-axis (height). Does anybody know how high a cast I can make with vacuforming?

Chris Ellerby

The taller the piece (buck) you you are vacuum forming over, the more the plastic needs to stretch to envelope it.    How tall your piece can be depends on the thickness of the plastic you start with, and how thick you want the plastic on the final part to be.

Thicker plastics will require more heat, so you are limited by how much your heating elements can produce.   Thicker plastic will also smooth out and reduce details.

One general rule followed in vacuum forming is you want your heated plastic to slump down (sag) about the height of the buck you are pulling over before you make the pull.  So your Z axis needs to be a little more than twice the height your buck.

The width and length of the plastic sheet you are using also factors in, as the plastic can only stretch so far before tearing.  For taller pulls you need a larger sheet/machine.

There's no general limit for how tall a buck can be, as it all depends on the shape/detail/volume of the buck and the size and thickness of your plastic.

/Chris

Joseph Humpston

I honestly think that the tallest cast I'd make would be about 12 inches high. I intend to use 1/16" Poly-carbonate sheet. My machine is about 24" by 24". Right now, the heating element is basically parabolic dish heater at the bottom of a wooden box! I might eventually upgrade to using am IR heater. What do you suggest?

Chris Ellerby

You should be able to go as far as 12 inches, but that depends on what the buck you are using looks like.  Any sharp corners are where tearing would be likely.  You may want to test from 1/16 to 1/8 polycarb.

/Chris

Joseph Humpston

Thanks for the tip Chris! Also, is possible to do a vacuform cast of an Oil-clay sculpt?

Chris Ellerby

That would be problematic for 2 reasons.  First, the plastic is very hot, so you never want to form over something that can be damaged or distorted with heat.  Second, the vacuum pulls the plastic over the form with some force, so that pressure would likely distort your sculpt.

You could try coating the sculpt in a few layers of spray paint to protect it a bit, and freezing it so it would take longer to heat up to the melting point.  That would give the plastic time to cool and stiffen before the clay deforms too much.  But even then you may run into problems.

I would test it a few times with a simple test sculpt.  But generally you want to form over rigid bucks that will not be damaged by heat. 

/Chris

Joseph Humpston

@Chris Ellerby Okay! So, casting sculpts in Plasticine Clay, not a good idea. What if I used wed clay instead? Actually, what kind of materials would you recommend vacuform casting?

Chris Ellerby

WED clay will still be a bit on the soft side.  While it does firm up when it dries, it also then starts to crack.

Making a mold of your sculpt and then casting up a buck in plaster is a common technique.  Plaster is great for vacuum forming over as it is nice and rigid.

You can also carve/sculpt your bucks out of rigid foam (like renshape), or MDF wood.

/Chris