vacform plug/plaster casting-what release agent to use?

[Sulla]

***EDIT***
I changed the title to reflect the direction this thread was heading - plaster casting. See posts #5 through #7 for the change of direction.

I want to vac form part of the inside of an acrylic dome that is too big to put on my little vac form box. I want to make some thin detail panels to apply to the outside of the dome.

My first plan was to press some clay onto the inner surface of the dome and pull it out (hopefully intact) and use it as a plug for my vac form box

Questions:
(I am trying to keep this down to what I have on hand if possible.)

1. I am plannning on using a medium sculpting clay I normally use for molding and casting. I am betting I will have trouble with it sticking to the inside of the dome when I want to remove it for use as a plug. Is there a 'clay realease' I can use for this, or some other trick?

2. This is not a clay that will harden with backing or anything. If left cold for long enough it will stiffin somewhat. I want to form .020" styrene over it in the vac form box. Will the heated plastic distort the clay when the vac forming proccess takes place?


3. Is it possible to pour resin into the acrylic dome using powder as a release agent and just let is swirl and pool to make the surface of my plug? It would be thin, but I would back it with lots of clay and can putty/sand the surface to get out any glaring omperfections to make the vac form plug.

 

[CustomCreations]

Re: Need help making a specific vacform plug

YOu may be lucky with the clay as the plastic is pretty thin, although personally Id avoid it and use the resin option.
If you want to keep it very cheap etc you can use a few layers of the SHINY brown packing tape as a release. If you really bed down the tape with your finger you can loose most of the wrinkles caused by the internal compound curve. If the resin cures fairly quickly (10 mins/15 mins?) it wont effect the tape at all- just give the dome two layers of tape- the last layer in opposite direction to the first layer. The cast 'should' just drop out once cured.
D

 

[Sulla]

Re: Need help making a specific vacform plug

Thanks. I appreciate your advice.

As with all good answers, there are a dozen more questions. Here's only two just spawned fresh:

1. If I use a commercial release agent (like a brushed-on liquid) on the acrylic surface, would the resin stick to the acrylic?
2. Will new liquid and un-hardened resin bond to already hardened and set resin?

Also, the dome is slightly flexible, so it can be flexed to release the resin.

 

[Sulla]

Re: Need help making a specific vacform plug

rmeyer over at astromech.net suggested I use Bondo to cast the inside edge of the dome. What say you all?

 

[Reactor drone]

Re: Need help making a specific vacform plug

For making parts that will go on the outside of the dome you'd be better trying to replicate the outer part of the dome for the plug rather than the inner.

I'd use plaster to mould part of the exterior of the dome,then use bondo or fibreglass to make the plug(after waxing the plaster of course).

 

[exoray]

Re: Need help making a specific vacform plug

Quick and dirty take a sheet of plastic, like a garbage bag, press it into the dome and then fill the void with Plaster of Paris, the sheet of plastic and shrinkage of the plaster should allow you to remove your plug as long as there are no undercuts... The bag should also release from the plaster short of any creases and such that are caught, but because it's plaster you can simply cut or pull out any remaining plastic bag and then patch it up with a little more plaster...

In the end you should have a nice solid buck to form over...

Also I don't know how complicated the shape is but you might be able to get away with simply Vaseline'ing the hell out of the dome and then pouring in Plaster of Paris, do yourself a favor if you do it this way and mold in a handle, it can be as simple as a rope with a knot on it where the knot is in the Plaster of Paris so that you can tug and pop the plaster out...

Caution should obviously be taken as you could get the buck stuck your dome and then have to deal with getting it out... Any undercuts even small or minute will create havoc with a solid plaster buck...

 

[Sulla]

Re: Need help making a specific vacform plug

Thanks everyone for your great ideas! I really appreciate them. Man I love this forum.

Casting plaster is rather cheep isn't it? A few containers of it from Hobbytown USA should work out for me, then. I can do a long section like in the pic posted earlier in this thread, and I can do a separate casting of the apex of the dome its self to vac-form a 'bowl' for the difficult panels I want to make up there.

I'll review my Molding and Casting Handbook again as I've never done plaster casting, need to figure out a release for the plaster, though that plastic bag idea sounds cool; should be fun actually. 'Bout time I learn anyways.

 

[Sulla]

Had some time to read the plaster casting section of my handbook. Looks like I can use Vaseline for a release agent.

I called my local Hobbytown USA - they don't carry Plaster of Paris, but they do have Hydracal in 1 gal. containers. I'll pick some up this week.

 

[Skaught]

I used Vaselines on my plaster over plaster mold/plug. Worked just fine, but the brush strokes in the Vaseline showed up in the plug.

Scott

 

[Sulla]

I used Vaselines on my plaster over plaster mold/plug. Worked just fine, but the brush strokes in the Vaseline showed up in the plug.

Scott

Thanks for the heads-up Scott. A little sanding is what I'll need then.

 

[Reactor drone]

Spray on cooking oils work fairly well as release agents for plaster,especially on clay/plasticene.

With the dome having a little flex it should be fairly easy to pop off a plaster part since it won't grip tightly to a smooth plastic surface.

 

[Fulcrum]

Spray on cooking oils work fairly well as release agents for plaster,especially on clay/plasticene.

With the dome having a little flex it should be fairly easy to pop off a plaster part since it won't grip tightly to a smooth plastic surface.

I have seen pam "cooking oil used aswell.:thumbsup

Also,I have heard of using WD-40 but you need to ask
around on that from some of the more knowledged members.I havent used it.

 

[Sulla]

OK, not vacuum formed, but I did use the frame and the oven. I need to make a plug to form panels for my R2 dome. My oven is not big enough to form the whole dome, so I need to make a plug of part of it.

Now, I may be going about this backwards, but I decided to form the part of the dome I wanted by just pressing the hot plastic over it. Then I can use this to make a plaster plug by pouring plaster into it. You can probably see the little bumps and imperfections in the plastic caused by the tape on the dome. This will have to be fixed on the plug using putty and sand paper.

Here's shots of what I did. The dome is mounted on a scrap wood base to keep it steady while I formed the plastic over it.

 

[Sulla]

I poured a hydracal plug last night into the vac formed shape last night. I used petroleum jelly for a release. I'll pull it tonight after work. It'll need some TLC to smooth out just right, but it should work for a great plug.

I also started a blog about all this. So far it's the same thing I posted here:
http://www.sullasdroid.blogspot.com