Vacforming question

[Tim Horne]

Hi all, hopefully a quick question. I have made a form to vacform a canopy for my latest model. When I tried to do the forming the plastic was not hot enough to form properly but it WAS hot enough to cause the sanding sealer/ dope surface of the mold to bubble and melt.
 What finish to a mold will be heat proof enough for this to not happen please?
Thanks in advance.

[Squirrelnet]

Hi Tim

 I have had this usually when I haven't left enough time for the sanding sealer to fully dryout. Once dried out fully It works much better and doesn't stick. I use a silicon mould release agent too. General Silicon spray works well for this.

 I recently did a canopy for Andy B (|of this parish) with just an untreated balsa plug. The first one pulled the grain up a bit and bits of Balsa fibres stuck to the plastic, but the others worked fine with just silicon release spray. I have used painted moulds too but again the paint has to be well and truly cured. I guess it depends what sort of plastic you are using too. I've been using PET or PETg if you can find any thin stock. When you heat it, it goes floppy and forms wrinkles but keep heating it and smooths out .

 I use the reflection in the strip light in the workshop to see when it goes smooth again. It looks like it's tighten up but in reality I think it's probably sagged but in a smooth uniform way. This is the point to put on the vac mould. I use a varaible heat gun so I can add some more local heat when it's on the mould if looks like it needs more softening help on the mould. Get it too hot and you'll see it has a white milky haze on the stressed parts


This is the spray I use, I think it's from Amazon ?? 

[nickhawk]

Hello Tim

I suppose the mold is wood???I have a vacuum former and though I usually pull bigger canopies I use two materials to ake a mold. The first is plaster for art with the highest possible containment of porcelain. The second is epoxy but the so called tooling epoxy which can withstand up to 400 Celsius temerature. The best aterial so far is the pet g if you can get it in acceptable thickness or even ABS transparent, or acrylic .The problem is that ABS and acryliv are yellowing with time and exposure to light.

[chris46]

Hi Tim
I used HDF boards. With this material, no further processing is necessary; fine sanding is sufficient.
Chris

[Garry]

I have had this problem when I tried to heat the plastic on the mould. Heat the plastic away from the vacuum former and when it nice and hot plunge it onto the mould with vacuum on. I haven't had the problem since doing this.
Garry

[Tim Horne]

Thanks for all the replies. I shall try again and report back.

[g_kandylakis]

Hi Tim,

Garry's point is an important one. Did you try heating the plastic while on the mold?

I once got a 50% sucked piece so I had the brilliant thought of reheating it while on the plug with the vacuum cleaner on, to go to 100%.

As soon as I saw enough black marks on the plug I stopped...Did not wait for flames...

My experience with hot plastic away from the plug has been zero problems to the plug.

[Tim Horne]

Hi George, no I didn't do that. I did exactly as I have done before, although I haven't done many canopies. I heated the plastic over an electric hob and then moved it to the vacformer as soon as it looked ready. The moment the hot plastic touched the mold the surface started bubbling. That's why I assumed it was something to do with the sanding sealer and dope that I had used to get a nice smooth surface.
I think I will try to finish it with some acrylic varnish and try again. I don't really want to carve a new plug.

[Garry]

Hi Tim

I just noticed something I had forgotten about.
I do not use anything to finish my forms. I use a harder wood like basswood and finish it with 600 grit sandpaper or lately I have been using plaster to make moulds from kit canopies and it works very well. If I think I will have trouble with it releasing the part, I will dust the form with baby powder.
Hope this helps

[Crabby]

One thing that has helped me in this endeavor is the use of canned compressed air.

[Tim Horne]

I can now finish this topic as I have a useable canopy!!

Thanks to everyone for chipping in with useful tips. Crabby, you will have to elaborate on your comment above. I am not sure how you use canned compressed air for vacforming?

I had to do a lot of smoothing of the mold surface after the initial overheating and resulting bubbling and cracking of the finish. I used grey primer this time and just kept coating and sanding until smooth.

It still took a few attempts to get a decent canopy as I either under heated or overheated the plastic a few times.
The pics show the mold with the second best piece on the right and the best one on the left.

I still had a problem happening in that I kept getting an air pocket on top of the mold as I formed it which left bulges on the top. You can see it in the way the light reflects off the top of the finished canopy. The earlier attempts were worse but the last one still has a small bulge on top. It will have to do.