vacuum pot

[jasmils]

G'day René,
I use both vac and pressure when I cast. And if I do get a bubble it is usually a large one. Not small pin holes. I dont recommend Polyester resin for models. One it stinks to high heaven, 2 it is a lot more toxic the Polyurethane and 3 is a bastard to clean up.
When I used to "drop pour" or home cast, I would at times use talcum powder in the mould but not often. Don't use a needle to try to pick out the bubbles, you will only cut and tear the mould. Use the back end of a small paint brush handle and push it to all corners of the mould. This will help break the surface tension between the resin and the rubber and reduce the number of bubbles on the surface.
One thing about Polyurethane, it will thicken with cold or thin with heat. Problem with warming it first is that the time it takes to exotherm (go off and harden) is reduced dramatically. The other thing you can do is (I didn't tell you this) is to add no more then 10% turpentine thinners. BUT, and I say again BUT, it will make the resin brittle and reduce the life of the mould. I don't do this to any of my kits or any custom casting.

Here is a article of a workshop I did for my model club.
http://www.ammsbrisbane.com/newsletter.htm
Click on the August 2009 newsletter and see "Jason's Casting Workshop".

Indecently I now offer to make the moulds for people and also supply a small but detailed video on how to drop pour their particular mould. I have only started to do this service, but in the 5 days since I placed an advert at my local casting shop, my phone has been running hot.

This image is of one of my figure kits. These figures are cast with vac/pressure. The second photo is a close up of a 1/35th Austyer drop poured. And it shows what can be done with out all of the expensive machinery.

Cheers Jason

 

[Renéduret]

Jason,

thanks a lot!!

good information for everyone!

regards,
Rene

 

[ausf]

I use pressure (30 PSI) and until last week, never had any trouble with bubbles. A high pour with a thin stream usually won't trap air. Then I switched silicone and had this strange bubble collection on the soles of boots I was casting. It sort of created a large void that thickened the boot sole in the final cast. The collection was on the highest spots of the master (opposite the pour) and this silicone is 40,000 cps, much thicker than I was used to. I thinned the silicone for the next attempt and let it sit out of the pressure pot for the full 25 minute pot life. Zero bubbles or issues this time, but it prompted me to look into vacuuming, silicone is too expensive to take chances.

I found a good vacuum pump (Mastercool) that can pull 29 inches Mercury is about $120 (in the US). A chamber seems pretty straightforward to build, so the whole setup should be less than a gallon kit of silicone.

I also decided to up the pressure to 60 PSI to get the resin into the tiny parts of 1/48 weapons I'll be casting soon. I had to tear down the pot and reassemble all the fittings to maintain 60 PSI (Aves is a great thread sealant by the way) as well as pour a silicone gasket.

I definitely want to experiment with vacuuming the resin in the mold, sounds very interesting and certainly has good results from what I've seen.