Need some pressure casting opinions/help/info

[Kylash]

So Im casting up a few add-on parts for a Transformer action figure, using smooth-on smooth-cast 1:1 urethane resin. Im getting pinhole bubbles on the surface and have done everything i can regarding vibrating, shaking the mold etc, and I still get them.

Im thinking my next step is to go to pressure casting, but know nothing about it, dont know what type of vac-pump i need, and am afraid id end up imploding something and injuring myself.

The pieces are quite small and have thin areas, I have a mold to make 2 at a time. Anyone have any tips of what I could do, and cheap?

 

[TK570]

PM sent.

Lonnie
Tk570

 

[Peter P.]

Are you using a silicone mold?What release agent are you using?
If you have a silicone mold, use babypowder or talcum.
smooth on has very good spray release agents.
one thin coat in the form,let it dry and then a thin layer babypowder (this eliminates the surface tension of the resin.
Forget abou shaking the mold.put the resin in and then rotate it slowly with your hands.this prevents the trapped air bubbles from sticking on the silicone.therefore a rotocast machine is nice,but they are expensive, but it will work with your hands too!im working for 25 years with resins and epoxis, and this is the most practicable way (and cheapest of course!)

 

[Kylash]

Yeah its just a small silicone mold. I dont ususally use a release agent, but i do have spray mold releas I use when making the 2 part mold.

 

[clonesix]

Pressure casting is done with a pressure pot and and air compressor. You can find a pressure pot on ebay.

Also with pressure casting, you need to have bubble-free molds. So the RTV needs to be degassed prior to pouring. If there are air bubbles n the RTV, they will compress under pressure and distort the mold.

As a last bit of advice, change resin to a slower cure (30 min) resin, and degass prior to filling mold for pressure pot.

 

[Kylash]

yeah that would be my problem with pressure casting, would need to make a new mold because it does indeed have bubbles within it.

 

[punisher]

have you tried brushing a thin layer of resin on the inside of the mold? I had the same problem and this seemed to work. are you using the same resin that comes with the starter kit that also included oomoo?

 

[LeMarchand]

If the molds fit in your pressurepot, you don't have to degass your silicone per se. When i make molds i will be using for pressure casting, i let the molds cure under pressure. (i cast under 5 bars, and let the silicone cure under 5 bars as well. Little trick i learned from Rob (replicaprops) and works like a charm

 

[franz bolo]

Can you post close ups of the pinholes?

I got the same thing. I had to fill them with Bondo Glazing Putty.

FB

 

[exoray]

As a last bit of advice, change resin to a slower cure (30 min) resin, and degass prior to filling mold for pressure pot.

That would be my advise, vacuum degassing the resin after mixing and before pouring into the mold... It makesa world of difference in the amount of small air bubbles, not only on the surface but throughout the entire casting...

Pressure casting also works as well, but sometimes requires a new mold as you have to have a reservoir of 'extra' resin as the pressure reduces the volume of the resin in the mold...

 

[punisher]

I still think brushing a thin coat of resin onto the mold would be a cheaper alternative.

 

[exoray]

I still think brushing a thin coat of resin onto the mold would be a cheaper alternative.

If you can get into the troubled areas that can work, not always an option depending on the mold design... And you can still get the pin holes as the air is still in the resin and will rise to the top surface...

 

[micdavis]

If they are small surface bubbles, I'd go with the baby powder idea. Also extends the life of the mold.

 

[darthchromer]

Moisture is you biggest enemy when casting urethane resin. Your silcone mold creates moisture as a by product of the cure process. Read your technical sheets and see what the silicone post cure instructions are to get rid of the moisture. Usually a bake 3-4 hours @125 Degress.

Urethanes are moisture sensitive. Moisture in the resin will cause it to gas during the cure which creates the tiny bubbles through out the part, there is nothing you can do about it after moisture absorbtion except pressure cast. I have used old resin that would normally foam up in casting due to moisture under regular atmosphere but it cures just fine in the preasure pot. I have three preasure pots. A small 1 gallon preasure cooker style pot, a bigger 4 gallon pot I made from a crafstman paint pot I got on Ebay and a large 60 gallon pot which used to be an old industrial canning preasure cooker I also found on Ebay.

Best advise without using a preasure pot is: post cure the silicone, use a new bottle of resin and cast as much as possible as soon as possible, and use a gas blanket like extend-it every time you open and close the container of resin. This removes moisture ladden air from the container and less water is absorbed. Hope this helps.