Resin casting bubbles

[LV LOU]

Maybe there is some sort of reaction between the tape's adhesive and the resin which generates gas?  Since the gas will get generated anyway, the 70 pis is simply ambient pressure at that time and the gas still gets generated, creating bubbles. Not sure if that makes sense but that's all I can think of.  Maybe try Scotch frosted tape instead?

You can't cast thin,open flat sections with resin.I'm doing loads right now.I'm making copies of the masters for my molds.The bottoms need to be perfect.If I pour the entire thing one shot,and then put a flat piece of RTV on top,I get a huge bubble.I have to pour half way,let it set up,then pour the other half and put the cover on.If I put in too much on the first pour,it looks like a psychotic swiss cheese,bubbles all over,and it foams to the point of lifting the cover.If I pour the second layer as a thick pour,no problems..
  Actually,for windows,THE best thing I've ever found is JB Weld.It levels out like glass...

[LV LOU]

Lou,

I can't visualize this.

Pete,the Harbor Freight pot is only maybe 12" in diameter,but much greater in height.Lay it down on a stand and put a platform in it,and end load it.

[jimmo]

You can't cast thin,open flat sections with resin.

You can with pressure. I do it all the time and the parts come out perfect. Before I got pressure it was hit or miss.

[LV LOU]

You can with pressure. I do it all the time and the parts come out perfect. Before I got pressure it was hit or miss.

It could still be hit or miss.Some really thin sections won't even harden,and I've had foaming even in the pot.Besides that,the JB Weld is SO superior for stuff like filling in windows,I never bother with anything else.
  Lately,I've even been using JB Weld for my small castings.I have 2 A-B-A sets of old Con Cor F3's I'm painting orange & blue CNJ,and O&W.I wanted to put real pilots on the shells,and I needed the fuel tank covers,only had one.A while back,I bought a little hotplate that came with it's own coffee cup.I never used it much.I made molds for what I needed.I put the mold on the plate,and turn it on.I already have the JB mixed,I pour it in,and go around the edges with a toothpick.The JB gets water thin as it warms up,all the bubbles come right out because the JB is so heavy.I close up the molds,and in less than an hour,I end up with perfect small parts.As an added bonus,it's heavier than resin,so,heavier loco.

[peteski]

It could still be hit or miss.Some really thin sections won't even harden,and I've had foaming even in the pot.Besides that,the JB Weld is SO superior for stuff like filling in windows,I never bother with anything else.
  Lately,I've even been using JB Weld for my small castings.I have 2 A-B-A sets of old Con Cor F3's I'm painting orange & blue CNJ,and O&W.I wanted to put real pilots on the shells,and I needed the fuel tank covers,only had one.A while back,I bought a little hotplate that came with it's own coffee cup.I never used it much.I made molds for what I needed.I put the mold on the plate,and turn it on.I already have the JB mixed,I pour it in,and go around the edges with a toothpick.The JB gets water thin as it warms up,all the bubbles come right out because the JB is so heavy.I close up the molds,and in less than an hour,I end up with perfect small parts.As an added bonus,it's heavier than resin,so,heavier loco.

Lou, sure you can cast very small or very thin parts - all you need is heat.  I had to chuckle seeing that you are already doing that with JB Weld.  If you let the resin cure while heated, it will set even in very thin sections.  But if your workshop is 60 degrees or less (that is how my workshop is in the winter), the small resin castings will not cure.

I pretty much exclusively cast very small parts in open molds (no pressure casting either).  How small?
N scale golf carts.



N scale bumper cars



The finial (or whatever the proper name is for it) on the N scale signal mast


Probably the smallest casting I've made is the cone-shaped base of the N scale bar stool in this model.  Only the base is cast - the post is made from a straight pin.



I don't think any of these would have cured without additional heat.  I simply put the resin-filled molds in a food dehydrator (which runs around 110 degrees F) - that helps the curing process.

Going back to Mike's problem, here are similar window plugs I cast for blanking out rear windows of a FedEx Sprinter. IIRC, I used SmoothOn Smooth Cast 321. I did what Mike did: I backed the window openings with a piece of tape, then poured resin into the openings.  These castings then became my master patterns for a RTV mold to make more castings.  Here is the master pattern with RTV mold made from it.  I had no problem with bubbles or with the resin curing (in the dehydrator).

[jimmo]

It could still be hit or miss.Some really thin sections won't even harden,and I've had foaming even in the pot.

I'm curious as to what brand or type of resin you're using. Foaming is something I've never had happen unless my resin had been sitting a long time and expired.

It sounds to me like you should just stick to what works for you. If JB Weld will do the job, why waste your time with anything else?

Peteski, I think you could make beautiful castings out of white glue. Like yourself, I cast for several years without pressure (or heat) and I had good success by just being really careful. I'll never go back to unpressurized casting.

[peteski]

Peteski, I think you could make beautiful castings out of white glue. Like yourself, I cast for several years without pressure (or heat) and I had good success by just being really careful. I'll never go back to unpressurized casting.

Thanks Jimmo!  I don't doubt the benefits of pressure casting. I even own a Harbor Freight pressure pot. I just haven't started using it.  To be honest, when I'm casting half a dozen of 1/4" square window plugs (2 at a time), and open air casting works ok, I don't feel like setting up the pressure pot, just for that.  Plus, I don't have unlimited workbench space available.