Resin casting bubbles

[spr1955]

I know this has been discussed and I know that pressure casting would solve most of the issue but until I get a pressure pot what else can be done to reduce bubbles in squish mold.
I have heated the mold, talc, fresh smooth-on 321 and get bubbles. I looks like they are forming as it cures. Pouring in the resin and using a pin in the corners to get out any stuck bubbles, the resin appears free of bubbles and I put on the top, rolling it on from one side to avoid trapping air. Demold 30 mins later and bubbles can be seen in the resin.

I have had good success with Aluminte but the pot life is really too short to pick the corners for air bubbles before putting on the top.

David P.

title edited for clarity. -gfh

[Rasputen]

It sounds to me like the humidity is too high when you are making the castings.  Wait for a nice cold day when the furnace is running.

[peteski]

That usually means that the resin absorbed moisture (its shelf life has expired). Urethane resins are infamous for that.  Placing uncured parts A and B in a vacuum chamber could remove some of the moisture, but I never had any luck making such resin bubble-free.  But yours is supposedly a fresh batch?

Also if there is any moisture (even traces) in either the mixing container or even the mixing implement (often a wooden stick which can have moisture in it) will results in moisture transferred to resin while mixing, resulting in bubbles while curing.

[spr1955]

It is humid now (atlanta). This was a batch made in may this year and just opened it.
I painted one and you don't see holes but it does bug me that they (the castings) don't seem as crisp as the first ones I did with some rather old Aluminite resin. I'll pick up a fresh batch of that and wait for a really dry weekend and try both resins. I do store my resin around 70 degrees and in a dark cabinet.

Thanks for the comments.

David P.

[LV LOU]

 Sorry to say,but made in May,it might as well have a long gray beard on the can,LOL!!
 Casting resin is crazy stuff.If it's really humid,or you have really old resin,you'll still get bubbles in the top of the casting even in a pot.
   Pete,I've never successfully vacuumed  resin,it will foam,and the foam will almost immediately set.As soon as you open a can of resin,it starts to absorb moisture.It really helps to use resin extender spray in the can every time you open it.Could you post a pic of the mold? If I try to do open topped castings in damp weather,there are almost always bubbles on top.I'm working on load castings today,the back looks like a bubble bath.Luckily,the back[bottom..] doesn't matter for what I'm doing...One thing that helps a lot is to have somewhere for the bubbles to go on top of the mold.

[pnolan48]

How big are your pours?

Do you have entrained air bubbles (small) or trapped air (larger)? I assume you are talking about entrained air bubbles.

When I'm making a big pour for a ship hull (let's say 22 oz total) I just assume I'm going to have bubbles, probably of both kinds. My smallest pour is probably 2 oz total, for many small parts--either ten of a kind, or ten small parts in different molds. My typical pour is about eight oz, for one or two hulls, plus any small parts molds lying around. I haven't had many problems with entrained air bubbles.

I used to vacuum the two parts separately, but that required large containers for small amounts of materials, to contain the foaming. I'd use an 18 oz cup for 3 oz of material; otherwise I'd lose material over the edges of the cup due to foaming, and be cleaning out the vacuum pot constantly--and resin and catalyst do not foam uniformly! So, once things foamed over the edges, who knew what proportions I ended up with?

I've had good luck just letting the two containers sit for ten minutes before mixing. If there are any remaining bubbles along the edges of the cup, I squish them with my favorite stirrer, a hardwood stick used by nail beauticians, just stirring slowly along the edge--before mixing. I always use fresh plastic cups, and fresh sticks. Then I pour the two quite slowly together into another fresh plastic cup. I stir quickly, looking to squish any bubbles forming along the edges of the cup. Then I pour, very slowly. I try to find a place where the resin will flow over a flat surface. Bubbles do break up coming over the lip of the cup, and then again when flowing from a flat spot into a crevice.

I use resin with a pot life of about 15 minutes, and a demold time of 3-4 hours. It's that 15 minutes that might allow air bubbles to form and escape, not the 3-4 hours. My best practices are to allow the components to sit for ten minutes in a plastic cup (paper dixie cups do not work!), then pouring very slowly. Or, if I'm injecting a two part mold, inject very slowly, from the bottom.

[Plumcrazy]

 Before I started pressure casting , I went to a faster curing urethane (smooth-on 300) the slower stuff was affected by moisture in the basement . I only use plastic cups and plastic stir sticks

(paper cups and wood stir sticks retain moisture) , when I use mold release I spray alight coat on the mold and let it dry at least 5 min. ( I have not had a lot of luck with talc ) Sometimes the

shape of the mold or the way it is put together traps air , for this I coat both sides of the mold with resin and I make sure I cover all surfaces , this will trap the bubbles  deeper into the part .

another method is to cut small troughs on the top of cavity to give the air bubbles an escape route . I hope this helps

Alex

[randgust]

Get yourself a humidistat and a thermometer and pay close attention to both.

Unless I can hold the room under 65% humidity at 70 degrees, I don't bother casting at all.   The same material that bubbles and foams above that comes out like glass below that (CR-600).

[jimmo]

Do whatever it takes to cut down on the moisture and get yourself a pressure pot. Especially if you're talking about using a squash molds, there's no way to get at any forming internal bubbles in order to pick them out.

Before I started pressure-casting I had the same problem with bubbles forming in the cure. Pressurization cured all of my problems. Taking advice from DKS, I pressure-cast all of my molds as well--otherwise you will get little balls and teets of resin on the surface of your casting. You might be surprised at how many bubbles get trapped in the mold rubber. I know all the "experts" say to vacuum the mold rubber but David's technique works damn well!

Fortunately I live in the Mediterranean climate of Southern California and humidity here is rare, so my casting can be done almost any time.

[pnolan48]

Can I ask the pressure-mold folks: how big are your molds, and what is the volume you are casting? Pressure pots are only so big . . .

[jimmo]

Can I ask the pressure-mold folks: how big are your molds, and what is the volume you are casting? Pressure pots are only so big . . .

Can't help you there Pete. 2-1/2 gallon pots are more capacity than I'll ever need. Unless I decide to join you nautical types.

I'll bet that there's a way to make a long, skinny horizontal pressure pot with pipe...

[peteski]

Can't help you there Pete. 2-1/2 gallon pots are more capacity than I'll ever need. Unless I decide to join you nautical types.

I'll bet that there's a way to make a long, skinny horizontal pressure pot with pipe...

...or a nice pipe-bomb!  BOOM!
Seriously though, Jimmo is right, there are probably long and skinny pressure pots which could be made out of some sort of pipe.

[spr1955]

My molds are 2-1/4" x 4-1/2 x 1-1/2 high. I am casting Gondola bodies for my steel mill, and I need a lot. I have 2 50' and 1 40'.
Randy, I am certainly going to get a humidistat as I believe that is causing the internal bubbles as the bubbles from pouring I get out with a needle just fine.
I use plastic cups and plastic stir sticks. No paper or wood. My pours are only a couple of ounces at best. After mixing I let it rest and look at the clairity of the mix.
What is really strange is I have a few flat one piece molds and pouring those they don't get any internal bubbles forming.

[Sokramiketes]

I've been having bubble trouble this week too and don't understand why.

I've been casting various details this week, and use a pressure pot with no bubble problems.  Until I decided to try to fill in some windows in the side of a passenger car body... with resin.

All I'm doing is putting Tamiya masking tape on the outside of the body, and using some resin to fill in the window itself.  I had bubbles just letting it sit on the workbench to cure.  So I pulled those plugs out and tried the next pour in the pressure pot.  Somehow I still got a handful of bubbles!  At 70psi! 

And other molds in the pressure pot for that cycle turned out bubble free as usual. 

I don't get that one. 

[pnolan48]

I've considered making a long pressure pot out of a piece of steel pipe. I think I'd need a larger compressor. I am afraid of the BOOM!

I let my materials sit before mixing. It seems to me that, even with long-setting materials, letting the mixture sit just invites problems, as the mixture gets progressively thicker every second. The 310 materials I am using have a supposed pot life of 15 minutes; I find them pretty useless for pouring after four or five minutes.

Flat pieces don't have bubbles because the entrained air can escape before the resin sets.