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why is my resin cast full of massive air bubbles after degass in chamber ?

9 posts in this topic


Resin casting,  I mixed equal parts A and B of a fast cast polyurethane resin using weight. stirred well, placed beaker into chamber, all levers correct, turn pump on, and degass, saw resin rise a bit, small bubbles surface etc, then as chamber reached max air lossage of 24 on gauge, wont go higher, , time now 6mins since mix A to B started, the small bubbles have declined but a few large air bubbles are now appearing, and more, so more degassing to go, then the resin looks less like a liquid liquid,  8 mins elapsed now, so turn off pump, let air in and remove lid, resin has massive airbubbles visible through beaker, like a swiss cheese, as well as part in and out of surface, thin skins over bubbles set as such, frozen in time !!!  and its set solid.


What use is that ? 

This beaker could have been my mould as such. Why the larger bubbles in the latter stages , as time goes on, how long did it need to degass if they were still rising after 7 mins or so ?


Just what went wrong ?


Time wise its impossible to mix A and B by weight and pour into mould then degass so it seems.



Best to weigh out equal parts separately then add together, saves some time.

start the clock as B poured into A. 00:00

stir well 00:20 elapsed

pouring is finished into mould at 00:40 then chamber it and seal is achieved in 1 minute.

24Hg takes 4 mins  and 24.4 Hg is 4m51 secs, no progress of needle after that,  so thats 4 mins~5mins  including mix and pour time. and the resin cures in 4 mins or so.


I was told this pump and chamber and resin would be a workable combination, just bulk out chamber with bricks etc for smaller moulds.


I am told best it be out of chamber for mould pouring in 2m 30 secs, but it takes 4 mins to get to vacuum, and thats with a 13L chamber (25 bulked out with solids) and an ECVP425 pump.


even if one doesnt let it get to 24, there were still bubbles appearing at 6 mins +. big ones by then.



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first of all, your pump is I think quite a bit under-powered. Yours pumps 42.5l/m, while I use a 6cfm pump which equals 170l/m and even at that power I don't attempt to de-gas resin (only silicone rubber) as I think it's still too slow for most casting resins.

Secondly resins attract moisture from the air and unless you have brand-new, unopened containers, totally fresh off the production line, there will be some contained in your components. This moisture  causes very small bubbles to form in the resin while it is curing. Normally, this is negligible, the bubbles are too small and too few to matter, but as you're under vacuum those bubbles expand to many times their size and then your resin sets and you have locked them like that.


A: find the appropriate time spot (trial and error) where enough trapped air is removed from the resin and let the atmospheric pressure back in to crush all remaining tiny bubbles and prevent the foaming from becoming a problem.

B: don't use vacuum, but pressure casting instead. You can displace most trapped air by good casting technique, good mould setup and gentle mould manipulation during casting, the rest will be crushed in the pressure chamber at 60psi.




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Screws you might turn:


- avoid moisture like the plague. A friend of mine casts bases for miniatures. He showed me what happened when he used a form that wasn't completely dry after being washed for some reason. The piece looked like soft foam.

- try another resin with longer open time

- stir gently to avoid getting bubbles in

- pour gently to avoid trapped bubbles.

- vibration plate instead of or in combination with vacuum. Said friend does everything with just a vibration plate.

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brand new unopened part A and B containers. (well , had them 1 yr but unopened stored in cool place), told makes no difference might increase working time a little.

exposure to moisture was the time it took to undo the top for the first time and pipette out the liquids, a couple of mins.

Stirred it fast as time was ticking and lost time to the measuring out., but surely degass would see those come out.

No pouring done at all, as degassed it in the mixing vessel.


Having bought this pump/chamber combination and liquids by explaining what I hoped to be casting, if this is 1/4 of the power it needs to be, then thats a total waste of my money, mis-sold etc etc...I cant go buying though a pressure chamber and pressure pump.


The mould I have must not be pressed as the scalpel cut side will open out and resin will get out. Squeezing moulds ???..surely that would encourage resin seepage ?


I needed a vacuum pump/chamber to degass the silicone but cannot afford pressure system as well.


Letting the inrush of air diminish remaining bubbles sounds useful, but it wouldnt have tackled those massive bubbles, 5mm diameter they are.


But why did I have massive bubbles forming after 5 mins and beyond, they are down in the resin like swiss cheese. Still not sure why. Imagine if that had been my mould !



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Posted (edited)

Those large embedded bubbles may indeed happen because of the vacuum. Have you ever seen what happens to a chocolate foam kiss under vacuum? The gas inside expands but cannot leave the cream because its' viscosity is too high.

This is what I mean:




You may get better results without vacuum.


And some clarification: A pressure chamber does nothing normal atmospheric pressure wouldn't do to your mould or the resin. The pressure is equal all around, so it is not like pushing and squeezing just from the top. It is more like pushing from all sides equally. The effect is the inverse of the example above: The gas bubbles get compressed instead of expanded. You may run your vacuum chamber as pressure chamber with the pump connected in reverse, depending on the construction of both.

Edited by Schwarz-Brot

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I never degass the resin, only silicone rubber.

As the parts don't need extra structural strength, pressurizing is enough to get a fully filled mould and smooth skin on the casting. 

Also, I like fast casting resins, gel time is barely long enough to fill all the moulds.



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For most castings simply mix up the resin then pour in. Fast Cast doesnt need de-bubbling. Lesson learnt :-)


Q1. If an object is such that as the resin is poured in, it traps air into part of its shape, and that shape cannot be air vented with a channel etc, what resin should one use for this ?

Q2. Also what vacuum achieved time on the chamber should be aimed for ?


My pump 42.5L/M, ( all I can afford brand the next pump price that does vacuum on 26L in 30 secs is £800 ), having added more solids to the 26L chamber, so its now in theory 10L of air space, sees the following stages reached on the gauge:-

27           1:23

28           1:36

29           1:57

29.5        2:13

29.7        2:27

30           2:42

30.3        3:10

30.4        3:25

30.5        3:44


and no go beyond 30.5


Q3. is it in fact possible to bulk out the 26L chamber to achieve 30 sec vacuum reached time with a 42.5L/M pump ?


Q4. at what stage is this going to be degassing and when should I let air in ?


The one fast cast I have has pot life 2 to 3 mins.


Another I have is supposed to be 6 mins.  That set with big bubbles in 5m:16sec


By the time I have added in part B then stirred it, then pipetted it into mould, placed lid on and switched on pump, I am at setting time it seems before vacuum has had chance to do its thing..


Pipetting Part B to get 7.0 g of B into A, sees three squirts and a careful 4th where drip by drip I achieve the same weight, allowed 1% variance at most, one drip too many can break the rule.



Here is a timed session on Fast Cast polyurethane resin pot life 2 to 3 mins, with 10% black pigment added to A :-


00:00 as part B first hits part A.

1:35 end of stir of 30 secs duration, 18mm wide stirring stick for greater effect, any wilder and it flicks out of beaker at me !

3:35 all is now into film canister (Pipette is slow method of getting mix into mould, need to rethink that ! )

Lid on, flip handles, start vacuum pump.

10           4:22     

15           4:31

20           4:47

22.5        5:00

25           5:12

27           5:31 appears to have slowed, let air in,

result, 4mm at base of canister is solid, the rest , totalling 20m height is a resin bubble mix ! and its hot to touch !

6 minute one no better, having also added 10% black pigment, in fact it had a whitish substance in small areas across the moon cratered bubbled surface, and was dark grey at best.

Q5. What was that white stuff, it can be poked away with cocktail stick, but doesnt just wash off. Incompatability of black pigment or side efect of 'boiling' ?

Q6. The longer I leave it in there the bigger the bubbles get. One will never get to no bubbles it seems, so if one needs to get air out from a shape , is a 30 sec vac chamber pump at £800 going to be the answer or will this stuff 'boil' and bubble ?

Q7. Does resin get hot under vacuum and forever generate bubbles, as these bubbles get bigger and time passes !



Edited by Merlin

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I won't touch anything with gel time less than 10 minutes.


Q1: Get the thinnest resin you can, consictency of milk or similar. Mix. Fill the mould. Vaccum. After a few seconds stop the vacuum and let the air in. This should first suck the air pocket into the resin and second backflush the resin into any cavity sou it can form a smooth unbroken skin. 


I do all my mixing in disposable plastic syringes. Open it, plug the bottom, pour in the A and B components, put the plunger back in, shake a few times until you get an uniform colour, squeeze into the mold.

If any additional pigment is used, it gets mixed into the component A before the component B is added.




Edited by dragonlanceHR

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@Merlin: a vacuum pump for £800?? Where are you shopping?


...and lots available on Ebay as well.

As for your Q7: longer "vacuuming" has two effects: 1 you create a better vacuum and thus any gas contained in your resin will expand more ("bubbles get larger") 2 the longer you wait the more time the resin has to react with the small amounts of moisture contained within and will continue to form bubbles until it sets. As I said before, under atmospheric pressure those bubbles are tiny tiny tiny and not noticeable, but under vacuum they will become huge.

10% pigment is quite a large amount. You also better pre-mix your pigment into your polyol (usually "part A", NOT the hardener). I use large syringes for component dispense (and short-term storage of the components).

Don't forget to use a dry gas like Polypurge etc. to seal your polyol and isocyanate containers after decanting, otherwise your components will draw more and more moisture, rendering them useless very quickly.


@Vedran: personally, I haven't come across a resin that's thin enough (i.e. low viscosity), has suitable material properties (hardness, elasticity, shrinkage..) for scale model parts AND has a pot life of 10 minutes or even more.

The best I could find was something around 6-7 minutes, not compromising too much on either cured properties or viscosity. Note that generally, lower viscosity goes along with faster curing time and pot life.



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