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

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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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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 new...as 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 BIGGER...as 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?

Examples: http://www.hvacstore.co.uk/acatalog/refrigeration_and_air_conditioning_vacuum_pumps.html


...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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Hello all, just my $.02 worth:


First off, I use CR900 resin from Micro Mark both because of it's longer pot and working life, but also because it is super thin AND extra hard. (I cast a lot of thin pieces, and many other resins won't stand up.)  If you are casting larger thicker pieces, then the strength part is not as critical, but then the resin will also catalyze more quickly.


Second, plus one  to JeffreyK, about using pressure, instead.


Third, I use a hybrid design set-up, wherein I can first pull a vacuum and then switch to the pressure quickly: This process is described in an article I wrote earlier in this forum.






Fourth, when used vacuum, the molding "gate" (or hole you pour into, or casting "stub") has to be large enough to contain the "overflow", otherwise the bubbly resin will just overflow the mold rather than being returned to the mold when the bubbles collapse.


You might contact Micro Mark and see what info they'll give you about their CR-900 (or at least elicit some specs if shipping is too expensive). Warning -- the CR-900 itself is expensive!


My feeling is that "if I just gotta have it" and no one else make a resin part for me, then I'll just have to make one myself. At that point it's a matter of desire vs. expense. My other feeling is "budget restraint first and foremost -- unless it revolves around modeling"!


Good luck,



Edited by TheRealMrEd
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I've done quite a lot of casting both in resin and white metal. I started out casting about 50 years ago with liquid Araldite resin but silicone moulds didn't last long with Araldite.  I have a home made vacuum chamber built out of half an upturned calor gas cylinder (about 9 inches diameter). It's upturned so it still has its original gas connection which is now the vacuum connection. It has with an upturned pyrex mixing bowl for the top (with rubber seal) so I can see what's going on. I use a sheet of quarter inch perspex as a safety shield between me and the pyrex just in case the pyrex implodes under vacuuum. It hasn't happened yet but better safe than sorry. However, I have now stopped using it for resin casting, as I've found it's better to mix and pour quickly but use a paint brush to make the resin go into all the detail of the mould. I still use the vacuum chamber to degas silicone rubber but I also use a paint brush on that to eliminate surface bubbles on detail. I use Poundland brushes and throw them away after one use. I suspect that prolonged vacuuming of silicone rubber can change its physical properties because of the different vapour pressures of its constituents so some will evaporate away more than others.


I also now make my moulds, especially one part moulds, with a bund, as found in central heating oil storage tanks so that they can catch any overflowing resin which sticks like mad to just about anything you don't want it to. I'd agree with an earlier comment to avoid water like the plague with polyurethane resin, unless you want it foamed, but I'd also avoid cliches like the plague.


As an aside, I once left the top off a bottle of Part A and it absorbed quite a lot of water from the atmosphere. I weighed it and by comparing with the weight of the bottle of part B I got a rough idea of how much water it contained. I then vacuumed it for a few hours and was able to remove most of the water and found it would work OK when mixed with part B. I wouldn't recommend this for a really important highly detailed casting but I couldn't bring myself to throw away a nearly full pack of resin. 


As another aside, I managed to resurrect my one week old Motorola Moto G mobile phone after its adventure in my trouser pocket in the washing machine, by an hour or two in the vacuum chamber. Two and a half years later it's still working.



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