in need of advice ( Vacuum chamber newbie)

[RageofAchilles]

Hey guys,
I just got myself a degassing kit with pump and 7.5ltr chamber. I get absolute vacuum in about 15 seconds and the vacuum holds well.

My problem is resin puking out of the moulds due to the violent degassing, I end up with hollow pieces.
I have tried 3 types of resin, and also tried letting air back in the chamber when the degassing gets too aggressive and then starting the pump again. a side effect of the degassing appears to be a marked acceleration in cure time from 10 mins outside the chamber to about 3 mins when I use it.

Any advice would be much appreciated

[rattrap]

Would you not be better to degass your resin in a container first, then do a slow pour into your mould?
Then apply pressure if you have that set up.

I am pretty much a newbie too, that is what I think would be the best thing to do. There are, however, much more experienced guys on this forum who may disagree and advise you differently.

[RageofAchilles]

Thanks rattrap,
I think I will try degassing the resin before mixing it. Unfortunately I don't have a pressure setup and the resin cures so quickly I don't think I could use it before it cures anyway (3mins).

It may be that my moulds weren't made under Vacuum, so air is filtering through the rubber causing the resin to froth like shaken cola.

[rattrap]

Are you using a fast catalyst with your resin? 3mins curing time sounds very quick!
If so, try a slow catalyst, that's what I use while I am experimenting. It just gives you that extra pot life.
If you degas your resin before mixing in the catalyst, when you mix the two together afterwards, you may cause air trapment, so degassing first would be a waste of time.

[RageofAchilles]

The resin I'm using is smooth-on 327 with a grey pigment. for some reason the degassing is rapidly speeding up the curing process.

I tried to mix the resin together in a container and degas that first and had no reaction, so it must be the rubber. the reaction was more pronounced in a soft shore A 20 cold casting silicon, while the reaction was less aggressive in a shore A 55 heat resistant RTV silicone. but the results were the same, hollow casts.

could it be air trapped in the moulds that is fizzing through the resin?

[Nostok]

I am in a similar situation, I must say though, the hollow casts are pretty cool looking and give a good idea of the level of detail. I understand that this is one of the trickier ways of casting resin and if I wanted to make my life "easy" I would invest in a new compressor with tank and a pressure pot ASAP.

I have followed Mango's split mould technique tutorial and I must say it is brilliant compared with the 2 part moulds where you make each half separately... and the cutting process is nowhere near as difficult as it sounds. Knowing I would be vacuum casting I have been moulding with a large reservoir and what I thought were good sized gates. My primary reason for casting at present is parts replication to make full use of plastic kits I have bought.

I am using Smooth-On OoMoo 25 and Smoothcast 300, I was originally gravity casting. My results were good but annoying bubbles made me invest further because everyone needs ever expanding expensive hobbies.

I have a 5CFM (7.5 micron) 2 Stage Vacuum pump and an 4.2 litre chamber. I have degassed at all stages of the mould with hilarious mistakes (do not release the vacuum to quickly - that is messy) but eventually with good mould making success and now the owner of better safety gear.

When casting I am coating the mould with a fine talcum powder, I am degassing the resin separately prior to mixing and then due to the pot-life I have been just pouring it (also using a syringe) and vacuuming it and getting the same effect as RageofAchilles. Now something I have noticed is that it sets before all the bubbles are gone but the bubbles are not allowing the thickening resin to reenter the mould from the reservoir via the gates once it has bubbled out of the mould. So my casts are solid at the base and work themselves up to hollow.

I have two thoughts on this, firstly make the gates larger and/or add vents, and secondly use a slightly slower setting resin such as Smoothcast 305. My question would be has anyone else who has had this sort of issue remedied it with either of my suggestions or are there other suggestions that people can make that do not involve buying a pressure rig just yet. The other threads I have read on this site with similar subject matter normally have the cast already owning a pressure rig and just going back to that for the final stage.

[RageofAchilles]

It appears that this issue is more widespread than I first thought.

Nostok, your describe the exact same results I have been getting. At first I was overjoyed when I opened the mould and found my bubble free cast , only to find it hollow half way up.

oh I also have a tip for newbies. Remember to shut of the pump valve after you've turned it off. I didn't the first time I used it and while I watched my resin froth a load of oil from my pump got sucked into the chamber right on top of the mould lol.

been talking to johncast about this , seems it may be an issue with trapped air or moisture in the mould rubber, not a resin issue. we are trying to degas empty moulds and lightly bake them to dry them out.

[Nostok]

Thankfully I read about the switching the valve off to the pump before trying this out.

I've not understood completely this:

"been talking to johncast about this , seems it may be an issue with trapped air or moisture in the mould rubber, not a resin issue. we are trying to degas empty moulds and lightly bake them to dry them out."

Can you or Johncast elaborate further as to what is actually happening with the mould and why it creates the hollow casts?

The mould is only a few days old and I read somewhere they can take up to 7 days to be fully cured. I think the baking of them has two effects, speeds the final cure but also releases oils in the silicon to the more surface parts.

[RageofAchilles]

well we theorised that the issue was either air or moisture in the silicone mould that was being sucked through the resin, thus causing the frothing. all of my moulds were poured in normal atmospheric conditions and have been kept in a damp garage for months, so it was a possibility that it was one of these two factors.
so drying ( at 30-50oc) and degassing empty moulds before pouring resin into them was a line of experimentation.

I will also try to make a mould in vacuum , so that I can see if there is any improvement. it's a process of elimination.

[Nostok]

If it helps, my moulds were made in a vacuum at each stage degassed. Premixing, after mixing, and after pouring.

Obviously I'd be interested to know if baking them off helps. Unfortunately it just got "hot" where I live (between 25-35 degrees C) and I do not know if this makes a difference on pot-life - I would imagine so.

EDIT: Also more humid and I guess that really does matter.

[RageofAchilles]

If you made your moulds in Vacuum and are getting the same issue , I don't hold out much hope .
thanks for the info.

just found this on the net:

Moisture contamination in urethanes causes foaming. If it foams quite a bit, most likely the moisture is right in the Alumilite. The moisture is usually in the A-side. There is not a full proof method of removing moisture from your system. Typically the B side will crystallize when moisture is present. The A side, however, will not look any different. You can try to use a molecular sieve in the A side. Mix the sieve into the resin and allow to settle to the bottom for a couple days. Then test the resin to see if the sieve was effective in removing the air. You can also try to vacuum the A-side to relieve it of moisture. Vacuum the A side for 20-30 minutes until it completely stops bubbling. If you don’t have access to vacuuming equipment or a molecular sieve, the last chance would be to boil the moisture off. This is possible because water has a lower boiling temperature than any of the other liquids in the A-side. To boil off the material, we recommend that you put the Alumilite resin in a glass or metal container and place the material in an oven at 250°F for 2 hours. This will draw the moisture out of the system and evaporate it. If you are getting a bunch of little pinholes, the moisture is probably coming from some materials you are using (mold, cups, stir sticks, filler, etc.).

also backed up on the RPF ( replica prop forum)

Just figure I would post this up here for anyone else that might experience the issues...

I have several gallons of resin that have been sitting for about 2 months, as I took a break from the hobby... Somewhere along the line they got contaminated with moisture and it was like casting with expanding foam, an eruption of foaming resin right before it kicked...

Anyway there is hope...

After a little research, two DIY ways to fix it popped up, boil off the moisture on a hotplate... From what I understand as long as you hold the temp at about 250° F it won't hurt the resin but the water will boil off... Just cook until it stops boiling at that temp... Seemed messy and I don't have any 'disposable' cookware to do it with...

Second option vacuum boil the resin...

I went with option two and after 20 minutes of boiling under vacuum the resin is back to normal, just like brand new... As always expect 3-4x expansion of the liquid when under vacuum to be safe

I can't necessary say boil A or B as that varies by resin company, the side you want to boil is the size that doesn't form the yellow crustys around the lid, the yellow crustys is actually indication of that side having moister as well but it forms the crust and takes out the moisture on it's own... The "clear" side or the one that is generally pigmented is the one you want to boil in most cases...

[Nostok]

Now that is interesting... I have noticed one of my resin pots has the crystallisation around the lid (opened for 1 - 2 months). I have been Vacuuming the resin prior to mixing and I do see water vapour come out of the pump venting. I will try again this evening but for longer and see what happens.

[RageofAchilles]

I had yet more catastrophic failures today. I degassed the moulds and unmixed resin for 30 mins in full vacuum and tried again. everything looked good until I hit 50% vacuum then it kicked off as usual.

I tried again and this time slowed the degassing to be less aggressive , but I just ended up with deformad castings.

what I don't get is why the resin only does this when in the mould. If I mix it in a pot and put that in full vacuum it just cures with no frothing......?

really starting to feel I would have been better off getting a pressure pot instead, I just don't have the money for both. And I'm binning everything I cast atm!

[Nostok]

I wonder if it is to do with what we are coating the mould in prior to pouring and degassing. Could the talcum powder have an effect?

[Nostok]

Wish me luck.... I'm going in.