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Problems with mixing investment

Hello everyone. I’m hoping that you guys can help me with some investment mixing issues that I’m having. I’ve scoured the forums, and while I’ve found a bunch of threads discussing investment issues, none of them seem to address my particular problem. So here’s the issue:

When I mix investment powder and water at a water/powder ratio of 40:100, I initially get an okay consistency (like pancake batter). However, when I put it under vacuum the mixture thickens and boils sluggishly. When I remove it from the vacuum after only 30 seconds, it has thickened up substantially, and at this point there is no flow to it, so pouring it into a flask isn’t possible. The vacuum pump appears to be working properly - it boils water in just under 40 seconds and pulls at a normal 28.5 Hg. I have made a quick 1 minute video and posted it on youtube to show what is happening:

Here is what I have tried:

I am mixing batches at a 40:100 water/powder ratio, weighing both water and powder. I have tried both distilled and deionized water at very close to recommended temperatures of 73 degrees for the water, and 75 degrees for the investment. After adding the investment to the water, I mix it with a mechanical mixer for 3 minutes. I have tried using different bowls/mixing utensils. I initially thought the problem could be the investment (newly purchased R&R Plasticast), so I ordered some Kerr Satin Cast, but I am still having the exact same problem with the Satin Cast.

So, this leads me to think that there’s an issue with the vacuum pump. The vacuum pump is only 6 months old, but it is regularly used to degas epoxy resin. I am unsure how using it to degas epoxy would affect the degassing of investment, but I admittedly know very little about vacuum pumps.

The vacuum pump is rated at 5 CFM. The vacuum table that I am using is the popular little one that Riogrande sells, and I believe they recommend a 3 CFM pump. From what I understand, a pump’s ability to suck air is limited by the size of the connected hoses, so there shouldn’t be much of a difference between the two as far as how much air is being sucked, with the small hose sizes that are being used. But, perhaps I am wrong and that extra 2 CFM is creating problems?

I am just about to purchase a new vacuum pump that is rated at 3 CFM, but first wanted to make this post to see if any of you are able to chime in with your experience.

Thanks for taking the time to read this rather long-winded post!

If you take a cup of plain water and put it under vacuum and it boils when it gets to full vacuum or shortly thereafter then you are getting an adequate vacuum for investing. When you are vacuuming investment does it rise inside the mixing bowl and then collapse? That is also a good indication you are getting a good vacuum. What I mean is you should get a pretty active boil and some spitting if your investment is under good vacuum
Hope this helps

I reread you letter and if your plaster is setting up so thick so quickly then something is probably wrong with it. It is usually liquid for nine minutes before it glosses off. I am thinking your plaster Is the issue. Mix a batch and time how long it takes to set maybe

Thanks for your reply, scb. I’m going to test a batch tomorrow morning to see the set time without the vacuum. The strange thing is that the mixture can be pretty fluid, but 30 seconds in the vacuum chamber turns it into Play Doh!

Vacuum pump is not the problem.
Use cold water.
Mix 1 1/2 minutes (electric mixer or
whisk) vacuum 1 1/2 minutes,
pour into flask, vacuum 1 1/2 min.
How does it turn out?
I cast 500-1000 pieces a week for 12 years
using that method.

I have tried both distilled and deionized water at very close to recommended temperatures of 73 degrees for the water, and 75 degrees for the investment.

The setting of investment is a chemical reaction. Chemical reactions happen faster at higher temperatures, slower at cooler temperatures. You are working at the high end of the range. Using colder water and investment may not completely solve your problem, but it really should help.

This is from the Kerr website:

“Work Time:
Work time is the time that has elapsed between adding the powder to the water, and when the investment thickens.
Water Temperature:
Water should be 70ºF / 21ºC to 75ºF / 24ºC. Colder water extends work time, warmer water shortens work time.”

How are you doing the mixing? Could that be adding some heat?

I don’t see how using a lower capacity vacuum pump could help in any way.

Neil A

Another thing they warn about is that your mixing bowls, tools, etc. are clean of cured investment. It is possible that cured investment acts as a chemical trigger to accelerate the set time. Or they are worried more about inclusions that would get stuck in your model and cause defects. :stuck_out_tongue:


You are mixing for too long. Your investment only has a 5 minute working time and you are taking too long. I mix by hand literally with my fingers so I can feel the lumps. Not more than 45 seconds maybe even less. Vacuum for 1 minute after it just barely begins to bubble. Poor over waxes then vacuum twice more. This time for 45 seconds again start a timer just before it barely begins to bubble. I don’t let it do that rolling boil for very long. If you are taking more than 5 minutes from adding water to setting it down to harden then you end up with exactly the problem you describe.

"Another thing they warn about is that your mixing bowls, tools, etc. are clean of cured investment. It is possible that cured investment acts as a chemical trigger to accelerate the set time. "
This is not true.
I have made mistakes in
measuring, and I have used
previously hardened investment
along the side of the flask to raise
the level of the investment to
cover the wax models.

Thank you kindly for the replies everyone. The earlier batches that I was mixing had about a 5 minute working time when left alone, but that seemed to lessen when I put the mixture under vacuum. I had batches that I mixed for only 1 minute, then put it under vacuum for 30 seconds and it would thicken up.

In any case, I tried what richard and neil mentioned and used cold water. That seems to have done the trick. I am now able to boil the investment without it thickening, pour it into a flask, and then boil again with no problem. Thank you everyone for your suggestions!

yes 3 minutes mixing time seems excessive and using your hands to mix is faster and you can feel and pulverize lumps cool water is also good

We all have our on best practices for investing. I have been using satin cast 20 for many years and have followed Kerr’s recommendations. I weigh water and investment to the gram at 38/100 ratio. My work time is 9 minutes. Mix for 3, vacuum and hold boil for 1 minutes, pour into flasks and vacuum to 9 minutes total.

Here are some interesting recent developments. As noted here and on other forums Kerr will no longer produce jewelry investments. My last 100lbs tub of investment was to thick at 38/100 and I had to go to 40/100 or slightly more to make work. I also put additives into water for flasks that have resin prints which thicken investment so fast it was unusable even with a 6 minuted work time. My fix for these resin prints was to chill water in refrigerator to 38 degrees which gave and 8 plus minute work time.

I did contact Kerr with no reply. My guess is that for what ever reason their recipe changed and now after 100 years they are not in this business anymore.

Your 5 CFM vacuum is adequate. When my 3 CFM died I went with a 7 CFM and very happy I did. When using PT investment like Doc’s which has 4 minute work time, the extra horse power ensures quick boils.

So my suggestions would be to try upping water ratio. If that fails try also chilling water.

Hope this helps

Plaster does not set faster when exposed to already finished plaster. But inclusions can ruin a casting if they fall in the wrong place and block the flow of investment around a detail.

I had this problem today on new SRS Silk investment. Used a 40:100 ratio and was so thick it was strange. But worse, it vacuumed oddly and when aI put it into the flasks, it froze up just as the investment was at maximum expansion and did not ever collapse. This was 5 min out. It left pockets in the flasks near the waxes, so 9 waxes wasted. Next time I’m going back to “by feel”. And cold water if necessary. So I am wondering, should I test every new batch of investment? Glad I did not try to cook my really important waxes!

Well, plasterers DO use powdered set plaster as a catalyst, to get the new mix to go off faster. Also water from washing plastering tools will cause plaster to “go off” faster. A lot of the investment plasters are unique blends of different materials
to deal with lowering expansion/contraction in the kiln heating, ease of removal from the finished casting, bubble release, etc. DO NOT use chunks of fired investment in a new mix, it MUST be well ground and “clean” of any chunks. Yes would be to use the
mix as received from the manufacturer and not try to speed up or slow down the set times. If you need such changes, buy investment that has those features have been formulated into the mix.

Just my 2ç and worth about as much……

John Dach