Pressure Casting In Resin

[garyo37388]

Homemade equipment... a little over top maybe

Here's some links to photos of my homemade vacuum chamber. It is 17 inches in diameter and 10 inches deep. The lid is 5/8" aluminum with 3" ports. The ports are covered by a piece of 3/8" plexiglass. This is plently thick because of the aluminum lid. I use a Welch 1402 for evacuation.

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Complete vacuum chamber: http://www.willowproducts.com/media/images/Tools/Vacuum_chamber_with_lid.JPG
Lid: http://www.willowproducts.com/media/images/Tools/vacuum_chamber_lid.JPG
body: http://www.willowproducts.com/media/images/Tools/Vacuum_chamber_body.JPG
seal detail: http://www.willowproducts.com/media/images/Tools/vacuum_chamber_seal.JPG
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Other vacuum related home projects:

Vibrating vacuum table: http://www.willowproducts.com/media/images/Tools/vibrating_vacuum_table.JPG

Multi-port small chamber freeze drying machine ( yes it actually works )
http://www.willowproducts.com/media/images/Tools/Homemade_freeze_dryer.JPG

Homemade vacuum centrifuge pull vacuum and spin casts material into mold at 40 mph. Yes it works:
http://www.willowproducts.com/media/images/Tools/Vacuum_centrifuge_complete.JPG
http://www.willowproducts.com/media/images/Tools/Vacuum_centrifuge_inside.JPG

You can find out the complete details on this monster here:
http://www.myheap.com/book/chapter-02/spincaster/spincaster.php


Also from myheap.com:
http://www.myheap.com/book/chapter-02/vacuumcaster/vacuumcaster.php

 

[garyo37388]

Thanks Gary. I'll work on it. The vacuum, actualy is not the problem. The increase of size of the bubbles is what I'm trying to fix.
T50, the equipment in the page that you indicated seems to be wonderfull but must worth a fortune!
For a while I'll have to use my "atomic beans" pot!

Sorry , but this post contains info on both vacuum and pressure:

---------------Vacuum----------------------------------------------

I've been meaning to post this and keep forgetting....

Make sure that your release agent is completely dry before vacuum casting your RTV. If not, the release agent can outgas in the vacuum under the RTV creating thousands of tiny bubbles.

If in doubt, vacuum the mold after you've applied release but BEFORE you pour RTV in mold. The release should evaporate quickly in the vacuum. You can take this opportunity to pre-vacuum the mixed RTV in a tall container at same time. THEN pour the RTV in mold and vacuum as noted below.

I have not seen out gassing occur in pressure system.

Now back to vacuum process.

In theory, if not in practice, if you achieve maximum vacuum and manage the vacuum cycle properly , you should not have bubbles remaining.

At 50 microns or less, the bubbles should turn to froth, "crest" as a foamy head, and then collapse as they bust into the voided chamber. Then you should break the vacuum and let the atmosheric pressure slam the remaining ones laying on the surface. The goal is to make the minutes ones so big they must escape. It takes a strong pump and a tight system to pull this off. Otherwise you'll end up with bigger bubbles than when you started.

All this must be done before the material begins to thicken as it "sets".

---------------Pressure----------------------------------------------
Now back to pressure:

This is why I prefer pressure. To achieve a bubble free cast I just set the poured mold into chamber, tighten the 4 screws, open the air valve and in less than a minute I'm done.

To speed the process of the lid on the paint pot: I replaced the thumbscrews with hex head bolts ( careful...usually metric...mine were 11mm ). I use a powerdriver with torque setting. Zoommmmmmmmmmmm it's fast.

If you switch to pressure, don't use that pressure pot. Probably safe for the 15psi that vacuum can create, but not so for for 40 and up of pressure casting.

 

[T50]

I have another question regarding vacuum process while we are at it
My vacuum pump pulls 0 to 29 inches of mercury in 25 sec from my chamber.
However, I leave the mold in full vacuum stage for a minute or two.
Is this unnecessary? or does it help?

TS

 

[garyo37388]

I have another question regarding vacuum process while we are at it
My vacuum pump pulls 0 to 29 inches of mercury in 25 sec from my chamber.
However, I leave the mold in full vacuum stage for a minute or two.
Is this unnecessary? or does it help?

TS

I think it helps. Especially in thick solutions where the tension on trapped air is high.

My process is actually a little more involved...kinda old school.

Once I reach full vac. I jiggle or vibrate the whole chamber. Many commercial chamber actually come mounted on a jiggle plate ( springs on 4 corners )

Then I break vac and repeat.

Probably over kill, but a little extra effort bets a bad mold!

gary O

 

[T50]

Then I break vac and repeat.

You mean you pull vacuum twice? I do that with my 1/16 scale figures
as the cavities in the molds are big and the pour openings are small.

TS

 

[garyo37388]

You mean you pull vacuum twice? I do that with my 1/16 scale figures
as the cavities in the molds are big and the pour openings are small.

TS

Yup. With an "atmosperhic push" between them to help burst any surface bubbles.

Did you see the note about outgassing mold release?

 

[T50]

Outgassing mold release? I never used any mold release because
I use silicone mold.

 

[garyo37388]

Outgassing mold release? I never used any mold release because
I use silicone mold.

Then don't worry. But outgassing can occur as any "finish" dries. An example might be a painted finish on a model than is having a mold made. Or mold release sprayed on surface to prevent a 2 part mold from sticking to itself.

Even in silicone i like a thin release....but that's just me.

 

[dinovision79]

Hi All,

very interesting topic in deed. I have been thinking about the theory for long and will stay at that (wish not to). But I wanted ask, how many casts can you make out of a mold? I know, depends on materials used, but still?

Thanks
Denes

 

[T50]

Yes, it depends on many different issues:
the master, the way you design the mold,
the way you tree up the parts, the way you
demold the parts, the temperature of the
curing resin, how long you age the mold before
castings, and the quality of the rubber/silicone, etc.

 

[dinovision79]

Thanks for the answer, let me put it in a different way: if you try to optimalize everything, what is the average, or how many cast you make out of a mould, Taesung? Just approx.?
By the way, could you tell me more about ageing? Is it just let silicone stand for a few days? Or some kind of special treatment? What is the reason for this?

Thanks

Denes

 

[T50]

I get any where between 25-50 castings depends on what it is.
Some molds just seem to last longer than others...
If you talked to the tech people at the silicone companies,
they recommend aging silicone molds at least a week or heat it
at 150 F in an oven for 4-5 hours. Whenever I can, I age my molds
about 2 weeks. Aging silicone seems to bring up the quality of
the material and elongate its life span.
Talk to the tech support of the company you buy the materials from.


TS

 

[dinovision79]

Thanks Taesung, everything is clear! I heard about silicon in the owen.
No more questions for now

Cheers
Denes

 

[dbang1988]

So may anyone, whether he uses vacuum or pressure, show some pictures of their procedures? So we starters can learn from you folks.

A picture worth thousand words!

I am desperately need those information. I just started with vacuum on Polyurethane resin, and I have been failing for two months. So please help

 

[ausf]

Wonderful thread, excellent info.

I was wondering about making the mold under pressure. Is there any change in the actual cure time of RTV under pressure and could I pressure cast into a mold that wasn't pressurized while made?

I have a pressure pot that is rated for 80 lbs, but in practice, it doesn't seem to reliably hold the pressure (over time) above 50 or so. I can lock it down and have it hold overnight below 50 lbs. The reason I ask is, I'm afraid to put the mold under pressure, then have it distort if the pressure drops before full cure. Running a compressor overnight is not an option for me due to noise considerations at this time.

Thanks again for all this info, I was thinking of long days of trial and error to figure this stuff out and this thread probably cut that down quite a bit.

 

[Calvin]

Is there any change in the actual cure time of RTV under pressure

Pressure generates cold which may delay the cure time (while heat speeds it up) but considering the small amount of required pressure that is not an issue.

could I pressure cast into a mold that wasn't pressurized while made?

No, that may break the small air bubbles trapped into the mold (if any) and if that happens into the internal surface of the mold (not just inside) the piece you cast gets a lot of small imperfections.

I have a pressure pot that is rated for 80 lbs, but in practice, it doesn't seem to reliably hold the pressure (over time) above 50 or so. I can lock it down and have it hold overnight below 50 lbs. The reason I ask is, I'm afraid to put the mold under pressure, then have it distort if the pressure drops before full cure. Running a compressor overnight is not an option for me due to noise considerations at this time.

There is a common misunderstanding about vacuum and pressure. Both have the same effect, which is popping up the air trapped into resin or silicone. Now due to the fact that pressure pushs (while vacuum pops) people think the air bubbles simply get compressed into the deepest of the mold or the casted piece, which is wrong. The only effect the pressure has is to pop up the trapped air without the mess generated by vacuum.

Another common mistake about pressure is the more the better cause that way air bubbles will go deepest inside the casted piece. Wrong again. The more pressure you use the more distorsion you get. There is really no need to go over 20/30 psi, using pressure causes air popping up so it is all about mold (and vents) design. Getting the trapped air bubbles out of the mold while casting exclusively depends on how you design your mold and how you do the pour, not by how much vacuum or pressure you use. Most of people do not get it and that is the reason for which most of resin castings on the market are so bad.

 

[ausf]

Thanks Luca, that answers my questions.

I have a good deal of experience with rigid molds for foam latex so undercuts and venting are burned into my head. I just have to relate all of this to the size of a 1/35 figure when I'm used to much bigger projects (some molds so large the 'oven' had to be built around it). I've also used RTV a bit in the past, but never under pressure and for anything more than a one off casting.

This should be interesting.