Silicone election for small detailed casting and pressure pot problems.

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udog

Active Member
I finished the mold with the T4 40d shore Xiameter silicone I have. As temps are rising here to hot summer I put in the mold to cure in the fridge to slow down the curing time which seemed to work ok, as I got a bubble free mold. I can see some bubbles around, but none touching the model. Good.
Also, the thin parts seem to stand better till the moment, better castings in general. And the back seam line is tighter with the higher shore too. The downside is that demolding is tough as it´s a block mold with a slit cit in the back and the silicone is much harder. All in all good, and I save some money having some kgs of this silicone here. And no pimples!
The other thing, and this is what I have settled on is to shake the resin really good an hour or two before I will be pouring.
Good advice, thanks. I´ll try that. But I´m still not confident because of the colour, Not sure that mixing one or two hours before will avoid the tint going down again (it does so fast).
I´m tinting "drop by drop", which I know might be risky some times, but it´s working till now.
You can also pour the silicone in the mold box and vacuum degas again if your mold box is big enough.

I could make a bigger box but not sure my degassing chamber will be big enough, specially for the 2kg molds (big model). I have to degas the silicone (prior to pouring) kg by kg or it will flow out of the container. I do have a bigger chamber, but yet to be prepared to work a as a degassing chamber,

I have one question about pressurizing. How long do you have your resin under pressure.?. I try to turn it off when I see it starts gelling. ¿At what pressure?. I´m in five now, lowered it down from six to five due to the softer silicone. Seems to work fine, although sometimes when sanding down the "pimpled castings" an anoying small bubble appears here or there,
Have to try degassing the resin too, haven't done so yet. What worries me about this is high temperatures plus degassing boiling might shorten the working time, which is already happening due to summer itself, makes me have to move fast. Might have to cool the resin down in the fridge to do so. Must test.

Thanks for all the feedback and interesting comments.
 

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udog

Active Member
Also, does anyone put two molds at a time in the pressure pot?. I am thinking of doing so to speed up the process a little. I´d build a structure to keep the molds separed.
 

Duncanator

Sr Member
When casting under pressure, I leave the resin in until it hardens. Pulling it when it is soft (gelled) can allow any air bubbles that the pressure was squishing down to re-expand and distort your casting. Once hard, the tiny pressurized bubble is trapped and can't hurt anything if you leave it under pressure till it cures.

Pressurizing doesn't get rid of the air bubbles, it just makes them tiny and pressurized. They want to grow big again when you return your casting to ambient air pressure, but a rigid casting resin keeps them small. (Flexible castings are another story.)

Vacuum makes the bubbles expand so that they can rise to the surface making it easier for them to escape. On rare occasions, I have had to vacuum degas my resins prior to pressure casting. It gives great results because you are covering all the bases: vac out the bubbles, pour, pressure squeeze any that might be left. The one thing you need is a resin with enough working time to do all these processes before it starts to thicken.


Lastly, Yes, you can put as many molds in your pressure pot as you have room for, so long as your resin isn't kicking off before you can get it under pressure. I do it all there time. :D
 
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sjanish

Sr Member
RPF PREMIUM MEMBER
Good advice, thanks. I´ll try that. But I´m still not confident because of the colour, Not sure that mixing one or two hours before will avoid the tint going down again (it does so fast).
I´m tinting "drop by drop", which I know might be risky some times, but it´s working till now.

Good that the new mold/process is working out for you.

I do a vigorous shake ahead of time. I forgot to say that I gently swirl/shake the resin right before I pour. On my last round I started to tint my whole jug. With the shake ahead swirl before method I could not tell a difference in the color of the castings.

I have one question about pressurizing. How long do you have your resin under pressure.?. I try to turn it off when I see it starts gelling. ¿At what pressure?. I´m in five now, lowered it down from six to five due to the softer silicone. Seems to work fine, although sometimes when sanding down the "pimpled castings" an anoying small bubble appears here or there,

The Smooth Cast 305 that I use has a cure time of 30 minutes. I leave it in the pot at least 30 minutes from when I first combined A and B. Sometimes longer. With the small, thin parts I normally mold I give the resin at least an hour from the mix time to harden before I demold.

I normally pressurize to 50psi. I think you are talking 100xkPa. 50psi would be 3 to 4 100xkPa. Going past that makes me nervous and does not produce better results for me. I have actually had parts come out perfect at 30psi(2 100xkPa).

Have to try degassing the resin too, haven't done so yet. What worries me about this is high temperatures plus degassing boiling might shorten the working time, which is already happening due to summer itself, makes me have to move fast. Might have to cool the resin down in the fridge to do so. Must test.

305 has a 7 minute working time. I have degassed before mixing, but once I mix I pour into the molds and then put in the pressure pot. Since I have started the shake an hour before I don't even bother with this anymore. If for some reason I forget to pre-shake I probably would.

Also, with resin (and RTV) don't over do the degassing. Smooth-On tech support has told me in the past that you can actually remove other gases beside air if you degas too long. They say it could lead to the material not curing. When I first started I thought I should degas until there were no longer any bubbles coming up. Sometime half an hour or more. Those molds cured and worked great. I guess I got lucky.

Also, does anyone put two molds at a time in the pressure pot?. I am thinking of doing so to speed up the process a little. I´d build a structure to keep the molds separed.

All the time. I normally fill it as full as possible. Might as well maximize product since wear and tear on the equipment is the same for one mold or twenty. I hate doing all that work for only one or two parts. I thought about building a fancy multi-level table, but never got around to it. I just stack the molds in starting with the biggest and then set any smaller molds on top of them or into any voids around the big molds.

The one thing you need is a resin with enough working time to do all these processes before it starts to thicken.

I thought about switching to a 20 minute work time resin, but tech support told me that it will actually wear the molds out quicker. They say any damage the resin does to the mold is when it is still in a liquid state. Any thoughts? I've stuck with the 7min work/30 minute cure. I normally have no problems pouring about 8-16 ounces into several molds and getting them into the pot with time to spare.
 
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Duncanator

Sr Member
[/QOUTE] I thought about switching to a 20 minute work time resin, but tech support told me that it will actually wear the molds out quicker. They say any damage the resin does to the mold is when it is still in a liquid state. Any thoughts? I've stuck with the 7min work/30 minute cure. I normally have no problems pouring about 8-16 ounces into several molds and getting them into the pot with time to spare.
[/QUOTE]

In my experience, it is the faster cure resins that are harder on molds because they produce more heat as they cure. The heat is what accelerates silicone mold degradation. Slower curing resins create less heat, so they are gentler on your molds.
 

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udog

Active Member
Thanks for all the interesting feedback and sorry for the delay answering, very busy days.
Well, things are going much better with the 40D shore silicone. Thin parts stand better. Also putting the silicone in the fridge to get longer working and curing time seems to work as I´m getting bubble free molds.
Demolding with the harder silicone is a bit of a pain (I need assistance to do it), and getting the spray excess out is also a hard one, hard to get my hands in there, and small brushes wont do. But that´s what comes along with a harder rubber I guess.
When I started with this I was discarding about 30% of the castings (horrible), now I must be in 10-5%. I still get some issues.
Part of the discarding is due to worn out molds, which I assume as normal. The mold is discarded and a new one is made. But there are two or three other unknown reasons still bugging around.
Sometimes a small bubble appears when sanding, I guess it could be due to many reasons. Probably I took too long before pressurizing, lots of castings, it can happen. Also we are now fully into hot summer here, so less working time. Might be the time to use the fridge for the resin too.
On the other hand I also get these pinholes once in a while, that don´t look like a normal bubble, but like a straight hole.
Probably it has to do with Duncanator´s coment:
 
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udog

Active Member
When casting under pressure, I leave the resin in until it hardens. Pulling it when it is soft (gelled) can allow any air bubbles that the pressure was squishing down to re-expand and distort your casting. Once hard, the tiny pressurized bubble is trapped and can't hurt anything if you leave it under pressure till it cures.
I started leaving the castings in the pot ´till completely hard and haven´t seen that issue any more, or not much more.
But I still get this once in a while too, which is annoying-I attach a picture.
Itś'a major distortion, generally in the back part.
DISTORT.jpg


I can think of some reasons for this:
-The resin is going into the pot too late.
-There is something going wrong with the pressure (ups and downs), but the gauge doesn´t seem to reflect that.
-I´m using too much pressure. I´m at 6, thought that with harder molds I could take it to the limit. But on the other hand, it pnly happens once in a while.
-I´m failing to remove the silicone spray excess (hard to do) and it´s pooling or something to do with the talc. I try to make sure the spray is dry before powdering. This is the reason less probable I think.
I normally pressurize to 50psi. I think you are talking 100xkPa. 50psi would be 3 to 4 100xkPa. Going past that makes me nervous and does not produce better results for me. I have actually had parts come out perfect at 30psi(2 100xkPa).
The gauge measures in the "bar" scale. So now I´m using 6 bar pressure, or at least that´s what the gauge says. I´ll lower down to 5 to see if these distortions disappear.

Just to finish. Is it realistic to expect an 100% perfect castings in a 200 pieces series?, or a 5-10% of failure is what I can expect and I should just stick to it and stop worrying?

Thanks again for all the feedback and braindrops here. Really appreciate it and of great help.
 
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udog

Active Member
In my experience, it is the faster cure resins that are harder on molds because they produce more heat as they cure. The heat is what accelerates silicone mold degradation. Slower curing resins create less heat, so they are gentler on your molds.
In my experience with resins and molds I would say so too, in fact when I have worked large series castings with fast resins a good thing to do is to reduce the number of castings per day for a mold as much as possible to preserve the mold form heating one after another. Sometimes I had to use fans to cool the molds down between castings.
But I assume the chemicals in resin also do their job. Solvents make silicone swell and "unswell" (if that´s a word). I think this could have to do with the mold wearing out too, and using longer setting resins means that the chemicals sit in the mold longer.
In fact, when I´ve casted technicall parts I have experienced thinner castings due to swelling and had to reduce the castings in a day to allow the silicone get back to it´s place.
Also, I was once recommended by an experienced moldmaker not to store silicone molds with Pu foams inside (big tin molds, to prevent distortion) as the chemicals liberated along time would shorten the mold´s life. Not sure this is so, but followed the advice along the years just in case.

But I´d say heat is the major reason too.

Just thoughts.
 

Duncanator

Sr Member
6 bar seems pretty high to me. I usually use about 2 bar. I’ve gone up to 3 on a few rare occasions, but that’s as high as I have ever needed to go.
 

udog

Active Member
6 bar seems pretty high to me. I usually use about 2 bar. I’ve gone up to 3 on a few rare occasions, but that’s as high as I have ever needed to go.
Well, 2...I have never tried so low with this. 4 is the lower I went and still got tiny random bubbles.
In any case the distortions in the last pic I uploaded have ceased to happen as soon as I started leaving the mold pressurized until the resin is hard.
 

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