resin casting and molds

[Rick L]

One note if you’re using clay. Be sure to use sulfur free clay or it will not cure. Sulfur inhibits mold rubber.

[robdebie]

The curing inhibition only applies to addition cure (platinum) silicone rubber. The list of cure inhibition materials is quite long. The following materials are listed by
manufacturers:

- condensation type silicone rubber (especially its tin-soap catalyst)
- unsaturated hydrocarbon solvents
- sulphur (in vulcanized natural and synthetic rubbers)
- phosper
- epoxies containing strong amine catalysts
- isocyanates of urethane resins
- tape adhesives
- metallo-organic salt-containing compounds (especially tin salts and heavy metals)
- plasticizers in plastics (especially vinyl)
- some epoxy and polyester resins
- nitrogen containing materials
- some modelling clays
- solder flux
- wood
- leather
- chlorinated products (such as neoprene rubber)

But still I very much prefer this type over condensation cure (tin salt) silicone, even though the latter is cheaper. I just don't like the shrinkage of condensation type. The amazing thing is (quoting from manufacturer, Shin-Etsu I think) is that the shrinkage never stops!

Rob

Edited August 30, 2021 by robdebie

[Rick L]

On 8/26/2021 at 12:09 PM, Bainford said:

Thank you much!

Another question if I may; Does room temperature effect the process in any significant way? Is humidity a factor?

Humidity can only be a factor with the release agent. If the release agent is still wet when pouring the rubber, it could cause the rubber to inhibit.

[freightshaker2]

This is an older thread I know, but still a very interesting read, as well as super informative.  Thank you guys for this explanation and tips and tricks to it!

[robdebie]

3 hours ago, freightshaker2 said:

This is an older thread I know, but still a very interesting read, as well as super informative.  Thank you guys for this explanation and tips and tricks to it!

Since then I've made a web page with a lot of information on my method of vacuum resin casting: https://robdebie.home.xs4all.nl/models/casting.htm

Here are a few pictures from the page.

Rob

[NOBLNG]

15 hours ago, robdebie said:

Since then I've made a web page with a lot of information on my method of vacuum resin casting: https://robdebie.home.xs4all.nl/models/casting.htm

Here are a few pictures from the page.

 

 

 

Rob

Thanks for this info and the link to your site Rob. Vacuum degassing makes a huge improvement on the mold quality indeed. I have cast the parts themselves under pressure but not vacuum. Have you tried both or know if vacuum is better than pressure?

Thanks, Greg.

[robdebie]

24 minutes ago, NOBLNG said:

Thanks for this info and the link to your site Rob. Vacuum degassing makes a huge improvement on the mold quality indeed. I have cast the parts themselves under pressure but not vacuum. Have you tried both or know if vacuum is better than pressure?

No, I haven't tried both, it just developed that way. Plus, like I write in the introduction of my webpage:

"There's an interesting cultural aspect to resin casting. It seems that vacuum casting is the European way of resin casting. In the USA pressure casting is the preferred method. I'm describing vacuum casting here. "

I'm in Europe 🙂

Rob

[peteski]

Vacuum casting seems to be a better idea to me.  Pressure casting leaves highly compressed air bubbles in the resin, while vacuum casting eliminates the air, leaving the resin air-free. 

[Chariots of Fire]

On 8/28/2021 at 3:09 PM, ChrisBcritter said:

I thought of a trick to suspend a tire in the mold box so I could do the pour in two parts: I drilled holes in three sides of the box at the same height that were just big enough to insert straight pins. I put the tire in the mold with the pin points touching the tread just enough to hold it steady, and poured the rubber to the top of the sidewall. Once it cured I smeared on some mold release and made the second pour. Once that cured, I pulled out the pins and the mold came out in two neat pieces. I managed to get a couple usable tires from it: 

One thing I wonder about two-part molds: Will a pressure pot help, or does the mold seal up and prevent the pressure from popping the air bubbles?

I tried this method once with mixed results.  The biggest problem is suspending the master perfectly level so that the parting line is not running across the tire tread.  Doing an initial pour and then a second pour to essentially rest the tire on eliminates this problem because the RTV seeks its own level and remains a flat surface.  It is an extra step but the results I think are better.    

[Chariots of Fire]

1 hour ago, peteski said:

Vacuum casting seems to be a better idea to me.  Pressure casting leaves highly compressed air bubbles in the resin, while vacuum casting eliminates the air, leaving the resin air-free. 

Pressure casting should be reserved for making the mold so that the RTV is forced into every crevice and the bubbles are forced out.  Vacuum casting in essence collapses the mold pieces in on themselves assuring a clean casting.  Tom Coolidge who did my '66 DODGE W300 used a pressure pot.  The results were amazing.

[peteski]

3 hours ago, Chariots of Fire said:

Pressure casting should be reserved for making the mold so that the RTV is forced into every crevice and the bubbles are forced out.  Vacuum casting in essence collapses the mold pieces in on themselves assuring a clean casting.  Tom Coolidge who did my '66 DODGE W300 used a pressure pot.  The results were amazing.

Yes, pressure casting will force resin into all the crevices in the mold. But if those crevices had voids in the when resin was poured, they were not empty.  The voids were full of air. Pressure pushes the liquid resin into the void by compressing the air that was in it in the first place. It does the same to all the air bubbles within the resin itself.    Since the compressed air doesn't just disappear, it is still there in a tiny bubble under high pressure (60 psi or whatever the pressure pot has in it.  When the hardened resin part is removed from the mold the compressed air from the void will be vented, but any bubbles which were within the resin itself, will remain trapped and under pressure.

At least that is how I understand the pressure casting.  Vacuum casting process sucks any air that is in the mold's voids and the resin itself, so no compressed air remains in the resin after it hardens.  Mold itself is air-free RTV material - it doesn't collapse since it is solid.  That's because the RTV itself was also vacuum cast.  Robs illustrations show all this quite clearly.

[robdebie]

6 hours ago, peteski said:

Vacuum casting seems to be a better idea to me.  Pressure casting leaves highly compressed air bubbles in the resin, while vacuum casting eliminates the air, leaving the resin air-free. 

I agree that vacuum casting makes more sense. But pressure casting works very well too 🙂

Maybe there's another mechanism with pressure casting: I have a feeling that small bubbles do not get compressed to a smaller volume, but that they are actually absorbed by the resin. Why do I think that? I have never seen tons of tiny bubbles in cured pressure-cast items.

It's interesting to look at the numbers. A 1 mm diameter bubble has a volume of 0.52 mm3. Compress it to 5x ambient pressure, and the volume becomes 0.10 mm3, and that gives a 0.6 mm diameter. So, the bubble size reduction is not very impressive. So one should see lots of tiny bubbles in a cured part - but you don't.

I'm sure there are technical papers and studies on this subject..

Rob

[robdebie]

5 hours ago, Chariots of Fire said:

I tried this method once with mixed results.  The biggest problem is suspending the master perfectly level so that the parting line is not running across the tire tread.  Doing an initial pour and then a second pour to essentially rest the tire on eliminates this problem because the RTV seeks its own level and remains a flat surface.  It is an extra step but the results I think are better.    

I don't know if it's of use, but here's how I copied a truck tire with single-piece mold and vacuum casting. I filled the master tire with Apoxie Sculpt to make it solid. Worked perfectly.

Rob

Edited February 15 by robdebie

[peteski]

21 minutes ago, robdebie said:

Maybe there's another mechanism with pressure casting: I have a feeling that small bubbles do not get compressed to a smaller volume, but that they are actually absorbed by the resin. Why do I think that? I have never seen tons of tiny bubbles in cured pressure-cast items.

I'm not sure Rob. Maybe I'm looking at it the wrong way.  Yet, I have doubts that air (mostly Nitrogen in the ambient atmosphere) gets absorbed by the liquid resin when under pressure. I know carbonated drinks (like Coco-Cola) have CO2 dissolved (?) in the liquid when under pressure, but then the gas escapes when not under pressure (when the sealed bottle is opened), but I'm not sure similar process can take place in resin.  But my opinion is just an opinion. I have not done any scientific research about it.

[robdebie]

In the past, I did quite a lot of vacuum infusion of composite parts. The biggest was 10  by 3 meters 🙂 We would always de-air the epoxy resin thoroughly, and if the injection went as planned, we produced totally void-free parts. But if you would get a small dry spot, with the injection encirling a spot, the tiny amount of air present would often be absorbed, solving the problem.

I know that research was done on the de-airing process, but I did not participate in that. Here's one example of such research, but as far as I can see, it does not discuss 're-absorption' of air, after de-airing that same resin. But if the gasses got in the resin before de-airing, they surely must be able to 'get in' after de-airing too.

https://www.sciencedirect.com/science/article/abs/pii/S1359835X20300567

The paper does not discuss the effect of pressurizing the resin-air combination. But that must have been studied extensively for autoclaving of composites, so I expect scientific literature on that too.

All in all, I still can't prove my assumption, but I'm pretty sure that I witnessed air absorption in practice, with epoxy.

It should be fairly easy to test the assumption, by curing polyurethane samples at increasing pressure, then cutting the samples and study for air bubbles. Actually, that might be a good question for Robert Tolone, who has a great Youtube channel: https://www.youtube.com/@RobertTolone/videos

Rob

Edited February 18 by robdebie

[robdebie]

Robert Tolone did a video on using different pressures, but it doesn't really answer our question. I would even dare to say that he did not consider the air absorption theory.

Rob

 

Edited February 18 by robdebie

[peteski]

Thanks for doing the research Rob, even if inconclusive.  As for Polyurethane resin, we know that the liquid resin absorbs moisture from the ambient air, and when it hardens the moisture causes bubbles to form.

[NOBLNG]

I have used about 60 psi for casting, but using relatively fresh resin. Those plastic fittings on his pressure pots are pretty scary looking! If they fail they can explode.

[peteski]

Yes, using PVC pipe for pressurized gases is a big no-no.  I've seen what happens, and it is not pretty. A BIG Kaboom!

[Rick L]

3M makes static mixing nozzles to alleviate the problem of introducing air into the plastic. 
 And WOW that pvc pipe at 100 psi. over time is going to be scary.

Edited February 18 by Rick L
Added comment

[NOBLNG]

Now that I look closer, I guess the PVC is only an exhaust baffle of sorts for when he releases the pressure. The supply must be piped in at the back or somewhere.

[peteski]

Well then, using PVC as a baffle for diffusing the released air is ok.  Looks like the pressure pipes on the bottom are iron.

[Ace-Garageguy]

1 hour ago, Rick L said:

And WOW that pvc pipe at 100 psi. over time is going to be scary.

 

22 minutes ago, peteski said:

Well then, using PVC as a baffle for diffusing the released air is ok.  Looks like the pressure pipes on the bottom are iron.

While I generally don't recommend it for a permanent installation, PVC schedule 40 pipe is routinely used in both mechanical and body shops to route compressed air to workstations. 

Air coming out of a shop compressor is usually around 120-140 psi in my experience.

Pressure ratings for schedule 40 PVC start at around 120 psi, and after 5 decades working in many shops that used PVC for air lines, I've seen zero failures other than poorly installed fittings working loose.

Keep in mind that the water pressure in a house can be as high as 80 to over 100 psi too...though it shouldn't exceed 40-60 or so.

The chart below shows both working and burst pressures for schedules 40 and 80 PVC pipe.

 

Edited February 18 by Ace-Garageguy

[Rick L]

You are correct my friend. I did not go back to look at the set up in the video and assumed it was the supply end.

[Ace-Garageguy]

Please note that published sustained working pressures for 3/4" and 1" PVC schedule 40 pipe, the most commonly used for low-buck air supply lines, are 480 and 450 psi respectively, at 73 F. 

Though manufacturers of PVC pipe usually warn against using it for compressed gasses, as I've said, I've seen it used in many shops over many years with zero failures, and I wouldn't hesitate to use it within reason on a little resin casting rig.

EDIT: But y'all do you. Might want to wear aircraft helmets and body armor too. 

Edited February 18 by Ace-Garageguy
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