Casting urethane rubber cowl - first time - I have a few questons

TheKingaSwing said:

We use both of those rubbers at our shop. PT Flex 50 for face masks and smaller items, Vytaflex 40 for cowls.

I use 3% of part B Kick It per layer, don't pour either rubber in one go as it'll kick into a big lumpy mess in the mould.

Figuring out layer and casting thickness is trial and error, what works on one piece may not work on another, you'll also learn where thicknesses of castings need to be adjusted piece by piece. If a piece is too thick, that can also bring its own risks in terms of tearing. 2-4mm seems to be a good thickness depending on the piece.
If using vytaflex, you can speed up the curing a bit once the rubber is in the mould with a hair dryer or heat gun on a lower heat. Just before a layer can't be moved anymore, heat will also flatten out most runs or lumps in the rubber's surface. I tend to dump the rubber in, roto cast it all over while it's thin and then once it thickens up, spread it around by hand until it's done.

We use the PT Flex for injecting our face masks as when it cures, it "snaps" quicker than Vytaflex does but you can hand lay it with good results. Both are great rubbers. The PT Flex cures in 60 minutes which is nice but the stretch isn't the same as Vytaflex.

We've found that both rubbers are patchable as long as you don't leave it days after demoulding, hell we've thickened up castings in vytaflex with PT Flex and it's stuck just fine.

I hope that helps a bit.

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regisaugusto said:

I would like to make some questions too. I'm familiar with latex castings, and I want to try urethane for cowls. I don't have vytaflex 40 readily available where I live, but I have other rubbers with the same specs (Shore, curing time, pot life, etc..). However, probably it'll be a labor-intense task, since this mentioned rubber is thixotropic. So, my question is: what material I should use on the negative mold? Silicone? Or fiberglass? I have seen people making perfect cowls without seam lines. I can clean up seam lines on latex (an sometimes, dependind on the piece, avoid them completely). However, I have no idea of the process for urethane.

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TheKingaSwing said:

What products are they exactly? You can't clean a urethane rubber seam like latex or silicone, once it's there, it's there and you can't fix the surface around it either. A one piece silicone mould would be best way to get a seamless piece in the end.

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KJC said:

So, the title should be self explanatory. I am going to be casting my first urethane rubber cowl and I need advise. I have scoured all over and can't find a good walk through on the full process for casting urethane rubber cowls/masks.


What I know so far:

I will be roto-casting or "slush" casting.

Vitaflex 40 seems to be the preferred rubber (this has a stock pot life of 30 mins with a cure/demold time of 16 hours)
I can use Kick-it to make the urethane cure quicker.

I have just found a rubber from Polytek called PT-Flex that is available in varying shore hardness (though they don't have a 40 shore). This is described as low viscosity and quick curing which seems ideal for roto-casting.


What I want to know:

If I go with the Vitaflex, how much Kick-it should I use? I found the guide on the Smooth-On site, but what is the recommended ratio?

Does anyone have experience with the Polytek PT-Flex urethanes?

How thick does a cast have to be to reduce risk of tearing?

Should I try to cast in a single pour or how many layers should I cast?

Is there a way to clean up the lumps or run marks on the inside after casting or how do I avoid these?

Is there a way to correct thin spots if there are any in the final casting?


I'm sure I'm not the only one who would benefit from a nice full guide on this. Like I said, I have been searching and have not found one.

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TheKingaSwing said:

We use both of those rubbers at our shop. PT Flex 50 for face masks and smaller items, Vytaflex 40 for cowls.

I use 3% of part B Kick It per layer, don't pour either rubber in one go as it'll kick into a big lumpy mess in the mould.

Figuring out layer and casting thickness is trial and error, what works on one piece may not work on another, you'll also learn where thicknesses of castings need to be adjusted piece by piece. If a piece is too thick, that can also bring its own risks in terms of tearing. 2-4mm seems to be a good thickness depending on the piece.
If using vytaflex, you can speed up the curing a bit once the rubber is in the mould with a hair dryer or heat gun on a lower heat. Just before a layer can't be moved anymore, heat will also flatten out most runs or lumps in the rubber's surface. I tend to dump the rubber in, roto cast it all over while it's thin and then once it thickens up, spread it around by hand until it's done.

We use the PT Flex for injecting our face masks as when it cures, it "snaps" quicker than Vytaflex does but you can hand lay it with good results. Both are great rubbers. The PT Flex cures in 60 minutes which is nice but the stretch isn't the same as Vytaflex.

We've found that both rubbers are patchable as long as you don't leave it days after demoulding, hell we've thickened up castings in vytaflex with PT Flex and it's stuck just fine.

I hope that helps a bit.

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Friko said:

im glad glad you asked, I can't find crap out either...




That at sums it up, this is some of the most straight forward and helpful info so far.
i do have a question about vytaflex, I have read the material overview and it didn't mention adding multiple layers. Form my understanding with urethane rubber is is that once it has cured it doesn't want to stick to itself, is there a particular time frame when a second coat should be added?
thanks.

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barbquebutt said:

Since we're asking cowl-casting questions, I have one that I can't seem to find an answer to.

If I want to cast a cowl that is form fitting (particularly in the face) from my own head cast, how do I achieve that? It seems like roto-casting leaves a lot of room for gaps and the last thing I want in my cowl is a sunken, floppy look.

Or should this even be a concern? Is it likely to be form-fitting on its own?

Thanks!

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TheKingaSwing said:

If it's sculpted over your own head, the face/head should fit well against you as long as the casting is a decent thickness. Injecting a piece with a core is the only way to make something truly form fitted but you really don't want to have a skin tight urethane cowl, it would be the most uncomfortable thing to wear. It would be so thick in places like the neck, that you couldn't even get it on, even in a soft shore rubber. The moulding/injecting process is quite involved and requires experience.

If you cast the cowl thick enough it won't sag when being worn.

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TheKingaSwing said:

You have to cast things in layers if you're not injecting. Add a layer, let it kick a little until you can't move it anymore, then add more until desired thickness is achieved. If you demould a piece and it has a thin spot, backing it as soon as possible will always be better but we've added more layers a couple of days later not had de-lamination issues as long as the piece is dust free. Can't guarantee it'll work the same for everyone but as I said, we've never had issues backing pieces post cure.

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