Molding urethane gaskets

LOTT · Apr 10, 2020

Anyone have experience molding resin? We were doing some product development work for a company, CNC machine stuff, and they mentioned needing a spacer and gasket to go with some of what we were doing. I happened to be experimenting with casting vacuum forming molds at the time and said we would give it a shot, and maybe make a sealing spacer out of flexible urethane. So I milled a low tech injection mold out of aluminum and shot the urethane in with a syringe.

It worked, and now they need about 1,400 of them. Which is fine and all, but my prototype setup does not lend itself to mass production.

The part is rectangular, 1-3/4"x3/4"x1/8" with some 2D features (not just flat that could be stamped out). Anything soft enough to seal I'm expecting to be too soft to machine, so I'm thinking molding is the only way to go? Anyone know different?

My first thought is to find a recovering junkie that will happily sit and inject these molds for about three weeks. I don't think the quantity is enough to justify a "real" injection mold set.

What's a good system for short run urethane molding? Some kind of semi-automated setup to speed things up would be great. I did some reading and most of the other posts seemed to be a few of larger gaskets where labor time wasn't so much an issue.

Limy Sami · Apr 10, 2020

Water jet from sheet material?

adh2000 · Apr 10, 2020

Limy Sami said:
Water jet from sheet material?

He said it wasn't flat and therefore couldn't be cut from sheet.

I've done a fair amount of urethane casting. Never tried a syringe, sounds like a mess but whatever. I was always able to pour into a metal or even wood mold. Wood has to be coated with several coats of lacquer. Gosh I'd hate to do 1,400 of them. Sign up the recovering junkie and see if you can make 5 or 10 molds so he can do several at a time.

as9100d · Apr 10, 2020

adh2000 said:
He said it wasn't flat and therefore couldn't be cut from sheet.

I've done a fair amount of urethane casting. Never tried a syringe, sounds like a mess but whatever. I was always able to pour into a metal or even wood mold. Wood has to be coated with several coats of lacquer. Gosh I'd hate to do 1,400 of them. Sign up the recovering junkie and see if you can make 5 or 10 molds so he can do several at a time.

We so lots of casting here. What questions bdo you have specifically

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LOTT · Apr 10, 2020

It needs to be flat and a fairly accurate thickness on both side, it could be poured into an open top mold but the surface tension means that it doesn't come out flat, it's more bubbled up. Hence the syringe injection, and yes it's a mess.

BugRobotics · Apr 10, 2020

You can go somewhere like Protolabs to get a small run completed. May make sense for your numbers. You can send in your model and get a quote within a day or so on their website.

Protolabs Part Upload

LOTT · Apr 10, 2020

Are you casting parts you can just pour the resin into, or injecting resin into a mold? Is your process automated at all? These are small gaskets, finished weight of these things are 1.25 grams, and the flange gets down to .015. Which it why we're injecting the resin, which is a slow manual process that I want to improve.

Limy Sami · Apr 10, 2020

Limy Sami said:
Water jet from sheet material?

My mistake please ignore this post.

Conrad Hoffman · Apr 10, 2020

I've done some of this kind of thing using silicones and resins from Smooth-On. Results have been great. You need a 2-part silicone mold, assuming the precision would be good enough. Can't imagine doing 1400 of 'em. Could you make a silicone mold with 5-10 cavities that you fill all at once? That would ease the pain a bit. See the videos at the Smooth-On site if you haven't done silicone molding.

LOTT · Apr 10, 2020

BugRobotics said:
You can go somewhere like Protolabs to get a small run completed. May make sense for your numbers. You can send in your model and get a quote within a day or so on their website.

Protolabs Part Upload

I haven't tried Protolabs, but just for fun I ran it though Xometry and they wanted $91 a piece. Which is probably how as9100d is able to be buying planes and Haas's by the truckload...

Punkinhead · Apr 10, 2020

There's bound to be at least one small run rubber mold house near you. We've got 3 in our medium size town that do everything from one off prototypes to 10,000+ piece runs.

LOTT · Apr 10, 2020

Conrad Hoffman said:
I've done some of this kind of thing using silicones and resins from Smooth-On. Results have been great. Can't imagine doing 1400 of 'em. Could you make a mold with 5-10 cavities that you fill all at once? That would ease the pain a bit.

I'm using a Smooth-On product, and you're exactly right about 1400 being a bunch to do by hand. The mold has three cavities, but because of the limited pressure we are filling the cavities one at a time. Some kind of high pressure injection system would be great, but since it's a time curing resin and not a melting plastic I'm not sure how it would work.

LOTT · Apr 10, 2020

Punkinhead said:
There's bound to be at least one small run rubber mold house near you. We've got 3 in our medium size town that do everything from one off prototypes to 10,000+ piece runs.

Not that I've found. We're in rural North Carolina, which is great in a lot of ways as far as low overhead and being generally left alone, but there isn't much in the way of industry. It's like a country song, if there's a traffic jam it's a tractor...

as9100d · Apr 10, 2020

Smoothon is way too expensive anyways. We use polytek here and we have some home made tools that do metered mixing under pressure for injection.

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LOTT · Apr 10, 2020

as9100d said:
Smoothon is way too expensive anyways. We use polytek here and we have some home made tools that do metered mixing under pressure for injection.

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That's what I'm looking for! Is there any non-proprietary info on metered mixing under pressure you can share? The Smooth-On may be more expensive, but for the sizes we're doing now the cost of resin is immaterial compared to labor. Also for someone new to the process Smooth-On makes it easier to find what you need then PolyTek, but I'll give them a try if this continues.

as9100d · Apr 10, 2020

You would be surprised.... Polytek applications engineers would tell you which product to use, estimate part cost, and show you machines to use. That's how we learned.

We go through about 200 gallons of 80a polyurethane per month.

Google a machine called "minirim" from dispensingliquid.com

That's what you will need, or we can quote the parts for you.

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BugRobotics · Apr 10, 2020

LOTT said:
I haven't tried Protolabs, but just for fun I ran it though Xometry and they wanted $91 a piece. Which is probably how as9100d is able to be buying planes and Haas's by the truckload...

Haha, yea that is a tad high [emoji1787]. Never tried Xeometry before

as9100d · Apr 10, 2020

[emoji23][emoji23] xometry work is good for sure! Lately I've been dominating the stock market and buying less Haas and more dmg Mori machines.

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adh2000 · Apr 10, 2020

as9100d said:
Smoothon is way too expensive anyways. We use polytek here and we have some home made tools that do metered mixing under pressure for injection.

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Smoothon is a lot cheaper than Polytek. I’ve been buying the 16 lb kits from Reynolds Advanced Materials for $92. The same 16 lb kit from Polytek is $143.

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as9100d · Apr 10, 2020

adh2000 said:
Smoothon is a lot cheaper than Polytek. I’ve been buying the 16 lb kits from Reynolds Advanced Materials for $92. The same 16 lb kit from Polytek is $143.

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I'd give polytek a call, order via phone and email not their website.

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