Ontopic The new car-seching thread

Jehannum

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We cast Flexane urethane in disposable 3D printed molds at work, for overmolding underwater cable joints and whatever.

Print the mold out of PLA with thin walls (1-2mm) and coat it with a suitable mold release first. Cast your part and let it fully cure, then heat the thing up in an oven to 100C to soften the PLA, put on some Ove-Gloves (seriously) and peel the PLA off by hand.

Also, pull a vacuum on your urethane after you mix it, but before you pour it, to get the air bubbles out of it. It'll "boil" when you do this and possibly make a mess, look out. About 1/3c of mixed Flexane 80 will almost overflow a standard red solo cup.
How long do you pull a vacuum on it?

The urethane I ordered only has a working life of ~10 minutes, so I can either pressure-cast it, or run the risk of not being able to pull vacuum long enough to boil out the water.
 

Mr. Asa

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You aren't boiling out the water, you're lowering the pressure enough in the system for the urethane to boil.

At my job before this one, we would design the molds to have a large excess reservoir, then we'd vacuum the entire mold with the liquid urethane inside it, all the bubbles would move to the reservoir and we'd cut that off when we were done. With 3D printed molds this can kind of be a bad idea as the urethane will flow into the voids in the print, but you could seal the surfaces of the mold to prevent that.
 
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Jehannum

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You aren't boiling out the water, you're lowering the pressure enough in the system for the urethane to boil.

At my job before this one, we would design the molds to have a large excess reservoir, then we'd vacuum the entire mold with the liquid urethane inside it, all the bubbles would move to the reservoir and we'd cut that off when we were done. With 3D printed molds this can kind of be a bad idea as the urethane will flow into the voids in the print, but you could seal the surfaces of the mold to prevent that.
What's the advantage to boiling the urethane?

I'm not dealing with critical tolerances here, since it's just a bushing. The molds I jiggered up last night were all open-topped, so there's plenty of overhead space, lol.
 

Mr. Asa

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I should have used quotes like gee did.
It isn't boiling boiling, but it will look like it is. The vacuum is pulling the air pockets from inside the mix out so you have solid urethane, otherwise you've got a heterogeneous mix
 

Jehannum

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I should have used quotes like gee did.
It isn't boiling boiling, but it will look like it is. The vacuum is pulling the air pockets from inside the mix out so you have solid urethane, otherwise you've got a heterogeneous mix
OK, so if I give the mold a good coat of release agent and keep the bottom open, I can just keep the whole thing in vacuum while it cures?
 

Mr. Asa

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I'm not sure about holding the vacuum the entire time, maybe just long enough to get it to clear the air bubbles. @gee, input on that?
Back to the previous job again, we'd pull vacuum for a minute or two, release to let them sink back down or pop, then do it again for another minute or two.
 

gee

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How long do you pull a vacuum on it?

The urethane I ordered only has a working life of ~10 minutes, so I can either pressure-cast it, or run the risk of not being able to pull vacuum long enough to boil out the water.
Only a minute or two. It bubbles up, sits there bubbling for a while, then it collapses and it settles back down into the cup. Once that happens we release the vacuum and pour it into the mold.

Molding under pressure instead will definitely do the job especially if you're tooled up for that. In our case, watertightness is absolutely critical so we don't want air. Not so much an issue for a car bushing.
 
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Jehannum

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Only a minute or two. It bubbles up, sits there bubbling for a while, then it collapses and it settles back down into the cup. Once that happens we release the vacuum and pour it into the mold.

Molding under pressure instead will definitely do the job especially if you're tooled up for that. In our case, watertightness is absolutely critical so we don't want air. Not so much an issue for a car bushing.
I'm much more equipped for pressure molds than I am for vacuum, but I can go either way for not a lot of dough.
 

gee

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I'm much more equipped for pressure molds than I am for vacuum, but I can go either way for not a lot of dough.
Pressure cure would probably be easier and it'll definitely be adequate. Mix urethane, pour into mold, put mold in pressure vessel, apply pressure (60psi from your shop compressor is fine, nothing fancy needed) and let it sit for however long the cure time is.

Whatever you do, buy a commercial pressure vessel, don't try to make one :)
 
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Jehannum

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Pressure cure would probably be easier and it'll definitely be adequate. Mix urethane, pour into mold, put mold in pressure vessel, apply pressure (60psi from your shop compressor is fine, nothing fancy needed) and let it sit for however long the cure time is.

Whatever you do, buy a commercial pressure vessel, don't try to make one :)
yeah, at 60 PSI it's p. much a given to buy and not build.
 

Mr. Asa

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Thoughts on how to get this bushing out? I can't get any damn purchase on it. Been using picks and all sorts of similar tools.

I'm almost at the point where I am going to cut it out and buy a new one for $10







 

Jehannum

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Thoughts on how to get this bushing out? I can't get any damn purchase on it. Been using picks and all sorts of similar tools.

I'm almost at the point where I am going to cut it out and buy a new one for $10







I'd probably start by cleaning that filthy hole up.

Then maybe a blind hole puller? That's probably more than the $10 for a new bushing though.
 

Mr. Asa

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I'd probably start by cleaning that filthy hole up.

Then maybe a blind hole puller? That's probably more than the $10 for a new bushing though.
Never run into one of those before. Kinda neat. Looks like even the HF versions are $30ish though.
It isn't actually a blind hole, I can get to it from the other side, but its so long that I cant hold it hard enough at an angle to keep it from slipping
 

gee

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If it's not a blind hole, can you find a piece of pipe or socket or something and drive it out?

If you value your time, $10 is pretty cheap though plus you end up with a new bushing.

Any reason you can't just leave it there?
 
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Mr. Asa

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Not a blind hole, but the ID is flush with the rest of the shaft in there. Probably less than a 16th of clearance between the metal and this
Trying to disassemble the entire thing to clean, strip, and repaint.
I found one on eBay that I know from prior experience is just over in Clearwater. Tolls getting there will be about what it would cost to ship it, but it'll be worth it to check out the shop the guy has.
 

Jehannum

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If it's not a blind hole, can you find a piece of pipe or socket or something and drive it out?

If you value your time, $10 is pretty cheap though plus you end up with a new bushing.

Any reason you can't just leave it there?
Even better: print a dildo that you can whack on with a hammer that will drive it out!