Author Topic: Casting polyurethane components  (Read 815 times)

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Offline drums'n fins

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Casting polyurethane components
« on: October 25, 2021, 10:45:32 PM »
Hi, I'm wondering if anyone has had much success casting parts out of polyurethane? I have taken some time to design and fabricate a fibreglass mould to make an air boot that goes between the air box and carburettor and after some initial set backs I perfected a good process and well finished conical boot. Problem is that when I fit it to the carb after a day or so it splits! Funny thing is that it is the lightest push on fit and isn't being stretched, nor is it being stressed or distorted where it sits against the airbox - it just splits??
I'm very much a novice with this stuff, I rang the store where I got the mix and am told polyurethane can be funny like that? Suggestion is to use silicone rubber but I haven't had any exposure to that either?
Any help would be most appreciated.
OMG,NOS,OEM,RMT,CDI,NLA,WTF!

Offline brent j

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Re: Casting polyurethane components
« Reply #1 on: October 26, 2021, 12:09:06 AM »
There was a guy on here quite a few years ago who made a range of polyurethane parts.
He ended up having everything wiped out in a fire.

I don't like to post peoples names online without their permission so I'll send a PM
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Offline pokey

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Re: Casting polyurethane components
« Reply #2 on: October 26, 2021, 12:29:42 AM »
Its been a while so im working from memory. i used the products for casting small parts and for tricky fibreglass moulds and plugs..

Splitting in urethane is because of a not quite right mix ratio or temp. Now the hard part is working out what the correct ratio is as it can and will change depending on humidity , the size of the part and the temperature on the day. Its all trial and error untill you get the feel for the medium and if your slow and stubborn like me it can take some time and lots of product. a controlled atmosphere would be the best situation.
RTV Silicone is much more forgiving  but, It doesnt last as long. Again how long it lasts is the same as urethane , if the catalysing process produces too much heat its going to be a rubbish part with warps and poor elasticity. Dont thin it too much with Dimethyl as it will flow nicely but weaken the product.

Cant say Ive seen too many parts made from those products  as they are usually only for casting moulds products.

 Best of luck with it.

Offline Tomas

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Re: Casting polyurethane components
« Reply #3 on: October 26, 2021, 03:10:40 AM »
made some simple bits out of urethane that is used for soft forms for casting concrete ,plaster etc. it worked well no splits etc. important is to mix just enough because it absorbs moisture from air. also mix it carefully and try not to mix too much air into it. can get quite foamy. there are various types of urethan for casting. i had one made by dawex chemicals(made in czech)just need to mix part a and b according to instruction. i used digital scales. it was roughly as hard as tire rubber but flexier when cured. good luck

Offline sleepy

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Re: Casting polyurethane components
« Reply #4 on: October 26, 2021, 10:18:02 AM »
Don't use silicon if it is near petrol. Silicon is the preferred material for methanol fuel but is attacked by petrol.
Not much knowledge in poly but is it possible to add some sort of fibrous reinforcing into to mix to give it more strength.

Offline drums'n fins

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Re: Casting polyurethane components
« Reply #5 on: October 26, 2021, 12:33:20 PM »
Thanks all, I'll post some pics that show the problem better

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Offline Momus

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Re: Casting polyurethane components
« Reply #6 on: October 26, 2021, 05:48:26 PM »
Hi, I'm wondering if anyone has had much success casting parts out of polyurethane? I have taken some time to design and fabricate a fibreglass mould to make an air boot that goes between the air box and carburettor and after some initial set backs I perfected a good process and well finished conical boot. Problem is that when I fit it to the carb after a day or so it splits! Funny thing is that it is the lightest push on fit and isn't being stretched, nor is it being stressed or distorted where it sits against the airbox - it just splits??
I'm very much a novice with this stuff, I rang the store where I got the mix and am told polyurethane can be funny like that? Suggestion is to use silicone rubber but I haven't had any exposure to that either?
Any help would be most appreciated.


Not really an answer but a suggestion for an alternative.

There is a huge variety of fabric reinforced sicilcone radiator hose and turbo ducting fittings available.- straight,  bends, expansions, expansion bends and so on. They're inexpensive.
 I've been using various as intake and filter fittings  in a petrol environment for years with no problems. You may find what you want.
If you love it, lube it.

Offline Gears

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Re: Casting polyurethane components
« Reply #7 on: October 26, 2021, 09:37:06 PM »
Hi, I'm wondering if anyone has had much success casting parts out of polyurethane? I have taken some time to design and fabricate a fibreglass mould to make an air boot that goes between the air box and carburettor and after some initial set backs I perfected a good process and well finished conical boot. Problem is that when I fit it to the carb after a day or so it splits! Funny thing is that it is the lightest push on fit and isn't being stretched, nor is it being stressed or distorted where it sits against the airbox - it just splits??
I'm very much a novice with this stuff, I rang the store where I got the mix and am told polyurethane can be funny like that? Suggestion is to use silicone rubber but I haven't had any exposure to that either?
Any help would be most appreciated.

You might be able to find what you need here....  https://dalchem.com.au/

Offline Hoony

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Re: Casting polyurethane components
« Reply #8 on: October 27, 2021, 04:50:13 PM »
There was a guy on here quite a few years ago who made a range of polyurethane parts.
He ended up having everything wiped out in a fire.

I don't like to post peoples names online without their permission so I'll send a PM

Yes i remember that guy, i sent him some NOS CR250/500 chain strips for swing-arm and he did some for me,
they were excellent, i felt sorry for him as he lost everything and i think it was close to Xmas.
i sent him $100 on top of what i owed him to help him out.
Long time Honda Fan, but all bike nut in general, Big Bore 2 stroke fan.    http://www.youtube.com/watch?v=XJoKP6MawYI
1985 Honda CR500RF "Big Red"
1986 Honda CR250RG
2005 KTM 300EXC "The GruntMeister" ( I love that engine)

Offline Slakewell

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Re: Casting polyurethane components
« Reply #9 on: October 28, 2021, 04:37:55 PM »
You can buy the materials from 3M , these days 3D printing has taken over.
Current bikes. KTM MC 250 77 Husky CR 360 77, Husky 82 420 Auto Bitsa XR 200 project. Dont need a pickle just need to ride my motorcickle

Offline Oldfart

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Re: Casting polyurethane components
« Reply #10 on: October 28, 2021, 05:36:03 PM »
 Bought a few parts off him before his misfortune.  His work excellent and still going strong today.
From memory he was from around Brisbane

Offline Hoony

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Re: Casting polyurethane components
« Reply #11 on: October 28, 2021, 05:56:05 PM »
Bought a few parts off him before his misfortune.  His work excellent and still going strong today.
From memory he was from around Brisbane

is he still doing it? did any moulds survive ?
Long time Honda Fan, but all bike nut in general, Big Bore 2 stroke fan.    http://www.youtube.com/watch?v=XJoKP6MawYI
1985 Honda CR500RF "Big Red"
1986 Honda CR250RG
2005 KTM 300EXC "The GruntMeister" ( I love that engine)

Offline Oldfart

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Re: Casting polyurethane components
« Reply #12 on: October 31, 2021, 11:38:11 AM »
Hoony ... sadly he had a fire which destroyed everything. Donations were sent forward, but nothing came of it.

it was a pretty simple set up, from what I saw.  fabricating  the molds would of been the most time  consuming part.