DIY Cast Polyurethane Mold

[xirforever]

Okay, so I made some delrin bushings for my subframe a while ago (SC400), and while they were and improvement, I don't feel they are exactly what I wanted - a hard poly insert. So I have my entire rear end pulled apart and I thought while I was repainting everything, I would pull those delrin inserts out, and make some poly ones to go in. Well my last experience machining poly for my engine mounts was not my favorite, so I thought, why not machine a mold, and cast them? I had some extra delrin laying around and I decided to draw up a mold, and start making it. The mold will be two or three piece in design so I can actually get the urathane out when done. I made the lower half today.

I figure I will buy some of the polyurathane resin from McMaster and try that out with a hardness of 80 shore A. Id prefer 85-90 but its not available and I just bought some 100 and you might as well call it delrin it is so hard. I will probably buy some mold release and pour it in very slowly when I am ready. Does anyone have any experience with this sort of thing? Any advice?

 

[4U2QUIK]

delrin is too stiff and shatters under shock. use uhmw instead. it has give and will never shatter.

 

[xirforever]

delrin is too stiff and shatters under shock. use uhmw instead. it has give and will never shatter.

Right now my differential ear mounts, and all four subframe mounts are delrin, and while I don't like them because I don't think they have enough give in them, I did shatter a rear wheel a while ago (long story) but never damaged any of the delrin bushings. That's just my experience FWIW. So I personally am not too worried about them shattering, if they are constrained properly.

 

[4U2QUIK]

I just know from 20 years of experience machining delrin. You cantakea 3" diameter rod of delrin and swing it into a metal pole and it will shatter. Do the same with uhmw and you are likely to get smacked in the head from the rebound. I use it for all of my motor mounts, and whats nice about uhmw is that it has the rigidity like a solid mount andat the same time it has deadening properties as well. So it does a great job of killing shock and vibration

 

[4U2QUIK]

and on a side not uhmw and nylon are the two most common materials used in dead blow hammers.

 

[4U2QUIK]

another thing you can try also is the polyurethane rod from McMaster. I made a bunch of suspension bushings out of the stuff about 15 years ago and the stuff is still like new. (the bushingsare very similar to the ones you used for your motor mounts. ironically we used the extras I made for motor mounts much lie yours for my friends nova)

Also, nasco sells a pourable urethane that you mix yourself and you can mix any durometer dependin on houw you mix the componants. (be sure to have an accurate scale available.) Mould should be made from aluminum, I usually make them from severalpieces. and be sure to polish all of the componants very well as the stuff does not like to let go even with mold realease compound

 

[xirforever]

I just know from 20 years of experience machining delrin. You cantakea 3" diameter rod of delrin and swing it into a metal pole and it will shatter. Do the same with uhmw and you are likely to get smacked in the head from the rebound. I use it for all of my motor mounts, and whats nice about uhmw is that it has the rigidity like a solid mount andat the same time it has deadening properties as well. So it does a great job of killing shock and vibration

You must have a pretty mean swing! I just decided to take my old bushings and take a 3lb sledge hammer to them and I was able to get some small pieces to fly off pretty violently. I hit it good enough on the concrete floor to make it bounce straight up about 6". Although I did hit it with a sledge hammer lol.

another thing you can try also is the polyurethane rod from McMaster. I made a bunch of suspension bushings out of the stuff about 15 years ago and the stuff is still like new. (the bushings are very similar to the ones you used for your motor mounts. ironically we used the extras I made for motor mounts much like yours for my friends nova)

Also, nasco sells a pourable urethane that you mix yourself and you can mix any durometer dependin on houw you mix the componants. (be sure to have an accurate scale available.) Mould should be made from aluminum, I usually make them from severalpieces. and be sure to polish all of the componants very well as the stuff does not like to let go even with mold realease compound

I thought about going down the road of machining four pieces out of poly rod, but when i machined the motor mount bushings that were 65A I think, it was not fun at all and i used liquid nitrogen to get it to machine, and I had to make my own tool to even be able to turn it. I was thinking of machining it once in a negative, and casting it four times.

Do you think I will be okay with the delrin material for my mold? I made it as smooth as possible, finishing passes were incredibly slow and the delrin really liked it. Getting 3.5" round aluminum is not cheap, not like spare delrin! Have you used the nasco stuff? Any better than the brand McMaster uses?

 

[xirforever]

Does anybody have any idea how much to expect the insert to shrink after being cured on a 2.75" OD part with about a 1" wall thickness? Trying to decrease the number of times I have to cast, measure, then enlarge my molding so I get the right interference fit on the bushing. Will it be less than 10 thousandths on OD?

 

[xirforever]

Finished the second half today. I am thinking about making a top to it so the surface finish will be exactly what I want without have to do any finish machining to it but I am worried about air getting trapped on the bottom of this top, I will update with what I will do. I bought some 80A urethane off mcmaster Sunday night, so hopefully it will be here soon!

 

[4U2QUIK]

I've been meaning to get back to you on all of this but I keep on not having time. Delrin will probably work fine. I know you can get a killer finish with it. It's late and im barely typing right, so goodnight and ill reply soon

 

[xirforever]

Okay guys, cast my first part tonight and waiting for it to dry. It only took one time for me to realize that bubbles are totally unacceptable to me and after trying to vibrate them out with little success, I have decided I will put a hold on the other three castings until I have a vacuum chamber. I have done some poking around online, and I think I want to build one. I saw several cool ones made entirely of acrylic, and I think that is how I want to make mine, but wow acrylic tubes are expensive!

http://www.youtube.com/watch?v=HN1bNDExW0c


Here was my progress tonight. I used a grease to create the seal between my two mold parts, then I sprayed the mold with a release agent. It seems that the release agent is just a silicon lubrication spray. Then I mixed, making sure to weight the chemicals properly, and poured. I tried best I could to vibrate as many bubbles out as possible. I hope the ones at the top are the only ones in the mold. I poured the extra into a cup and I will cut that one in half to see if any bubbles are found.

 

[youngcnc]

little trick..

Hi xirforever
Years ago when helping a mate to pour clear resin beer handles we used to use mig wire to remove air bubbles until one day after mixing in the catalyst the phone went. Pouring the mixture was a little later than usual but all the bubbles had gone.
Maybe with a little luck you might be similar...

Cheers Craig

 

[4U2QUIK]

use a heat gun on low setting on your mold to get the bubles out. this is why aluminum is preferred. You also have to be carefull when mixing the urethane. It's very easy to accidently whip air into the mixture.

 

[xirforever]

Hi xirforever
Years ago when helping a mate to pour clear resin beer handles we used to use mig wire to remove air bubbles until one day after mixing in the catalyst the phone went. Pouring the mixture was a little later than usual but all the bubbles had gone.
Maybe with a little luck you might be similar...

Cheers Craig

How did you utilize a the wire to help with the bubbles?

use a heat gun on low setting on your mold to get the bubles out. this is why aluminum is preferred. You also have to be carefull when mixing the urethane. It's very easy to accidently whip air into the mixture.

Are you saying the heat will lower the viscosity to release the air bubbles? Or that the bubbles will expand and become more buoyant when heated?



Okay so today I tried to pull the mold apart, and I was hoping to be able to do it by hand but I could not. I milled a flat spot on the lower half so that I could grip the lower mold in a vise and then get a rubber strap wrench on the upper half a twist. It came off with moderate force. Then I was left with the bushing in the upper half, and I pressed it out in the vice, not too hard. I think I used a little too much mold release as the mold release liquid can be see as pools in the urethane now lol. Anyway, success! I am ecstatic that it worked! The urethane seemed to only shrink by 0.001" so that was very pleasing as well! The instructions that came with the urethane said it was a low shrink compound and to expect about half a thou for ever 1" of material. Pretty darn accurate. I will wait a little for the material to fully cure before I attempt to machine some chamfers on it and machine the face that was not in the mold. 80A is pretty hard, but I think I will buy some dry ice to freeze the stuff before i attempt to machine.

Also, I just bought a vacuum bell jar to degas my next one best as possible. I think I will hold off on buying a vacuum pump as apparently they can be rented at autozone. I think I want to rent one before I decide to buy one. I think I will degas in a cup, then pour into the mold and degas one last time. Should come out perfect.

Oh, on a side note, this stuff barely smelled at all, I was expecting it to stink but you could do this in your home and people in the next room would have no idea.

 

[4U2QUIK]

raising the temp does indeed lower the viscosity, and also causes the bubble to expand.
But I'm thinking the desiccator (vacuum jar) Will work as well. adding a little heat probably wouldn't hurt either. you culd throw a heat lamp over the jar.

good progress BTW!

 

[youngcnc]

mig wire uses..

Hi xirforever
We just picked at the air bubble with the mig wire, sort of lifted the bubbles to the surface and then popped them.

I am interested in the price of the product and how much you get for that price?

Great job by the way, looks factory made...

Cheers Craig

 

[stevechumo]

Very nice! I've been thinking about making my own bushings for the rear of my SC for awhile including subframe, LSD, control arms and whatever.

 

[xirforever]

Hi xirforever
We just picked at the air bubble with the mig wire, sort of lifted the bubbles to the surface and then popped them.

I am interested in the price of the product and how much you get for that price?

Great job by the way, looks factory made...

Cheers Craig

Craig,

Here is my shopping list:

McMaster.com
urethane 1 lbs can for $32 - 8644K11
mold release aerosol $12 - 1409K34

It seems quite expensive for the volume that I received, I was expecting a little more. I want more than 1lb and less than 10 for sure. I wish now that I had made the mold from aluminum or steel because I would prefer to use injection molding urethane pellets, they are far cheaper and if i make the mold right I shouldn't have to worry too much about shrinking (this is more concerning with injection molding pellets as they have to be heated and melted, and when cooled it shrinks).

Very nice! I've been thinking about making my own bushings for the rear of my SC for awhile including subframe, LSD, control arms and whatever.

I plan on doing both front and rear subframe bushings and front and rear differential bushings. I will be happy to share prints when I know my tolerances are right.





Right now I am just waiting for my vacuum bell jar to arrive and I will pour the second half. In the mean time I will face the bushing on the lathe, and turn the two bolt spacers that will be inserted into the bushing. I have some stainless laying around, should be nice : )

 

[youngcnc]

Cheers xirforever..

Thanks for the info..

Craig

 

[xspsi6]

Looks good so far, I have been making the diff bushings and rear sub frame bushings for a few years now and I can say through the trials of getting it right I almost threw in the towel but hung in there and have sold over 200 sets now but poly shrinks a bit when it dries so hopefully you made the mold a few thou bigger to compensate for this as this was our biggest problem as we had to re make the mold 4 times to get it right.

The results of having both the rear sub frame and diff ear bushings is night and day and really wakes these cars up.

Also there is no need for those wondering about the rear diff bushings to replace those or the front sub frame bushings as those do not carry much of the load/stress that causes wheel hop (we have gone mid 9's) on just our 2 bushings plus the rear diff mount is vulcanized rubber to the metal center and outer shell so that adds mega cost to building them.