Casting rubber -- hints/tips?: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Nov. 19, 2010 8:28 p.m. Treb Reader

There's a simple part I need for my wife's Fiat Spider -- the old pieces crumbled in my hands. 35-year-old rubber, not so durable.

The usual suspects have them backordered.

And I'm stubborn sometimes about making things.

Simple enough piece -- it's the bumper between the door and the door jamb. Basically, it's supposed to soak up some of the vibration caused by scuttle shake, I think.

Kind of a triangle, rounded on one end. 16mm thick. I used the fragments of the originals to figure out the dimensions & made a model of the original in balsa.

My plan, to the extent that I have one, is to make a mold in plaster and then cast new pieces in urethane. I'm all right with the plaster moldmaking; but any suggestions on the rubber/urethane stuff?

Methods/suppliers/material properties/warnings/etc appreciated.

Thanks Matt
Nov. 19, 2010 8:38 p.m. RoosterSauce Reader

I used a urethane kit from McMaster-Carr to make myself an engine mount. It worked great, and was easy to use. As a bonus, the stuff smelled like grape. I used the Flexible Urethane #8644K18. Of course, you can choose whatever hardness and tensile strength you think is appropriate for your bumper thingy http://www.mcmaster.com/#urethane-casting-compounds/=1l1oz5
Nov. 19, 2010 8:58 p.m. skeze New Reader

also if you are super cheap and wanna go at the home depot way you could always fill your mold with that tool grip dip stuff it comes in many colors,or you could even use that blue gasket silicone stuff...I've made silly balls with both of these products to great affect so a little door stopper should be a no brainer
Nov. 19, 2010 9:02 p.m. Knurled HalfDork

I've made rubber bits by making a mold out of JB Weld and then using Permatex Right Stuff to make the part.

Ziploc bag material makes a great mold release agent.

Not great for really complex pieces, but worked great for what I needed to make, which was, oddly enough, a rubberbumper. I cast a wingnut in place for something to thread onto.
Nov. 20, 2010 12:22 a.m. procainestart Dork

Get this book? http://www.amazon.com/cast-small-metal-rubber-parts/dp/0830698698
Nov. 20, 2010 1:08 a.m. benzbaron HalfDork

Silastic J is a silicone RTV that you can cast. My pops used it in jewelery makings so it must be easy to use.
Nov. 20, 2010 10:56 a.m. Treb Reader

The plan, right now: plaster mold, johnson wax as a MRA, and starting with RTV/handle dip type things. (i.e. locally available and cheap). Depending on results, I might try the McMaster-Carr urethane.
Nov. 23, 2010 5:24 a.m. Treb Reader

First try: Right Stuff.

Not bad -- though there are a lot of voids in the part (poor form on my part -- ptobably better not to move the nozzle very much, just let the stuff flow a little.) Also, 36 hours later, it's still liquid in the center, so that will take a while, I guess.
Nov. 23, 2010 6:36 a.m. 44Dwarf Dork

Treb. Where are you from? Your profiles empty... I have a few gallons of 2 part RTV i'll never use but it not the lightest and considered hazardus in non cured form so shipping sucks.
Nov. 23, 2010 8:48 p.m. Treb Reader

Ooh, I'll have to fix that.

Lexington... Kentucky. (Named for the battle of, but that's a different story)

Thanks for the offer, 44Dwarf.

Matt
Nov. 23, 2010 10:04 p.m. 93gsxturbo HalfDork

If you have access to a bell jar and a vacuum pump, once you pour the part, put it in the bell jar and pull a vacuum. It will draw all the air out and leave you a part with no or very small voids.

We used to make silicone molds for casting low temperature alloys like pewter. The vacuum method looks crazy because the silicone will actually appear to boil as the air is sucked out, but it works great!
Nov. 24, 2010 6:15 a.m. 44Dwarf Dork

Vacuum degassing will work but only with two part mixes.
Nov. 25, 2010 2:20 p.m. motomoron HalfDork

I'd find some big rubber suspension bump stop in a junk yard, bandsaw it into shape and finish on a belt sander.

If I had to make a bunch I'd carve a plug, make a silicone mold, and cast them in urethane.
Nov. 25, 2010 2:53 p.m. 02Pilot Reader

3M Window Weld is a handy material for stuff like this. I've used it to fill voids in motor mounts and other rubber bushings. Cheap and readily available, plus one-part and pretty easy to work with.