Urethane question... creating molds....

[tor]

I need help making a mold for urethane. I build xylophones and have been purchasing urethane heads, but I desire a better shape, as well as lowering the cost.

This is what I have now:

A doughnut shaped cylinder 1.25" diameter and 5/8th" deep (or however deep you fill the mold) in the center is a hole where one pounds a dowel. The hole is 5/16th in. The urethane is roughly 60 durameters in hardness and is amber in color. The walls are strait.

I buy a few hundred of these a year at $1.00 a head and seek to make them myself. How do I do that?

Thanks so much!

Tor Clausen

[Andy Dingley]

Talk to Smooth-on chemicals. They (or their distributors) sell the stuff and they're helpful to small-volume makers. You'll probably make one or more flexible moulds from similar materials, then pour into that. It's an easy process to work with. Vacuum debubbling is worth rigging up for (I just use a Vac-U-Vin coffee jar)

[marks542004]

If what you are making is like a thick washer you should be able to use a forstner bit to drill a series of holes in a piece of wood. Then drill the center hole for the dowel.

Smooth-on can sell you the urethane and release agents.

They may tell you to seal the mold and can tell you what to seal it with.

One of the urethane materials does not stick to anything and is stretchy enough to remove from the molds.

The easiest to use is one of the one-one by volume mixes. No weighing involved.

[Andy Dingley]

I find weighing easier. Digital scales are cheap these days and it's easier to use them than try to measure viscous liquids in a measuring cup.

[xstrange]

Hello Tor;

I'm also in the musical instrument business, and I cast many of my own parts out of urethane and epoxy. The basic technique is to first machine up an accurate "master" from aluminum or hard plastic, make silicone molds from that master, and then cast your parts by pouring the urethane into the silicone molds. Because the silicone is flexible, straight walled parts like you describe can easily be popped out of the mold. The silicone molds will wear out after maybe a few hundred parts, but then you just pour new molds from the master.

Since your part is cylindrical, it would be a simple job to machine up the master on a lathe. If you don't have access to a lathe, you should be able to find a machinist to make it for you. Everything else you can do yourself.

The urethane casting resin is very easy to work with. I typically use hard urethane for casting parts like bobbins for pickups, but resins are available in many different durometer ratings.

Bruce Johnson Johnson's Extremely Strange Musical Instrument Co. Burbank, CA

[tor]

Thanks Bruce,

Your comments set me on a clear course. I wonder how many I should gang up on a group mold? If the Urethane goes bad, I should make a few of these silicone molds, eh.

t

[xstrange]

Hello Tor;

A multiple group mold isn't really necessary, and it's a lot more work to make multiple identical masters. It's better for consistency to make up one master, and make a bunch of individual silicone molds from it.

For example, with the pickup bobbins that I use in my basses, I need 8 bobbin halves per instrument. I machined up an aluminum master, then made up 8 silicone molds from it. Now I can mix up one little cup of urethane and pour a full instrument set of bobbins in one shot. If and when any of the molds get damaged, I'll make up new ones as I need them. The important thing is that all of the molds came from the same master, so that the parts will always be identical.

The hard urethane that I use is very easy on the silicone molds. I generally use mold release spray for good practice, but it isn't absolutely necessary with the urethane. Casting epoxy is another story. Without a good coating of mold release, epoxy will pull chunks out of a silicone mold.

I don't remember having any urethane "go bad". It's a 50/50 mix and it isn't that fussy. If you really screwed up the mix, it probably just wouldn't harden. I use a pair of little stainless steel kitchen mixing spoons, and count out so many spoonfuls of each part into two paper cups. I visually check the levels of the two cups, side by side, pour one into the other, stir for 15 seconds, and pour into the mold.

Urethane is great for all kinds of small parts. It sets up very quickly, typically less than 10 minutes, but it takes about 30 minutes before it fully hardens. After casting, it machines easily. One drawback to hard urethane: it's naturally white, and you can use pigments to make it any color, except black. Adding black pigment will make it smoke grey, but it won't go black. I'm not sure about the softer urethanes. I make my pickup bobbins smoke grey, and then finished pickup coil goes into another mold and gets overcast with black epoxy.

Bruce Johnson Johnson's Extremely Strange Musical Instrument Co. Burbank, CA

[Oleg Lego]

The [e-mail] entity posted thusly:

I'm interested. I am familiar with a plastic I used to know as "casting resin" or "casting plastic", and for all I know, this urethane might be the same thing. Do the Borgs carry urethane? Do you have a brand name? A URL for a web site I could visit?

Thanks,

Larry Larry

[tor]

I am new to this, but

seems to be a very good source.

tor

[xstrange]

Hello Larry;

The casting resin that you're familiar with is probably clear polyester resin. It's essentially liquid plexiglas; it casts completely clear and can be polished up to a gloss. It's commonly used for embedding objects, making clear trophies, etc. Yes, you can certainly cast parts out of clear polyester if you want clear parts. You can also add dyes to it to make beautiful transparent color parts. As compared to the urethane (official name: pourable polyurethane casting resin), the polyester is more expensive, takes longer to cure, is harder, and is more brittle.

The urethane that I use is naturally opaque white and is similar in characteristics to polystyrene (as in the plastic in model airplane kits). It's very easy to machine and very impact resistant. I particularly like it for parts like pickup bobbins in my instruments, because it's very unlikely that it will ever shatter or crack.

Thanks, Tor, for the link to Polytek! I wasn't familiar with them, and it looks like they have a nice selection. I get my resins from a Los Angeles area chain called Plastics Depot. They have a line of various resins under their own label. The urethane resin that I use is probably the same as Polytek's EasyFlo 95, listed under Pourable Polyurethane Resins. Some of Polytek's other resins sound useful too.

By the way, I'm no expert on plastics...I've learned what I know by reading catalogs (and newsgroups!) and experimenting.

Bruce Johnson Johnson's Extremely Strange Musical Instrument Co. Burbank, CA

[Oleg Lego]

The [e-mail] entity posted thusly:

That's the stuff. Used to embed coins, pictures, etc. Fun stuff.

Ahh... thanks. Will check it out.

Sounds like just the ticket.

And thanks from me too. I should be able to find a source of it or something equivalent around here

Larry