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
www.musicalfurnishings.com

On 18 Dec 2005 23:22:36 -0800, "tor" wrote:

I need help making a mold for urethane.

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)

tor wrote:
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
www.musicalfurnishings.com

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.

On 19 Dec 2005 08:52:08 -0800, wrote:

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

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.

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

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

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

The entity posted thusly:

The hard urethane that I use is very easy on the silicone molds.

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
---
There are 10 kinds of people --
those who understand binary, and those who don't.
-- Uncle Phil

I am new to this, but http://www.polytek.com seems to be a very good
source.

tor

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

The entity posted thusly:

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.

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

. 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.

Ahh... thanks. Will check it out.

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.

Sounds like just the ticket.

Thanks, Tor, for the link to Polytek!

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

Larry