Subject: Re: Casting metal in rubber

From: [email-masked] (Carl Sutton)
Date: 5 Apr 1995 23:59:39 GMT

In article <[email-masked]>, [email-masked]
(Ron Ginger) writes: > In article <[email-masked]>
[email-masked] (ErnieLeim) writes: > >From: [email-masked] (ErnieLeim)

> >Subject: Re: Casting metal in rubber
> >Date: 1 Apr 1995 03:53:11 -0500
> 
> >What are you talking about ? I have never heard of anyone pouring hot
> >metal into a rubber mold. The rubber molds are for the replication of
> >waxes to be used in investment casting. I can't think of any rubber that
> >can withstand the heat of molten tin let alone bronze.
> 
> .......rest deleted.
> 
> Yes, you certainly can cast SOME metal in rubber molds. Mostly pewter and 
> similar metas. Its widely used in the model business- like model boat kit 
> fittings, and in some jewelery and 'craft' products.

   Never heard of this happening in the years i've been in the jewelry
business 

> The rubber is supplied in sheet form, about 1/2" thick. The parts are laid out 
> on it, a parting dust applied, and another sheet of rubber placed on top. The 
> whole sandwich is then vulcanized in a hot press.

      Yes, and then the mold is carefully cut apart so that a negative image
is retained.  The rubber mold is then sealed shut and wax is forced into it.
The wax is then taken out and put in a flask where investment is poured around
it.  The wax is then burnt out in an oven and the plaster filled with the
desired metal.  The plaster is then quenshed and breaks apart, leaving the
desired piece.

> See any jewelery supply catalog for materials.

     See most jewelry making instruction books.

> Ron Ginger
> my signature is http://www.ultranet.com/~ginger
>

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From: [email-masked] (-Mayo,H.H.)
Date: Mon, 10 Apr 1995 18:49:41 GMT

>For years I have been casting pewter (tin/antimony) into silicone rubber
>molds made of the ordinary caulk which comes in tubes at Walmarts. It
>works fine for short runs and prototype castings and the detail is
>excellent. The only thing to avoid is causing stresses when the metal
>cools by designing molds that have sections of rubber that are surrounded
>by metal. 
>
>I have no idea how long you could use such a mold for long runs. 

How do you do this? What do you use to keep the stuff from sticking
to the pattern? I'm all ears.

                                              Larry Mayo

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From: Flint Smith <[email-masked]>
Date: 11 Apr 1995 02:52:06 GMT

[email-masked] (-Mayo,H.H.) wrote:
>>...
>> The only thing to avoid is causing stresses when the metal

What do you mean by this?  Does having rubber surrounded by metal cause stress
or avoid causing stress?

What do you think of the earlier post suggesting using the high temperature
"copper" RTV rubber surrounded by the regular rubber?

How do you melt your pewter?  Do you have an easy way to keep the temperature
in a range that wont destroy the mold?  I guess it would be easy to overheat
the metal used in these rubber molds.

Sure sounds like a hacker-friendly method.

Thanks,
Flint Smith  [email-masked]

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From: [email-masked] (Lawrence Boul)
Date: Fri, 7 Apr 1995 08:44:29

In article <[email-masked]> [email-masked] (Carl Sutton) writes:
>From: [email-masked] (Carl Sutton)
>Date: 5 Apr 1995 23:59:39 GMT

>> Ron Ginger

I think you are at cross purposes here.  Ron is talking of the investment or 
lost wax process which is good for precious metals and high temp allows such 
as brass etc.  The  Process for pewter that is alluded to is quite different 
and generally produces a lower quality of casting (but not much lower).  This 
is a spin casting process using Rubber moulds as described previously.  If you 
want a lot of parts it can be very economic.  Here in NZ we get a 6" radius  
(I think) rubber made for around US$35.  Pewter is c. US$8 per kilo and there 
is a per spin charge of a couple of dollars.

Both processes are comonly used for producing model railway kitsets.

Lawrence

---

From: [email-masked] (-Mayo,H.H.)
Date: Mon, 10 Apr 1995 18:49:41 GMT

Larry Mayo

---

From: Flint Smith <[email-masked]>
Date: 11 Apr 1995 02:52:06 GMT

Thanks,

---

From: [email-masked] (Lawrence Boul)
Date: Fri, 7 Apr 1995 08:44:29

Lawrence

---

From: Hank Kaminsky <kaminsky@comp>
Date: Thu, 13 Apr 1995 19:01:55 -0500

_____________________________________________________________

> to the pattern? I'm all ears.
 
Ah, good question! 

The pattern must be sealed (if it is porrous) and lubricated before 
applying the rubber. I use a 50/50 mix of lacquer thinner and lacquer for 
the sealer and a 50/50 mix of lacquer thinner and Johnson's Paste wax for 
the lubricant. One or two coats of the sealer (if needed) and when the 
volatiles have quit coming off I coat with a very thin coat of the wax 
mixture. When the wax is not sticky any more I buff with a shoe brush (or 
equivelant) and coat another layer. Wait for the smell to quit (volatiles 
have gone) and buff again. Now it's ready for the silicone.

The mold must be planned to allow for pouring the metal. I use runners and
pouring cups and such made of clay (silicone won't stick to wet clay) or
wax and keep the plan simple. The silicone is applied straight out of the
tube in a gob to a point on the surface and then spread with a finger
dipped in soapy water. Push the silicone into the surface of the pattern
until it is 1/8 inch thick.  I use clear silicone so I can see any bubbles
under the surface. I let the first layer cure overnight and then apply
another and let it cure overnight. The last thing to do is to apply 
ridges to divide the mold into keyed sections for cutting apart when it 
is cured and as guides for the plaster mother mold to hold the floppy 
silicone mold in place. 

Once the rubber has cured and the plaster mother is in place, peal off 
the silicone (or cut along the ridges if the mold is complex.) The mold 
must of course be dry to pour metal. Remember the prohibition against 
trapping any rubber inside a metal form. I use pewter made of 92.5% tin 
and 7.5% antimony and it seems to work fine for short runs. 

I take no responsibiltiy for any misuse of this information resulting in 
injury. The whole thing may be a fiction for all you know. 

Hank Kaminsky ([email-masked])
543 Assembly Rd. (Home) 501-443-2077
641 West 6th St. (Studio) 501-442-5805
Fayetteville AR 72701
_____________________________________________________________
Sand-Matrix Design Portfolio at: 
http://wuarchive.wustl.edu/edu/arts/metal/Gallery/Kaminsky_H/kaminsky.html
or anonymous FTP to same address
_____________________________________________________________