Chad Wedding wrote:
>
> Hello, my name is Chad Wedding and I work for R.J. Robinson at the
> University of Kentucky. I am working with low melt alloys from MCP to
> form the core inserts for RTV molding. The core mold is also RTV. The
> alloy is Tin-Bismuth and melts at 70 degrees C. The problem we are having
> is that we the parts have a very porous surface. If anyone has any
> experience with casting thes low melt alloys, I would appreciate any
> recommendations concerning temperature, rate of fill, etc.
>
> William Chad Wedding
> wcwedd01@engr.uky.edu
>
> For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
Chad,
Recently, I have been doing some work on spin-casting of low-melt alloys
into an RTV-31 silicon rubber mould, to gain reproductions of an SL
master pattern. The rubber is capable of accepting materials with
temperatures up to 350 degrees C.
I used a range of low-melt alloys from Woods Metal (used for bending of
small sections of copper piping etc..) which melts at 70 degrees C, to
Lead-Free Pewter which melts at 245 degrees C.
MCP - 70 which you are using has similar properties to Woods Metal:-
Melting Point 70 degrees C
Constituents Lead/Bismuth/Tin/Cadmium
Density 9.6 g/cm^3
Brinell Hardness 12.2 - 12.6
Specific Heat 0.15 J/g degree C
Thermal Conductivity 0.18 W/cm degree C
Electrical Resistivity 4.2 %IACS
The metals at the higher end of the temperature scale were initially
casting to an acceptable standard, but not brilliant. Those materials at
the bottom end of the scale were producing parts with obvious surface
porosity. Eventually, I found that heating the material to no more than
+50 degrees C above the melting point before pouring produced a better
surface. An added bonus was to coat the rubber cavity with a light
dusting of Talcum Powder. The rubber mould was left to cool for approx.
5-10 mins before removal of the cast part.
Another problem I had was when casting the Woods Metal into a two-part
steel tool. Again surface porosity was a major problem, with the
porosity reaching a depth of 1-2 mm on each side of a 25mm cross
section. By heating the tool to a temperature of 50 degrees C before
casting, and again adding a light dusting of Talcum powder, the porosity
was greatly reduced. The tool was left to cool for approx. 15 mins
before removal of the cast part.
A factor to remember is that when heated, Bismuth expands approximately
3% by volume, so will shrink again as it solidifies.
Hope it helps, and let me know if you come up with a different method.
MacCheers,
Allan.
--Allan E W Rennie Research Student Centre for Rapid Design and Manufacture Buckinghamshire Chilterns University College Queen Alexandra Road High Wycombe, Bucks HP11 2JZ, UK
Tel: +44 (0)1494 605085 Fax: +44 (0)1494 538593
http://www.buckscol.ac.uk/tech/crdm/crdm.html
For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
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