AP learns some new tricks with white metal

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Paul Townsend
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AP learns some new tricks with white metal

Postby Paul Townsend » Mon Dec 17, 2012 5:44 pm

I finally found a missing RoundTuit and got stuck into white metal casting. I have wanted to try this for decades and was finally incentivised by a desire to replace some missing castings for the Broad Gauge Society as cheaply as possible. The BGS owns the patterns and/or copyright so no worries there but sometimes the cost of getting new professional moulds made is prohibitive for low-cost items if few required.

Google showed me Nigel Lawton's helpful recipe and a guy in USA has some good tips too

http://www.nigellawton009.com/Whitemeta ... Theory.htm
http://www.donjarrs-place.com/07-how-to-metal

Nigel's notes implied that an open faced single-sided mould was OK for my first simple job which has no through holes. Hobby's supplied the gunk for mould making so I had a go then recycled some white metal offcuts.

This first trial showed me that the SS mould was a bad idea, and I was underwhelmed by my cardboard walls to contain the mould rubber. At wm pour time I could not control the top face of the casting and got results which were too thick and domed; in part this is due to high surface tension of the wm. However I learnt a lot and got about 10% useable samples after heavy fettling. Most went round the melting pot again and again!
Best results require the mould pre-heated as much as possible...well worth cranking the oven to max...220C here...and reheating between pours.

Second trial was with a double sided mould for the same master pattern, 5 planted and two of another simple pattern with a large through hole. I had bought some more silicone rubber at Warley from Sylmasta who were encouraging and showed some nice samples.
I liked the Yankee Lego walls idea but not the use of Lego for registering the two halves...too hard to separate without damage, and rather greedy on use of expensive rubber. Maybe the release agent wasn't good enough. Next go will use Lego walls and Nigel's 4 corner dowels for re-registration. This will also use less of the expensive rubber by cunning design of the pour-in duct area.

As well as controlling thickness, the two sided mould provides a large pond of metal at pour time which helps gravity feed into the corners and retains heat better so tapping to dispel air is more effective.
I have poured twice and of the wanted 7x2 copies I have 6x2 which are pretty good with the flat base and desired thinness. Detail is better preserved too. The failed 7th is a bit too far from the pour duct so metal reluctant to go in.

I want a better way of scraping the scale scum off my ladle at pour time.

However, success!

Next task will be a 4mm wagon side ...quite thin, no holes but finely detailed...I reckon with my new expertise this should succeed, we shall see.
I can't yet say how cost effective this exercise will be, but must when I have done the wagons.

Then the real challenge... a complete side of a BG loco...perhaps just after Christmas.

I attach 5 pix showing what I did:

1. Good results (from patterns on the right)

AUT_7202.JPG
Result from DS mould.
Original patterns on right.


2. Poor result from SS mould on left shows excess thickness. Centre is pattern, good copy on right is from DS mould

AUT_7204tweaked.JPG


3. Moulds, SS on right superceded by DS to left and centre.

AUT_7206.JPG


4. The DS mould clamped in hardboard sandwich for pouring.

AUT_7201.JPG

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