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Wednesday, June 7, 2017

Castings! Part II, making the first resin castings

In the set from Micro-Mark they include a casting resin. It is the medium type of what they sell in terms of speed to cure and hardness. I found it to work pretty well for working out techniques with these molds.

The first castings were not great. One big adjustment was that compared to when I make metal castings you need to vent the mold more, to allow air bubbles to escape. I think the weight of the molten metal tends to push out the bubbles better.

Prior to casting you spray on the mold release material for the resin, which is a different release than used when making the molds. The resin material said it had a working life of 7 minutes but in reality, the material was the most liquid – almost like water – for the first 3 minutes or so. Pour it quickly!

My very first test casting I made in an old mold that I had actually never used (a Famoco baggage car end) and it came out pretty well. So, I forged on. I colored the resin using some brown colorant I had from when I tried to make castings in high school, not always the same amount, so the color ranged from white to the darker hue seen.

Another item to note is you have to make wood blocks to clamp the mold tightly with rubber bands. I tried a couple other methods, but in the flat wood blocks and rubber bands really work the best.

My initial work focused on the parts for the Nason sand-cast boxcar.

But with things going fairly well I got to thinking back to the Nieter molds that I had purchased, in particular two large parts, his version of sides for the Graceline hopper and also the Limco P54 (MP54) passenger car. I found that I needed to use a heavy coat of mold release but they otherwise worked fine with some extra venting! This was particularly exciting with the hopper sides, as I have a half dozen of the wood block bodies that are for this car but have no sides and few ends. The actual sides are pressed cardboard, which Nieter modified to make a mold from. I may try his idea with some of the other Graceline sides too, if I can ultimately get castings of high enough quality.

As to the Limco car, I have molds for the sides, ends, underframe, and trucks. Not sure there is much demand for this car, but I will try to make at least one complete model of this too.

In terms of time this all takes some time but not that much also. Basically, there is a setup to do, and you have to have a clear working space for it, but at any given step you won’t spend much over 15 minutes at a time. After that it is just a waiting game for the material to cure.

The good news is many of the castings are usable in terms of not too many bubbles. The bad news is the casting material is a bit too flexible in the thin sections that I am working with to actually use them on a car. So, for now I things are stalled and I have a harder resin on order. It should work better, but will slow production down too as the cure time is 16 hours.

In any case though I am happy with how things have gone so far and be looking for a part III with hopefully some castings that can be used on models.

Continue to Part III

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