Following a discussion in the RepRap forums I decided to find out what happens if you microwave PolyMorph. I placed about 25g in a glass dish together with a mug of water in our 650W microwave oven.
I tried 30 seconds of defrost first which had very little effect. Next I tried 30 seconds of full power. That raised the temperature of the water to 40°C but had little effect on the PolyMorph. A further minute got the water to 60°C and the PolyMorph to 30°C. After another minute the water boiled and the PolyMorph got to 41°C. I replaced the water with cold and heated it for another 2 minutes. At that point the PolyMorph was at 80°C and most of it had melted.
I formed it into a ball and made a new pen holder. At the point where I took this photograph it was still too molten to support the pen.
It took quite a long time to set, longer than my first attempt using hot water. As I kept it pressed against the metal while it was setting it ended up a lot flatter. The result was not pretty because some of the unmelted granules are visible. Also the pen is welded in, whereas it was removable from the other one.
I must admit it looks almost as curved as the first one.
After drilling and bolting the holder to the metal plate the pen is held rigidly again but the holder has shrunk so that the back is about 1mm away from the back of the plate. It is the front edge which locks it in position.
Conclusions: Well I wouldn't recommend microwaving as a way of melting PolyMorph because it is not even enough. The outer edge, where it is in contact with the glass dish, is a lot cooler. Also it does not absorb microwaves anything like as well as water does.
I think most of the shrinking and curling occurs when it first cools down but I can't explain why the first one can no longer be locked in place. Maybe there is some further shrinkage over the long term.