How to mold a paraffin grain
Following my previous post about preparing a test combustion chamber and preparing the paraffin grain molding, last Thursday I actually molded the grain.
First, after a quick discussion with fellow members working in the mechanics room, I discarded the idea of screwing the machined bottom part to the aluminium tube: the tube wall is about 4mm wide, which would imply drilling aluminium with a 2mm drill, on a length of 3 to 4cm (to hold the 1cm thick bottom part), and then using a tap to prepare the threading. That would have too much chance to break the drill - which was exactly what happened when I tried that last week. Instead, F4GRX submitted a far simpler idea to link the two parts for the molding: using a clamp. And indeed, that's what I did.
Someone at the lab had melted paraffin in an oven for an entirely different purpose, using a simple empty can in which he had put candles bits. There was a surplus of the resulting hot liquid, so I used it to make a molding test.
The overall affair was fairly simple: I first set the steel pipe in the hole I had made in the bottom part.
Then I poured the liquid paraffin in the gap between the steel pipe and the aluminium tube.
I then let the wax cool for about 10 minutes. I was wondering if the steel pipe would come out easily, or if the paraffin would stick to it, causing the grain to be made irregular in the process. It turned out that the steel pipe came out without effort, and the grain shape inside appeared perfect.
Next step: testing the ignition process, with the steel wool and cotton blend stuffed inside the grain. To be continued!