How to mold a paraffin grain

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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!


ryan.pulkrabek's picture

Nice work. Certainly it is more refreshing to see these results than the ones I am getting with the nozzle.

Damien's picture

You have to take into account that it's always easier to see the results of others than one's own. Your own results are very interesting and motivating for me - and they make our project, and the "global knowledge base", move on! Of course, everything has to be documented on the Wiki to make sure all we try, where we fail and where we succeed, can be used by someone else to build upon. That's the magic of open source.

I love this quote from Mickael Jordan: "I've failed over and over and over again in my life and that is why I succeed." ;o)

Damien's picture

A great milestone for the project happened last Tuesday at the lab: the first combustion! Based on the above grain, which was stuffed with the steel wool + cotton blend at its beginning and then cotton at the end, I managed to start a real combustion inside.

First with simply blowing on the burning cotton (ignited with the spark gap igniter), and later in the evening using compressed air available at the lab. There was a nice 10-15 cm flame getting out of the exhaust, while the grain itself was glowing in orange, that was thrilling ;o)

Unfortunately no photo or video of the combustion - it took at least a dozen trials to get to the end result expected, and after the 2-3 early failures, we stopped taking pictures or videos. That will be for the next time - anyway, it does seem to validate our ignition system!

ryan.pulkrabek's picture

That is really great to hear. That's real progress. Congrats. It keeps me motivated to continue designing. Now, let's attach a nozzle to the end. This is one point I would like to discuss during tonights meeting.

ryan.pulkrabek's picture

Speaking of combustion with nozzle, I have put in the post for you a concrete nozzle. It is not optimal, but it can be used for our MVP.

Damien's picture

Thanks Ryan, it arrived today, it seems to be in good shape. The PLA casing does protect it! I will fit it in the aluminium tube next week.