I didn't get my welder working properly, but I managed to attach the head of the piston with some really awful looking welding. Then it was time to test! Don't laugh now, it was just a test to see if things work. It was successful. This is the squeezed(injection molded) bit on the right and a big blob of leakage on the left.

Now for the details. Here is the poorly welded piston.

I set it up with the heating tube just sitting on a very roughly made wooden mold. Don't laugh. Then I stuffed in a medium-small bag and compressed it down to the height shown. It sticks up too far above the heating coil, so the top might not melt as quickly. I'll have to consider this problem.

It took 63 seconds to reach 180C. I then let it sit for about 2 minutes before pressing the piston. It took quite a bit of pressure, but should be no problem if properly levered. I was just pushing down with my hands. When the piston would move no further, I shut it down and let it cool just a minute before disassembling things. This was what I saw.

I was pleased that not much stuck on the piston or tube. I was less pleased by the leakage, but that was to be expected. Upon opening the mold I had to laugh.

I cut it open to inspect the quality. Not bad. It does have bubbles, but it's reasonable and expected. Overall I'm happy with the test and learned some things. Next I will make a proper metal mold and weld it onto the heating tube so there will be no leakage. Actually, wood turned out to be a much better mold material than I expected. It didn't stick as I thought it would. That's cypress by the way.