This is a followup to my Build a water mortar
instructable. It shows some design alternatives tried this weekend.
Step 1: Piston cap alternative
I had some great feedback on the original instructable and wanted try out a suggestion by thinkahead
for a different way to construct the piston. The suggestion was to get rid of the wooden plug at the upper end of the inner pipe, substituting a cap at the lower end instead. I also wanted to try filling the inner pipe with expandable foam. Here are the ingredients.
Step 2: Trying the end cap
First thing I did was remove the wooden cap and put the end cap on the lower portion of the inner pipe. This worked pretty good. You do have to do a "priming" step initially where you suck water up, point the mortar at the sky, and push the air out, but then you're in good shape and things function pretty much the same as before. I like this better than the wooden plug overall.
Step 3: Trying the foam
I added the expanding foam to the inner pipe and let it set for a couple hours, then sanded the excess off flush. I set it aside for a while more but couldn't stand waiting the whole 8 hours you're supposed to for a full cure! :)
This type of foam starts curing when exposed to the moisture in the air, so the outer layer hardens quickly, but the parts that are buried take a lot longer. When I tried it out I got a couple good shots, then water was able to make it down past the cured foam to the part that was still uncured. The uncured foam worked its way out and fouled up everything resulting in a total loss. Sigh...
This clearly didn't work out like I expected. The idea still has merit, but I'd use a two-part expanding foam if I did it again, so that the cure is not dependent on exposure to air. On the other hand it works okay without the foam, so try it that way first.