First go at using expanding foam

Well, that was a learning experience... aiming for extreme light weight, I had the idea of not using plywood for the floor of the basin, instead just the insulation itself. The problem though, is plywood would be required when applying the insulation, so as to get a flat surface, then afterward the plywood could be removed.

Expanding foam, that comes in pressure cans, is polyurethane, which is a thermosetting plastic, and can be expected to keep its integrity at the expected maximum temperature inside the solar still -- that is, won't sag.

I also found information that silicone sealant will stick to it. So I decided to go with this idea. The insulation is going to be 35mm thick (except toward the back, where it tapers to a bit less, as you will see in the photos). I have a piece of 12mm MDF, so cut that to fit snuggly inside the still. Needed some supports to hold the MDF:

I read on the Internet that expanding foam will not stick to baking paper, due to its wax coating, so I put that on the MDF, held in place with sticky tape, and inserted into the still frame:

OK, the idea seems simple enough, apply the expanding foam, let it set, then use a large saw to trim. I am a complete novice with expanding foam, so watched some YouTube videos first. I took the recommended precautions, face mask, latex gloves, and sprayed the surface lightly with water before application.

I bought a 500ml can of Sika Boom AP expanding foam, claimed coverage 25 litres. The area that I need to fill is about 0.6*0.7*0.035, which is 0.0147 metre-cubed. One litre is 0.001 metre-cubed, so that works out as 14.7 litres. That can should be more than enough...

...that is what it looked like after I arrived back from Bunnings with another can. The second can is 340ml Parfix, the cheapest and smallest.

Another spray of water, then finished it off. then waited until the next day. Cutting the foam was easy:

...it will need to be planed a bit more even.

Now for the other side, which will become the floor of the basin. I pulled off the MDF, and found that the surface is not flat, it has waves:

The waves are up to 5mm deep, not acceptable. I think that what has happened is that as the foam expanded, it lifted from the base. It was not stuck to the base, due to the baking paper.

I read about using baking paper on the Internet, but no one said anything about this. I could spread something over it, to make it even, however, I have gone off this technique.

I will give it some more thought, but I think that I will pull the foam out, and instead install a thin plywood floor with some structural support, then apply the foam -- it should stick to the wood and not lift off.

Or, I could use fibreglass batts. But, grumble, I would then have to buy an entire pack of 9 batts, cheapest at Bunnings is AU$37.45. Weigh that against 850ml Sika Boom expanding foam, at AU$18.96. I might let this rest for awhile, feel that I have given Bunnings enough money recently! 

EDIT:
After reflecting on the situation and an afternoon snooze, I decided to keep the foam fill as it is, and fill the waves. I have lots of partly-used acrylic sealant tubes in the garage, plus a couple of full tubes, so this looks like the best choice to fill the dips in the basin surface.

Most acrylic sealants have a "service temperature" of 80 degrees C, and there are some fire-rated ones up to 90 degrees C. For example, Sikaseal Joint & gap, Sika Caulk, and Full Caulk In Colours are all rated at 80 degrees. One exception I found is Sikacryl 100, interior use only, rated at 70 degrees.

Using the wide spreader, I have applied the first layer. It will need at least two layers I think, partly because acrylic sealant shrinks as it sets.  First layer was less than half of a tube, so the situation isn't so bad.

Of course, this workaround is not suitable for the final DIY plans. When I do finally get to publishing such plans, will probably specify a plywood floor for the basin.  

EDIT 2019-11-28:
I receive emails in response to blog posts, don't normally forward them to a blog post, just the occasional one. Like this comment from Sage (Puppy Forum name):

Expanding foam: you'll pardon my sniggers - I've used this quite a lot for building works, including garages and garden sheds. It runs wild not always into intended interstices and seems to have a mind of its own. Voids abound. Ended up cutting off more than exuded! A crude, if cheap, solution best suited to builders and desperate householders.

Yes, it does indeed have a "mind of its own". I received advice from Rick, that it would have been better to have forced the foam to stay flat by using 3mm plywood on the basin floor and back of the distiller, and injecting the foam into the cavity. 

Tags: nomad