Posted on June 24, 2011
I've been interested in experimenting with some resin casting for a while now ... mainly as a method for producing salad bowls for my "Forest Treasures" business. I sell salad servers through my market stall at the Port Douglas markets (North Queensland, Australia) but simply cannot come up with enough wood salad bowls to make a difference due to the time involvement in each article.
So, my reasoning is that I'd like to employ a method of replication to produce limited runs of contemporary salad bowls based on one-of originals. Initially, the casting material should be resin, although I think I'm ultimately interested in metal ... notably pewter, which I can cast myself, in-house.
During that my past obsessions, I've had a fair bit of exposure to resins and thermoplastics through several large boatbuilding projects in which I've been involved. The idea of casting resin and mould-making is therefore definitely not foreign to me. Interestingly though, I've never actually had the need to cast un-reinforced resin ... all my work to date has been with fibre reinforced plastics ... fibreglass.
Yesterday, I did a small trial resin pour ... just a simple slab approximately 5 mm thick using normal ortho waxed polyester laminating resin (supplied by FGI). I also have polyester casting resin on hand, but wanted to trial the less expensive option first. The laminating resin provides a much longer pot life ... especially right now in our cool winter conditions.
The aims of yesterday's trial pour was to establish if the polyester laminating resin maintained enough strength at thicknesses of only 5 mm to be suitable to my needs ... and examine for brittleness, etc. I wanted to assess the shrinkage at room cure working temperature, and the gel time ... without resorting to refrigerating the poured mould (which may be necessary to control exotherm during summer). For this trial, I also added some colour just to see how well the non-clear laminating resin would colour up and if this affected the plastic cure. Finally, the trial pour was really a bit of a shove off the blocks to actually get this idea moving.
I made a very simple wood mould and calculated the volume for a depth of about 5 mm of final pour. This gave me the required volume of resin which I weighed, coloured and catalysed at 1%. The room temperature was about 23 C and humidity 28% relative. At this temperature, 1% is well within the bounds of producing a controllable exotherm ... in a laminating situation. In a casting process, where there is not the heat concentrating properties of the fibre, I'm not sure.
The resin mixed surprisingly bubble-free and poured evenly, levelling out to an approximate 5 mm depth slab.
Exotherm build-up was agonisingly slow. It took well over half an hour for the slab to gel and it was probably an hour before I could say it was hard, although still soft around the edges. The surprising thing though, was that even at this very shallow exotherm profile, the shrinkage of the slab was substantial. I suspect that shrinkage might be even more for casting resin because the pot life is so much shorter ... suggesting a higher exotherm.
After several hours, I popped the slab out of the mould to let the styrene vapour off overnight. The night temperature would have dropped to about 15 C in the workshop, so I expect the cure might be a bit retarded ... and might have to consider post curing.