After some careful thought I now had a plan of how to cast the investment plaster blocks in a rubbery bucket without the sides expanding and the plaster becoming stuck.
Idea - Line the sides of the bucket with polystyrene packing chips and plastic.
Using a slightly larger bucket this time, I prepared for my next batch of investment plaster.
Step 1 - Line the bucket with a sheet of thick polythene then pop in a smaller bucket inside so that the polythene is trapped between two buckets.
Step 2 - Carefully fold the polythene sheet over the edge and into the inner bucket to leave a small gap around the inner bucket.
Step 3 - Fill the small gap with packing chips with the idea that when the plaster expands horizontally it will squash the chips and not stretch the bucket.
Step 4 - Carefully unfold the polythene sheet out of the inner bucket and over the sides of the outer bucket, and slowly remove the inner bucket leaving behind a bucket lined with chips and polythene sheeting.
Step 5 - Pour the plaster mix into the bucket and vibrate to remove the bubbles.
Step 6 - Leave to set for 2 hours, then simply lift the polythene sheet out of the bucket which should pull out the plaster block with it, simple?
Well it sounded like a good plan at the time and it all went smoothly until step 5, once the bucket was placed on the vibrating plate and it was switched on all the plaster chips vibrated up the side of the bucket and escaped over the top.
After the 5 minutes of vibrating was over I had to swiftly collect all the escapees and squash them back into the bucket trying not to disturb the plaster.
This method did however have some degree of success, once the plaster was set it was very easy to lift the cast block out of the bucket using the polythene sheeting. Take a look at the pictures, the sides of the plaster look great but unfortunately they will need to be carved away.
There must be a better method than this, something that can be squashed but doesn’t shake out of the bucket.