Hello fellow addicts,
I thought I would add my two cents worth to this thread.
Firstly, all praise Mangozac. I have found this technique to be far better than two part moulds for fiddly small parts.
Also, all praise Blind-Pig, forthe tip about talc and bubbles.
Over the weekend I have made a number of split moulds of parts that have been producing poor casts from two part or plug moulds with results that have stunned me because of their detail and ease.
My technique and materials are listed below. I know a lot of this has been covered already but I thought some other patient might get some benefit from another perspective. I really hope this dosen't come off as pompous.
Mould creation (vacuum and pressure):
I make my mould boxes from LEGO, as I can calculate the exact volume of silicone needed for each mould. (each 'pip' on the lego = 0.61 ml).
I base the mould box with 'klean klay' (from my silicone supplier, compatable with the silicone I use) to seal the base of the LEGO mould box. I also use this clay for the pouring plug that the parts are pushed into.
I degas my 'Pinkysil' silicone (usually takes about 27mm Hg to really kick off) using 200ml of mixed pinkysil in a one liter tall plastic beaker (again, from my supplier). Usually the silicone bubbles will collapse before it overflows the beaker.
I pour the silicone into the mould and then put the mould into my pressure pot at 40psi to set for half an hour.
I use 40psi because it does the job and isnt too scary a pressure, I know some other patients use higer pressures but 40 is what I am comfortable with.
Once the mould is set, I disassemble the LEGO and remove all the clay, leaving the mould with the parts inside. I then use a scalpel to cut the split in the mould, I am a bit untidy at this but it produces a mould that is easy to align.
Mould Pouring (vacuum and pressure):
Firstly, I dust my mould with talc (thanks Blind-Pig) and blow it out with compressed air (1,001 uses for an air compressor), this leaves a VERY fine layer of talc on the mould surface.
I use 4PU (from Barnes, again, thanks Blind-Pig, and Mangozac for your suggestions), as it is a very fluid resin and gets into the mould well.
I use Mangozac's squeeze technique to get the resin into the mould and leave a resevoir of resin in the pour plug.
Then the mould (I often do 3 or 4 small moulds at once) goes into the vacuum pot to get the bubbles out. I take the resin down to ~25mm Hg to get the bigger bubbles out and stop when the resin gets lots of small bubbles. This takes about 30 seconds.
I'm not sure if the small bubbles are gas in the resin or something important bubbling off, can anyone comment on this?
As soon as I have released the vacuum, I apply 40 psi pressure to the resin and let it go off for an hour.
I should comment at this point that I use the same converted paint pot for my vacuum and pressure applications, with different lids, this saves moving moulds full of liquid resin too often.
I am very happy with the results I have got from this method. Its something I should have tried a while ago.
Thankyou to all who have contributed to this thread so far. I have found it very heplful.