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Thread: casting under pressure
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5th April 2014, 02:40 PM #1
casting under pressure
well its starting to cool down here in the southern states and i think i will have to start casting under pressure
last weekend i did one small batch in the pressure pot which was sitting on a table in the sunshine, it was about 22 degrees, the blanks came out spectacular after about 5 hours
one night through the week i decided to whack a couple more in because i was home early, took them out after 5 hours and they had set but not fantastically, it was much cooler and no sunshine to warm the pot
i am pretty sure that air under pressure gets very cold so i think if i am going to cast under pressure for the next few months i am going to have to come up with a way to heat my pressure pot
i am thinking of getting an electric blanket which i can wrap around the pot, they usually have a couple of heat settings to pick from, trial and error should find the correct setting
has anyone tried this or had/have any other ideas i would greatly appreciate your thoughts and input
thanks
MIKNo Result Without Effort
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5th April 2014, 03:09 PM #2
Under pressure resin takes a lot longer to set, and the opposite for vacuum, it takes less time to set. Temperature certainly helps and your idea of an electric blanket would be good although sometimes I like it to take longer so it has more time for the resin to penetrate. Here in Qld there are not as many cold days so I can just pick my times. I try to cast under pressure when it is ~30 C and then in the pot it is equivalent to ~25 C (not a conversion, merely a guess, but you get my drift). I also tend to leave my casting overnight and recommend 24 hrs anywhere where it is less than 30 C.
Over 30 C is fine for most applications, except as mentioned previously, I want to penetrate the timber as much as possible. In these circumstances I release pressure, top up resin, then re-pressurise sometimes a few times to enable any bubbles to rise and release. If too hot, the resin just sets too fast and releasing the pressure can cause debonding.Neil____________________________________________Every day presents an opportunity to learn something new
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5th April 2014, 03:30 PM #3
thanks for the input Neil
for the next 6 months we probably wont get a day over 25C and the evenings soon will down around the 10C mark
i am not doing the WW casts so penetration is not an issue, i am using the pressure to compress any small bubbles that might occur when casting on uneven surfaces
as an added question what pressure do you all think is needed just to compress bubbles, i have been using 50psi it seems to compress the molds a little, i think i will dial it back to around the 35psi mark as an experiment next time
MIKNo Result Without Effort
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5th April 2014, 07:39 PM #4
I always take the pressure the pot can withstand, in my case, 45psi. Whilst at George's place at Mt Compass I got to use his pot he takes to 100psi (I stood around corner ). It was a modified pot and he has had it tested to 150psi, but it still has a thin top with bolt marks on the rim (where they fail), so I wasn't confident of the safety at only FoS 1.5.
On my next trip to Melb I am picking up a huge pot under my mum's house I didn't know was there. Heavy duty steel reinforced tank ~600 deep that I am hoping once cleaned up can take a higher pressure. At 100psi the resin even penetrates rotten timber and has less chance of debonding and bubbles.Neil____________________________________________Every day presents an opportunity to learn something new
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5th April 2014, 08:08 PM #5
When I cast under pressure I go up to 45 psi, I always leave under pressure overnight. I try to keep heat down to a minimum as I prefer a slower cure so as to reduce the chances of cracking or debonding.
Regards Rumnut.
SimplyWoodwork
Qld. Australia.
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27th April 2014, 07:25 PM #6
i tried out my new idea on tube in casting today, i just have to let the resin go off completely before i can demold them..... tomorrow night.... if it works i will be able to do 30 - 40 tubes in one go of the pressure pot
here's hoping
where does everyone in OZ buy their silicon stoppers, i found only one place on ebay and they don't have a very big range, i will try the local bearing shops sometime this week, but they don't tend to go out of their way for the hobbyist which is a pity
all the best
MIKNo Result Without Effort
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28th April 2014, 07:01 AM #7
I got some stoppers from the local home brew shop.
They werent real cheap at $2.50 each, but they do the job.Brad.
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28th April 2014, 11:56 AM #8SENIOR MEMBER
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You could try lab suppliers. They would probaby stock a range of sizes.
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28th April 2014, 10:51 PM #9
I got mine from Ebay, but I do NOT use this type of mold under pressure. I know you can, I believe 20psi max is recommended, but normally things cast with the tubes in don't need pressure.
Neil____________________________________________Every day presents an opportunity to learn something new
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30th April 2014, 10:34 PM #10
I used to cast under pressure. When casting SWMBO applied constant pressure on me to mow the lawn instead
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