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Vacuum Chamber or Pressure pot?

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1 hour ago, MikeMaben said:

OK I'll ask this of any of you experienced guys.


If the goal of vacing molds and pressurising resin is to eliminate

bubbles, why couldn't either vacing or pressurising work for both ?






Sorry Mike it’s been a long day. So, yes, either could work, and as I think has been pointed out, there are outfits that do vacuum casting. It requires serious equipment and also mold design that allows for the “boil” of the resin. In my view, at the level of where I was at, there was no need. Vacuuming requires a constant pump running to maintain vacuum (there may be chambers that hold a vacuum for the 4-6 hours required for resin cure but that’s specialized equipment I think). Conversely, pressurizing a mold while it cures (for 24 hours) is not feasible without serious specialized (i.e. leak-free) equipment. I’m likely not explaining this very well. 


Case study: when I began casting (about 24 years ago) I did not have vacuum equipment at first. I mixed my RTV at ambient pressure and poured the molds. Once cured, the entrapped bubbles in the rubber, while unseen on the surface, created “pimples” for lack of a better term, in the pressure cast parts. The pressure distorted the air voids and that was reflected in the cast parts. That was all it took for me to invest in vacuum equipment as shown above. Once I started vacuuming the mixed RTV, removing the entrapped air, the RTV was smooth and void free. Then pressure cast parts came out equally smooth.


To pressurize RTV for at least 24 hours while it cures was not feasible, the equipment I was using leaked enough that the compressor would cycle once an hour or so. Not a good thing in my basement shop. I never really considered one way or the other, the experienced casters I learned from all did vac molds and pressure casting. Good enough for me, why reinvent the wheel and all that. 


As for talc v. Mold release, that also depends. Graham makes a valid point about talc drying out molds, but in my experience the silicone loses its properties no matter what you do. Talc simply helps break surface tension and helps resin “flow” into voids much better. Tomorrow I’ll try and post some photos of the types of parts I was casting where, without talc, even under pressure I would end up with voids in the finished parts. Remember - pressure does not force resin into voids, it simply reduces air bubbles into “suspension”, I.e. nothing. Hope this helps!



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As some of you know I've done a bit of casting over the years (started 1966) apart from my stint in mining then started MDC 25 years ago and we do a bit of casting.

On the talc side I agree with Graham as you are introducing a powder this will result in the mould not closing properly causing mould lines and flash, for casting were are 100% vacuum but may have to go into pressure for clear:BANGHEAD2:

On the materials side with the fastcasts a couple of cycles in the tank and demould in 45 minutes ( I only have the resin under vacuum for about 90 seconds total) there's even a new silicon 8 minutes pot life and 20 minutes demould so you can turn a part round in about an hour

Only ever use a one part mould and cut the part out.

On the companies side Graham is correct most reps are not the best help, they tend to tell you who is using their resin and I have been asked to help a company out by training at my place

nice thought but think of the phone calls :notworking:

If you are buying a small vacuum set up use a proper vacuum engineering company and not a tool company that has one or two items, as the engineering company will normally have an entry level unit or a second hand unit that has been traded back, these guys know vacuum so are really helpful and sometimes know more about application than resin reps.



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