terryt Posted August 14, 2007 Share Posted August 14, 2007 For those of you that are making silicon molds and need to de-gas (de-air) your molds, I had these in the cupboard and never thought about using them. http://kitchen-dining.hsn.com/rival-seal-a...attr=&o=-RI You can use the canisters with this hand pump: http://www.harborfreight.com/cpi/ctaf/disp...temnumber=92474 Or this vacuum pump: http://www.harborfreight.com/cpi/ctaf/disp...temnumber=92475 The canisters are a lot cheaper than the one I bought from Edmunds Scientific. http://scientificsonline.com/product.asp?p...hcd2=1187106205 I have tested the canisters to -25 inHG with no problems. Quote Link to post Share on other sites
Blackcollar Posted August 14, 2007 Share Posted August 14, 2007 (edited) While we're on the subject of casting, got a question for you, do you use straight epoxy resin/hardener or do you add a filler as well? Castings I see have a yellowish tint to them, which suggests a filler. Would you have access to sites which deal with casting, set up, vaccum, mix, forms etc? -Al Edited August 14, 2007 by Blackcollar Quote Link to post Share on other sites
terryt Posted August 14, 2007 Author Share Posted August 14, 2007 I don't use epoxy resin or filler, I use urethane resin. I'm currently using this stuff: http://smooth-on.com/liqplas.htm It cures white or off white, depends on which urethane resin you use. You can use filler with the resin, but I haven't used it yet. I also get my silicon rubber from them. Quote Link to post Share on other sites
DevilsChariot Posted August 15, 2007 Share Posted August 15, 2007 Good Find Terry! About to do my first mold/castings myself. Quote Link to post Share on other sites
DevilsChariot Posted August 20, 2007 Share Posted August 20, 2007 Did my first castings this weekend. Didn't pull a vaccum, but gave em the squeeze, since the rtv silicone is so squishy, made some antenae for a hind in 1/35, lost one of two, so made a mold and now I have 2 yay! Made a kk-1 ejector seat from on I had sitting in my box, came out great. Totally excited! I also used the smooth on resin and rtv, they were easy and worked great. Quote Link to post Share on other sites
terryt Posted August 20, 2007 Author Share Posted August 20, 2007 For casting you should use a pressure pot, it eliminates voids and air bubbles. http://www.harborfreight.com/cpi/ctaf/disp...temnumber=93119 Print the page out and bring it to the store to get the Web price. Quote Link to post Share on other sites
dsmith Posted August 21, 2007 Share Posted August 21, 2007 Just be sure to add some WD-40 to the clamps on the pressure pot :D -Doug Quote Link to post Share on other sites
Blackcollar Posted August 21, 2007 Share Posted August 21, 2007 I was wondering about how much pressure is used on the pressure pot? At the same time, what would be better vacuum or pressure? -Al Quote Link to post Share on other sites
doctorpepper Posted August 21, 2007 Share Posted August 21, 2007 Just be sure to add some WD-40 to the clamps on the pressure pot <_<-Doug i used.. cooking oil! as i couldn't find any WD40 in the house at the moment. but the screw threads on my clamps did start to go bad, and i had to resort to a fair amount of filing.. in any case, casting is usually done at 40 psi or so. it's good enough to ensure there are no bubbles, and that the resin is hard enough drPepper Quote Link to post Share on other sites
BOC262 Posted August 21, 2007 Share Posted August 21, 2007 I was wondering about how much pressure is used on the pressure pot? At the same time, what would be better vacuum or pressure?-Al Vac is used to remove trapped air and gasses from liquid RTV prior to pouring it over your masters. If you don't remove the air, the RTV can cure with voids and bubbles in it--if any of those voids are near the cavity in your mold (in other words, near the part you are casting) the casting could come out with a lump where the bubble was. Even if the bubbles in the mold aren't right on the surface of the cavity, if you are casting under pressure the pressure will deform the resin toward the "soft spots" in the mold caused by the proximity of entrapped bubbles. Does that make any sense? It's kind of hard to write about this stuff--a picture would show instantly what I'm trying to say. Regarding the casting side of the operation--you want to use PRESSURE!! Pressure will drive the bubbles in the resin either completely into "solution" (think of what happens when you observe a sealed bottle of beer--no bubbles--the pressure has forced the CO2 into solution, but as soon as you open the bottle, the pressure is released and the CO2 bubbles immediately appear--they have come out of solution) or the pressure will drive the bubbles toward the center of the casting, away from the surfaces where they could appear as flaws such as "pinholes". The only time you may want to use vac during the casting phase is to help fill a complex mold. In that situation, you would fill the mold with liquid resin, pull a vac on it for a few minutes, than place the filled mold under pressure to cure. I used to run a business doing mold making and casting, and the above is what I learned from a lot of experimentation and experience. I would highly recommend to anyone doing mold making and casting to keep a journal. Make note of the things you do, resin and RTV formulas, temperatures, pressures, times, etc . After a while you will have enough data to help refine your techniques, finding out what works, what works better, what doesn't work, etc. HTH & happy casting! Karl Quote Link to post Share on other sites
Blackcollar Posted August 22, 2007 Share Posted August 22, 2007 Thanks Karl for the info, Would you say 10 psi a good pressure for casting? Quote Link to post Share on other sites
BOC262 Posted August 22, 2007 Share Posted August 22, 2007 ...Would you say 10 psi a good pressure for casting? It depends. Personally, I never used such a low pressure when casting. I usually used from 40 to 60 psi. Of course, the psi you use depends first of all on what your tank is rated for (minus a healthy safety margin!). After being sure you are not over pressurizing the tank (which could result in an explosion), next you want to determine the minimum amount of pressure needed to get consistent results from a particular mold/resin combination. If you get good results at 10 psi, super, you nailed it! If not, try going up another 10 psi and see what results that produces. The viscosity of your particular resin formulation, and the idiosyncracies of your particular mold will determine how effective different pressures will be. This is why my previous suggestion of keeping a "shop journal" is so important. Mold making and casting is as much an art as it is a science. Though there are some "rules of thumb" there really are not that many absolute rules. Without experimentation, you will never know what the most efficient procedures are for your particular set of circumstances. This is vital in a production setting, such as I was involved in, but it can also help the "one off" hobbyist. HTH and happy casting! Karl Quote Link to post Share on other sites
dsmith Posted August 28, 2007 Share Posted August 28, 2007 I find that between 40 and 50 psi works well. -Doug :D Quote Link to post Share on other sites
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