Thanks a lot. Good info. Alden Danny Miller wrote: > Pressure does not de-gas, what it does is force the bubbles into > solution but AFAIK you still need to de-gas first or there will be too > many bubbles, and too large. I'm sure it depends on what resin we're > talking about. High viscosity resins are difficult to get bubbles out of. > > You need to vac in a different container typically. I tried to pour > into a mold and vac and there were bubbles coming up that were clearly > too many to have been from the already degassed urethane I just put in > (pouring often traps small bubbles between the resin and mold and I > wanted it out, plus I needed it to flow into an undercut which kept > trapping air). The silicone mold itself had trapped air that was coming > out apparently, and it seemed to have no end. Plus it'll loft up and > overflow your mold. > > So you generally can't vac it after it's in the mold, but your > previously degassed solution is no longer completely bubble-free after > pouring. That's where the pressure casting comes in, pressure is not a > problem for the mold. > > Do not use tin-cured silicone molds in a heated oven. It breaks down in > a few cycles.. You need the more expensive platinum-cure. > > http://www.smooth-on.com/ are the guys to deal with. Lots of products, > very knowledgeable, carry mfg specs, good prices. > > A lot of other suppliers buy their stuff or the same stuff from their > mfg, relabel it, and don't know crap about it and have no specs. > > Danny > > Alden Hart wrote: > >> Fantastic advice. Answers a question I've had for a long time. Have you >> done any casting resin curing this way? >> >> I'm looking for a way to cure water-clear casting resins at low >> temperatures. The typical process (if you talk to the "deal toy" guys) >> is 6 atmospheres of pressure in an oven, with about 2 hours of 375 >> degrees (F) along with some gentle ramp up / down time. They seem to >> de-gas using pressure, not vacuum. Supposedly some dental molding >> processes use a UV process instead of heat. I'm trying to come up with a >> process that I can do myself that will not require excessive equipment >> or hazard. >> >> Any thoughts are appreciated. >> >> Alden >> >> >> Danny Miller wrote: >> >> >>> Lemme tell you EXACTLY what to do. >>> >>> Go down to Hobby Lobby and they've got these clear hard plastic >>> containers with a hinged, rubber gasketed lid. The hinge is made of >>> bent chromed wire and forms a wire bail latch (snaps down, like a lager >>> bottle). They're fairly thick and I believe acrylic. >>> >>> Drill a hole in the bottom of the container. Get a big rubber stopper- >>> bigger than the hole, it's not going to stick in there like a stopper- >>> drill a hole all the way through that, jam a barbed hose coupler in the >>> rubber, and connect your hose to it. Remove the wire latch/hinge and >>> discard. >>> >>> Set the lid on the table with its gasket. Put your target item on it. >>> Set the container over it as a "bell jar", it's upside down so the hole >>> in the bottom of the container is at the top. Put the rubber stopper >>> with the vac hose sticking out of it over the hole and it will seal >>> itself tight. In fact you don't want the stopper to be permanently >>> attached to the jar or you'll need to find some other way to break the >>> vac to open it because that lid ain't separating from the container. >>> This way, you just grab that rubber and peel it sideways to pull it off. >>> >>> I vacuum urethanes, silicones, etc that I'm making like this to de-gas >>> them. You really need a clear chamber for that because if you pull vac >>> too fast they expand wildly and overflow a container even 5x bigger than >>> the resin in it. >>> >>> Just what are you trying to do, anyways? >>> >>> Danny >>> >>> Rich wrote: >>> >>> >>> >>>> Thank you for the reply, Marcel. I need a glass or steel bell Jar with a >>>> vacuum pump But your approach is certainly creative. >>>> >>>> ----- Original Message ----- >>>> From: "Marcel Duchamp" >>>> To: "Microcontroller discussion list - Public." >>>> Sent: Friday, November 21, 2008 12:23 AM >>>> Subject: Re: [OT] Bell Jar >>>> >>>> >>>> >>>> >>>> >>>> >>>>> Rich wrote: >>>>> >>>>> >>>>> >>>>> >>>>>> I am looking for a very low cost bell jar and vacuum pump. Does anyone on >>>>>> the list have one that they would like to sell? I don't expect to pump >>>>>> down to ten to the minus 6 torr. But I want to get down to the lowest >>>>>> possible pressure (below 100 u) >>>>>> >>>>>> >>>>>> >>>>>> >>>>> Depending on what you need it for, you may find a pressure cooker works >>>>> for you. I used one for several years and you can probably find them in >>>>> second hand stores. New ones are probably in the range of $20 to $40 or >>>>> so. >>>>> >>>>> For mine, I used a canning pressure cooker that is something like 12 >>>>> quarts in size. A little vaseline on the edge and a rubber gasket >>>>> between that and a slab of 1" plexiglas for a lid will seal the deal. >>>>> >>>>> A hole was drilled and tapped and a plastic hose barb fitting for the >>>>> vacuum hose was inserted with teflon tape. Not quite as nice as a real >>>>> bell jar but it worked perfectly for de-airing two part RTV rubber for >>>>> mold making. >>>>> -- >>>>> http://www.piclist.com PIC/SX FAQ & list archive >>>>> View/change your membership options at >>>>> http://mailman.mit.edu/mailman/listinfo/piclist >>>>> >>>>> >>>>> >>>>> >>>> >>>> >>>> >>>> >>> >>> >>> >> >> > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist