cecrops props
Active Member
Now that I am getting closer to the molding step of my Borderlands 2 Jakobs pistol (http://www.therpf.com/f9/borderlands-2-jakobs-pistol-build-230069/), I find myself in need of a larger vacuum degassing chamber. The one that I currently have is way too small for larger quantities of silicone mold making resin. In order to assure that my new chamber will be large enough for all future projects, I have decided that it must fit a 5 gallon bucket, giving me plenty of room for the expansion of the silicone that takes place during degassing. For reference, a standard Home Depot 5 gallon bucket is 14.5 inches tall by 11.8 inches in diameter.
After looking into the construction details of several different chambers online, I see that there are four basic approaches to construction.
One of the most common approaches is to use a large heavy walled aluminum stock pot for the chamber itself. A 40 quart aluminum stock pot will barely fit a 5 gallon bucket. There are three or four large enough aluminum stock pots listed on Amazon for around $50, with free shipping, so this is an economical approach to building a vacuum chamber. This is also a nice and easy approach since you start with ready made chamber. The only disadvantage is that the 40 quart pot is a pretty tight fit for a 5 gallon bucket, and larger sizes of pots jump way up in price.
A second approach is to use large diameter plastic pipe, like 12 or 14 inch PVC drain pipe, or a large PVC pipe coupling fitting. This approach works well if you have a source for scrap pipe or fittings, but the pipe is usually sold new in ten foot length for a painful amount. The fittings are also expensive. The other downside is that the plastic plate for the bottom, since it is flat, needs to be quite thick to withstand the full vacuum, probably 1 inch thick for a 12 inch diameter chamber, and that adds quite a bit to the cost of this method, unless you have access to scrap plastic of the appropriate size.
A third approach is to use an appropriately sized section of steel pipe, and weld on a flat plate bottom. This approach may be cost effective if you have access to large diameter scrap pipe, but purchasing the pipe new is very expensive. Also, purchasing, cutting and welding on the flat plate bottom greatly adds to the expense unless you have access to scrap and someone to cut and weld it for you. Given that the bottom plate is flat it will have to be fairly thick (1/4”?) to withstand flexing during full vacuum. The whole chamber will end up fairly heavy.
The fourth approach is a modification of the steel pipe approach, a steel walled tank with a formed dome bottom. This makes for a much lighter tank. Of course, forming and constructing such a tank is beyond the capabilities of most people, as the forming of the domed bottom requires a large die press. Fortunately, you can purchase and already constructed tank in the form of a compressed air tank, and cut off one end to form the opened top of your vacuum chamber. There a several options for acquiring a tank. Lowes carries a 7 gallon compressed air tank for $60, Walmart has a 10 gallon compressed air tank also for $60. A tank from a used air compressor is also a good option. Using a compressor tank, you can move up in to a larger sized tank, as portable compressed air tanks seem to top out at the 10 gallon size. A used compressed air tank can often be found on craigslist in the form of a complete or damaged air compressor. I have seen older compressors with 20 or 25 gallon tanks sell there in the $50 to $80 range. The disadvantage of using a compressor tank is that you may have to remove various welded on brackets and plug one or two air outlets to get a clean tank, but that’s not too much work. In this regard, a vertical compressor tank is easier to use than a horizontal tank, as most of the extra brackets are removed when the top is cut off.
After looking into the construction details of several different chambers online, I see that there are four basic approaches to construction.
One of the most common approaches is to use a large heavy walled aluminum stock pot for the chamber itself. A 40 quart aluminum stock pot will barely fit a 5 gallon bucket. There are three or four large enough aluminum stock pots listed on Amazon for around $50, with free shipping, so this is an economical approach to building a vacuum chamber. This is also a nice and easy approach since you start with ready made chamber. The only disadvantage is that the 40 quart pot is a pretty tight fit for a 5 gallon bucket, and larger sizes of pots jump way up in price.
A second approach is to use large diameter plastic pipe, like 12 or 14 inch PVC drain pipe, or a large PVC pipe coupling fitting. This approach works well if you have a source for scrap pipe or fittings, but the pipe is usually sold new in ten foot length for a painful amount. The fittings are also expensive. The other downside is that the plastic plate for the bottom, since it is flat, needs to be quite thick to withstand the full vacuum, probably 1 inch thick for a 12 inch diameter chamber, and that adds quite a bit to the cost of this method, unless you have access to scrap plastic of the appropriate size.
A third approach is to use an appropriately sized section of steel pipe, and weld on a flat plate bottom. This approach may be cost effective if you have access to large diameter scrap pipe, but purchasing the pipe new is very expensive. Also, purchasing, cutting and welding on the flat plate bottom greatly adds to the expense unless you have access to scrap and someone to cut and weld it for you. Given that the bottom plate is flat it will have to be fairly thick (1/4”?) to withstand flexing during full vacuum. The whole chamber will end up fairly heavy.
The fourth approach is a modification of the steel pipe approach, a steel walled tank with a formed dome bottom. This makes for a much lighter tank. Of course, forming and constructing such a tank is beyond the capabilities of most people, as the forming of the domed bottom requires a large die press. Fortunately, you can purchase and already constructed tank in the form of a compressed air tank, and cut off one end to form the opened top of your vacuum chamber. There a several options for acquiring a tank. Lowes carries a 7 gallon compressed air tank for $60, Walmart has a 10 gallon compressed air tank also for $60. A tank from a used air compressor is also a good option. Using a compressor tank, you can move up in to a larger sized tank, as portable compressed air tanks seem to top out at the 10 gallon size. A used compressed air tank can often be found on craigslist in the form of a complete or damaged air compressor. I have seen older compressors with 20 or 25 gallon tanks sell there in the $50 to $80 range. The disadvantage of using a compressor tank is that you may have to remove various welded on brackets and plug one or two air outlets to get a clean tank, but that’s not too much work. In this regard, a vertical compressor tank is easier to use than a horizontal tank, as most of the extra brackets are removed when the top is cut off.
