How to build a vacuum pressure chamber for silicone degassing, mold making, and casting
How to Build Your Own Vacuum Degassing Chamber for casting Silicone Molds
In this "How to" presentation I will show you how to make your own vacuum chamber for degassing silicone. I picked up mold making as a hobby so that I could recreate some of my own sculptures for small time commercial use. However, since I am a fine artist, it's not like I have tons of cash laying around to spend on those fancy (and expensive) commercial degassing units so I decided to build my own. This tutorial will show you how to make a chamber for about half the cost. The most expensive part will be your vacuum pump, the chamber can be built for around $100 or less, depending on how much of this you may have laying around. Commercial chambers range between $500-$600, NOT INCLUDING THE PUMP, which I thought was way too expensive. The vacuum pump you will need is approx. $200, but you can always just modify an existing air pump with a venturi valve (that is if you already have an air pump instead of a vacuum pump); venturi valves sell for approx. $30 at Harbor Freight, if you don't want to buy a straight up vacuum pump. Instructions for venturi valve and air pump not included in this tutorial (google it, there's lots of info on pump conversion on many other craft sites). I included a link to a tutorial for Venturi pump conversion in the "Helpful Links" section at the bottom of the page. I also included links where you can purchase the materials to build this chamber. Hope that helps!
How to build the chamber
1x 16"x16" of 3/8" Lexan, (approx $50)
2x 3/8" male to male pipe nipples (approx $1.50/each),
2x 3/8" ball valves (approx $4/ea),
1x vacuum gauge (goes to 30" Hg, approx $6),
1x 6' reinforced 1/4" pressure tubing (approx $12 for 6 ft),
2x 3/8" to 1/4" male hose nib (approx $1.50/ea),
1x 3/8" female to female pipe fitting (approx $1.50/ea, used to secure hose to vacuum pump,
1x Refco ECO 5 vacuum pump (approx $175-$200 wholesale, purchased from a HVAC wholesale store),
1x high pressure paint pot (approx $80 at Harbor Freight Tools)
1x pressure cooker pot (approx $50-100, I used one that was purchased second hand for about $10, you can find them at Goodwill or other second hand shops). A smaller pressure cooker is used in the working model shown in the pictures.
1x Tube of White Caulk with Gun (approx. $10-12)
1x Plumber's Tape (approx $1)
WARNING: DO NOT USE REGULAR COOKING POTS, PLASTIC BUCKETS OR OTHER SIMILAR THIN WALLED CHAMBERS! I will post implosion pictures of the first regular cooking pot I used to give you an idea of the kind of forces you will be dealing with if you create this chamber. UNDER NO CIRCUMSTANCES SHOULD YOU USE A GLASS CHAMBER; glass chamber failure could result in dangerous shrapnel, severe to crippling injury and possibly even death, depending on how spectacular the implosion.
Also, all joints have been sealed with caulking silicone (approx $10 with caulking gun). The same material was used to form a rudimentary gasket between flat lid and chamber. A healthy amount is needed (applied to both sides of the lexan and the pot rim) to prevent air leaks and allow a vacuum to be formed.
Leave protective adhesive paper on lexan for drilling holes. In center of lexan sheet line up your two ball valves and the vacuum gage close to where the middle of the pot would be (just eyeball it). Make sure your valves and gauges are spaced several inches apart to avoid lexan failure when vacuum is turned on.
Take one 1/2" drill bit and make the holes for the ball valves with the paper still covering the lexan (this will make your drilled holes cleaner). Use a 1/4" drill bit to make the hole for your vacuum gauge. The holes will be slightly smaller than the nips on the end of the valves and gauges so you may have to widen them slightly with the drill bits, remember the tighter the fit the less chances of leakage when the vacuum is turned on.
Once holes are drilled remove the protective paper sheeting from the lexan. Place lexan sheet on table, upend pot onto surface of lexan and draw an outline with a marker. Flip sheep back over and using some plumbers tape wrap ends of valves, 3/8" male to male pipe nibs, and gauge in tape and screw into holes (use 3/8" pipe nibs on bottom ends of ball valves). Take silicone caulk and seal the areas around the valves and gauge (on top of the lexan sheet, leave holes on the underside clear. Flip lexan over and place in a bowl or other surface that will support the lid without crushing the valves.
Take silicone caulk and run along the marker line you just traced. Make sure it is very thick and extends at least 1/2" inside this marker circle to ensure you make a good gasket. Do the same thing on the rim of the pot you have selected to use as your chamber. Let dry for at least 24 hours, preferably more to ensure that the silicone has completely cured.
Hook hose up to one ball valve and attach the other end to your vacuum pump. Line up gaskets and press firmly to create a seal. Turn on vacuum pump to test seal (making sure your other ball valve is closed and that the ball valve that is connected to the pump is open). You may have to press firmly on the lid to ensure a seal forms. Once a seal is formed the pressure should rise pretty quickly so stand back in case something fails.
PLEASE NOTE: You should always ensure that you wear safety equipment such as full eye goggles and gloves when operating this type of equipment. If chamber failure occurs there could be shrapnel. Use at your own risk!
To degas silicone:
Make sure the container you put the silicone into is at least 4x larger than the amount of silicone that you will be using, you will be amazed at how this silicone inflates when it is degassing. The second ball valve can be used to "burp" the silicone if it expands to close to the lid. It is also a good safety feature to use in case the pressure builds too high when degassing. My chamber pulls about 28" Hg and it takes approximately 4-6 minutes to completely degas the silicone.
I hope this helps and good luck model making!
LIABILITY DISCLAIMER: Don't try this at home, if you do so it is at your own risk. I am not an expert in engineering and using any pressurized container can be dangerous. If you do make your own chamber and use it ALWAYS WEAR SAFETY GEAR! The reason the commercial chambers are so expensive is because they have to pass all kinds of regulation safety tests, this is a cheap and probably unsafe alternative, be advised!
Photo Gallery of Working Chamber and Failed ChamberClick thumbnail to view full-size
Amazon Spotlight: Scale for Measuring Silicone
This is the scale that I use for measuring silicone before putting it into the degassing chamber.
I bought this scale so I could measure the exact weights of Part A and B of my Smooth-On.Com's Mold Max XLS II - 1 Gallon Unit (10014859) = $91.94 (used with Universal Mold Release - Aerosol Can (10001661) = $11.48). Silicone mold making is very finicky and you need EXACT measurements if you want it to cure correctly (it's too expensive of a material to screw up your measurements, trust me from experience on this one). It is ridiculously easy to use and has automatic tare feature so you don't have to factor in the weight of your measuring containers and you don't have to worry about getting improper measurements. Measures up to 11 lbs, which is more than enough for my small scale molds and also displays ounces/lbs/grams/kgs so you can use whichever unit you prefer. Overall, best bang for my buck and I would strongly suggest you get one if you plan to do silicone casting.
Order Needed Parts on Amazon
These are some of the parts that I used to make my chamber, the vacuum pump is not the same one but it is a similar model to my Refco 5 CFM pump. The valves and gauges are the same though. The pressure cooker is not the same model as mine but something similar would work.
Smooth On Silicone for Molds
Caulk for sealing
- TAPP Plastics, custom lexan sheeting
This is where I purchased my lexan sheeting. I would not suggest getting a sheet any thinner than 3/8" thick unless you want to have a massive chamber failure and possible injury. The thicker the lid, the better!
- Smooth-On, Inc. Liquid Rubber and Plastics
I buy my casting materials from Smooth-On.com because they are highly reputable; most professional artists use them exclusively. I have never had any problems with them. The current materials I cast with are shown below, prices reflect cost at time o
Please let me know if you have any questions that need clarification or if you have any pertinent suggestions. Constructive criticism is appreciated but please avoid flaming. Thanks and have a great day!