A Homemade Vacuum Bagging System

Although you will not save very much money by building a vacuum pump system over a commercial unit, particularly if you can't find a used main pump, the tinker toy mentality lives on. Vacuum bagging is used to apply uniform pressure to parts to be laminated together, in this case model airplane wings made from carbon fiber and kevlar.

CAUTION! I have not included any instructions for the electrical wiring of this system, if you are not comfortable doing the electrical at 120 Volts, find someone who can do it for you!.

Schematic

 

What you need

Main pump – phone around to the used refrigerator dealers and repair shops, ask for a decent used (or discontinued) refrigerator compressor. Select one that is filled with oil and has the inlet and outlet tubing still attached. The grey box on the left side of the compressor with the wiring is also important, it contains circuitry to start the pump. You should not have to pay more than $20 for one, some people I know have begged one for free. DO NOT take a old fridge and strip out the compressor, unless you go to the trouble of having the Freon properly drained. That’s just good for the planet. If you are paranoid, get two and keep it around as an emergency backup. I also made a cheap vac pump from an aquarium air pump, but it would only pull 9" Hg of vacuum.

The compressor will spit out oil when running, collect the oil from the outlet in a small cup and then pour it back into the inlet side every so often. Note that Team Ariane has a CNC mill probably worth more than 20 k$, but uses a refrigerator compressor for their vacuum pump. The pump below has two suction lines on each side, and the single outlet sprays oil into the red cup.

the heart of the system

Recently I decided to get a backup pump as the fridge pumps are known to fail on occasion. The composite store had a surplus Gast O523 rotary vane pump in the back room. This pump is rated for continuous service but is noisy. So the pump is in the garage and a vacuum line is run to the basement workshop. Also a 10 gallon pressure tank has been added to line as a reservoir. Filters prevent debris from entering the pump and carbon dust from entering the garage. This pump has a bleed valve to regulate suction pressure and no vacuum control switch. Using the hot box only requires that the part be under vacuum for 10 hours so the pump is not running for days. I use the fridge pump for little jobs and maximum suction, the Gast pump is used for all the big jobs, preset at 14" Hg.

Vacuum switch – Surplus is good, otherwise buy one from a electrical supply store or Aerospace Composite Products (ACP). A switch is better than a continuous run pump with a bleed valve, the bleed valve may plug overnight and then your wing is subjected to max pressure which will probably crush the foam, or at least distort the part very badly. Barksdale industrial vacuum switches come up on Ebay pretty regularly, new retail industrial switches are too expensive. You have to be really sure of what you are buying though, many of the industrial switches are not rated for full vacuum, but they are usually rated for full current without the relay. I purchased some full current rated switches for less than $7 each but they required installation of an intermediate actuating lever, the deadband was too small for our purposes. Do not buy a differential pressure switch rated for 3000 psi, the sensitivity will be unusable to set in any reasonable range.

Relay - I did not bother with the relay when the pump was first run, and the switch is fine after about 100 hours of run time on the system. Last week the relay was installed, Radio Shack has them for about $7.00, Part #275-217.

Check Valve - $2.00 from ACP and helps to hold vacuum when your pump can leak air backwards into the suction side. Not at all necessary. For some weaker pumps that have trouble starting with vacuum on the suction side, you can install a check valve and bleed a small amount of air into the line between the pump and the check valve. This way there in no differential pressure on the pump when it starts up due to your intentional leak. The check valve holds vacuum on the part while the air bleeds into the line.

Vacuum gauge – I had a brake bleeder hand pump with a gauge attached, so I used it. You will be pulling between 7 and 18 inches of Hg, so find a gauge with this range. More than 20 in Hg is just plain excessive. Blue foam can take 20, white foam can take 7 in Hg. Crush a few pieces of foam for fun.

Vacuum reservoir – a reservoir will reduce cycling of the pump when you don’t get a perfect seal on the bag. I made one from 6 feet of ABS pipe, 2" diameter, with screw on end caps. Just seal the end caps in place with ABS glue. Get three brass barb fittings (1/4" size) from the plumbing store and drill the pipe to fit. Epoxy the brass barbs into the pipe.

Fittings – you will need a few Tee fittings, some tubing, and valves. The plumbing store carries " inside diameter plastic tubing, and a large selection of valves, plugs, and couplers. Auto supply stores like Pep Boys are also a good source of tubing and fittings. I like the clear flexible vinyl tubing rather than the hard white tubing. The hard white tubing is needed for inside the hot box, the clear soft tubing will collapse under high vacuum with the high temperatures. You can get away with plastic fittings for the tubing runs, which is a lot cheaper than the brass fittings. Pick up some Teflon sealing tape to use on all the plastic joints. The quick connects to the vac bag are handy, again from ACP.

Misc. film and bags – lots of choices for bags and film pick whatever suits your wallet. Nylon is good, no resin I’ve ever used has stuck to the bag and with reasonable care, it can be used over and over. Wings need mylar, peel ply, breather etc, which is subject in itself. Plasticine (children's modeling clay) works well for patching accidental punctures in the bag.

When you lose a lot of oil from the pump, it will start to overheat.  I have used replacement oil of synthetic 20W-40 automobile oil without any problem. Pour the oil into the inlet with the pump running until the amount of spitting at the outlet increases.  I do this about every 200 hours of bagging time or when there's 5 cc's of oil in the red cup.

If you are using the new high strength epoxies, they will require a heated post cure at elevated temperature. A simple hot box that fits over the work bench is an easy way to do this, look at the workshop page.

For more information on vac bagging try these links:

  1. Andy MacDonald's Page
  2. The Workbench.

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