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Vacuum Bagging Fiberglass Components

By Ben Owen (originally published in EAA Sport Aviation, May 1988)

If you will check some of the advanced articles on fiberglass components, you will see that many of the early articles and textbooks on fiberglass show . . . "a good ratio by weight of epoxy to fiberglass cloth to be about 50/50." However, with today's material the best for strength is approximately 40% epoxy and 60% cloth. This is very hard to do with a hand lay-up procedure without vacuum bagging.

For vacuum bagging, you will need:

  • A supply of light plastic such as used to cover produce in the field. The 2-1 /2 to 3 mil. (.0025 to .0003 of an inch thick) material is available from farm supply stores, Sears, etc.
  • Burlap bags or a fairly thick polyester cloth to use as a breather cloth can be obtained from a fabric store. The purpose of this material is to absorb the excess epoxy.
  • Polyester or nylon cloth to use as "peel ply." This can also be obtained at a fabric store.
  • Caulking material in a tube can be purchased inexpensively at about 77 cents per tube at K-Mart and other suppliers.
  • Tape to help seal in addition to the caulking.
  • A vacuum pump. The usual home vacuum cleaner will not work for large pieces. A friend has a good one he bought in 1980 for $180 from a farmer, and this is from a former "surge" milking machine. This, and other commercial vacuum pumps, can be purchased in farm supply stores and should be able to draw a minimum of 15" of vacuum.
  • A vacuum gauge for the above vacuum pump so you will have some control on your procedure.
  • In the vacuum line you should have a "catch glass" that you can build inexpensively from components you may have at home or in the shop. The primary purpose of the "catch glass" is so that you don't pull epoxy into the pump itself and so that the epoxy will drop by vacuum into the bottom of the glass if it is ingested into the vacuum line. The usual vacuum reading should be around 8" of vacuum.

You should prepare a board about 1/2" and about 1' square by pounding in nails every square inch. This small version of the old "Fakir's Couch" is used to punch holes in the cloth.

You should prepare sufficient (2-1/2 to 3 mil.) light plastic with holes punched every 1" to cover your entire part to be fabricated, both top and bottom. The first item that is put on the recently laid up part is the "peel ply" of nylon or polyester, covering the entire surface top and bottom. This is then covered with the light plastic in which the holes have been punched. Following this is burlap. A thick layer is desirable and may require more than one, as this is where the excess epoxy goes when you start the vacuum pump. You cover the entire apparatus with a solid sheet of light plastic. Use the cheap caulking material and tape to hold it down while sealing.

At this point you need to be careful, as you can extract more epoxy than you want to. This is more an art than a science. It would be suggested to try several trial pieces and to weigh the cloth before you start. Then weigh the combined cloth and epoxy later to see how the vacuum is working. The following formula will apply for finished pieces. Weight of cloth divided by total weight of cloth and epoxy should equal 60%.

In practice, the lay-up has to be done with the normal speed - you have to cover with the four layers of vacuum material, put in the vacuum pump tube and all the epoxy you are going to take out should be removed within 15 minutes. As soon as you have taken out sufficient epoxy, you can remove all the materials, leaving the "peel ply" until the surface epoxy has set up well. This is a successful method used by builder Larry Fitzgerald of Bristol, WI.

According to Larry, it is best to use a female mold, well waxed with 3-4 layers of wax, either Johnson's Wax or another suitable lay-up wax. Clear this process with the designer before doing large lay-ups of wings, etc.

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