Vacuum bagging

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 This is a build log of  vacuum bagging

Here the vertical fins of a swept  flying wing are being vacuum bagged.. The same technique can be used for any part.. (just that this is the one I documented)

Of the different methods, this one uses a thin PET sheet as the finishing surface. This is different from using a peel ply.

materials:- thin PET sheet, mold release wax, vacuum bag/ polyethylene sheet, vacuum bag sealing tape, some plastic pipe, vacuum pump/ brake bleeding pump.

The foam blanks are readied.. making sure that surface finish is good and smooth and the surfaces are cleaned .. it needs to be understood that ANY surface imperfections more than the thickness of the FG cloth will be visible after glassing.. therefore though this step may seem trivial, it is quite important


The pet sheet is cut in the shape of the part to be glassed, one for each side. At this stage it is not important for it to be absolutely accurate, I keep  around 1cm excess all around... 

One side of the sheet is then waxed.. I use 2 coats of mold release wax..... it is lightly coated, making sure that the complete surface is covered ... some people use PVA. I found wax to be effective, cheaper and easier.

After the was coating, give it about 30 mins to dry, and then it can be painted , I use normal spray paint. .. but watch the weight. some high pigmentation paints can be heavy. Generally two coats are sufficient, making sure that the first coat is dry before applying the second one.  In the finished product the paint will adhere to the epoxy and lift cleanly from the wax. ..  you may want to have some indication/marker for the coated side (a piece of masking tape stuck on the other side)


The FG cloth is cut slightly larger than the PET skins.. while cutting the cloth make sure the bias of the weave is taken into account. A 45 degrees bias significantly increase torsional rigidity and strength.

Some people wet the skin with a roller and then drape the FG on the skin.. I first drape the FG on the dry skin, making sure it is aligned properly and the wet the FG from top. .. below is the FG draped on the dry skin, making sure that the bias is correct , no wrinkles and the whole skin is covered. 


epoxy is then poured over the FG and the skin and spread with a scrap foam piece , making sure that the FG is completely wetted. Do not worry about the excess epoxy.. focus on getting the FG wetted properly



next lay some paper towels over the skins and apply pressure from top with a roller.. the paper towel will absorb all the excess epoxy. This needs to be done properly to ensure a lightweight part. I normally do it twice with a fresh set of paper towels. Pay attention while removing the paper towel, so that the FG cloth does not move... 

A light film/coat of epoxy is then applied to the foam and it is then sandwiched between the skins. While applying the epoxy to the foam, make sure it is only a light coat, and pay special attention to the edges. Once sandwiched , trim the edges of the excess FG and tape up the whole sandwich with couple of  masking tape, so that it does not move... now it is ready to be put under vacuum


The foam FG sandwich is the put in the vacuum bag. A piece of breather cloth is used to cover the parts. The breather cloth ensures than air is vacuumed across the whole bag.. (breather cloth can be substituted with  layers of paper towel or cheap felt / unwoven fiber blanket).. Make sure the wrinkles are smoothed out and the bag sealed properly  (without any leaks).. The air is then extracted. I initially suck it out and then use a brake bleeder hand pump to build up the vacuum.. I keep it between 15 and 20 mm of Hg.... Now the whole thing is covered with a blanket to raise the temperature, and it is left to cure for generally 24 hrs  .. rest it on a flat surface and put some weights to ensure that it is straight





After 24 hrs open the bag and carefully take out the parts. If your waxing has been done properly the outer PET skin will peel of very easily


Trim of the edges carefully without damaging the part and keeping the sealed edges proper.. The part is  now ready, it is strong and stiff, and has good finish and painted... and light





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