Testing epoxy glass over plastic sheet as an inflatable pattern (mold).
Saturday 08 of November,
Inflated patterns can potentially be quite useful. Paper mache over balloons used to be an old favorite. Buildings can be made by spraying cement over inflated domes. Igloos can also be made over an inflated dome. Inflated patterns allow a variety of practical and/or useful objects to be fabricated.
I've tried many composite fabrication methods over the years but only tried an inflated pattern once (a plastic ball) and that was a dismal failure. I was trying to make an underwater camera housing by laying epoxy glass over a plastic ball and had serious problems because the ball expanded and contracted as the air changed temperature. I ended up with a wrinkled mess.
Now that I've worked out how to weld plastic sheet (see Welding for plastic parafoils) I can potentially make some interesting things.
Before building secret project-Y I need to test the basics - baby steps.
The material.I'm using "builder plastic" to make my test pattern. This is cheap, strong, a little stretchy and I know I can weld it. I think it is high density PE. PETE (mylar ) might have better resin compatibility but is isn't as readily available, doesn't conform well and is harder to work with. Low density PE might be a little to stretchy. Ditto for vinyl.
The Resin.I'm using epoxy. I'm found polyester/vinyl ester doesn't play well with plastic film. The film tends to wrinkle on contact with it. Also the fumes give me health problems.
It would be nice to have the option of gel-coating the pattern but don't think this will work ( I might try one day).
It may be possible to seal the film with a thin layer of epoxy glass and then use a cheaper resin over this for the bulk of the material.
The fibre.Initially all my tests will be with glass. My immediate needs don't require carbon or kevlar.
The pump/regulator.The air pump is a cheap K-mart aquarium pump. To regulate the pressure I use a long drinking straw in a bottle of water as a shunt. As well as being a simple, cheap, reliable adjustable regulator you also have a visual and audible indicator the all is well. I was expecting to need more depth but 200mm seems plenty.
Above is my first test rig. The tin on the right has clear "builders plastic" stretched over is and inflated. This was covered with some facing tissue and two layers of chopped strand mat.
This is a close up of the excess air bubbling off. After everything was stable I clamped to tube with a g-clamp to reduce the flow to a minimum. I left it overnight and it was still working nicely ext day. The fibreglass looks good but I haven't removed it from the plastic because I may try infusing a few thick glass layer over the top.
I vacuum infused a couple of layers of glass cloth over the CSM and the results were fairly good. There are a few small defects but no show stoppers. The curved surface is even accurate enough to focus light to a point.
My first test pattern was a simple six pointed start cut from light builders plastic/drop sheet.
The seams where double welded and all went well until I cut a hole in the fat end to invert the bag to place the seams inside. After that I couldn't seal the bag well enough to glass over it.
My next attempt used thicker plastic and overlapped seams. I was again plagued with small leaks until I realized I could simply put another bag inside mine. I also placed a oval plate made from a dead CD inside to make the end conform to a shape which would lend itself to adding a hatch.
The pattern was successfilly covered with epoxy glass. The shape has corners which should be more rounded but I think this will improve when made to a larger scale.