Hi.

I have designed  and built a UAV which is operating quite successfully. The prototype fuselage was made using a ‘lost foam’ technique which is neither repeatable or pretty. I am considering making a mould for further units. It is a ‘twin boom’ type design so the fuselage is fairly stubby, about 50cm long.

 

It would be a split mould and I would like to use a pressure/ bladder system.

Clearly the mould would have to not distort under the internal pressure. Im guessing at least 20mm thick.

Question.

Could I layup the first layers of the mould using epoxy,then build up some thickness with cheaper poly/CSM?

Could I laminate in steel box sections/ribs or might this cause internal stresses/distortion?

I couldn’t find much advice on getting a good surface finish on the plug. Can you point me in the right direction?

Also any info on bladder materials welcome.

 

Thanks.

Vince

 

 

EDIT...................................

‘Lost Foam’ vacuum bagged prototype fuselage. The insidefinish is better than the outside! Need to make a mould now.

Hi Vince,

My first impressions are that it is quite a large fuselage which will cause several headaches using a pressure-bladder technique.

For comparison the fuselage that I make is for a small "Discus launched glider" it is approx 1m long and the maximum diameter is approx 4cm.

My mold is 5cm deep, (tooling epoxy, 4x 25gsm glass, 4x 80gsm glass, 6x 160 gsm glass, then backfilled with poraver + epoxy + foaming agent then backed with 8x160gsm glass)

It does deform slightly under pressure (approx 75psi) which means I need a LOT of clamps to keep things together. Remember that if a mold does deform too much (especially at the seam), then the bladder will most likely try to escape don the crack and burst.

I think the issues that you will see are

1. having a mold strong enough to take any decent amount of pressure over such a large amount of surface area.

2. making the bladder

3. inflating the bladder without pinching and distorting fabric

I think you would probably gett better results making your fuselage by creating a sandwich for each half and then join using a wet seam as you have plenty of space to get in there and roll down the seam.

Im having doubts about the advantages of applying pressure in this case. It was mainly because I wanted to lay in some 5mm foam core to stiffen the large flat sides, which works great if you can get a vacuum on it.I could vacuum each half in its mould and then join when cured (Is that what you meant?). Otherwise just do a normal layup with a bit of Kevlar between the carbon to thicken it up.

I may do a wooden plug which will split in half to make forming the flange easier, on a flat surface.

 

Hi,

interesting project, mine is very similar though not building a UAV

Why dont you build a wooden plug, get it sprayed with 2k etc. Then get 1mm wax sheet and cut to shape, lay over the plug. Make a 2 part mould. Remove the wax. Lay up into the two and sandwich them around the plug.