Hi everyone,

I want to Form a narcelle cowl for a scratch build 1/6 scale DH2. I found a lot of Videos on youtube about vacuum forming and have already build a vacuum box. Which plastic is best for forming a cowl in terms of stiffness and weight? What is the maximal tickness to get a positive result?

Best regards,
Andreas

There are a lot of technical details to do this right but I will give you a few suggestions. I dont know what the equipment is you have, what is the table size?

The master model/tool should be made of a material that can handle heat like aluminum, aluminum filled epoxy, ceramic, plaster. For just a single copy from a master model the material can be a hardwood with a hard shell like fiberglass. You can damage the master model if the material is soft due to heat and compression from the vacuum.
The surface finish of the tool does not need to be polished in fact a high polished surface will stick to certain materials. Its better to have a lightly sanded surface to allow air flow. The plastc will not transfer the sanding marks if they are very light.

The tool should not be taller than 6in for a sheet thickness of about 0.030in. If the tool is taller then the sheet thickness has to be thicker.
A good choice for your cowl material is polycarbonate/ Lexan (same thing) or ABS Styrene. These sheets in general are about $5-6 in quantity and about 20x18in for example.

For a round cowl oriented vertical to the table you should make a 0.75-1in tall ring that is 0.5-0.75in from the tool on all sides. This will control the sheet and webbing which will ruin the part. Make this ring from a hardwood and tapered so it will release from the sheet.

The base of the tool should have a 0.25in minimum base to allow the plastic to pull past the part. So the part line/cut line should be 0.25/6mm up from the table.

In the center of the cowl on top (front) where you want to pull the plastic into a pocket you have to drill a few 1/16in holes through to the base to draw vacuum. Place these holes in the edge/corner of the pocket to draw the plastic tightly into the detail.

The plastic sheet must be heated to the point when the sheet droops down from the frame evenly. Only then is it ready to suck down on the tool.

Once you get a few parts done to practice you will understand the behaivior of the tool and the machine.

Post photos!

Myself, and a number of people that do that sort of thing, use a plastic called PETG. I don't remember what that stands for, but it is universally used. It does not outgas like some of the more common plastics.

Les

+1 PTEG aka Polyethylene Terephthalate Glycol


That depends on the size of your plug, I typically would use .030 for small parts or .040 for larger parts

As a simple alternative to "vacuum forming" you can mount the plug on a down and then just stretch the heated plastic (in a frame) down over the plug. Vacuum forming is more useful when you have to make multiple identical versions of the same part.

Another alternative for a "one off" part is to carve a form out of foam, then fiberglass over it, and then use "dissolve" the foam away.

40 thou styrene sheet would be fine for that size nose bowl. Styrene is weak. Since you have the plug for the pull go ahead and 'slick' it up. Paint and wax it. Attach to a parting board. Apply mold release. When dry start adding glass and resin (I like polyester) to make a mold. Repeat the wax and 'release' with the mold and lay up your nose bowl. Sounds complicated but isn't. The part will be stronger than styrene, you need the nose weight and all your friends will wanna build one now they know where they can purchase one of the more difficult (in their minds... hehehe) parts!

Hi Guys,

thank you so much for all the great solutions. I first tried vacuum forming, but the styrene cowling went much too thin. I decided to go with the fiberglass/epoxy solution then and it turned out very well, it is strong, not too heavy and (hopefully) will accept paint very well. I post some pictures of all the DH2-Parts done so far, next week I'll begin constructing the narcelle. Best regards.

Nice work

Nice plane. What size? Is it your plan?

Nice looking work so far!