Making your own composite parts: looking for tips

[ChrisH]

Originally Posted by lukevanoort

If ChrisH comes across this thread, you'd probably get the best information from him. I don't really know much about working with composites, but have you considered wood? I imagine you could make some nice and light sheets with CF and balsa (or birch or hard maple if you want some more strength). The low price requirement kinda puts a bit of a limit on what you could use (polycarb and garolite, for example, get rather pricey in large amounts). Industrial cardboard can also be quite strong... I imagine a healthy diet of CF would make it even stronger.

Did I hear somebody take my name in vain?

My professional curiousity wants to know what resin and fiber you are using and who manufactured it. This information would tell me just how "custom" the material is. As you can see from the above posts this is an important consideration in figuring out whether or not a material is "legal".

For core materials, anything that will handle 250F is not going to be terribly cheap. Rohacell will certainly work in the proper grade. It will go over 350F if you keep it from picking up moisture and give it the proper "heat treat". You can also form it if you handle it right. There is another product called Divinycell F from DIAB Products that will also work. DIAB also sells an end-grain balsa product called ProBalsa that wll probably work.

Rohacell and DIAB poroducts are readily available, but you have to know where to look for them. When I go to SAMPE (Society for the Advancement of Materials and Process Engineering) shows DIAB is always there handing out samples. If I had to pick one company to approach for "sponsorship" of materials I'd start with them. They seem to be happy just to have thier materials out there. It seems like they give out tons of 6"x6" samples.

All these materials come in various thicknesses and densities. There is a direct relationship between density and strength. For sandwich cores the most critical property is typically shear strength, followed by compression. If there is a significant amount of tension on a face sheet you are doing something wrong. Check the manufacturer's data sheet to find a material that works for your application.

Here's a rough fabrication sequence:

1) trim core to manufacturing size. Composite panels are generally built oversize and then trimmed back. this is so all the fiber goes all the way out to the edge. The Stress guys don't like it when a ply stops short of where it is supposed to.

2) Apply the proper release agent to a clean metal or composite mold or tool. This can be a flat plate for a flat part. Do not apply release to the outer two inches or so. Follow any cure instructions from the release agent manufacturer.

3) Lay up the plies of the outer skin. Depending on how sticky the prepreg is and how much contour your mold has you might need to "debulk" periodically during layup. Since this is expired material it is likely to be pretty dry or "boardy" so count on at least a couple of debulks. For a debulk you construct a vacuum bag over the part. First cover the part with a release film such as teflon film. Then put a breather material such as fiber glass cloth over the release film and out to the edge of the released area of the tool. Apply vacuum bag sealant to the edge of the tool. Bag sealant comes in rolls and is typicaly 1/2" wide x 1/8" thick. There are high temp and room temp bag sealants. For room tempertature debulks you can get away with the cheap room temperature stuff. Place plastic bagging film over the part and tool and stick it down to the bag sealant. Make sure you have vacuum probes that go through the bag. Hook the probes up to a vacuum source and let the bag "pull down" onto the part. The bag will compress the part and help the plies stick together.

4) Repeat the lay-up/debulk sequence untill all of outer skinn plies are laid up. Do a final debulk and then position your core. Debulk the core into place and repeat the lay-up/debulk cycles for the inner skin.

5) Now rip off your trusty debulk bag and throw it away. Rebuild it with high temp capable bag film and sealant tape. Make sure the bag does not leak.

6) Cure per the resin manufacturer's instructions. You can cheat a little bit here. Sometimes you can cure at a lower temperature for a longer time or a higher temp for a shorter time and get the same results. But you need to check with the manufacturer before doing this. You will need at least an oven big enough to hold your part and the mold it is made on. You might need an autoclave, which applies pressure as well as heat, to properly cure the material. I do not recommend using a home oven for curing parts. Too much nasty stuff comes off to think about mixing it with food preparation.

7) Let the part cool and remove the bag. Throw the bag materials away, they are not generally re-usable. If this was the aircraft biz the part would then go to Non-destructive Evaluation (ultrasonic and x-ray inspection) to see if it was any good. Depening on the location, voids or other problems as small as 0.25" dia can cause a part to be scrapped. But whatever you get will probably work just fine for FIRST.

8) Trim and drill the part to final dimensions. Trim with a high speed rotary grinder like a radiac. Carbide spade drills should be used for holes. These are drill bits designed especially for composites. You can use conventional bits, but then you are talking drills/hole rather than holes/drill.

As you can see this is an expensive process in terms of labor. The ancillary materials like the teflon release film aren't cheap either and the mold can be more expensive than all of the above. While things are better than when I started 26 years ago, there is still a sgnificant mount of "art" to composite fabrication. One giant hole in the cost rules is they don't address the cost of ancillary materials required for fabrication. Of couse if they did you could probably forget custom fabricated composites. This stuff just isn't cheap.

For work I just finished a graphite/epoxy test panel with a sandwich core about a week before Christmas. We used 1.5" thick titanium core. Ti core would certainly work for this application and it was the easiest core to work with that I have ever used. But the cost of the core block could fund at least one team for an entire season. So it is probably out of your price range.