Thanks to all for your help and suggestions on cf layup. I tried playing with
it
this weekend on a simple item, and it seemed to have turned out okay. Not the
best
looking, but this was strictly a trial, and I didn't have the vacuum bag setup
either. I also have not tried trimming the final part, which I should at least
practice before I try my "real" project. From the sounds of it, I may be best
off
using a cutoff wheel and a sander to do the final finish. FWIW, I got my
materials
locally at a place call TAP Plastic; they were very helpful. My layup consisted
of medium wt. cf fabric (single layer) and epoxy resin; I plan on trying vacuum
bagging next, using an old refrigerator compressor as the vacuum source (I don't
think my shop vac is going to cut it.....).
Mike
From: Keith Kaplan <keithka@microsoft.com>:
>(I don't do composites for a living, but I did a bunch in school, and a
>friend of mine is a full-time composites engineer at a local aerospace
>joint. What follows are my opinions, hard facts would require that I
>consult with the expert who's had his 240z taken apart on my lift for the
>past 3 months.)
>
>Prepreg is the only way to go. The beauty of prepreg is that just the right
>amount of resin is already very evenly distributed, and the resin was very
>accurately measured out and well mixed, too. It's also much less messy than
>wet layup.
>
>The vacuum bag helps get the fiber pressed into exactly the shape of the
>mold, and also makes sure the various layers are pressed together tightly
>enough to bond well. All you need is some heavy clear plastic, something to
>the seal the edges with (weatherstip, or better yet that zinc chromate putty
>strip stuff often sold as "roll caulk") and a vacuum pump.
>
>A vacuum pump can be one of those venturi deals you use with your
>compressor, or, like mine, a bastardized compressor from an old
>refrigerator. (The freon had been emptied in an EPA approved manner.)
>
>Autoclaves are fun, but not necessary for home use. Obviously, they require
>that you vacuum bag the part, or else all the pressure in the world isn't
>helping you. If you get the bag sealed well and pull a full 14.7 psi
>vacuum, that's really pretty good. Understand that if you have any kind of
>honeycomb core in the part, you may need to go easy on the vacuum or else
>you'll crush the core.
>
>Now all you need is an oven to cure parts in. This can be an old toaster
>oven for small parts. For larger parts, build a box out of rigid styrene
>insulation and cut a hole in one end to stick a heat gun in. Make sure you
>don't melt/ignite the insulation, and yes this duty does wear out heat guns.
>
>Parts can be trimmed with sabre saw, die grinder, band saw, etc. Unless the
>part is really thin, or you're a real he-man, tin snips/aviation shears
>won't cut it.
From: "John Mikes" <jmikes.gt4.38@worldnet.att.net>:
>You might try www.bryanf.com/
>There's a link to his carbon figer page on the left column.
From: Todd Walke <racertod@ix.netcom.com>
>I haven't used them yet, but I have a catalog from Fibre Glast
>(http://www.fibreglast.com/). They sell all the materials and tools you
>need.
From: "Simon & Heidi Young" <mothra@bigpond.com.au>
>> I'm wondering how similar handling this stuff is to dealing with fiberglass.
> I'm
>> comfortable with making a mold for fiberglass stuff, and I imagine that the
>process
>> is similar for cf. The layup seems to be similar too, except I don't think
>I'd be
>> using a gelcoat, and would start straight off with an epoxy resin (gotta let
>'em
>> know it's cf... =8^).
>
>Lay-up is like fibreglass, lay out rag and squeeze resin through it before
>rolling into mould
>
>>Except for small pieces, autoclaving would be impossible for me; is this
>>always necessary, or can I avoid it by using pre-preg cloth. I'd think
>>it's not absolutely necessary, as Aircraft Spruce doesn't mention the
>>need, and they
>>are catering to people putting a lot more stress on the finished product.
>
>
>Autoclave is used with pre preg to activate the resin already in the rag,
>with wet lay-up autoclave is not needed.
>
>
>> I also wonder if vacuum bagging is absolutely needed if I stipple the resin
>> enough; I believe the reason is to use less resin and save weight, but I
>think the
>> trade-off in complexity would cause me to avoid it.
>
>Vacuum bagging is nice but not vital, roll well to force the air out and
>squeeze the resin through. Finish isn't as pretty but adequate. Backing
>cloth can be used to pull excess resin out as the part cures.
>
>> Finally, I'm wondering how the parts are
>> finished trimmed. Most of the commercial parts I've seen look like they are
>finished
>> trimmed after they are made; I don't see how the parts could be molded that
>accurately.
>> What is used to trim the parts? I'd think a saw would leave a messy cut
>line,
>> and I don't think scissors would cut it....
>
>Trim roughly while wet, good scissors will do it. Finish trimming with a
>roll or line sander or just sand by hand. Avoid drilling large holes, use
>a rotor-broach where possible. Snips can be used to rough trim hardened
>parts but leave plenty of meat to sand as they tend to split the edges.
From: Smith Timothy M CIV AFRL/VASE <Timothy.Smith@va.wpafb.af.mil>
> Your best bet is a wet lay-up, not pre-preg. Pre-preg is primarily used in
>the autoclave process. It requires the substantial and prolonged application
>of controlled heat. Unless you have an oven that can handle the size of part
>you're making, pre-preg is out. In addition, the inherent stiffness of these
>materials makes the use of an autoclave essential for any but flat or simply
>curved parts. You could successfully process pre-preg flat panels with a
>simple oven, a platen under the part and a platen on top of your lay-up
>with weights on top of that.
>
> The "pros" process pre-preg in an autoclave. The lay-up is covered with a
>bleeder cloth to soak up any excess resin during the cure. It is also vacuum
>bagged. The vessel is pressurized ( typically with nitrogen) and the
>temperature is controlled via computer. Thermocouples are installed in the
>laminate at points which will be trimmed off later. This ensures that the
>lay-up cures throughout it's thickness.
>
> Even with a wet lay-up, vacuum bagging is recommended to keep the stiff
>cloth conformed to your mold. However, you may be ok with the copius use of
>weights! You may be able to find an "open" weave of cloth that will conform
>more easily to compound curves.
>
> These materials are difficult to cut or machine-especially once cured.
>Commercially, abrasive CNC waterjet cutters are used to trim the finished
>parts. Diamond saws are also used extensively. For a low-buck operation,
>carbide tools may be your best bet. Drilling is best done with special spade
>drills that minimize fraying or delamination of the far side of the hole.
>Final finishing may best be accomplished with standard abrasives.
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