Dash01

Anybody used carbon fiber cloth and epoxy to fix a cracked front bumper cover?

Does epoxy bond well with the Porsche bumper cover plastic?

I have a SEM kit for fixing cracked bumper covers, but the cloth reinforcement frankly does not seem very impressive.

So, having some carbon fiber cloth left over from a composites class I took, and some West System marine grade epoxy used for boat building and repairs, I wonder about using that stuff on the back side of the bumper cover. Hence the questions above.

Rich Sandor

Its the epoxy thats going to crack or delaminate wayyyy before the backing material strength becomes a factor

Dash01

That's one reason I'm curious about marine grade epoxy.

The stuff the car parts places sell, like SEM, instruct to slather it on pretty thick, reinforced by not much fiberglass tape. The tape is open weave stuff which looks like that used for drywall. In other words, resin:fiber ratio is high, which means a stiff, brittle section of plastic with not that much in it for strength, and, being thick, not very flexible.

Everything I learned in composite construction of homebuilt aircraft and boats says just the opposite, i.e., to have relatively less resin and more reinforcement. That way, it will flex rather than break, with much better strength. The repaired section is typically stronger after repair than before it broke.

OTOH, the car repair kits are designed to act as both structure and Bondo-like sandable and formable stuff to smooth and paint so it does not show, i.e., may have cosmetic advantages.

Unfortunately for me, this is a ~4" break in the bumper cover left upper corner, that extends to the edge. I drilled a ~3/16" hole at the inner end of the crack to relieve stress, and will overlap the crack by at least 2-3" on either side with the repair patch FWIW, fiberglass repairs in light aircraft and boats typically entail reinforcement of 20 or more times the thickness of the part, so if the part is 1/10" thick, use a 2" backing, etc.

Perhaps I'll apply a patch of carbon fiber and marine grade epoxy to an unbroken part of this bumper cover, and test its adhesion to the plastic.

What type of plastic is this bumper cover? It may be PUR, but does anybody know for sure?

Ronin-951

Not sure what it's made of, but I'd give Plasti-Welding a shot before any resin based repair.

Dash01

I don't think that works with this particular material. I'm pretty sure this stuff has internal fiber reinforcement, unlike some plastics, which can be heated and melted together and so rewelded.

3M makes a patch kit that reportedly works. Not cheap, and you have to buy a box of a dozen or so, as I recall.

Years ago, Burt Rutan of homebuilt aircraft fame told us he'd seen patches done by the Australian air force on some of their Mirage jets, which he said had developed hairline cracks in the skin of the wing root, a highly stressed part. He said they didn't even drill holes for stress relief, but put a carbon fiber patch on the back of the aluminum skin, bonded with epoxy, and it was stronger than the original new skin had been. However, any such patch depends on very strong adhesion between epoxy and skin, which must be surgically clean and chemically compatible.

I don't know if West System marine grade epoxy has such compatibility with the plastic in these bumper covers, hence my post above. What works, what doesn't?

CurtP

I use Lord Fusor 101EZ for plastic repairs. If it's a tear or rip, I don't use any backer. If it's structural (e.g., mounting point) or if it's an excessively long break, then I use a backer of the same material. I rough sand both the backer and back of the repair with 40 to 60 grit.

Even though it says heat set, I allow it to slow set at room temperature. It can be accelerated with a heat gun if desired. Clecos make the job a lot easier.

http://www.lord.com/products-and-sol...roduct.xml/298

YZ250

I've successfully used regular fiberglass and expoxy from the auto store.

Dash01

Good. Thanks. How long has it been on the car, without delaminating?

Grandpa#3

Plastic Weld Kit will do the job or you can use what is linked below.

http://www.crosslinktech.com/product...pound-kit.html

Cheers,
Larry