Plastic bumper repair

So. The BMW took a bit of a bump on the front yesterday (not me, don't ask) and the bumper cover/spoiler is cracked. The pieces fit together very tightly, to the point where the cracks aren't really visible if I hold them together in a couple of strategic spots. On the backside of the bumper, beside the BMW part number, it says >>ABS<<. So I'm guessing it's made of ABS plastic

Anyone have experience in repairing something like this? Since the fit is so tight, I'm wondering about using model glue to stick it together with a couple of riveted-in metal straps on the backside to provide extra mechanical support at the ends of the cracks - they'd be hidden behind other plastic panels.

Thoughts?

Model glue works by the action of solvent softening the base plastic enough so that the edges melt together, it's really 'solvent welding'. You must determine if the ABS is affected by the solvent in the glue, test it behind where it wont show. Alternative is to look in Home Depot/Lowes plumbing dept for ABS specific solvent. If you are very careful with application, ie use a Q-tip or small artist brush to place it only on the crack edges, the crack repair may be nearly invisible.

I know with softer bumpers, you can buy the rubberised stuff that bonds bumpers.. worked well on the soft plastic of my saab.

BMW's use a more rigid plastic that tends to crack easily (Trust me, I know) Model glue might hold up for a while.. what you might want to do is get some plastic and reinforce it from behind as well. While the glue joint will be plenty strong.. it will be more brittle than the surrounding plastic.. so if you can reinforce it, it will last longer.

If you can't find the ABS specific glue at the big stores, try a mobile home repair place if you know of one. ABS is not used in regular home plumbing, but older mobile homes used it for drain lines.

I've got some model glue left over from a previous build, and it specifies ABS and polystyrene plastics. I've just done a couple of test fixes on some of the cracks and it does seem to be gluing like model glue does - as erohslc said, it melts the two pieces together. So far, so good. I'll see how it looks in a couple of hours. It certainly is a brittle plastic, a very different stuff from the soft flexy covers you usually see.

I'd like to reinforce the backside with some sort of epoxy. I'll see what's out there and I'm open to suggestions. I have a few, like PC-7 and the usual JB Weld options in the toolbox. I'm tempted to try the former.

For anyone else who's looking, there's some basic information in the April 2009 Car Craft and a good chart on discovering what kind of plastic you're dealing with at Urethane Supply Company. Of course, finding that big ABS stamp on the back really helps...

There is epoxy at the hardware store here that says its meant for plastic bumpers. I think it would be appropriate for reinforcing behind the cracked area after you glue it.

I could swear that Tommy was doing a similar repair in a semi-recent issue. Or maybe an online article.

make sure you pay attention to what kind of epoxy you use - some will not bond to plastic at all, and others will dissolve it if you use too much hardener. You can melt HDPE plastic (grocery bags, laundry soap containers etc) on the stove top if you cover the container to keep air out - it melts at I believe ~285°F. You could smear a bit of that onto the Glued joint - it should bond to ABS pretty darn well.

You can use some glass cloth with whatever will bond with the bumper. I've had decent luck (not on bumpers) using PVC plumbing cement as it has a fair amount of MEK in it and will act as a solvent for a lot of plastics. It also has enough "body" to it that it can saturate the cloth and bond it sufficiently.

Any auto body supply place will sell a tube of plastic bumper repair epoxy. It comes packaged in two plastic syringes that are bonded together. You push the plunger and it dispenses equal parts of the two part epoxy. Mix and spread. Works great. I used it on my wife's Mazda after I forgot to set the ebrake and her car rolled into the tailpipe on my truck.

mistanfo wrote: I could swear that Tommy was doing a similar repair in a semi-recent issue. Or maybe an online article.

Seems to me I remember that article. Not sure if Tommy was doing the work but it was in GRM earlier this year. Almost made me want to take my bosses old 500SL with a cracked under-bumper ground effect and try to fix it.

tr8todd wrote: Any auto body supply place will sell a tube of plastic bumper repair epoxy. It comes packaged in two plastic syringes that are bonded together. You push the plunger and it dispenses equal parts of the two part epoxy. Mix and spread. Works great. I used it on my wife's Mazda after I forgot to set the ebrake and her car rolled into the tailpipe on my truck.

that works well on soft bumper cars.. BMW's use a very stiff and brittle plastic on their bumpers. When I hit a dog not long after getting my Ti.. I shattered the bumper cover

wlkelley3 wrote:

mistanfo wrote: I could swear that Tommy was doing a similar repair in a semi-recent issue. Or maybe an online article.

Seems to me I remember that article. Not sure if Tommy was doing the work but it was in GRM earlier this year. Almost made me want to take my bosses old 500SL with a cracked under-bumper ground effect and try to fix it.

Yep, I did the work and wrote the story. It was a couple issues ago. I'll try and dig it up.

Thanks Tommy. I thought I remembered a GRM article, but when I found the Car Craft one (I subscribe to both magazines) I figured that was the one I was recalling. If you know what month it is, I'll dig it up. How's the repair holding up?

Tommy had a pro do it (the bonding part) IIRC. The two part bonding agent is really the way to go. The rest of the finishing process can be done by mortals. I really, really doubt the model glue will prove adequate. For most of my model construction these days, if I want something to actually stay together I use an ACC adhesive like Zap A Gap. Styrene glue really isn't very strong or durable. While the fusion theory is it's basis, the level of 'melting' is really pretty small. I've had too many projects fail after transporting to have much faith in styrene glue.

Update: plastic bumper repair epoxy is the bomb. Make sure you've got some good ventilation going, but it allowed me to reassemble the bumper perfectly. I've also used it to reattach a couple of broken tabs and whatnot that were possibly not related to this incident. Definitely a better choice than model glue. I put one metal strap on the backside of the repair but I don't think it was needed at all. The resulting piece feels rock solid.

Next, I have some good filler for, umm, filling in the scratches. We'll see how that goes. Then the paint.

In reply to Keith:

We specialize in plastic welding... There are various tools you can find online. If you check out our website http://www.ultragloss.biz we have some pictures of the type of repair you're talking about... With cracks in the bumper. Or you can view our facebook page with some more recent images http://www.facebook.com/ultraglossamerica . Feel free to e-mail us and we can give you some more information about what tools, etc...

Keith wrote: So. The BMW took a bit of a bump on the front yesterday (not me, don't ask) and the bumper cover/spoiler is cracked. The pieces fit together very tightly, to the point where the cracks aren't really visible if I hold them together in a couple of strategic spots.

Dude - zip ties for that super tyte jay dee emm doridori dorifto king look!

Thanks for the offers of help, but I fixed it last summer :)

So that means this was just a canoe trip?