mx bikes with tank plastics does anyone know how to repair these

[heartlandcruisers]

as i am sure alot of you have heard i currently am the proud owner of a heavily used flying o qualifier bmx that needs some love.
i got it looking ok and i added some wheels that give it an agressive look however, i found an issue i need to adress.
the plastics are cracked in several places aong the top and now i just found another one on the side.
the only reason i am rebuilding this bike is for the plastics so does anyone know how to repair these or even have one they would be willing to part with?
i tried to jb weld the first crack and it looks worse and the jb weld did not hold so i need to repair it again.
another option i thought of was use the originas as a mold and make a fberglass replacement but i am not sure that would work.
so if anyone knows how to repair these or has any ideas let me know the cracks are clean and even so repairing could be done i assume but thats why im askign i do not know.

 

[RustySprockets]

I don't know this bike specifically, but I suspect the parts you are trying to repair are made from a thermoplastic such as polypropylene. These plastics are tough and resilient but, as you've discovered, once broken they cannot be bonded by glues--heat is needed. There are "plastic welding" kits available to do this job, but the results can be unsightly, unless you practice.

Were it me, there are a couple of approaches I might try. First, you can buy plastic weld rod cheap ($5) from Harbor Freight. I don't own the proper tool, but would attempt to melt this sacrificial rod into the the repair using a low wattage soldering iron. Doing this on the back side only will help keep the repair hidden. As an alternative, one might cut the rod into short lengths and chuck it into a Dremel, then use friction to melt the rod against the damaged area. Rumor has it that a hot glue gun will sometimes work, too, especially if the fix is just cosmetic.

Bottom line: heat must be used to recombine the plastics in the affected area.

 

[RustySprockets]

 

[horsefarmer]

BITD I used to use woodburning tools or a soldering iron to melt the plastic "trees" that came with model car kit parts to "weld" plastic - sometimes worked.
Use with lots of ventilation (outside) tho!

 

[heartlandcruisers]

I don't know this bike specifically, but I suspect the parts you are trying to repair are made from a thermoplastic such as polypropylene. These plastics are tough and resilient but, as you've discovered, once broken they cannot be bonded by glues--heat is needed. There are "plastic welding" kits available to do this job, but the results can be unsightly, unless you practice.

Were it me, there are a couple of approaches I might try. First, you can buy plastic weld rod cheap ($5) from Harbor Freight. I don't own the proper tool, but would attempt to melt this sacrificial rod into the the repair using a low wattage soldering iron. Doing this on the back side only will help keep the repair hidden. As an alternative, one might cut the rod into short lengths and chuck it into a Dremel, then use friction to melt the rod against the damaged area. Rumor has it that a hot glue gun will sometimes work, too, especially if the fix is just cosmetic.

Bottom line: heat must be used to recombine the plastics in the affected area.

ugh that's what i was afraid of it makes sense now that you described that i will take it to a guy that plastic welds and have him fix it up i know it will cost but this bikes worth it to me thanks for the info rustysprockets this is the bike that needs the plastic work done the "gas tank" area is cracked and the side where the "tank" meets the rear part has a crack in it back in the day these bikes were ridden hard so im surprised it is in this good of shape

 

[Raoul Dude]

Were it me, there are a couple of approaches I might try. First, you can buy plastic weld rod cheap ($5) from Harbor Freight. I don't own the proper tool, but would attempt to melt this sacrificial rod into the the repair using a low wattage soldering iron. Doing this on the back side only will help keep the repair hidden. As an alternative, one might cut the rod into short lengths and chuck it into a Dremel, then use friction to melt the rod against the damaged area. Rumor has it that a hot glue gun will sometimes work, too, especially if the fix is just cosmetic.

Bottom line: heat must be used to recombine the plastics in the affected area.

This and the video immediately took me back to a "toy kit" that Mattel sold in the 1970's -- the Spinwelder!

There are plenty of articles (and videos) that can be found with Google -- just search for "Mattel Spinwelder" -- but the concept was pretty much identical: a short length of sacrificial plastic rod that was inserted into a battery-operated Dremel-like tool and used to weld plastic girders and sheets together to actually BUILD model cars, helicopters, and skyscrapers.

Like most cool toys from the 1970's, this one was potentially dangerous, what with melting plastic and tiny parts, but it was fun AND educational. The only "problems" that I can remember was the short learning curve between "not enough pressure to actually melt the rod" and "too much pressure and burning a hole through the material being welded" and the bigger problem of "JUST enough materials and welding rods to build about one and a half models." Part of the fun was that you could then tear the finished product apart and re-use the parts -- or better yet, buy more parts and "welding rods" from Mattel -- but my parents were too smart (or too cheap) to fall for that marketing ploy.

The Discovery Channel apparently reintroduced something similar and sold it on their website and via Amazon -- there are Amazon reviews from 2009 and 2013, but it's currently unavailable.

Now I want to buy some plastic welding rods and build something again!

 

[CrazyDave]

Plenty of plastic welders out there, they are fairly inexpensive and work fine. I would practice on something else first and get the hang of it...

 

[KJV]

5 second fix.
Usually in , as Sen on tv section, of most stores.also, marketed as,,
Lazer Bond USA.

 

[Duchess]

It is most likely a weldable thermoplastic. It depends on the type of plastic and how badly age (UV, in particular) may have made it brittle over the years how easy it would be to repair. I have the electric style welder which is kind of like the result of if a soldering iron and a glue gun found themselves alone in a barn one night in a thunderstorm and however many months later that these things gestate for, gave birth to a new species of tool. Anyway, I'm not an expert, but I have welded polypropylene, ABS, and polycarbonate. The latter I find to be a real pain as it is very temp sensitive (didn't help that it was a custom lens replacement for an "aero" headlight lens that also required forming the polycarbonate lens in a small compound curve). The polypropylene was easy, but has a kind of greasy feel and the ABS was pretty straight forward, though it's been years and I forget the particulars of that one. None of the plastics I welded were old and devoid of their elasticizers, which I imagine would make it trickier. The weld rod comes in all kinds of colors, too, though a new color probably won't match something old and UV-damaged. I like to grind a V shape into the back side of the crack, then fill the V with the weld. If you get it just right, you might not even see much of any evidence on the visible side. Of course, it you paint, that doesn't matter.

 

[horsefarmer]

5 second fix.
Usually in , as Sen on tv section, of most stores.also, marketed as,,
Lazer Bond USA.

Have you used that stuff? I've seen the ads and wondered if it is any good, online reviews say it does not work.

 

[KJV]

Not personaly , yet.
Have sen, before/after result..
On a repair , where a friend used it.
Just like anything else: proper prep is key.