Repairing Polyporpylene

Groundhog

Groundhog

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A brass (?) thread insert pulled out of the plastic air box on my motorcycle. (Pics #1 and #2). I think the air box is made from polypropylene judging from the oily like texture and the "PP" in the manufacturing codes (Pic #1) (any idea what all the symols, letters & numbers mean?). (Side Note: My camera kept going to portrait/face recognition mode while taking this picture!)
I attempted a repair and ground a depression around the top of the hole to give the glue more contract area. The adhesive I used didn't hold at all - all of its data must be on the packaging that got tossed 3 months ago when I bought it. After I discovered it wasn't working too good I looked it up on-line. Instructions say NOT to use it on polypropylene. Go figure.
The adhesives I've found that say they will work with polypropylene are pretty expensive and not available locally. $20 up to $65 for a minimum amount. The air box is about $60, but I bet it will have to come from Italy and I don't want to wait that long to get the bike back together.

My only other thought is to make a similar insert from aluminum only a few thou bigger (and knurled). Heat the sucker up and melt it into the existing hole. But I'm afraid of trying that because I think if I get it too hot it will melt too big of a hole and then I'm really screwed.

So, anyone have any other good ideas?

Pic #1
0625.JPG

Pic #2
0627.jpg

molding codes / data
0646.JPG

Insert and corresponding screw M5x.08
0630.jpg

air box, pulled insert and 3rd screw go in the vicinity of blue circle
0643.jpg

Thanks!!
 
I'd try hot melt glue - not much will stick to PP! Get the brass insert ouchy hot and part-fill the hole. assemble while the glue's still liquid? If the worst happens and it doesn't stick the hot glue should peel out pretty easily.
Alternatively, a sleeve nut / "sex bolt nut" fitted from inside, that would have the flange to spread the load (if you have clearance inside the airbox)
I had a similar problem on my ZZR's airbox, one of the rain-drain pipe nubbins broke off, I ended up hot gluing a piece of ally pipe in its place, which has lasted about 5 years so far...

Just my ha'pennorth
Dave H. (the other one)
 
Forget any kind of glue unless you work for NASA. The hole could be welded up with pp and tapped but this would only return things to the orignal weak design.
Inserts are the way to go, what about Riv-nuts?
 
What about maybe trying the harbor freight plastic welding kit. maybe you could use one of the rods as a glue not sure exactly if it would work I have one but haven't used it but they sell a pack of the welding rods for 5.99 and it says in the description they have PP rods which is think is polypropylene. just an idea maybe worth a look
 
In my experience with welding plastic, when the failure is due to inadequate design, welding it up is only a VERY temporary fix. If the same plastic failed once under normal use it will fail again.

It does not take much to exceed the strength of plastic parts especially PP ones. If the over stress is accidental then welding can be a valid fix. But I do not see any point in reusing the original insert if it failed once already. Again, what about Riv-nuts?
 
I'd probably go with a length of steel or aluminum that bridges the hole and is pop riveted or screwed in place.
If steel a nut or threaded barrel could be welded to that plate in the original position.
Seal the plate to the box with right-stuff.
 
I know of no adhesives that work with polypropylene, polypropylene co-polymers, the polyethylenes, Nylons, or acetals.

I usually repair break by welding using a soldering iron. It doesn't work as well as I would like because the iron isn't the best geometry for building the weld. I try to get to the root of the break and use a weaving technique to build up weld on either side. The other reason it isn't as good as I would like is the lack of filler material. Some material is always lost in the welding process so the build up isn't enough. If I can scrounge filler.material from a non critical area, I will and that helps. They used to sell plastic welding tips for soldering guns that had a flat blade for the surface smoothing which would help.

Generally though the welding process works and the item is put back into service. I have welded some high performance parts like fishing reel bail levers and trolling motor propellers. It is often the only way to salvage a failed part.

In the case of your failed insert, I would most likely try to replace it with a custom insert. If the back side is accessible, I would put the insert in from that side. Here is a fairly simple design to make. It has a relatively large flange on the back side to resist pull out. a rebated middle section which has a square cross section to resist rotation and to key the insert in place. The rebating isn't critical. Some hand file work would do the job. I would put the insert in place and backfill with polypropylene weld. All of the adhesive used in the previous repair attempt should be removed.

If you only have front access, a similar approach could be used. Just reduce the diameter of the back flange. The welding procedure would be the same. A little more tricky holding the insert in place but with patience, achievable.
Plastic Insert.JPG
 
The original insert seemingly failed due to over tightening (electric impact / screw gun?). The insert had obviously been spun in the plastic, then there was evidence of a thread sealer repair attempt.

This is the cover you remove to get access to the air filter so it may have been used more often than most other plastic parts. The insert is surrounded by a lot of plastic in the form of a "post" inside the air box. I believe that the original structural design was very good

Riv-Nuts are the similar to the existing (usable) insert or similar to anything I would make out of aluminum - makes RJs design worth a hard look.

I know that I've seen/read about some new hot glues that work with hard to bond materials but didn't really pay attention at the time. With just a quick search I haven't found any hot glue to work with PP. Maybe I need to look a little harder here if my new plan fails.

There is no way to access the inside diameter of the hole to get a good plastic weld on it and very limited access the inside of the airbox. Although I might use a plastic welder or soldering gun to fill in the bevel I cut yesterday.

Proposed fix - if anyone finds a flaw with this or has a better idea please let me know - thanks.

I just found this stuff that claims to be good for PP; http://www.loctiteproducts.com/p/sg_plstc/overview/Loctite-Plastics-Bonding-System.htm. sg_plstc.png
This Loctite plastic kit is in stock locally (Menard's and True Value) for $5.
So, I'm going to try the Loctite Bonder in conjunction with the original insert or a bastardization of RJs idea and an aluminum knurled thingie. I may try mixing some PP sandingdust (or baking soda?) with it to thicken it up. Just need to get out and buy the adhesive and do some testing.
But I need a nap first. I stayed up all night watching the F1 race from Melbourne, then went out & tinkered with the bike. I'm way too old to do that sh*t anymore. Both Hass cars dropped out mid-race due to wheels not being secured properly after pit stops. Bummer.
 
A: repair will only return to original weak design, as mentioned above
B: hot melt works rather well if enough heat used ....u kind of mix the original surface with the hot melt glue Have had lasting seal/repair that way.
C: make a better fitting !!
 
RJ,
We did a lot of plastic welding at the ski areas. Chair lift seat covers, ATV fenders & bodies, snowmobile parts, sno-cat tiller covers, etc.
I was never very good at it but one of my guys was a pro. Besides a professional plastics welding setup and a supply of different types, diameters and colors of plastic filler rods he would form copper wire (10 ga - 12 ga?) to the shape he needed and bend the 2 legs to fit a soldering gun. I always thought his custom building soldering tips was a great idea.
 
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