Let's talk plastics

[Paul Fithian]

Many have questions on "plastics" used in chainsaws and repair techniques. I wanted to start this thread to share my knowledge and help others here who have helped me. Most of my professional career has been application development of plastics in new applications, much of it with nylon. I'm one of they guys who convinced the US automakers to switch from aluminum to nylon for intake manifolds in the early 90's. Great fun, I spent several days per week for years at Ford Engine Engineering.

There are many types of plastics. For the most part, chainsaws use nylon for covers, shrouds, etc. Technically called polyamide, abreviated to PA. It is the least expensive option for parts that require heat, oil, and fluid resistance. It's the material used these days for injection molded automotive valve covers, intake manifolds, radiators, timing gears/guides, etc. Chainsaw covers and parts have similar requirements. It's the same material used in nylon apparel, which is why it is so easy to dye black.

To increase performance, it is often reinforced with glass fibers or minerals. Think of how straw makes a brick stronger. Designed and molded properly, it's an excellent material. Here's a mineral filled nylon carburetor that my 1976 Lawn Boy was built with. I still use it all the time to cut my grass, I've never touched the carb except for the tuning screw.


Here's the tank/handle from a G372XP I'm porting, note how it's marked :


Let's start with the weld seam, this one appears to be hot plate welded, like this:

Note material ID, this is found on a lot of parts made after the mid-90's, adopting the nomenclature used in the automotive world. Here's another example on the top cover for a G372XP:


Here's what that material ID means, PA6GF20
- PA = Polyamide, or nylon
- 6 means nylon 6, which is the most common type of nylon. Another type often used, especially in the automotive world, is 66, a different type of nylon with higher heat capability
- GF means glass filled, <1/8" length glass fibers compounded into the nylon. Think of fiberglass insulation strands, but real short.
- 20 means 20%, or 20% of the weight of the part is glass fiber. This is used to stiffen and reinforce the molded part

OK, so how can we fix one of these that breaks? Here's an example of PL Premium used to fix a cracked cover on my SIL's MS 251:


For nylons, PL Premium is an outstanding choice of adhesive. It's a moisture cured urethane that expands slightly when curing, filling any voids in the joint. It also is impervious to oils/solvents after 24 hours of exposure to humidity in the air. It's the same basic material used for truck bed liners, very tough stuff.

 

[ammoaddict]

So you are partly responsible for the plastic intake on my 5.4L F150 that cracked and cost me a lot to replace? Thanks a lot man.

 

[Paul Fithian]

What year/model?

 

[Shane¹]

So you are partly responsible for the plastic intake on my 5.4L F150 that cracked and cost me a lot to replace? Thanks a lot man.

That's your fault for buying a Ford... Those intakes did have some issues definitely changed more than 1

 

[ammoaddict]

What year/model?

2002 F150 5.4

 

[ammoaddict]

That's your fault for buying a Ford... Those intakes did have some issues definitely changed more than 1

I agree. I only bought it because it was a nice clean truck at a good price. But I have spent a lot in repairs on it.

 

[Paul Fithian]

2002 F150 5.4

Didn't work on that one. Can't comment on what others did after me.

Note my earlier statement above:
Designed and molded properly . .

 

[chiselbit]

This is good stuff. Thanks Paul. I tried to fix a leaking wedco gas jug the other day and ended up making it worse. I was using a woodburner tool , the kind that heats the tip and then you can draw or write on wood,, and I tried melting some weed trimmer string to fill the gap. Since I’m not a welder of any sort and know nothing about it, what would have been the best way to fix a puncture on a gas jug?

 

[Paul Fithian]

Thanks CB!

A Wedco gas jug is polyethylene. String trimmer line is nylon, which melts at a much higher temperature than polyethylene. That’s like trying to weld aluminum with a steel rod - doesn't work.

I don’t have a suggestion for repair of that jug, except to discard it and get a new one.

 

[drf256]

Great write up Paul. I didn’t know what those designations meant, though I saw them commonly on AM plastics.

So PL premium is pretty much the stuff to use for Plastic saw repairs?

 

[drf256]

@MustangMike

 

[nbbt]

This is good stuff. Thanks Paul. I tried to fix a leaking wedco gas jug the other day and ended up making it worse. I was using a woodburner tool , the kind that heats the tip and then you can draw or write on wood,, and I tried melting some weed trimmer string to fill the gap. Since I’m not a welder of any sort and know nothing about it, what would have been the best way to fix a puncture on a gas jug?

I've used coffee containers before. The red Folgers ones are nearly the same color as a gas can and have worked well for several repairs. Some have held up for at least 3 years and counting...

 

[Paul Fithian]

Thanks Doc! I've used PL Premium in a lot of applications, adhesion depends on the material. For nylon, wood, PVC, it is excellent. It doesn't work with polyethylene, Delrin, or acetal molded parts.

Make sure the parts are cleaned thoroughly and degreased - I use acetone. Parts also have to clamped for 24 hours to prevent the part expanding when the PL Premium expands while curing

 

[Paul Fithian]

I've used coffee containers before. The red Folgers ones are nearly the same color as a gas can and have worked well for several repairs. Some have held up for at least 3 years and counting...

Those red Folger cans are polyethylene, same material as the fuel jugs. It can be welded together with the right technique.

 

[Paul Fithian]

Another production option for welding the fuel tank parts together is vibration welding. These typically have a flange on the outside perimeter, the vibration welding machine grabs on to these flanges and moves the two parts against each other to create frictional heat, melting the nylon. Most intake manifolds are made this way.

Here's video showing how its done on a chainsaw tank

Not sure when Stihl started using this technique to make fuel tank/handle parts, the 024 AVS I purchased new in the mid 80's was done this way. It was at least a decade before the automotive world adopted it for intake manifolds. It was used to demonstrate this was a viable process, as a chainsaw typically has a much rougher life than an intake manifold on a car.

 

[Loony661]

I’ve melted the black zip ties for filler material to repair Powersports gas tanks before, specifically on a snowmobile. Worked great.

 

[Paul Fithian]

I’ve melted the black zip ties for filler material to repair Powersports gas tanks before, specifically on a snowmobile. Worked great.

Those zip ties are molded out of toughened nylon, and would work on a nylon tank with the right technique

 

[MustangMike]

I've been advocating using PL Premium for repairs for several years now. Not only is it a great construction adhesive (used on wood), but I've used it on the chainsaw milled wood projects I've done (tables, benches, gun cabinets). It is great stuff.

On chainsaws, I've not only used it to repair plastic parts but also to repair broken cases, etc. I've also successfully repaired steel file cabinets with them (the draw hinge came off ... flat metal to flat metal). For non-porous surfaces, sand a bit to make it rough if it is a smooth surface and always slightly dampen non- porous surfaces. I also used it to repair a broken recoil cover.

Thanks for the excellent information on the plastics, I did not know what that stuff meant.

PS - My plastic chainsaw handles have never broken, and the plastic intake on my 2006 Mustang GT (running a supercharger with 550 Hp) has never failed.

 

[Paul Fithian]

Cool @MustangMike ! In all the cars I've had, I have never had an issue with a nylon intake manifold. Maybe I've had cars where it was done right?

I've not had good experience with PL Premium metal to metal.

I go to epoxy for metals, like bonding a golf club shaft to an iron head. When I used to golf . .

 

[MustangMike]

Likely depends on circumstances that are beyond my expertise!

My file cabinet metal to metal repair has held up for years, and the chainsaw recoil + case repair (on a MS 460 for a tree pro) help up for years with daily use. His climber had dropped the saw out of a tree. As far as I know, the repair is still good. I'll see if I can locate my old pics later, I did post them when I did the work. (The repair was not pretty, but it worked).

I used a "good" recoil to "clamp" the case in the right place when repairing the case, then screwed the broken recoil to the case when repairing it. I cut a piece from another broken case to repair the broken section of the case. It was real "Frankenstein" stuff! I did not just put PL Premium in the seam, but also thickened the case with it in that area to give it extra strength.