Plastic Welding and Testing Poulan HDPE Fuel Tank

cprodave

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I am nearing my wits end trying to plastic weld a leaking black HDPE fuel tank on my Poulan 21inch self propelled model PR65Y21MA. This is a clamshell-type tank whereby the top 1/3 of the tank is apparently ultrasonic welded (or solvent cemented?) at the factory to the bottom 2/3 of tank. I got the mower used/free so am unsure how the first crack (3/8 inch vertical/perpendicular to the clamshell seam) developed. As I fixed the original crack I then noticed additional leaks along the factory weld. I am using a Harbor Freight plastic welder tool. I test for leaks by blowing my breath into the tank (fuel line attached to tank drain fitting) with cap in-place, spraying the outside of the tank with soapy water and then visually observing for bubbles. It seems no matter how I melt/move the welding rod/material, I "chase the leak" to another location. Whack-a-mole!!

On youtube (I know--be careful what you believe)I saw a guy whose technique seemed to make sense. First he melts at the leak location, then he adds material from the rod and "mushes around" the melted material at the leak location. Makes sense to me, but is there a better technique?

Also, is my soap bubble testing technique improper (too stringent, i.e. "false fails") for this problem? During my last failed test, there were 3 areas that produced bubbles. I filled the tank to the very top with water and sat the tank in a basin overnight. By morning there was approximately 1 Tablespoon of water in the basin, i.e. the tank leaked but very minimal. I could probably live with this minor leakage--just fill the tank to no more than 2/3 full before mowing, then if tank is filled slightly above 2/3 even with fuel "sloshing around" while mowing there should be little or no leakage/danger. Main caution would be to never store the mower with fuel above 2/3 level.

Any suggestions would be greatly appreciated.
 

bertsmobile1

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Plastic welding only works if the two parts are surgically clean.
I wire brush wit a stainless steel rotary brush, then degrease then solvent wash then acetone wash then blow dry.
The way it works is the heat activates the ends of the plastic molecules
However if there is anything on the surface the p\activated molecules will bond to it and not the filler rod.

The system is very much like soldering .
If the surfaces are not both clean & receptive then nothing happens no matter how much heat, solder & flux you apply.
Then there is the type of plastic itself.
Slight variations in the composition will prevent them bonding together.
I have limited success welding JD hoods that have gone brittle
Despite the fact that they are marked HDPE , they are a blend so I have to buy special JD filler rods.
Where as the petrol tanks are strait HDPE so they can be fixed with a lump of old milk bottle.

Trying to weld a seam is a very difficult job as the leaking fuel will have left deposits and even chemically attacked the plastic surfaces.
So you might have to V out the joint to get back to uncontaminated material.

From a cost position it would be a lot cheaper to weld the existing tank
From a effort perspective then replacing the tank is a no brainer.
 

cprodave

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Thanks to all for the inputs.

Yes I have considered replacing the fuel tank. Thus far I have been unable to even determine a correct p/n for the tank. I have repaired dozens of mowers/blowers/trimmers/chainsaws/outboards/compressors/generators with both Used and New parts but this tank p/n eludes me. If you find the p/n I would love to know what it is. However, my guess on Price is in the $30-$40 range which approaches the Market Value of this mower (want to resell it for $60-70).

Bertsmobile,
Thanks for the tips. I have already done some but not all of what you suggested. I will try again/differently and let you know results.
 

bertsmobile1

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When I come across an unknown item I clean a section the try to stick a filler rod on it.
Let it cool right down then try to break the rod free.
If it holds then I have the right rod
FWIW I now have 11 different filler rods.
Not as simple as it is made on on You tube and cable ties rarely work on anything.
 

Scrubcadet10

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Looks like That mower uses a Briggs Quantum. There are gas tanks (used but condition) on eBay for $20 < >
37$ brand new on amazon.
 

cprodave

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I am leery of buying a Used tank. Paying $37 (or more--if Shipping is extra) will cut into my profit margin too much --I want to resell this mower. IF I can't fix this 100% then it is probably best to find a buyer who understands and doesn' t mind running with tank 2/3 or less full.

Bertsmobile1, those are some great tips/tricks! Thanks very much. You seem to have more experience welding plastic (fuel tanks and "other") than anyone else I have found on this (and other) forums. Can you list more of the 11 types of filler rods you have used? Also, do you ever use a hot air welder or only soldering-iron type tool (such as Harbor Freight's)? Thanks again.
 

bertsmobile1

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I use both hot air & soldering iron type.
Like most people I started with a standard soldering iron with a nice new clean tip and still use it occasionally.
Next I bought a Kingchrome plastic welding kit which is just a soldering iron tip with a hole in it and a grab bag of different rods that don't actually work very well with anything.
I was getting better results with a std 2 heat hot air gun.
You can get a good job with both of them but it does require a lot of skill & judgement as to how much heat is in the plastic.

Following too many failures I bought a proper air heat gun with temperature control.
Not the best but I can now work without constantly worrying about melting the job into a useless blob.
After that , it was a better temperature controlled soldering iron type of gun.
Still have not mastered it but can do a strong enough job on most hoods to be able to reuse them

Mostly on Yardman hoods where they bolt on at the front & JD hoods where you rip out the lifting handle.
These are both types of polly carbonates, but they are a blend so the strait PC rods supplied with most cheap welders will not work for very long if they take at all.
JD & MTD both use Xenoy but they are different blends.
I got my rods from a mob in New Zealand, But then I am in OZ
They supply rods in both the correct JD green & Yardman yellow but warned me they are not a regular stock item so are only bached occasionally as orders arrive so I bought 5 kg of each
In the USA the go to people are Polyvance
Or Goodwoods.

The other tools I use a lot are a stainless steel roller and a silicon roller to get any air bubbles out.

How much kit you end up with will be a value judgement on your behalf but like everything the cheap & nasty kits are the hardest ones to use & do the worst jobs.

Most of the stuff on You Tube was bull dust and the description of a JD hood repair on the Polyvance page says it all.
Clean with the proper solvent
Prepare the surface with a CUTTING TOOL so you do not embed abrasive in the surface
Support the repair firmly

What they fail to mention is to walk away , hae a beer or two before you touch the finished job.
If you put a thick bead down it can take hours to cool down & set.

When I do JD tanks. I use thin strips of HDPE sheets rolled into the surface gradually getting bigger
I do only one pre day so a tank repair can take a week or better.

Down here a JD hood is around $ 200 bare & a Yardmachine hood is around $ 400 as you have to buy them complete.
 

cprodave

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Bertsmobile1,

Wow! that is a ton of useful information I can see you climbed quite a learning curve that would obviously take me quite a while (if ever) to catch up. I tried again using your previous tips/tricks but still No Luck. It is possible that I didn't allow adequate time to fully cool before pressure/bubble testing. I will either sell the mower as-is or (if buyer requires) install a new Tank--I do see one here in US for $37 USD including shipping. Your tips are useful for some other plastic items (that don't have to be liquid leakproof ) that I need to repair. So thanks again.
 

bertsmobile1

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What people forget is all thermo-plastics start off as a bag of pellets
They are just heated up then forced into the mould
If they get to the right temperature & ae clean they join to become a single item.
All that plastic welding does is mimmick the same process.
What I found out is most of the time I was getting them way way too hot.
Just right is when the surfaces turn glassy.
If you go much past they the plastic actually starts to burn ( well react with the air any way ).
With black t is hard to tell when you are going too far and the surface turns browny black.
Lke all of these things , practice is the key.
I have a really good working relationship with a JD dealer & he gave me a pile of busted up hoods to practice on.
When you are starting to burn them they start to give off the same smell as you get just before you burn a hole in your favourite shirt with the iron.
 

upupandaway

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I am nearing my wits end trying to plastic weld a leaking black HDPE fuel tank on my Poulan 21inch self propelled model PR65Y21MA.
The parts diagram says it has a briggs motor and have this tank - https://www.repairclinic.com/PartDetail/Fuel-Tank/699374/1567987?model=747339
R
I don't know about you, but shops where i live leave at least 1 a week complete with this tank out back that i can get the tank off of.....
Getting a replacement tank for my 40 year old Stihl trimmer is another problem.....
 
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