# Plastic welding a small fuel tank......



## brino (May 12, 2020)

Hi Everyone,

A few weeks ago, after I tilled the vegetable garden, I went to put the roto-tiller away in the shed and noticed it was dripping fuel.
I am sure it was NOT doing that while I was tilling.
The tank is fairly well protected behind a "bush bar".
I do not know the cause, but lets see if I can get it fixed.


The Crack:
=======

Here's a few shots of the crack from different angles.....

























Since the crack is right at the corner of that flange, I wonder if it's due to a defect or stresses in the original plastic moulding.


Step 1 - Research
============

The fuel tank is marked "HDPE" so I know it is High-Density Polyethylene.
I just happened to have some filler rod for that:



Princess Auto here carries several different plastic types for their repair kit:
https://www.princessauto.com/en/plastic-welding-kit/product/PA0008699134
https://www.princessauto.com/en/10-pc-polyethylene-plastic-welding-filler-rods/product/PA0008709982

I talked to a buddy of mine that had plastic welded a fuel tank on his outboard motor a few years back.
He recommended hot air gun and filler material from the insulation around the middle conductor in coax cable (no kidding!).

I also watched a bunch of youtube videos and did some internet searches for the range of tools available.
The videos and tools for sale were either fancy tips for a soldering iron, or a hot air blower.


Step 2 - Tool-Up
===========

Based on some of the tools I saw online I built a few new tips for my big-arse soldering iron.









I got this soldering iron at a yard sale many years ago for $3. Money well spent!
I bought it for big solder joints on old TV chassis(think CRTs), but my wife has used it since for her stained-glass work.





I really liked the idea of hot air, as long as I had good heat control. I did not want to melt so much of the tank that the bottom sagged in.

Luckily I also had a hot air option. Years ago I picked up this hot air surface mount rework station from Spark Fun.
(https://www.sparkfun.com/products/14557)
It usually lives on the electronics bench, but it might do the trick for this.......









Well this post is already getting long.
I'll be back shortly with the repair..........stay tuned!

-brino


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## brino (May 12, 2020)

Step 3 - Practice
==============

I am a firm believer in practice cuts, practice welds, etc. so I went looking for another HDPE object that was already broken, or that I could crack just for a test.

Our big blue recycling bin has been cracked across the bottom for a while. I could NOT find a mark on it to show the plastic type, however the old "sniff test" showed that it smelled the same as the fuel tank plastic when touched with a red-hot wire. 

Here's the bottom of the blue box after some practice:




and some close-ups:






The black filler is the real plastic welding filler shown in the first post. I did NOT care for it.
It seemed too big for the thickness of the plastic I'm trying to weld.
You can see it sat on the surface and did not become intimate with the base plastic.

The white stuff is the recommended insulation from coax cable.
I had found some pieces in the garbage at work, looked up the vendor and verified the insulation type in the datasheet.
After a few minutes of stripping off the jacket, the ground and then the centre conductor I had this:



It was thinner and melted into the base plastic better.

I used the hot air blower and warmed the base plastic first.
Then introduced the filler, dabbing until both were at a good temperature.
I then started laying the filler, pushing it into the crack as I danced the hot air around.
I just kept backing up the hot air blower as the two materials came together.

I then came back with the soldering iron and my new tips to flatten and blend.
It seemed to work well.


Step 4 - Dive In!
==============

One trick I read during the research was to rinse the tank with acetone to remove fuel residue and make the plastic weld go better.
So a few days before the attempt I gave the tank a thorough rinse, and left it in the sun to dry.

Next; expose the wound.... since the crack was right at that flange I worried that I could not get into the crack well.
Luckily that flange is only used on some other application, I do not need the flange.
My tank mounts with screws into the top on the other side.

So I cut off part of the flange:









and filed the area in preparation for welding:




...and then just like my practice, I heated the base plastic, brought in the filler, and heated both until they fused well.
I kept backing up the hot air blower but kept it pointed at the union between the base and plastic.
I managed to wrap around the corner of the tank.
I kept pushing the filler into the crack, and ended up here:




You can  see the glossy surface beside the weld where the base material surface flowed.

I then went about refining, smoothing, adding more filler and flattening with the soldering iron and triangle tip.














As you can see, by the time I was done, all the filler plastic was black.
Did I burn it or just get really good incorporation of the white filler with the base plastic.
I dunno, let's just say it was the latter......

I had also added the piece of the flange I had sawn off as filler.
Everything seemed to flow together very well, and I tried to make sure it was all really one piece, and not just a thin plastic veneer that would peel off.

The Test:
======

Unlike that blue box this tank mattered. It has to hold liquid.....flammable liquid.

I tested for leaks and was very happy to see NO fluid coming out of this corner of the tank!!

It's a good project where I can:
i) learn some new things,
ii) make some new tools, and
iii) have a successful repair.

(Note I did the leak test with acetone as well instead of gas, because if it did leak I did not want to contaminate it and have to rinse again!)


Thanks for reading!
-brino


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## C-Bag (May 12, 2020)




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## C-Bag (May 12, 2020)

Very cool, and interesting coax insulation is the right stuff. Did I not see it, but how hot did you set your heat gun to?


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## OTmachine (May 13, 2020)

About 45 years ago, there was a toy, I believe was called a “spin welder”,that used friction from rotation of the rod to weld.  It actually worked pretty good.  Somewhere, I have an automotive plastic welding set that uses a heat gun and has various welding rod materials.


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## Tozguy (May 13, 2020)

Good work Brino and it sounds like you had fun. Thanks for sharing. 
Do you have any idea of why it cracked in the first place?


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## brino (May 13, 2020)

C-Bag said:


> how hot did you set your heat gun to?



Good question! 
I had to run out to the shop, plug it in and turn it on to see where I left it.

It was set about 340 deg. C.
Also, the air flow was at full.

I really like this little hot-air rework station.
It was up to temp in less than a minute, and is very quiet.
It keeps the air flowing after turn off, until the unit cools down.

I see now they also sell nozzles for different component packages:
https://www.sparkfun.com/products/7925

We have a different kind at work, but their little fans scream when then run. They are hard to talk over.

If you ever use one on a Printed Circuit Board (PCB) here a couple tips:
i) keep it moving; this thing has more than enough heat to scorch the fiberglass board (and release those gagging odours!)
ii) don't set the air speed too high, you can blow components right off the board once the solder melts.
iii) find a dead board to practice on first!

-brino


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## brino (May 13, 2020)

OTmachine said:


> About 45 years ago, there was a toy, I believe was called a “spin welder”,that used friction from rotation of the rod to weld. It actually worked pretty good. Somewhere, I have an automotive plastic welding set that uses a heat gun and has various welding rod materials.



Yeah, I had the Spin Welder bi-plane kit. I got it from Santa:
https://www.picclickimg.com/d/l400/...attel-Spinwelder-Airplane-Factory-No-7692.jpg
 I remember being fascinated by the process.

Many of the videos and tools I found during my research were of automotive repair kits.
Most were bumper repairs.
There are some real magicians out there doing auto body work.

I hate painting! It never goes well for me, and the prep time is so long, and then the clean-up....... hate it.


Another thing I saw in my research was embedding wire mesh screen across a crack for strength.
Heat and press it in with the iron, then add plastic over top.

There are also a number of kits that allow you to embed small metal "staples" right into the plastic.
https://www.princessauto.com/en/detail/shark-industries-mini-tack-plastic-welder/A-p8734782e
https://www.princessauto.com/en/detail/shark-industries-pro-tack-plastic-welder/A-p8734758e
You cut off the tails once they are in.

-brino


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## OTmachine (May 13, 2020)

brino said:


> There are some real magicians out there doing auto body work.
> 
> 
> -brino


My father-in-law was one of those Magical Body Men.


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## brino (May 13, 2020)

Tozguy said:


> Do you have any idea of why it cracked in the first place?



I really do not know.

It is a Troybilt Horse model roto tiller (about 20 years old) with the wrap-around bush bar, so the tank is pretty well protected.
It was sitting in the sun for a couple hours just before I went to put it away.

My only other thought was that the pull-start handle hit the tank when I let go of the pull rope.
It does have a strong recoil.
But the pull-rope is way around front and this crack was at the back outside corner.

I do still wonder about a manufacturing defect as it was right where that flange meets the tank.
But still I guess I cannot complain, it has had a great run!

-brino

I do NOT have pictures of mine, but it is similar to the below:
(but my pull cord pulls out the same side as the fuel tank.....)


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## C-Bag (May 13, 2020)

brino said:


> I really do not know.
> 
> It is a Troybilt Horse model roto tiller (about 20 years old) with the wrap-around bush bar, so the tank is pretty well protected.
> It was sitting in the sun for a couple hours just before I went to put it away.
> ...


I'm sure John has a lot of stories about tank failures both plastic and metal. Probably they could have gotten around that if the tank was not attached to the engine but to the frame. There is just too much vibration with a single piston engine. I had to braze up the gas tank and air filter on the cheapo rototiller I enherited. They put bigger tanks on them than law mowers.


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## C-Bag (May 13, 2020)

Thanks for posting your adventure with plastic welding Brino. My favorite welding process has always been OA. Briefly got into OA welding aluminum and hope to get back to it. My FIL got approached by a company back in the '80's when everybody converted from wood field bins for stone fruit and citrus to plastic bins to repair them by welding. He was not interested because he could barely see to weld much less do something plastic. He also couldn't OA weld. I was intreagued but never saw the people again but was always interested in it. I'll have to look into the links you posted when I have time. I'm sure it's a really involved thing since there are so many different kinds of plastic. Still blown away that the best filler is coax insulation, whodathunk?


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## macardoso (May 13, 2020)

I've welded quite a few whitewater kayaks that met an early demise on some submerged rocks. Done similar steps to what you did. The biggest thing for us was to get full strength back since the hull is impacted often. I've never done steel mesh, but the club bought a hot staple gun that electrically heated "staples" (more like the lightning bolt shape from Harry Potter's forehead and no spikes on them) and pressed them into the hull. You'd do one of these every quarter inch or so, spanning across the crack. Finally you'd finish the weld with a heat gun, soldering iron, and filler material like you did.









Funniest one to date was my buddy bought a new boat that had a defect and the manufacturer would ship him a replacement boat for free (under warranty) only if he drilled thirty 1/2" holes into the bottom of the broken boat. He did of course and after the new boat came in, we painstakingly repaired every one of those holes and used the boat to teach our beginners. It never leaked but probably weighs a couple of extra pounds for all the plastic that was added.


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## middle.road (May 13, 2020)

Cool repair, and you added more knowledge to the ol' memory bank than what I had when I got out of bed this morning.
Now if I can just remember this if the need arises.  

Did you do anything to the mounting to prevent it cracking again in the future?


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## brino (May 13, 2020)

middle.road said:


> Did you do anything to the mounting to prevent it cracking again in the future?



Honestly, the tank is still waiting to be re-installed.
Work has been getting in the way of play-time, so it likely won't happen 'til the weekend.

Plus I am not sure what I could do to help the situation....suggestions are welcome.

There is engine vibration; maybe add some rubber isolation between the tank and the engine?

Maybe I could spin the plate that mounts the pull-cord so that it comes out the "passenger side".

Without really knowing the root-cause, it's a bit of shooting in the dark.

-brino


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## middle.road (May 13, 2020)

brino said:


> Honestly, the tank is still waiting to be re-installed.
> Work has been getting in the way of play-time, so it likely won't happen 'til the weekend.
> 
> Plus I am not sure what I could do to help the situation....suggestions are welcome.
> ...


We'll have to see some picts when you get back on it to see how it mounts.


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## matthewsx (May 13, 2020)

A few years ago when I was still building race motors I could have sent you a whole mess of those tanks. Hope your repair holds up but don't get mad with yourself if it fails again. It probably cracked from years of vibration and/or the fasteners holding it being overtorqued. I wouldn't try to re-engineer the mountings, if it still leaks check with your local small engine repair shop, they might have a tank in their junk pile.

If you want the pull start going the other way just take the bolts out and rotate it, that's how they work.

John


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## BtoVin83 (May 13, 2020)

I traveled around the US inspecting HDPE tanks and having them repaired. The techs used a high wattage light and reflector with powdered resin and had very good control of the heat process. Move it around in a circle an vary the proximity to the tank wall


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