Introduction: Automotive Plastic Welding Repair

Picture of Automotive Plastic Welding Repair

This is the inside of my boot lid.

I drive a commercial 4x4 and somewhere in its life, something heavy fell against the door and cracked the plastic door card.

The crack started small but over time and a large amount of off road driving through construction sites and quarries has made the crack so large that the door card has started to fall off.

I looked for a replacement and the cheapest I found was over E250, used!

Time for the Maguyver in me to come out, let's make a weld repair!!!

Step 1: Let's See What We Are Working With

Picture of Let's See What We Are Working With

The first job was to remove the door card, it would be nearly impossible to repair it in place and even though I wasn't looking to bring it back to looking new, I wanted to make the repair to the back as it doesn't look too great when finished.

Using a prybar, I removed the snap fasteners and dropped the cover off.

There was some sound deadening bonded to the inside so I cut this, I forgot to take a picture but I stitched this back together with zip ties before I replaced the part.

The crack was not aligned so the first job is to make the sides of the crack line up as best you can.

Step 2: Welding

Picture of Welding

We are going to use a soldering iron with a large tip to make the weld.

WARNING!!!! Melting plastic releases toxic gasses, depending on the plastic in question it may even release cyanide so work outside or with extraction and wear a mask.

Also, the soldering iron and ultimately the plastic will get hot so be careful of burns.

Start out with a low wattage soldering iron, maybe 25 or 30W, the hotter irons will vaporize the plastic and do more damage than repair.

Turn the iron so the flat is pointing to the sky and press very gently into the plastic. You want to melt about 50% of the way through.

Start before the end of the crack and continue past both ends, this will seal the ends of the crack and stop it spreading beyond the repair.

Keep moving the iron and pushing the newly melted plastic towards the last melt, this will produce the "stack of pennies" look you get with TIG welding and is very strong.

Step 3: Dressing

Picture of Dressing

Once complete, the plastic repair will be strong but the crack will still be visible on the other side.

To clean this up a little, I flipped the part over and using the flat of the iron, melted the joint a little.

This looked like a line of sealer in the end but it closed up the crack and leveled off 2 high points left from the repair.

It's never going to win a beauty pageant but it is fixed, strong and it was free!


ryniablackfox.inesory (author)2017-08-24

the timing of this showing up in my emails is impeccable. I bought a plastic bin that i didn't realize was cracked on the bottom, being too lazy to put it in the car and bring it back to the store i was about to look up ways i might be able to fix it. And here we are. I will have to try this out.

uhhh then theres duct tape.....

I'm glad the universe aligned my project with your problem :-) I hope it works, please post a picture.

GergelyD (author)2017-08-25

This is not welding. This is some sort of a local melting but it is proven weak.

Welding such plastics like that one requires a hot air gun with adjustable heat and airflow and a reduced diameter tip. And a "welding rod" which MUST be made of the same plastic you want to repair.

In this case it is a Polypropilene mix, called >EPDM<, it has nothing common with ABS.

You have to cut a V-groove from the front, having some support from the backside to hold the loose sides level and you have to heat up the filler rod, not the material you are welding. The techic the amount of heat, the angle of the filler rod varies at every single plastic type. You got to know the properties of the materials you are playing with.

Been repairing plastic (car) parts for years by trade, just please look around how professionals do it before you make a tutorial. Such "repairs" always ended up at our shop for a proper welding and been charged extra for having a ruined surface to deal with...

This is NOT THE WAY to repair plastic.

Firstly, there are many definitions of welding, many of which do not utilise a welding rod or filler. Welding is a process of heating any material to the point of melting and joining the parts, when cooled the parts are fused.

Second, if you have a moment to look at the comments, I actually note that I checked and you are correct this is a Polypropylene part and I understand that it is not, nor is it like ABS (or else I would have used an acetone binding method)

I have also stated that I have made this repair many times and have never experienced a fail if your experience is different and you have found it weak that's a different story, I did say that the heat of the iron was important and the lower temperature was required in this case as higher temperatures vaporised the plastic and made a poor repair.

I applaud you for this effort !

Plastic 'welds' do work if you are holding your tongue just right !

I have seen vids on YT of a squatting barefoot Indian with irons and open fire repairing 5 gal. buckets .... do the welds need to look nice ?.... Not for me ! If I find that old vid I will post it. You did a great job .

What is the melting temperature of PP and EPDM? How LONG you must expose the plastic to this heat to reach that point where cross-bounds release, but chain-bounds are not? -Just to reach that state where these plastics fuse.

Again, I spent a lot of time fixing people's such attempts for repairing their broken plastic parts...

All excellent points and ones which I cannot answer. I am not an expert I. The area and my knowledge with technique has been trial and error. I have experience with mig and tig welding and in a similar way I practiced making welds, some were good some were poor but over time I got a feel for when it was going right. Similarly with this project I made the repair, stressed it and it held well under far more pressure than it would experience in use so I feel the technique is a valid repair

Jasonw217 (author)GergelyD2017-08-25

I'm just going to leave this here,


Definition of weld

  1. intransitive verb
  2. : to become or be capable of being welded
  3. transitive verb
  4. 1a : to unite (metallic parts) by heating and allowing the metals to flow together or by hammering or compressing with or without previous heating
  5. b : to unite (plastics) in a similar manner by heating
  6. c : to repair (something) by this method
  7. d: to produce or create as if by such a process
GergelyD (author)Jasonw2172017-08-25

GergelyD (author)Jasonw2172017-08-25

You're right, but technically this won't work out too good. You can't control the heat (possibly overheat the surroundings of the crack but a few mms away still not hot enough, -this is how you make ABS brittle for example), and you can't apply any pressure, because you'd press through the hot tip, but without pressure you can't make a strong weld on thermoplastics. One thing the filler rod is used for...

Just look around on youtube for example how swimming pools are welded.

KB5RXZ (author)2017-08-27

For all the detractors, come of the contentions are accurate. Never the less, the panel is repaired and is not a structural component. I have repaired my 4 wheeler fender in a similar manner with great results. HINT: got a broken garbage can? Cut strips from it to make GREAT plastic filler rod that often matches the plastic chemistry and color. Also, somewhere they sell "texture" pads that mimic the dress texture of original interior panels. Simply use an iron to "melt" texture into the dress side of repaired interior panels.

you wouldn't happen to have a link for those pads?

Mea Culpa... I remember a texturing pad for vinyl when I was shopping for a plastic welder, oh, 25 years ago. I'm probably a victim of brain rot 'cause I went looking for interior panel texturing "stuff", but only found the texture paper for liquid type vinyl upholstery repair kits. (dog gon it!)
I got a DIY idea while thinking, though. If one were to use a hard rubber roller to roll some etch resistant paint onto an interior panel flat area. Then use a clean copper or brass piece of 1"- 1 1/2" dia. bar stock rolled over the freshly painted panel to create the texture of the panel onto the roller. Then etch said roller in ferric chloride (PC board etch solution) to desired texture depth and cut to length (roller width), clean the paint off of the roller and mount in a wooden frame handle. Soften the repaired, rear supported panel with hot air and apply pressure with the etched roller in a controlled rolling motion to put that texture onto the softened plastic.
I wish I could find the product I mentioned and my memory were better. I sincerely apologize, folks.

Wow, that's a well thought through "off the top of the head" plan!

IzNoGuD (author)2017-08-27

This Method result in a weak connection.
It would be better to repair it with a hot airgun with welding Nozzle and plastic welding rod. At first you should have tabe the front with masking tape together for a plane surface.

All good suggestions, however this part is under no load and 3 weeks on (after serious offroad driving) is still very much in tact.

D6equj5 (author)2017-08-26

Good instructable - I have to repair my partners Jimny inner mudguard on the front. Excellent timing.

thanks hope it helps. make sure it's good and clean first...

MaxH86 (author)2017-08-26

at one time, maybe still do, Harbpr Freight sold a plastic weilding kit just for things like this!

MarkP176 (author)2017-08-25

Would this work for a large crack in a bumper, any ideas? Obviously getting the thing off in the first place will probably be a trial unless I can get access to the inside from underneath (...seems to be a thin outer skin over a load of expanded polystyrene filler, is the thing).

Alternatives - hot glue gun (the "glue" itself might even be a similar kind of plastic?) having preheated the area with a hairdryer or careful use of a blowtorch? Polystyrene Cement as used in model aircraft kits (seems to be halfway between superglue and some kind of acetone-like solvent, bonds pieces together by acting both as a normal adhesive and by temporarily melting the edges somewhat, without heating)?

And what even to do about the foam filler itself, which is slightly broken up now? drill holes and use zipties, hold it loosely together with duct tape, something else?

GergelyD (author)MarkP1762017-08-25

Hot glue gun cartridges are made weakened (for a low melt point) Polyethylene. No bumpers are made of that plastic.

GergelyD (author)MarkP1762017-08-25

You can, from the front, as written above. From the back you might want to reach the cracks by drilling and apply some expanding PU foam. Will hold for a while.

One other important part: whatever welding method you choose, you have to clean the surface form the oxidated layer (seen black plastic parts turned gray? -that's it)

Most of thermoplastics can't be joined with glues, not solvent based glues, nor contact glues. Exceptions: ABS (since it has a high Styrene content, some solvent base glues might work)

Styrene -as above

I think it would. This is not my area of expertise so its just opinion. I would open the crack and spray in a small amount of expanding foam to capture the loose bits, not too much or it will push out the crack and you'll never get it level.

Then try this or the friction welding technique shared by Fisherman Luka:

This looks like something that may work.

I can't tell you how careful you need to be as this part will be visible. I'm also a little weary of bumper repairs as they are part of the safety system on a car in terms of impact absorbance. Try at your own risk I would say, sorry not to be of more help.

Fisherman Luka (author)2017-08-23

How about friction welding?

You could get a dremel and some 3d printing filament. Spinning the 3d printing filament with the dremel against the crack would cause the plastic to melt and cool down into a solid fix.

Thanks for sharing that one, it looks like a really easy and fast way to joint up some plastic.

ElizabethGreene (author)2017-08-22

Bonus information:

If you have a hot air SMD rework station, these can be used as a hot air plastic welder.

For filler material, look for the Recycling triangle marking and use the number to tell you what type of filler you need.

If your part is ABS

* it can be "glued" with acetone

* it can be reinforced or built up with ABS "putty" made by desolving ABS 50/50 in acetone.

Great looking repair. No complaints about that at all.

jaxboy (author)ElizabethGreene2017-08-24

ABS welding is child's play. It produces a joint that is as strong as the original, and if done correctly, is almost invisible. Take small pieces, hopefully of the same piece from an unseen area, and put them in a glass jar. Immerse them in pure acetone (Available in Walmart for $1 in the finger-nail polish remover section, or in the paint remover section) until they turn into a slurry. Then, with some kind of applicator (I use a popsicle stick), gently pry the joint apart slightly and then apply the slurry, continuing all the way along the crack. Let dry for a day or so. A hair dryer aimed from semi-close by will aid in the acetone evaporating. If necessary, you might want to use an acetate-resistant syringe to dribble acetone along the front side to further seal the crack. According to industry standards, acetone is NOT toxic.

I've done the acetone truck with abs 3d prints. Great when the part is too big to print on the bed in 1 piece

Gardenclaire (author)2017-08-23

Good repair! Would this work for those flexible rubbery/plastic buckets you get these days? I'd like to cut one down and re-make it into a different shape, and I've been wondering the best way to do that. Maybe this technique would work!

I'm sure it would just be careful with the temperature and don't over work the material. depending on the plastic it may get harder or more brittle with heating. I'd cut off some of the bucket rim and give it a try. Also post a picture

Of course, what a good idea! I'll experiment with the surplus bit. What I want to do is cut off the bottom of the flixible bucket and make the circular bucket into more of an oval shape, to fit into a specific place. Will come back with a picture when I (eventually) do it. Hopefully it won't be one of those "nailed it - not" pictures hahaha!

hammer on and give it a go I await results :-)

lyvwyr70 (author)2017-08-23

you dont want to weld on both sides? wouldnt fiberglass and epoxy be better?

epxoy doesn't stick to every type of plastic.

JudyP63 (author)2017-08-23

This is my absolute favorite type of Instructible! I love learning processes that can be applied across many situations. My previous laundry basket repair was: drilling small holes on either side of the crack, joining with twist ties, gluing over the spaces with plastic bread tabs, then covering the whole repair with duck tape. Ugly, but stayed put for about 10 years. NOW they're going to get welded!!!

cool, post a pic

dave.maunder (author)2017-08-23

As already stated further below, black cable ties or zip ties are a great 'filler' or the 'solder' (so to speak) when performing this type of welding/soldering. They might not work with all types of plastics, but have worked brilliantly for me over the years when using a number of differing plastics.

yeah I've used them in the past, this plastic was quite thick so I didn't see the need. Zip ties are also available in many any varied flavours

Eligong (author)2017-08-23

A good way to strengthen that joint would be to use a fine wire mesh.
I had to make some modifications on my car AC ducting, cutting and welding using a 40w soldering iron but the joints weren't strong enough.
I found a roll of braided mesh wire in my junk box, it was about 3/4" wide.
I laid it down the length of the joint and pressed it into the plastic just enough to allow the plastic to ooze up through the mesh, then I swiftly ran the iron over the plastic that came up through the mesh to seal and level the plastic.
And oh yeah...use a fan or work outdoors to blow the fumes away.

nice job

JohnD316 (author)2017-08-23

I know that it is nice to know how to do this procedure much would it cost to buy a liner like this in good condition from an auto wrecking yard? Most wrecking yards in my area will even remove the liner and have it ready for you when you get there. This is an insurance liability issue for most.

This was not covered under our commercial insurance and a liner from a breakers yard was cheapest 250 euros

june48bug (author)2017-08-23

The daughter of a lady I worked with smashed the grass catcher to the riding lawnmower into a tree & split it right across the middle. The price on a new one was around $500. I told her I thought I might be able to fix it but she didn't believe me. I looked around & found a fiberglass repair kit for boats. With a little help from my son, I matched up the broken seams inside the catcher, applied some resin with a paintbrush, then a fiberglass strip & another coat of resin. Then I placed some strips of wood in strategic areas to keep it from folding in on itself & to keep everything aligned. I left it out in the sun for approx. 4 days - it worked! Since the fiberglass was white & the grass catcher was black, I bought some semi-gloss black, waterproof spray paint & sprayed the repaired area & fanned it out lightly & you couldn't even tell it had been repaired or painted. I repeated this same process on the outside of the catcher.

That was over 25 yrs. ago & the grass catcher still looks like the day I repaired it. I am a female Mrs. fix-it & this was probably my best fix ever.

nice work!

mc2517 (author)2017-08-23

How about mixing a quantity of JB Weld and imbed some of the fiberglass mesh that's used in drywall joints. Would also emphasize the "stop drill" technique mentioned by others. Be sure to clean and rough up the plastic so the JB can key into it.

wrsexton (author)2017-08-17

Having endured this problem myself, I purchased a plasic welder kit. Good quality great instructions, but still a soldering iron (with a hole in the foot to push filler rod through) and adjustable heat. Best results with this pro tool are accomplished just as how you outlined it, heat the base material thoroughly and flow the two pieces together, adding filler as needed. Will last as long as it did originally, in my experience. Great job, and saved yourself the $150 I spent on my tool (in fairness it's paid for itself several times over!).

Thanks, maybe I should look at modding this iron tip by adding a hole to pass filler through. Depending on the material you are welding, I'm sure you'd find a zip ties in the same chemistry to use as a cheap fill rod

Zip ties are actually a bad choice for filler for 2 reasons the first is the plastic is actually a styrene polymer and is very toxic to us. And 2 when burning or melting it will boil or bubble it would be better to use a milk jug or hdpe. I was a supervisor for a plastic company for 2 years.

We use some in my job that are PP zip ties (more a strap with a zip tie closure). I take your point about the toxic gasses and would not recommend anyone work with plastics without full ventilation and breathing protection. As some advice, would you know with the wide range of 3D printers available, is there a filament to match most plastics now? Or is there a general overlap in compatibility between some plastics?

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Bio: I am an automation engineer but I will give anything a go. I don't know if you call if pessimism or just being an ... More »
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