Automotive Plastic Welding Repair





Introduction: Automotive Plastic Welding Repair

About: 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 engineer, but I look for problems everywhere, then I look for some weird, left field way to s...

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

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

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

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!



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    117 Discussions

    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.

    2 replies

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

    about 30 some years ago when i was living in idaho my friend had a small farm he milked a few head & had only i collector the part that the milkers attached the strap came off & the cow stepped on it and broke two of the pieces that the milkers attached to,he still had around 15 or 20 cows to milk his milker was quite old their was no part to be had,they didnt have all the fancy glues they have today.he asked if i could fix it for him.i didnt think i could but i would try,got out the old weller solering gun &did just like you did it worked for 3days till the new part came,he put it on the shelf for a spare.i did not know all the fancy stuff a everybody is talking about then it was plastic or metal &we did the best we could.good job glad it worked

    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.

    2 replies

    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.

    8 replies

    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

    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

    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.


    Reply 10 months ago

    It's still there...

    Screen Shot 2017-08-30 at 4.04.02 AM.png

    10 months ago

    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.

    3 replies

    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!

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