Automotive Plastic Welding Repair

35,975

188

135

Posted

Introduction: 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

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!

Share

Recommendations

  • Sew Warm Contest 2018

    Sew Warm Contest 2018
  • Gluten Free Challenge

    Gluten Free Challenge
  • Minecraft Challenge 2018

    Minecraft Challenge 2018
user

We have a be nice policy.
Please be positive and constructive.

Tips

Questions

116 Comments

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.

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

I'm just going to leave this here,

From https://www.merriam-webster.com/dictionary/weld

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

https://www.youtube.com/watch?v=VLz88Wsg2fA