Spot Weld Plastic




Introduction: Spot Weld Plastic

About: Putter (pŭt′ər) v. put·tered, put·ter·ing, put·ters 1. To busy oneself in a desultory though agreeable manner. 2. To do random, unplanned work or activities. 3. To occupy oneself with minor or unimport...

I'm a big fan of the plastic milk jugs. They're a strong, flexible, accessible, free material. There's hardly a jug that makes it to my recycle bin, without a gaping hole of some sort, in the side.

The only problem I had was the HDPE (high density polyethylene) plastic is, it's almost impossible to glue. Glue just won't stick.

I decided I wanted to develop a method to weld this plastic and make it even more versatile.
In his instructable, I'll show my process, successes and failures and I hope you'll find some tips for your own creativity.

Step 1: The Jug

I'd suggest starting off with milk jugs. The thinner plastic will be easier to weld. With more practice you can weld thicker plastic.

Step 2: Tools and Materials

1- Photo 1 shows a Bbq mat. These teflon fiberglass reinforced mats are used on barbecues, to keep your food from falling through the grill. They will withstand very high temps. 550F 288C. This is what you will use to protect the plastic from burning, but also allow the heat to transfer through and melt the weld. It stops the iron from burning the plastic, which is a real problem with other plastic welding methods. There's no toxic smoke with my method, but just to be safe, do this in a well ventilated area.

2- Cut 2 strips, from your mat. 4cm x 12cm.

3- This is an aluminum espresso tamper. Any smooth flat piece of metal should work.

4- This soldering iron is as cheap and crappy as they come. It's only 30 watts. I would recommend low wattage. Higher wattage, more heat, and more likely to melt through.

The tip has been removed, but you want an end that looks like the picture. Others I've seen, do not have the flat washer shaped surface, that helps make the weld. They are more like a tube, and would not have the surface area, to press the weld. Although, if you found the right size bolt to fit into the hole for the tip and flattened the head, it would probably work.

5- A piece of rubber to give a bit of cushion and heat protection.

Step 3: Do It!

1- Cut up some small pieces of plastic to experiment with. Heat up your soldering iron. Have your tamper close to your work.

2- You will be making a sandwich of the 2 pieces to be welded, between the 2 strips of the bbq mat. This will sit on top of the rubber block.

3- Once you have it lined up with your iron perpendicular to your work, lower it to the spot to be welded. Use a medium amount of pressure. Too much pressure and it will burn through, and squeeze the melted plastic up and out, too little, it won't stick. 1 or 2 pounds.

If you look on my cover photo, you'll see on the bottom 2 spots right side, there are tiny voids. This is the plastic starting to bubble/boil. You don't want too much of this or it will eventually break through the layers and weaken the final weld. Walk the fine line between time, temperature, and pressure.

My ideal time is 9 seconds. It will vary for everyone, so experiment to get the correct time for you. I find it useful to have a clock ticking in the background, so I'm accurate each time.

4- After 9 or so seconds, remove the iron and apply your tamper and press for a few seconds.

5- Now you have a hot little plastic sandwich, stuck together. Don't pull off the mats till it cools. It will only take a minute or so, but if you peel it off too soon, it will stick, and ruin your surface texture. If you are using different coloured plastics, be sure to use a fresh area of your teflon, the colour can transfer to lighter coloured plastic.

Step 4: Voila!

A beautiful strong weld.

The second and seventh picture, are the back views of their preceding pictures.

You will soon get the knack of knowing if it's a successful weld or not. Mostly by ripping them apart.

If you are successful, you won't be able to pull them apart. It's all about timing of your heat, so be accurate in your timing, so you can reproduce your success.

The nice thing about this method is, there is a small amount of fold over on the weld as a result of pressing, after the heat and it acts like a grommet and strengthens it at the weakest point of the weld.

This method would also work well as a reinforced hole, like a grommet.

The final picture shows a nice distribution of plastic within the weld.

Step 5: And Now for My Failures.

It doesn't take long to master the weld.

The first picture shows the timing from 8 to 20 seconds.

The last 3 photos are without the teflon mat. Lots of smoke and melted through very quickly.

I tried other heat shields, including silicone, which sort of worked, but not as neat, and there was colour transfer.

Step 6: Different Materials

I tried welding this thicker plastic. I'm not sure what type it is, but it's not HDPE. After a bit of experimenting, I had success. I found with the thicker plastic, you don't put pressure on the iron and let the heat slowly penetrate through the thickness. I found 2 minutes made a strong weld. I was able to rip, but with a lot of effort and even then, it ripped into the surrounding plastic. If it was HDPE, it definitely would not have ripped. The 5th picture shows 20 60 120 seconds. The 120 seconds had no pressure, and worked well.

Looking at the back of the weld is a pretty good indicator to a good weld. You want it to have melted through to the back.

KLARCK, posted below, and wondered about welding, coraplast corrugated polyethylene. I gave it a try, and it worked beautifully. You have to push down quite hard, and the teflon gets forced into the hole, but a very nice result.

(See pictures)

Thanks for asking.

I tried welding 2 layers of a 55 gallon barrel, total of 1 cm thickness. The photo shows 2 welds. Top weld, the iron was left for 5 minutes, the bottom weld, was 10 minutes. Only slight discolouration after 10 minutes. It did melt through to the second layer, but not enough to make a weld. I think a higher wattage iron, might work. (stay tuned)

Step 7: Thicker Plastic Experiments

A lot of people have been asking about welding thicker plastic. I have one of those 55 gal HDPE barrels, I've cut up and been using, over the years. It ranges in thickness, depending where on the barrel.

I tried the above method on 2 layers, total 1cm thick. I couldn't seem the get the melt much deeper than the first layer.

I thought maybe if I could get a probe down through the full thickness, and melt the full depth, it might work.

I made this tip, out of aluminum pipe. I thought the metal mass, and the good heat transfer of the aluminum, might work.

With the flat blade shape, I could take a small narrow piece of teflon mat, down the hole and just let it sit and melt, without rumpling the teflon, and weakening its structure.

It worked very well. I was able to get through to the back, with a good amount of pressure and get a good melt, in about 2 minutes.

I pulled out the teflon before cooling, and pressed with the tamper, and the hole closed in nicely, with the molten plastic.

I could, in no way pull this apart. Put a few of these in line and you would have a very strong patch. It might take some care, to get it water tight, but I think it could be done.

Step 8: Now, Go Make Something!

I hope you find many uses for this method of welding plastic, and I look forward to hearing all about them.

Step 9: B Movie

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    51 Discussions

    This was a nice instructable - thank you for posting it!

    It should be noted that the kinds of plastics this technique works best on are so-called "thermoplastics", because they are formed and molded using heat. The polyethylene family ( is best known for this, and consists of multiple types, HDPE being one of the very popularly known ones.

    There are other kinds of thermoplastics available too which can be heat welded and formed; but as always, be sure to research safety concerns before attempting unknown plastics.

    One type of plastic that extra care should be taken when heating it is PVC. While it can be softened and formed using heat, it should be done outside or in a well-ventilated area. The greatest danger is if it burns, because chlorine gas (among other bad compounds) can be formed, which is very poisonous. In the "instructable" and "maker" worlds, it is generally known that you should never cut PVC in a laser cutter for this reason (and most laser cutter manufacturers put this warning very clearly in their safety literature).

    Ultimately, before attempting any kind of bonding operation with plastic, research and knowing what plastic you are using is key.

    When attempting to use a glue or solvent to bond plastics, research can make the difference between wasted time and a successful bond (certain plastics, especially polyethylene and polypropylene, are almost impossible to bond with glue or solvents due to their chemical nature which gives them their useful properties).

    When attempting to bond a plastic using a heat source, if you don't know what the plastic is, use extreme caution.

    Also note that even if a plastic is marked as say "HDPE", each manufacturer can and does add different kinds of products to the plastics to give it different capabilities (and there's no easy way to know who manufactured each plastic unless you purchase it direct from the manufacturer). These differences can make two seemingly "identical" plastics incompatible in certain ways - whether bonding by chemical or heat. Experimentation, if possible, should be done to determine what works best.

    3 replies

    Thank you, very much, for taking the time to post this very informative and important information.

    If there's enough "meat" left on the flange or whatever you're trying to screw into, and you're willing to spend the money, I'd do this:

    1. Get some PVC pipe "glue" (solvent) and some scrap PVC bits, and mix 'em together to dissolve the PVC into a goopy cement of sorts. Do this in a small batch, you won't need much.

    2. Buy what's called a "nutsert" tool and nutserts - these are essentially "pop rivet nuts" - you put one in the tool, put it thru the hole, then squeeze the handles, and they are "crushed" against the material. You may want to try it first on a sacrificial piece of PVC similar in thickness to your flange; they are usually meant for metal, so I don't know how PVC would stand up to them.

    3. Assuming #2 goes ok, ream out the existing holes cleanly, then mount a nutsert on your tool that'll fit into the holes properly. Then take some of the cement goop and smear it on the outside of the nutsert. Put it in the hole, and mount the nutsert as normal. Take a screw to fit the nutsert, smear some grease or vaseline on it, and thread it into the nutsert.

    4. Now - let the whole thing cure; give it a good 12-24 hours. After that, it should be bonded ok.

    5. You may need to widen the holes on the grate to fit your screws; whatever you do, make sure the nutserts you use are stainless steel or zinc coated or something like that, and the same for your screws. Also, be sure to grease the screws before you put them in. Since it's going to be in a wet environment, you don't want it to rust - or if it does, for it to rust together.

    The nutserts should be pretty well bonded and clamped to the PVC flange. and you won't have to worry about them stripping out or anything again.

    Yes, it's overkill, and it's not going to be cheap - but anything else you might try will probably fail in the same way over time anyhow. PVC is a pretty tricky material to work with.

    If you think you can get the JB Weld to bond, you can try that - do it similar to the nutsert thing, but grease your screws, and put them into the JB Weld as well, so when it cures, it is "pre-threaded" - easier than tapping threads.

    Good luck!

    Hello; My shower drain opening appears to be white PVC and the screw holes for holding the drain cover in place are totally stripped out. After regrouting I tried the original screws, and 2 different size larger screws; and the holes just keep getting wider and wider without grabbing. Should I try to fill in with PVC and drill/tap new threads or would it be just as good to do same with JB weld? Or something else?


    je cherche depuis longtemps a coller les endroits abîmés de mes chaises tressées avec trame en plastique sans succès. avec votre système, pensez vous que cela fonctionne

    1 more answer

    Bonjour, C'est une question très difficile à répondre, sans voir la chaise, et savoir quel type de plastique. Je dirai que vous devez utiliser le même type de plastique pour assembler les pièces. La meilleure chose à faire serait de tester d'abord, mais ce n'est probablement pas possible dans votre situation. Si vous affichez une image des dégâts et de la chaise, je pourrais peut-être vous aider davantage. Pardon.

    Hi, That's a very difficult question to answer, without seeing the chair, and knowing what type of plastic. I will say that you must use the same type of plastic to join the pieces together. The best thing to do, would be to test first, but that's probably not possible in your situation. If you post a picture of the damage and the chair, I might be able to help more. Sorry.

    Thank you for posting, what would seem like a simple task or solution. But, in reality, it's far more than that. It can really be put to use as practical solution to many projects.

    Once again, Thank you.

    1 reply

    I appreciate both the eloquence and intent of your compliment.

    Thank you.

    Good Job!

    I make lotions and have purchased new tubes online but not the expensive equipment for sealing them. I found a cheap flat iron (like a curling iron for hair, but used to straighten hair). It is difficult to control the temp and pressure so I've made many goofs in attempting to seal. But same concept.

    1 reply

    Great idea, using a flat iron. If it's burning through, you might give the teflon mat a try. It's quite amazing at buffering the temperature, and giving you more control.

    Very helpful , have other methods of my own to repair ATV plastic fender tears ext.
    Looking forward to trying yours Thanks

    1 reply

    Thank you. I've added another method, for thicker plastic, that might be useful for you. Good luck.

    I've added another method, that might work for you.

    I'm not sure what type of rainwater tank you're referring to. The HDPE 55 gallon barrels that are common, round the world, might be what you refer to. They are usually blue or white. If so, yes, but as I mentioned, I have had limited success with thicker plastic. It was one of these barrels I tested this weld. The 2 layers were 1cm thick and my soldering iron was only 30 watts. I haven't tried a higher wattage iron yet, but in theory, it should work. In this photo, the top weld, I left the iron on for 5 min. and 10 min. bottom. Only very slight discolouration on the 10 minutes. It didn't heat through to the second layer, enough to make a successful weld. I'm going to do some more experimenting, with more wattage. There's also the idea of melting around the outer edges of a patch, and melting them into the barrel. I guess it depends on the hole to be patched. If it's just a small pin hole, you probably wouldn't need a patch, just remelt, or a thinner milk jug patch. There are some options, but they all take a certain degree of experimentation. Good luck.


    17 days ago

    Higher wattage would result in a higher quantity of heat, BTU's, Kilo calories , not an increase in temperature

    2 replies

    I see what you were refering to. Corrected, I hope.

    Thanks for the tip. I think that's what I need, for the thicker plastic.
    Quantity to penetrate to a deeper layer. A higher temp would be more
    likely to burn or boil the plastic. Thanks for the clarification.

    hey, this is great! i've always wondered how it was done. i've made a couple of unsuccessful stabs at it, but didn't think about the mats or foil... how thick can you go with this technique? a friend of mine had a cracked golf cart top that i was tempted to have a go at, but was afraid i'd only make it worse.

    1 reply

    i don't know how he cracked the top... i think it was just worn out. he traded it in for a newer one this past spring. i'm interested in the process... when i get time this fall, i'll give it some practice time and see what i can get going. thanks....