Welding Plastic

[RobRus]

There is nothing that makes me more crazy than waiting for glue to dry when I am trying to do some major modification like adding fender flairs or chopping a top.

What is worse is the model is much more fragile than before I cut it and the glue lines always seem to bleed through to the finish.

The reason is because we are using glue that softens and melts the plastic and it takes a long time to evaporate out.

I started welding plastic when I was about 15 and there is nothing that is stronger and faster than using heat to weld seams and joints. I use an adjustable temp pencil tip soldering iron. It has a very small tip that is perfect for welding plastic. It is important not to melt the surrounding area so a variable temp soldering iron is a must.

I have to make this warning and I want everyone to read it and understand that melting styrene fumes and smoke is not a good thing to breath in.

WARNING!!!!

VENTILATION IS THE MOST IMPORTANT THING WHEN WELDING PLASTIC.

AT THE VERY LEAST SIT BY AN OPEN WINDOW WITH A FAN BLOWING OUT TO EXHAUST THE FUMES. DO NOT BREATH IN THE FUMES OR SMOKE!!!!!

OK I have warned you... Don't be dumb and ignore this.

The first thing to understand in welding plastic is not to get it hot enough to smoke. If it is smoking and turns brown you are burning the plastic and it will become brittle and your joint will fail.

When you see smoke stop and clean the tip of your soldering iron with steel wool.

Practice is the best thing I can suggest to figure out how hot to get it and when to get off a spot before it starts to warp. But here is a quick explanation of how I do it.

I will put a good weld up against any glue as far as strength and the best part is when you are done welding you can get on with your customizing and do not have to wait for anything to dry.

Thanks and I hope this helps.

Bob

First make sure your area is clean and free of any flammable materials like paint or thinner.

Start by tacking the parts together to keep them in position.

V out a small section and weld in the rod to fill the V. Then create another V and repeat the process.

Notice when I create the V I push the plastic out to the sides of the V. This will be mixed in with the rod in the next step.

Start welding by laying the rod in the V and moving the iron very close to the end of the rod.

When you see the tip of the rod begin to melt start pushing the rod into the V. While it is soft I start to move it around and then pull the sides of the V in to create the weld.

The real trick is to get a very small spot hot but not melt or warp the surrounding plastic. I like to brace the part I am working on with a finger under the spot I am welding. If the opposite side is too hot to keep your finger on then you need to stop and let that spot cool (it only takes seconds).

After I have welded a complete seam I like to go back over it and clean my weld up buy dragging down both sides to move any bumps into the middle of my weld. Then I drag a diagonal line to create a nice smooth weld. If your joint looks smooth with no holes or burns then it will be a good strong joint.

Edited May 7, 2012 by RobRus

[george 53]

I just use Ambroid liquid glue. No hassle an unless ya drink the stuff, it's safer and MUCH easier to use. But that's just me. To each his/her own.

[Guest]

Well done tutorial Robert! I experimented with welding plastic also at one time. I used a pin point tip to do the "welding" to keep the size of the joint small. I repaired a roof on a pickup that had a sunroof opening in it. It does work well,but it's time consuming compared to using Tenax7 or something similar. It's kinda like the difference between using bondo and lead.

[Harry P.]

I agree. Liquid cement is so much easier and faster. But if you like doing it this way, more power to you.

[Scale-Master]

Seems like a lot of extra effort, and you still need to use putty, (which you need to wait for it to dry...).

How deep is the "weld"? Can you show us the other side of the two pieces where the seam is? Do you also fill that seam with welding/putty or just leave it?

Seems like traditional current cement technology would still be easier and quicker as well as stronger.

[SuperStockAndy]

I can see this being especially useful for body work, thanks

[RobRus]

@ Plowboy and SuperStockAndy, thanks for your comments relating to the topic.

I know most guys use glue of some sort and it doesn't surprise me that you voice your opinion about how easy/safe/fast glue is but it is clear you have not tried welding because you would find out that it is about 75% stronger and about 50% faster once you learn how to do it correctly. The one thing I will give you is that glue is easy and doesn't require any skill.

If done correctly the joint is much stronger than the surrounding plastic because you are using the same material and not introducing a solvent into the mix. I ask you do try an experiment. After you have glued a seam and you think it is dry (in what 12 or 24 hours?), grab your part like a 3 year old would and twist it. When (not if) it breaks apart take an exacto knife and scrape at the seam. You will find that it is soft and mushy inside. That is the solvent that has melted the plastic and will probably not evaporate out for weeks. And even when it has evaporated out completely you will still be able to snap the joint right along the seam. If I snap one of my welds it will not break the seam because there isn't any seam left. You can break it but it will be somewhere other than the weld. Very much like welding metal, the weld is much stronger than the surrounding metal.

As a testimonial of how strong the joints are. I am working on a project right now were I cut part of the roof and hatch off of a car and welded the sail panels back in. As I was just finishing up I made the mistake of setting it down right in front of my dust collector. I just bought that new dust collector you see in the pictures which draws air at 600 CFM and has a velocity of 5500 foot-per-minute at the opening. You can see I have not put a screen over the opening (yet) and when I set the model down it sucked it right into the collector. The hose is a 4" corrugated tube with the collector installed down stairs in my shop. The hose is about 25 feet long and it sucked my model all the way down into the base of the collector.

I was shocked at how quick it sucked the model in and I sat there thinking "all that work for nothing". I went downstairs and opened up the collector and there was my model all in one piece. It has a few nicks and scratches but the welds held up just fine.

@ Scale-Master, The weld is completely through the plastic so there is no seam anymore. If you look at the last 2 pictures I am showing both sides but one is ground down and ready for filler. I could actually sand it smooth and prime it if I wanted to but I prefer to use bondo. As far as you saying current technology is faster and stronger that is just not the case. I am guessing you do not glue a seam up and start filling it as soon as you close the glue bottle do you? And as far as strength... no comparison what so ever since there isn't a seam anymore if it breaks it will break beside the weld or somewhere else but not a properly welded seam.

I am not trying to convert anyone from how they work now. I just thought I would put this tutorial up here for anyone interested in learning about how to weld plastic. I don't think it is really necessary to make a post saying you prefer glue and why.

Now it all honesty, the weld I did for this tutorial is much more "finished" than I would do on a car. I wanted it to be "proper" looking weld for this example.

Here is what my welds usually look like. This is the car that was sucked through my collector. This stage I have already welded the seams and shaped the window opening and the welds you see are just filling in the shape so I don't have to use as much Bondo to smooth it off. It has not been ground down so this is a good example of welding an actual car.

[crazyjim]

Interesting Rob. At the June FAST meeting Grandpamcgurk is going to do a tutorial on plastic welding.

[RobRus]

crazyjim, I would love to go to that and see how someone else does it. I started playing around with it when I was about 15, as a matter of fact the soldering iron I am using is the very same one I started with. It was actually my dads and everytime I used it he would yell at me and take it away... He finally got tired of taking it back...

Back in the mid 70's and I was working in body shops they came out with an actual plastic welder that used hot air in a small jet. I talked my manager into buying one and I took a class on how to use it. The technique of making the V came from that class. At the time cars were going plastic on us with inner fender wells, radiator shrouds and bumpers which are the first things to crack in an accident. We made our money over and over by welding up a crack rather than replace a part.

For anyone interested in this here is a cool video that shows how to use an actual plastic welder. I am doing exactly the same thing but with a soldering iron. They don't show him doing the other side of the bumper but the process is the same, grind out a V and melt in the plastic.

Edited May 8, 2012 by RobRus

[crazyjim]

Well then come on down. The meeting is June 2. A real long time ago I used the Ungar woodburning unit and some tips from the original Auto World. The unit burned out but I still have the tips. One of them has an Xacto blade welded to it for cutting open doors and such.

[GrandpaMcGurk]

That's going to be quite a drive for Rob, Jim. Hang on to the old tips you have...you may need them. I did a tute about this on another site & my technique is similar to Robs.

I don't want to hijack his thread but if he doesn't mind I'll see if I still have the pics and post them here for those interested?

It does take practice so I wouldn't recommend that anyone try it on a model they want to hang on to without practicing on a junker first.

Some folks have mentioned plastic dissolved in solvent and I use that method also......but there are some things that as far as I'm concerned "welding" is the way to go....especially in larger scales.

However....head Rob's warnings........melting plastic not only stinks, will burn your fingers sooner or later and I'm sure chain smoking cigars would be better for your lungs.

Edited May 9, 2012 by GrandpaMcGurk

[RobRus]

@ crazyjim, I would love to come to see that but it is a pretty long drive from Michigan.

@ GrandpaMcGurk, By all means post anything you have on the topic. The more info the better.

[Jantrix]

The one thing I will give you is that glue is easy and doesn't require any skill.

This is the only blatantly incorrect thing you posted Rob. We have seen plenty of evidence to the contrary here.

Now I have to ask. You keep going on about strength. Exactly what are you doing with models where you need more strength than you get with a Tenax bond or your standard gap filling CA glue? Slot cars?

[RobRus]

I was not talking about artistic ability when I said that glue doesn't take any skill.

It takes plenty of skill to chop a car up and turn it into something else.

I was talking about the act of applying the material.

It isn't a matter of how much I abuse a model. It just makes a stronger joint and a stronger joint will last longer.

I still use glue and I am not cursing the glue gods...

Edited May 9, 2012 by RobRus

[Gothic Kustomz]

I use the Ambroid pro weld myself, then using a V file I file the seam, then add plastic using the Ambroid, works great, this is kewl tho,

[Ace-Garageguy]

This is a good alternative skill to have for some applications. It is also the standard technique for repairing thermoplastic (as opposed to thermoset-plastic, like urethane) bumpers on 1:1 cars.

[pandamonium2112]

I for one found this tut very cool and although i probably won't switch completely over to heat welding, i do know that if i'd have used this handy procedure in the past it would have saved me alot of headaches on some mod builds..

Thanks so much Rob, i will remember this one..

[1zebra3]

Great tutorial Rob, I've been using this technique since eary 70s, and as a matter of fact I'm using it right now on two projects.

[farmer1]

Thanks for sharing this, I have thought about trying this after having glue joints show up through the paint. I cut and spliced a roof with glue, let it dry for a couple months, painted it and it looked fine, went ahead and finished and foiled it, a few weeks later that glue joint started to show up . I am definately going to give this a try. I would have liked to seen the expression on your face when that model got sucked down the tube glad it survived the trip for you.

Randy

[Chas SCR]

Why not just use CA glue and kicker and then sand? To fill seems use and old xacto blad and take a drop of CA clue accross the blad and spred it over the seem to fit it in, Hit it with kicker and it turns sold and you can sand it right off with in couple of sec's of doing this. Then you can still use bondo or even what ever you want but if it's done in an open area like a door seem that you are relocating you can sand it smooth to the point you can not see it under the primer when painting.

[shucky]

Over the past 20+ years I've done the plastic "welding" thing myself on various builds that I felt required additional strength. Particularly useful when I chopped and sectioned a 53 Ford. Regardless of what others may think, this is much stronger than using any type of glue only. I tacked the upper and lower body pieces together first using CA then proceeded to plastic "weld" the seem (fumes are crazy bad though). I find this really useful when doing a load of body work that will require a lot of sanding and handling. It does indeed make the seem very strong, much more so than simply using any glue or even epoxy. Last year I chopped and sectioned a 32 Ford Sedan and DID NOT plastic weld the seems. Used CA only. I found myself being extra delicate with handling and sanding and still managed to crack the body 2-3 times whereas when I did the plastic welding it did not happen at all. Basically if you dont like it or dont want to do it, then dont LOL. But it does work well.

[Pro Wrench]

Ive never used this method on a model but I have used this numerous times on 1:1 cars and it works FAR better than any glue. In fact, there is a BMW 525i running around town that I welded the busted gas tank on that no glue on the market would hold. I definitely plan on using this method as much as possible on my models now.

As far as the tutorial, it was full of great information and very well done. I dont undrstand why people have to be rude and basically infer that the original poster is an idiot when its obvious they likely havent tried it so have no idea if it works. What ever happened to just saying thanks for the effort of putting the tutorial together or holding your tongue if you dont plan to ever use the posted method?

[mark wheat]

going to try this thanks