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Welding Plastic
July 19, 2008 2:57 PM   Subscribe

Most people are familiar with welding metal, but it’s entirely possible to weld plastic. There are a surprising number of ways to weld plastic, but first you will need to identify what kind it is. The smell of burning plastic is a particularly effective diagnostic. This man is welding with hot air. Many instructional videos are made by companies whose products are featured in the video, like this somewhat surreal demonstration of speed tip welding. Perhaps the most low-tech method is with a soldering iron.
posted by Tube (42 comments total) 18 users marked this as a favorite

The guy in the "using hot air" example isn't, actually. He just has a soldering iron attachment on the end of a heat gun. So he's still soldering, it's just the tip is being heated by air rather than the usual flame/electrical element.
posted by Brockles at 3:19 PM on July 19, 2008

On second look, it does look similar to the other air welding but either the guy is ham fisted and keeps bumping the air nozzle into the piece accidentally, or it's a solid tip. I can't see if it has a hole in the end or not. It looks very similar to a soldering tip that we have at work, so that is why it looks like 'normal' soldering to me. Maybe he IS just ham fisted, though, cos the action matches the wikipedia description pretty accurately. It's not the best quality video, mind you. It must be dull in (wherever that is) if that's the sort of thing they film each other doing. I expected to see an instructional video like the first link. Interesting stuff, though.

Although I'd rather they didn't find ways to make plastics more usable in engineering applications until they found a way to make the damn stuff biodegradable. I've had a massive aversion to teh stuff ever since I found out about that nasty ball in the ocean. It has made me rethink what things I buy in plastic, that's for sure.
posted by Brockles at 3:29 PM on July 19, 2008

This post wins my personal prize for making me interested in a topic that I would not only have never considered looking into, but felt a certain revulsion toward. I was hooked by the line "the smell of burning plastic is a particularly effective diagnostic," and I now see that you are referring to assessing the type of plastic in hand. I thought it might possibly have referred to the "smell of burning plastic" sometimes reported by patients with brain tumors. Does anybody know about this? (The demonstration of speed tip welding is not only surreal, it also illustrates "speed talk" narration. Who can understand that guy?)
posted by Faze at 3:29 PM on July 19, 2008

The soldering iron is great for forming new bits and parts if you're careful. It's not pretty, but very functional.
posted by IronLizard at 3:43 PM on July 19, 2008

Being as that brings up the 'fixing automotive plastics' question, does anyone know how welding and its suitability for use is affected by the relatively recent change to plastics more suitable for recycling that the motor industry is having to move toward using? Logic suggests that using a material more suited to recycling also suggests better suitability of welding.

Does that logic hold any water at all? Is welding modern (as in the new, new cars built to latest standards) plastic elements as easy? Or are these new materials actually moving away from plastics?
posted by Brockles at 3:49 PM on July 19, 2008

uh, wait a minute. inhaling the smoke from burning plastic is not a good idea. severe lung and/or brain damage can result from doing it.
posted by kitchenrat at 3:59 PM on July 19, 2008

Welding is merely the joining of pieces by merging them together (which differs from soldering, gluing, or brazing, which use a 3rd material to stick bits together). Lots of things you might not think of can be welded, included paper (the usual term for this is papier-mâché), or even glass.

Brockles: as I understand things recyclable plastics are generally easier to weld than non-recyclable, as they're designed to be re-formed. Most non-recyclable plastics undergo an irreversable chemical or structural change when setting, and so can't just be melted down and reformed (which also precludes welding).
posted by Pinback at 4:00 PM on July 19, 2008

i remember this stuff from a plastics course i took, back in the paleolithic ages. thermosetting, and thermoplastic...the two main types of plastic. kool!
posted by billybobtoo at 4:45 PM on July 19, 2008

Anybody remember the "spin welder" toy from the late sixties early seventies? This was basically a small child size battery powered drill that had replaceable tips made of softish plastic. You would take the supplied plastic sheet stock and plastic I-beams in the kit, lay them side by side or overlapped slightly and run a bead of MOLTEN HOT plastic along the edge, which was generated by the friction of the spin welder tip as you pressed it spinning against the hard plastic. I made some pretty rocking structures back in the day. My fingers still hurt thinking about the fumes and the heat though.
posted by HappyHippo at 5:23 PM on July 19, 2008 [1 favorite]

oh and nice post tube :) Thanks!
posted by HappyHippo at 5:24 PM on July 19, 2008

Ultrasonic welding of plastic
posted by Frasermoo at 5:40 PM on July 19, 2008

HappyHippo - I had one of those, waaaay back when. I remember it as more frustrating than fun. But, I've never had much luck with making anything requiring fine motor skills.
posted by bashos_frog at 6:01 PM on July 19, 2008

I made cool scars on my Six Million Dollar Man action figure with my Spin Welder.
posted by cropshy at 6:38 PM on July 19, 2008

Perhaps the most low-tech method is with a soldering iron.
I dunno... When I was a kid, I used to weld my kit-bashed toys by using a lighter...
posted by vertigo25 at 7:14 PM on July 19, 2008

Welding is merely the joining of pieces by merging them together (which differs from soldering, gluing, or brazing, which use a 3rd material to stick bits together). Lots of things you might not think of can be welded, included paper (the usual term for this is papier-mâché), or even glass.

It's using heat to merge them together. Glass can indeed be welded, but paper mâché would be a composite. The Wikipedia page in the first link lists "solvent welding" but that doesn't use heat, so I don't think it should be included.

As a child, I was always frustrated if a plastic toy broke, as I could never repair it. I went through many years of life assuming that broken plastic things couldn't be repaired. It was only a few years ago that I saw a hot air plastic welder for sale in the JC Whitney catalog. For some reason this really blew my mind, so I can only hope that this post might expose a few people who have never heard of such a thing.

I think a lot of hobbyists use adhesives to join plastics, and that's fine, but some slick plastics like polyethylene or polypropylene do not work well with adhesives. When I was in high school my friend Dave discovered that he could weld drop cloths with a clothing iron if the plastic had paper over it. We used this technique to weld seams on hot air balloon UFO's. (self link)

HappyHippo's toy sounds unbelievably cool! I've never encountered such a thing, though I would have been a toy user during that time period. Further research is warranted.

I'm glad people find this interesting.
posted by Tube at 7:22 PM on July 19, 2008

I have modified the handles of a Rubbermaid container with my soldering iron. It was smelly, but I got exactly what I needed. There is so much plastic out there, it's nice to be able to make something of your own with it.
posted by peeedro at 8:48 PM on July 19, 2008

I'm too sleepy to research the specifics, but as a person who does blue-collary work in theatre, where people are often searching for weird solutions like this to weird problems, look out. Heat-affected plastic can almost surely result in some serious life-altering carcinogens. Please, for the love of all intact DNA, do some research before you wander around melting things together.
posted by lauranesson at 9:12 PM on July 19, 2008

Yeah. Jesus. Urethane Supply Company, who sells the urethane rod like that used to weld plastic in the "speed tip" demonstration, has this to say about their Material Safety Data Sheets:

"Granted, nobody ever looks at these things, but some guy sitting in his air-conditioned office in Washington, D.C. has decided for you that you need to keep these on file. He feels like he is doing his "job". Anyway, if some inspector is at your place asking to see copies of your MSDS's, we've got them here for you."

MSDSs are the only means workers in the field have of judging the possible long-term effects of their working materials, and heating up anything generally leads to more dramatic health results. The sheer lack of respect you can see for anyone who would like to know how they might be affected is only compounded when you read the MSDSs from this company and realize they've left out legally mandated information about inhalation limits.

This is honestly serious stuff.

And for the record, I "look at these things" ALL THE TIME.
posted by lauranesson at 9:24 PM on July 19, 2008 [4 favorites]

Your somewhat surreal demonstration link wins as best YT clip of the month. The gal, the music, the narrator, the jelly-like plastic being squeezed from tubes*... perfect.

*um... eponysterical tubes.
posted by flapjax at midnite at 9:52 PM on July 19, 2008

The Wikipedia page in the first link lists "solvent welding" but that doesn't use heat, so I don't think it should be included.

I don't know why you wouldn't. Solvent welding is cleaner, easier, safer, and infinitely more awesome. It probably results in a better bond most lot of the time, too. (I understand that it doesn't fit the strictest definition of "welding", but that seems a little pedantic.)

I mean, I like melting plastic as much as the next guy, but I have also spent plenty of time repairing clear acrylic with Bondene, and let me tell you -- you can't do that with heat.

Tangentially, for those who like playing with plastic structures: I highly recommend trying out a cyanoacrylate (superglue) gel (Bob Smith IC-Gel is the one I've been using) and an accelerator spray. Instant solid acrylic -- for connections, for filling gaps or holes, for sculpture. Coolest stuff ever. And no, I don't work for the company.
posted by medialyte at 10:19 PM on July 19, 2008

This is honestly serious stuff. And for the record, I "look at these things" ALL THE TIME.

I printed out this MSDS and put it next to the coffee pot in the break room at a job ten years ago.
posted by mrbill at 10:33 PM on July 19, 2008 [1 favorite]

Brockles: that hot air tip has a hole about 1/10 of an inch, I have one just like it. I tried using it to weld a product that I originated but ended up using a soldering iron with a Teflon tip
posted by hortense at 10:38 PM on July 19, 2008

Reminder for those reading at home: Do NOT try to melt Thermoset Polyurethanes!!
posted by milnak at 12:05 AM on July 20, 2008

Many "plastic" consumer items are actually made of an amalgamation of materials, which include up to 40% by weight inert materials as "filler," or composite reinforcing fiber; talc, cornstarch, cellulose fiber and high strength materials like glass and silica beads, fiberglas, carbon fiber and metallic fiber are all routinely used fillers and engineering additives in plastic materials. Heating random examples of consumer plastic materials to find out if they can be welded is not a good practice not only for the reasons of volatile release to which lauranesson alludes above, but because of the potential for release of filler material. Abrasive operations or surface preparation to prepare plastic materials for cold welding processes can also release filler materials into the air, where they can be readily inhaled.

With the rising cost of petroleum feedstock for base polymer creation, more products are incorporating higher levels of such "filler" materials, as a cost control measure. These changes can affect the welding properties of the materials, as well as their suitability for joining by adhesives or by re-heating. In fact, identifying the actual type of plastic and the fillers and ratios in which they are incorporated is a major challenge in successfully welding plastic. As an example, low filler styrene is easily welded by a number of processes, but when processed with increasing amounts of cheap talc filler, styrene becomes increasingly difficult to spin weld reliably, and welded articles may easily stress crack at weld points even if welding is accomplished successfully.

Some products traditionally made of thermoplastics like styrene and polypropylene are now even being made of 100% cornstarch "plastic," which is not really a thermoplastic material at all.
posted by paulsc at 12:06 AM on July 20, 2008 [1 favorite]

Having just put together a cool water inlet to my solar panels out of 3/4" PVC pipe, I have to say that the "Solvent Welding" is pretty much welding -- the "cement" is really just a solvent that melts the PVC for 30 seconds or so until the solvent evaporates or migrates into your brain or whatever it does. That's why you have to apply it liberally to both pieces. Also, if you spill the purple primer on your asphalt shingle roof, be prepared to live with a blob of permanent new roof color.

I also bent some PVC pipe using a hot air gun (that I last used to re-seat some tiny surface mount components on my girlfriend's cellphone) for the sunshade frame on our golf cart. Maybe the fact that we use the electric golf cart instead of the gas powered minivan makes up for the fact that we just encouraged the manufacture of 20 more feet of PVC pipe.

After doing my share of plumbing and making stuff with copper pipe (and lots of other kinds of metal), this PVC stuff is incredibly quick, easy, and cheap. $0.10/foot instead of $2? Too bad I still have hot water coming out of the solar panels that requires copper.

I wonder if I can find urethane rods that will fit in my glue gun?
posted by Hello Dad, I'm in Jail at 2:08 AM on July 20, 2008

Yeah, don't try to melt urethanes. They can release cyanide. Polyethylene, on the other hand, is pretty innocuous.
posted by ryanrs at 6:33 AM on July 20, 2008

Man, there are some serious nerds in this thread. Of course, that's what I love about MeFi!!!
posted by tippiedog at 6:38 AM on July 20, 2008

On a related topic, this site will hook you up for gluing.
posted by StickyCarpet at 6:38 AM on July 20, 2008 [1 favorite]

Since it's vaguely related, I'll throw out a link to 3M VHB Tape. It's industrial strength double sided tape, and it's awesome.

Remember that scene in The Blues Brothers where Elwood says, "This is glue... strong stuff", then sabotages the Good Ol' Boys' Winnebago? With VHB tape, that would actually work.
posted by ryanrs at 6:53 AM on July 20, 2008 [3 favorites]

I have to say that the "Solvent Welding" is pretty much welding

That's an interesting one - edge cases always interest me. As Tube says, though, welding is using heat to fuse things together, whereas Solvent welding apes the action of welding, but uses a chemical reaction to melt the material, not heat - the result is melting, but by changing chemical composition not by applying heat. Even if it gets hot while reacting (as it surely does) the heat in this instance is a by product rather than the catalyst of the reaction.

I think to most people calling it welding just explains how it works better. To en engineer or someone who does this kind of thing it will grate as it isn't actually accurate, but rather describes 'bonding that looks like welding'. I don't know why they don't want to call it "Solvent Bonding". It's not like there is any stigma attached to bonding...
posted by Brockles at 6:59 AM on July 20, 2008

Likewise, licking a stamp is like spit-brazing.
posted by ryanrs at 7:09 AM on July 20, 2008 [2 favorites]

For those that maintain that a solvent-adhered joint is "welded", I beg you to take a careful look at this useful page.

Note the following passage:

The AWS definition for a welding process is "a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material".

Note that the Kabbalah-esque diagram above this definition lists "adhesive bonding" as an "allied process".

The AWS trumps Wikipedia as a source of information, as AWS is the source.

On a personal level, telling me that PVC is "welded" with purple goo is like telling a professional chef that frying is the same thing as baking.
posted by Tube at 9:09 AM on July 20, 2008 [1 favorite]

Weld seems to be similar to the term active, as in active vs. passive.
posted by Chuckles at 10:33 AM on July 20, 2008

and with or without the use of filler material

So, they are calling braze/solder a filler material? To my ears that is a strange use of the word filler..
posted by Chuckles at 10:35 AM on July 20, 2008

Chuckles: Definition #12.
posted by theclaw at 11:49 AM on July 20, 2008

"a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material".

Bah. The point of Welding metals, as opposed to Brazing and Soldering, is that you are using *the metal itself* to make the join. Indeed, most welding is less pure than solvent welding, because you almost always add material via a filler rod -- often, not the same material. But this material doesn't act like a glue -- it forms an alloy with the material you are trying to join.

This is exactly what solvent welding is -- you melt the plastic chemically, and force it into contact with another part. What welds steel to steel is steel. What welds PVC to PVC is PVC. The Solvent/Filler just helps the process.

The equivalent of gluing plastic is brazing metal, not welding it -- the joint is plastic-glue-plastic, just as the metal braze joint is metal-filler-metal. If we had a solvent that could melt metal enough to weld, then evaporate, leaving the metal behind, it would supplant heat-based welding in a heartbeat. The biggest problem with welding is all of the heat required -- which changed how the metal at (and near) the weld behaves, compared to metal away from the weld.

This is one reason most building joints -- and many other steel joints -- are mechanical. Bolts don't change how the metal reacts to stress. (The bolt holes do -- but we can make sure the metal can handle the load.) It's far easier to make a bad weld than it is to bolt up a joint improperly.
posted by eriko at 8:26 PM on July 20, 2008

Due to the sage advice given by various individuals in the comments here, I have reimagined my thoughts on what welding is. I have discovered that a bag of brown sugar in my cabinet has SPONTAINOUSLY WELDED ITSELF!!!

Tomorrow I shall be submitting this sucrose sample via FedEx to the AWS. Obviously they are DEAD WRONG when they say coalescence of materials by heating them to suitable temperatures... I expect to be duly revered and commemorated for my breakthrough discovery.

Hey, it wouldn't be the Internet if you didn't post clear, unambiguous, proscriptive definitions of simple concepts, and yet SOMEONE WILL ARGUE THE POINT.

For those who still don't get it, perhaps this technical instruction video will help.
posted by Tube at 10:11 PM on July 20, 2008

I have discovered that a bag of brown sugar in my cabinet has SPONTAINOUSLY WELDED ITSELF!!!

I think that would be sintering :)
(within the logic of this discussion, of course)
posted by Chuckles at 11:11 PM on July 20, 2008

Timely post! Repairing plastic goods is the American Home Ec handicraft skill of the future. In poorer parts of the world, people routinely repair broken plastic stuff at home by - I dare not call it welding - melting other plastic onto it. Plastic buckets, racks, and other housewares are all repaired with other plastic bits. By trial and error people have learned what types of plastic works well for various repairs. For example: The plastic molded handle came off my machete. I stuffed the hole in the hilt with blue plastic twine, heated the base of the machete over the stuff until red hot, and jammed it into the hole. It seemed ok, but after it cooled and I started using it, the blue plastic "glue" cracked and fell out. So on my maid's suggestion I repeated the process with a black plastic trash bag. It works wonderfully, and I save USD 3.50! And that's the second half of the post-Bush economy skill set people will need: distinguishing between different grades of household plastic and their properties.
posted by BinGregory at 11:25 PM on July 20, 2008

...over the stuff stove...
posted by BinGregory at 11:36 PM on July 20, 2008

I for one...
posted by flabdablet at 10:29 PM on July 21, 2008

If you must thermoweld thermoset polyurethane temperature control is crucial, a non wettable heater surface helpful, if you keep the temperature below 350 you can do it.
posted by hortense at 7:47 PM on July 22, 2008

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