Broken Plastic? No Problem!

When a computer case has survived several decades from being a new toy through being an unloved relic to being rediscovered and finding its way into the hands of an enthusiast, it is inevitable that it will have picked up some damage along the way. It will be scuffed, maybe cracked, and often broken. If it has faced the ordeal of an international courier after an eBay sale then the likelihood of a break increases significantly.

If you receive a vintage computer in the post and find it cracked or broken, never fear. [Drygol] has a solution, a guide to plastic welding with a soldering iron.

After a thorough cleaning, the technique is to hold the sides of the break together, run the iron along it to melt the plastic together, and scrape the overflowed plastic back into the resulting trench before it solidifies. With careful sanding, a spot of polyester putty, and some spray paint, the broken case can be returned to new condition.

There is a video showing the process, in this case repairing a crack on a Commodore 64 case.

Plastic welding is a useful skill to add to your arsenal for many more applications than old computer cases, but it’s safe to say it requires some practice to master. In the interests of safety we’d like to suggest it’s best done in an area with adequate ventilation.

We’ve shown you other plastic welding techniques in the past, including ultrasonic welding, and friction welding. If you do any plastic welding jobs you’re proud of, drop us a line.

34 thoughts on “Broken Plastic? No Problem!

  1. You might want to to also link to this, since it is relevant here.

    The plastic used on Commodore 64 machines is ABS. Plastic welding this is similar to “modern” 3d extrusion based printers in that it uses, well, ABS plastic to weld other ABS plastic. You can also buy dedicated plastic welding machines that act as large hand held 3D printer nozzles and take spools of material. Basically big MIG welders, just with plastic instead of steel.

    Also, Jenny…. PLEASE start putting commas in your articles. I can always tell when you are posting something without even looking at the author because the errors in your posts crop up as many as several times an article.

    “In the interests of safety (PUT A COMMA HERE) we’d like to suggest it’s best done in an area with adequate ventilation.”

    “Poor grammar revealed laziness and a lack of respect for the reader. It’s the literary form of bad manners and exposes the writer as someone who isn’t serious about the craft.

    If you’re an author, particularly a self-published author, you need to do everything possible to win your readers’ hearts and minds. When they are distracted by grammatical errors or confused by the meaning of a sentence, they aren’t likely to buy your next book — or finish the one they are reading.”

    Missing Comma After Introductory Element

    A comma should be used after an introductory word, phrase, or clause. This gives the reader a slight pause after an introductory element and often can help avoid confusion.

    Example 1:

    Incorrect: In case you haven’t noticed my real name doesn’t appear in the article.

    Correct: In case you haven’t noticed, my real name doesn’t appear in the article.

    Example 2:

    Incorrect: Before she had time to think about it Sharon jumped into the icy pool.

    Correct: Before she had time to think about it, Sharon jumped into the icy pool.

    1. Please, be polite, to correct supposed issues, or just send an email to HaD.
      The first four paragraphs were great, but the examples seemed superfluous.

      P.S.: How much of that was copy-paste from Wikipedia?

    2. I see proper use of commas in the article however you should perhaps take your own advice? E.g. what does the “….” represent? First it isn’t the proper ellipsis “…” and second the use of an ellipsis isn’t correct in the way you do it, it should be used to indicate that the text isn’t complete and is intentionally left out. In your case it is used as a pause before continuing. A comma would be proper, a “can’t remember the name” symbol could be used (ASCII representation –) a semicolon could be used (by that’s not really “okay”) and I sometimes use a colon (and that’s also not “okay”).

      Before anyone begin to point out errors in my posts: I know I don’t write properly at all times, I have trouble spelling some things (modern computers help a lot in those cases) etc., but then I’m not a grammar nazi…

      1. I agree on much of this, but we (commenters) don’t have the ability to go back and edit our comments. So when silly typos, grammar, or editing errors creep in, we’re stuck.

        As others have pointed out (and for a long time) Hackaday *articles* is riddled with spelling errors (often–embarrassingly in article headlines), and even when politely pointed out, they are seldom corrected.

    3. Rimmer: After intensive investigation, comma, of the markings on the alien pod, comma, it has become clear, comma, to me, comma, that we are dealing, comma, with a species of awesome intellect, colon.
      (Red Dwarf, Waiting for God)

  2. Whoa! Grammar aside, get some solvent at the level of acetone or MEK. Something that simply, in one drop welds clean breaks in most plastics. No bubbles, burns, gaps, just a nearly invisible line and original strength. Rat Shack used to sell the chemical and powdered plastic that one could build up repairs better than new. The powder works when things don’t fit neatly. The chemical is huff-able so access may vary. I used to mess around with a gun but only for a hidden job where filler plastic could be added. Avoid charring!

    1. Have you used solvent before to do this kind of thing? It would seem to me that dosage is crucial. If too much solvent is applied, I would expect a large sag or a hole.

      Please share any knowledge.

      1. For fine “dead end” cracks, use a non-viscous solvent applied with a very small artist’s brush. I’ve used methylene chloride (caution: can cause cancer, but I’m old enough that the extra micromorts are irrelevant).

        That is wicked into the crack, leaving none on the surface. Applying gentle pressure and movement then “melts” the plastic inside the crack.

        I’ve also successfully used it on a mixed plastic “ABS” case where one piece had become detached. Once repositioned the cracks were narrow but not fine.

    2. I’ve had problems with acetone e.g. when fixing C64 cases – either there are bubbles in the joint or the joint isn’t strong enough. In most cases (pun not intended) I use plastic welding inside the case (where it is invisible) and an acetone joint on the outside.

  3. Done this many a time, for 4 or so years at work. Essentially I was 3d-printing with a smoldering* iron before 3d printing was a thing:

    Aged bulbs would arc out on some equipment with ltn121xj-l06 LCDs or compatible. The plastics would become heated and age quickly during service. The only way when some of the stocks had ran dry was to melt the plastics into roughly the correct shape and file the rough printed-by-hand repair back into the correct shape, then drill the screw fixing hole.

    Done that until the customer requested all the units be scrapped as they finally upgraded to something a bit more modern.

    *Smoldering iron: An iron with a bad/worn tip and has been repurposed to “Smolder” plastics together.

  4. You can also grind up a lot of scrap ABS, or buy nice shiny new pellets and melt it overnight in Methyl Ethyl Ketone and/or Acetone, and use that to cement and fill.

    Though I have been wondering how useful those “freehand 3D printer pen” type things would be, locked and loaded with ABS for ABS repair.

    1. The ABS putty shrinks quite a bit when it dries up.

      For shits and giggles, try dropping a bunch into a bowl of water and knead it like dough. Acetone dissolves in water, so it draws H2O into the plastic and forms a rubbery material that conforms to any shape, sticks to itself without a seam, and dries up into a very hard substance over time. Kneading in a bit of filler, like starch or chalk, can help the curing time because ABS itself isn’t permeable to water and it takes ages to diffuse out.

      If you put it in an oven, it rises like a cookie dough as the water steams out, and leaves hard ABS foam.

        1. I’d have to get more ABS and acetone.

          I tried it once to test a hypothesis that water would draw the acetone out of the plastic to cure it faster, but that didn’t work out. Instead the water got drawn into the plastic as the acetone came out. The result was a very bouncy non-greasy substance that would stick to itself but not much else, like extremely thick silly putty.

          Then I tried to press it into molds, but found out it took days for the water to come out and it would shrink to a shriveled leathery appearance. I rolled a 1″ ball out of it and found that it was still slightly malleable two days later. I re-shaped it over and over until the shape took, and then couple weeks later I cut it open: it was hard plastic with a number of air bubbles throughout. One effect of acetone on ABS is that it becomes glassy and more brittle after curing, and this was apparent here as well.

          I tried it again and put some putty close under a 40 Watt halogen lamp to heat it, and it started rising like cookie dough. That experiment exhausted my batch of dissolved ABS, so I left it at that.

    1. Back in the day, there was a toy that worked that way. I’m pretty sure the piece being used as a “welding rod” must be straight or it will just flail around. Does 3D printer filament straighten out enough when removed from the reel?

    1. I’ve given up worrying since that city went hardass on emissions and banned a lot of private traffic and still had photochemical smog issues….. whereupon they called in the scientists again and it was eventually found that a huge chunk of the VOCs were coming from trees….. fresh air ain’t never been fresh.

  5. This is pretty cool. I’ve done this on large plastic pieces that need a strong structure after the repair is done. Doesn’t work well on glass fiber impregnated plastics. On bends, I’ve used bolts, screws or whatever I have handy, melt the plastic and shove in the screw (that should sound so wrong, but it doesn’t…)

    If you need to preserve the original beauty, use adhesives. I learned a long time ago that there is an adhesive for everything. EVERYTHING. You just need to know the names and how to work with the different requirements. For instance, there’s a red epoxy out there that only cures when you hit it with a blow torch.

    Here’s an example: a case has a crack on a flat spot. Clean the backside and the cracked ridge itself with STRONG solvents. Don’t leave the solvent on long. Some will melt plastic in less than a second (DCM) so test it first.. Oils will stick around in pores even after you think you’ve cleaned it with alcohol. Oil flows better when warm, so warm the plastic and the solvent if you can before cleaning. Use ventilation please…I’ve used heated up DCM (DiChloroMethane) before to instantly clean an entire carbeurator. The vapors are enough to knock you out cold. Scared of gluing something you shouldn’t? Use a ear swab and a tiny bit of Vaseline. Nothing sticks to that. Side note: Don’t use hard epoxy’s on plastic. The kind you mold with your hand like play-dough. It doesn’t flow into the pores very well.

    Get your plastic ready by harshly cutting up the back side. I use a big shop file if it fits. Otherwise, use a dremel and score he living hell out of it leaving threads of plastic jutting off the surface. Epoxy likes grabbing on to fibers for traction.

    Test fit the plastic to see if it’s seamless when pressed together. If not, break out an exacto or dremel and shape the bent plastic that’s causing an interference problem. Remember: the whole idea of doing it this way is to try and completely hide the damage..

    Take duct tape and tear off a strip. Go into the kitchen and hope you have wax paper (or just use vaseline on the duct tape).. Cut a small piece that is a little larger than the crack and put it on the middle of the duct tape. Get that Plastic welding cement out that melts plastic. . Tesssst iiiiit first! You might end up using a tiny bit of cheap acetone. if the chemical composition is right. Mix up some specialty plastic Epoxy. Not the quickset stuff. The longer it takes to cure, the stronger it is (more time to settle in to the pores).. Something like PC-11. Or just use JBWeld! JBWeld is my favorite general purpose epoxy but boy does it have a long cure time. The epoxy will form a bridge on the backside giving the plastic strength to support the new cemented joint. Sometimes I take a rod of plastic/wood/aluminum or even a deck screw and epoxy it as a bridge when I’m gluing a corner together or a part that acts as a structural component. Don’t put too much epoxy on! Epoxy is rigid. It’s resin based. You want the end product to be able to flex as much as possible while still remaining in one piece. .

    Once the plastic can be snugged together, break out the cement and put it on JUST the inside of the seam. Use a thin layer. Work fast. The plastic flowing effect of cement is nearly instant. Hold it there for 60 seconds or so. Give it a chance to dry just a little. Apply the duct tape so the wax paper covers thew crack on the outside. Make sure the duct tape is on there as tight as you can. Now you can turn the plastic around and take your time building the epoxy bridge.

    Done! The wife/husband will never know!

    Don’t use super glue if you care about cosmetics and strength. SG emits a nasty adhesive vapor over a period of several hours that ruins the look. Also, it’s sort of crystalline. The cement melts the plastic and allows it to weld without vapors.

    Side note, I’ve seen industrial adhesives that are so effective they stick to ceramics, glass, stainless steel, etc and are as hard as steel. If you want to become a multimillionaire play with adhesives to try and come up with a new strain that does the impossible…like gluing divorced couples back together.

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