Repair Cracked Plastic

[Maren Ruminski]

Plasticis the material of the 21st century. After learning how to fix broken plastic I realized how plastic is everywhere. A huge number of day to day things that we use are made of different polymers that most people just throw away as they get broken... and they break all the time. I've turned myself into an addict of soldering plastic. I often take a soldering iron in my back pack and offer to fix people's plastic everywhere. It's impressive the number of things that are just discarded and that could still be useful:

I will show you one of various techniques I've seen to fix plastic. This is the most generic and will fit most cases. This will fix plastics that have some thickness. You won't be able to weld plastic bags or other flexible plastic.

To finish this introduction I would like to ask everyone to correct any misspelled or misused words. As I'm not a native English person I have made mistakes. Just make a comment and I will fix it. Thanks! And I must say that many people have already sent suggestions and corrections. Thank you very much!

Sometimes, when the plastic is too thin or you might have lost some piece or need to seal it for liquids or if the piece suffers big tensions you might need those items. I'll explain better in further steps.

Note: some plastic are already in a decomposition state, they break quite easily, almost crumbling by itself. Decomposition is more common to see in plastic exposed to sun and rain, but can happen to indoor plastic as well. In these cases the welding process won't work at all.

There are many types of soldering irons and stations. Besides ordinary ones, which you just plug on electricity, you can choose one in which you can set the temperature. If the plastic reaches too high temperatures it can loose its original properties and get weaker, breaking easier. High temperatures applied on polymers also produces toxic smoke. See next step on toxicity.

When choosing an iron to buy, try to find a relatively flat tip for it or one that can get flattened. Usually cheaper irons have easier removable tips that are just a metal stick. If you think about making your own tip, those will make your life easier. See the video I've put here showing how I make my flat tips for soldering plastic.

More advanced irons or soldering stations may require more elaborated DIY tips and might be worth to buy a new one. You can wait to see if you will get addicted to fixing plastic as me to spend money on it ;) although they are really cheap.

Instead of making a flat tip you can also think about flatten the one that came with the iron. I've done that in some. A few hammered and a file or grinder can do the job. It's not a problem if you reach the internal copper layer.

I can't give an exact measure, but when the plastic is too thin, like 0,5 mm (0,02 in) or even thicker ones, and the iron is too hot, it just make a hole in the plastic instead of weld it. In this cases you will need some spare plastic parts to add material to the original plastic.

The best option is to take strips of a plastic of the same type of the one you will weld. You can identify your plastic type by the number printed on it inside 3 arrows, like shown in the image above or in this page.

In any case, to make the welding stronger you can use more plastic, as with the plastic strips suggested before, or you can insert some metal inside the plastic. Suggested metal parts may depend on the job to be done. I've already used staple (the one I use most), bent wire (on thicker parts) and small strips of metal sheet (in glasses arms). There was a time when common 1.5V batteries were covered with a metal sheet that you could cut with ordinary scissors. Surely aluminium cans can also be cut with common tools. Try to find the things you can find easier. For me the staples are the most accessible.

If it's hard to hold the parts to be welded together you can try to use some soldering arms, just like in tin electronic soldering, or you can use instant glue to put them in place before soldering. In this case be careful with the soldering smoke in your eyes. If possible, use a mask on them too. I'll talk more about it in the next step.

Many people will probably talk about the toxicity of the plastic and instant glue smokes. I'm not an expert on the subject but, from what I've read, even usual soldering like the one made with tin on electronics and also professional metal soldering smokes are all highly toxic. In all cases you should try to avoid breathing or get your face exposed to the smoke.

After reading some comments made here, I suggest that, if you are concerned about the toxicity, and you should, mainly if you think you will do this often, choose an iron with settable temperature. As you see smoke burning from the plastic, reduce the temperature. Some polymers are more toxic than others and people advised me to be extra careful with PVC, which stands for Poly Vinyl Chloride, listed as carcinogen.

Alternatively to the settable temperature iron you can make a plug connected to a dimmer to reduce the iron power and temperature. I had the spare items necessary to make this at home, maybe you're also lucky :) (don't really know if it's called "dimmer" in English. We call it "dimmer" in Brazil. It is a potentiometer. That in the picture is from a ceiling fan).

You can search the internet for the SDS (formerly MSDS - Material Safety Data Sheet) of the desired polymer. Some have the specification of the recommended respiration protection, like N95 Cartridge, for example. More details at Plasteek and Pgs070947 comments below.

I've already welded a couple of them. Be sure to protect the lenses. Remove them if you can. The heat can damage them permanently. Many lenses are made of polymers nowadays, which will melt with any accidental touch of the hot iron or even only with its proximity.

This is the main step and in many cases can be the only one. Once you get used to it you will can do it very fast. I'm saying that because the big tutorial can intimidate some people, but the main process is very fast.

I usually add more plastic when the fixed parts are too thin or to complete some missing part. Sometimes they start to shrink and some holes appear during soldering. You can cover it with additional plastic, just like people do with tin on electronic soldering. I've embed a video here showing plastic addition.

You can put a plastic strip over the piece to be fixed and weld them together with the iron coming from above the strip until it melts. You need to feel when the plastic below starts to melt. If it doesn't melt at all, then the soldering will be bad and the parts tend to separate after sometime. If you push it too much maybe you will open a new hole on the plastic.

You need to put the metal, let's say a staple, crossing the soldering line and press it with the iron until it enters the plastic by melting it. After finishing you may want to add more plastic over it so the metal don't stay exposed.

After doing the weld bead you can try to make the surface smooth again by passing the iron over it. A flat tip will do this job better. The glass on the first picture has been finished this way and it has a staple inside it. After smoothing I have painted the front with a permanent marker.

It's a good idea to test the work after finishing. It's also a good idea to wait for it to cool down completely before testing. Try to make as much pressure or tension as you think it will suffer when used.

Some plastic suffer great tensions, as plastic chairs. A good way to test them is to really use them in extreme situations, like putting all your weight on it, not letting your legs on the floor, and maybe also jumping a little bit over it. Just be prepared for a sudden break.

I've read, here on FPN and on eBay, of 'professionally repaired' cracks in a pen. What do you use to do a crack repair like that? I found out the hard way that super glue, while effective, will melt and scar most (many?) pen plastics. And how do you 'stabilize' a crack? I have a few 'beater' pens that I really like, that make good user pens, but that have cracks in the section that allow ink to leak out. For the time being I've been able to stop the leaks using silicone adhesive but over time that peels off.

BTW - I did try to search the archives before I posted this but while I found a bazillion references to cracks and repairs, I couldn't find one to answer my question (though I'm sure there are plenty of them out there somewhere).

- If the celluloid is quite thick and subsequently there is a large mating surface to both sides of the solvent by itself may work adequately for strength but do the slight melting of the plastic by the solvent you will have a slight depression to the edges of the crack and would then have to smooth it out to reduce noticeability.

- I have found that taking old broken clear plastic Parker Vacumatic plungers and sanding them with a 220 grit sanding block makes a nice fine powder that due to it's clarity will be unnoticeable with most semi transparent celluloids.

- I pile the plastic powder carefully into and around the ground out crack and then after drop the solvent onto the pile of powder with an eyedropper that has a nice small opening. (these can be found at most hobby shops)

- While the powder and solvent are still soft I then condense it and press it into the crack further by rolling a smooth piece of metal over the pile. This also helps to shape the mixture so that once it sets it is easier to get the natural shape of the pen back during finishing.

- Depending on how much material I need to remove and how rough it is I will start with either 2400 or 3600 micro mesh and wet sand it working down through 6000, 8000 and 12000 grit to finish it and give it that beautiful smooth new plastic sheen.

- If the crack was on the lip of a cap or barrel and the very edge of the lip is quite thin you may want to also reduce the edge of the lip slightly to make the edge a little thicker. This will help reduce the risk of future cracks both to your repair and to the rest of the original plastic and will also help make the lip even all the way around.

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