#0020: Plastic welding techniques

Preamble

This is an introductory tutorial on welding plastics. The goal of this tutorial is to be a rather brief yet sufficient guide, that will allow the reader to be able to weld shut cracks and holes in plastic containers to the point of being water tight. It will cover welding technique, tooling, plastic types, PPE, and best practices. Everything an aspirant will need to effectively weld plastic containers.

Tools and materials

Tooling:

• temperature controlled soldering iron or gun
• (optional) fan

Materials:

• appropriate donor plastic strips
• (optional) duct tape or electrical tape

Personal Protective Equipment:

• safety glasses
• filter mask
• thin rubber gloves

Core tool summary

As you can see the core tools and materials list is tiny. All you really need is a hot piece of metal to melt the plastic; and donor plastic material to flow into the various cracks and holes. This material is to reinforce and buttress the affected areas against any structural stress. That’s it.

Temperature controlled soldering iron

I specify a temperature controlled soldering iron (or gun) because you actually need relatively low temperatures to weld plastic; just enough to melt it, but not enough to burn the material. Most thermoregulated soldering irons would be too hot, because they are specced for melting solder. A material that generally has a higher melting point than many plastics. If the your iron is glowing red (even a little bit), then it’s probably a couple of hundred degrees (celsius) too hot.

Additionally, it would be beneficial if the soldering iron had a large thermal mass to enable it to maintain a stable temperature whilst it is in active use. I.e. actively transferring heat into the workpiece. A bit with a large surface area will also be useful, this is to effectively melt a good area of material at a time. Working with smaller bits makes the job more tedious. A smaller bit also concentrates the heat across a smaller surface area, which could cause heat spikes in the workpiece and consequently burn the local plastic. For the reasons above are why I chose to use a soldering gun with it’s widest tip rather than my usual general purpose soldering iron.

Plastic donor material

There’s not actually much to say about this. I like to make sure that the donor plastic is the same type of plastic as the item under repair is made of. This is to assist them in chemically bonding together more effectively. If you don’t want to purchase plastic donor strips like I have; you can alternatively just cut up similar items (i.e. items made from the same material) and use that. I have also seen many people use zip-ties as a donor plastic due to their convenient strip shape.

When selecting a donor plastic, look for a recycling symbol on the donor item (e.g. water bottle). Here it should have a few letters under it. These indicate what type of plastic the item is made from. Another thing to keep in mind when selecting a donor plastic is the food grade safety factor. To know whether or not a container is food safe, look for the knife and fork symbol. Specifically, when repairing a food grade container: do not assume that just because the donor plastic is the same type, as the food grade plastic container; that the donor is also food grade. Even if it initially is, the repair itself my change the chemical structure of the plastic to the point that it will now leech into any food (or drink) stored in it.

A good example is the container I repaired for this article. It was initially a food safe box, but now after the repair, even though it is now watertight again and I repaired it with the same type of plastic (PP); I would not use it for food, for fear of it leeching toxins into my foodstuffs.

common plastic types:

• PET (Polyethylene Terephthalate) typically used in water bottles
• PP (Polypropylene) general use plastic for containers
• HDPE (High-density Polyethylene) typically used in milk bottles

Additional tools

Apart from the essentials, a fan will also help; both to blow any toxic plastic fumes away from you and to help cool the piece quickly as you work on it. Sometimes I find that as I work, an entire section of the container can suddenly soften to the point of almost liquefying, meaning that I will have to wait until it reconstitutes somewhat before work can continue. This however is an excellent state for moulding the pliable material to seal any cracks. Other than that, some duct tape may be useful to make the repair look more presentable and to give it a little more structural strength after a complete repair. Tape can also be used to hold the piece in place, as you weld the cracks.

Recommended PPE

As for PPE: I highly recommend wearing safety glasses and a filter mask. You really don’t want to risk globs of hot plastic flicking into your eyes accidentality. Especially since, if you are anything like me: chances are that your face is very close to the workpiece, in order to see every little detail. You also probably don’t want to suck up all those toxic fumes from any burning plastic either, so a cheap filter mask will help avoid that.

I would also recommend wearing a pair of thin rubber gloves. Although the plastic shouldn’t reach any serious temperatures before melting, a pair of gloves helps you comfortably shape any jellied plastic by hand; should you wish to do so.

Plastic welding techniques

I usually start by placing any loose container segments back into the gaps and holes that they broke out from. In a similar manner to welding metals, I then tack the loose segment into place. This is done by melting small spots across the circumferences of the segments; i.e. melting little bridges across the cracks. Do enough to keep each break out piece in place, or to stabilise a crack (in the case where there aren’t any breakout pieces, just cracks). For any holes where the original broken out piece has gone missing, you’ll have to use the sections of donor material to fill them in. I recommend melting the complete area into jelly, and shaping the mixed plastic mass into form. This is in order to properly blend the plastics into a watertight seal.

As an alternative to tack welds, some sticky tape can be used to hold a breakout piece in place while you weld. But this can get messy when you introduce heat near that tape. Depending on the type of tape you used, just some residual heat can cause the tape’s adhesive to turn into a sticky treacle that cakes the work area. The heat resistant kapton tape may be useful here, but it seems like a waste of resources using it for this application.

The general technique I employ in the actual repair, is by melting the donor strip onto the crack of the container. Then with the broadest side of the soldering gun’s tip, I scrap the extra material into the container’s crevasses. Like a plasterer covering a brick wall; pushing the plastic deep into it’s valleys. Then once one side is completed (exterior, interior), do the same for the other side of the fissure. That’s the short of it.

One thing to note however, this type of repair is not pretty. Although it can be very effective structurally. This is due to the repair involving melting and blending away the cracks. Then adding material to help remove any residual weak points. Weak points which tend to linger after a repair that doesn’t use any donor plastic.

The main reason for using donor plastic is because (in my opinion) plastic shrinks in the presence of heat. In other words, as you repair it with your soldering iron, the repaired plastic is actually smaller than what it was prior. This means that the repaired crack areas are actually thinner than they were originally. So consequently bolstering it with donor material is often necessary. Although admittedly it does make it look awful.

Demonstration of initial fix and seal

Closing thoughts

In summary plastic objects can be repaired should you wish to do so. It’ll require a soldering iron set to a low temperature and some donor plastic of the same type. Take care not to burn the plastic, and for goodness sake don’t breath the fumes in. Also remember that ideally any repaired food safe containers, should no longer be considered food safe. (However that could just be my general paranoia speaking.)

That’s all folks. Best of luck with your plastic welding adventures.

Thanks for reading.