PLA Welding – How to Fuse PLA Seams and Cracks

PLA welding is a way to fill in gaps or “glue” 3D prints together.

Using a power drill with a small piece of PLA filament instead of a drill bit, the rotating filament is pushed against the 3D printed plastic. This generates friction, which heats up the filament and softens it. The filament is then free to fill in a crack or weld plastic together. After cooling, this creates a strong bond, as if it had been part of the piece all along. 

Inspired by friction welding, a welding technique typically used to fuse metals, PLA welding applies the concept of utilizing rotational friction to plastics instead. Friction welding is often used in industry to join dissimilar materials at more stable temperatures than other welding methods. Applications include the aerospace and automotive sector, joining together turbine shafts, car frames, and more. 

PLA welding takes advantage of the uniquely low melting point of thermoplastic filament. This means a standard power drill is more than enough to create friction for welding. There’s no need for any specialized tools, which is great news for everyone!

In the following, we’ll take a look at for which situations PLA welding is best suited and then move on to a step-by-step guide.

Though most people stick to superglue to connect printed parts, there are times where glue just doesn’t cut it. In these cases, PLA welding can really save the day. Some common situations include:

• Small surface area joints that require a strong bond
• Curved or spherical joints that don’t have a lot of area for glue
• Complicated shapes that make it difficult to apply glue
• Cracks in poorly printed or broken parts
• Seamless joints (that don’t have a nasty bit of glue sticking out)

Ultimately, there are always alternatives available (which we’ll cover later on), so the method you choose is a matter of personal preference. That said, PLA welding is a quick, versatile, and effective method when mastered – so it’s definitely worth a shot.

Another thing to note is that PLA welding works better when there’s some sort of channel for the filament to fill up, rather than trying to join two flat pieces. This way, the weld reaches deeper into the part and is able to make a stronger bond. To achieve this, you can design a bevel into your part or create one manually with a soldering iron.

For PLA welding, you’ll need the following:

• Power drill, Dremel, or similar rotary tool
• Collet to hold the filament, depending on filament size: 3/32″ for 1.75 mm, or 1/8″ for 3 mm
• Small piece of PLA filament (a few inches, depending on how much you’d like to weld)

Power drills and collet packs can be easily found in hardware stores if you don’t have one already. We don’t need anything too fancy for our purposes. Alternatively, most maker spaces may have a power drill available for those who don’t want to buy one outright. 

To set up, put your collet into the drill and insert your filament, tightening the collet into place. Cut the filament about 15-30 mm from the collet. As you’ll quickly find, longer pieces of filament can be very difficult to control. We want enough filament to work with, but not so much that we can’t wield it properly.

It’s also important that the filament is as straight as possible. Use your fingers to straighten any curves. A heat gun or even hair dryer may come in handy if the material doesn’t cooperate.

So, you’ve set everything up and are ready to weld. Now what? 

The speed of the drill can make a huge difference in welding. Too fast and you might find filament flying everywhere and things spiraling out of control. Too slow, however, and it may take a long time for enough heat to build up to melt the filament. Start out with a medium setting, and adjust according to your needs.

Another thing to watch is how much pressure you put on the seam. You want to create friction and penetrate into the plastic, but do it too much and you’ll end up snapping the filament. It helps to angle the filament at 45 degrees to prevent the piece from bending too much. 

Welding does take some technique. Especially for beginners, it’s recommended to try it out on some scrap pieces first. With practice, you’ll become an expert in no time!

There are many types of welds you can create depending on how you move your drill around. Here are the two main ones:

• Right-to-Left: Holding the filament perpendicular to the seam, drag the drill slowly across from right to left. Since the drill rotates clockwise, melted filament gets deposited directly onto the seam.
• Circular: In this weld, move the drill in a clockwise circular motion as you move down the seam, as if you were drawing tiny swirly circles. This keeps the weld neat and even on both sides.

For an overview of even more techniques, try this video by Polymaker.

Each weld has a slightly different appearance, and some find it easier to weld one way than the other. Welding is sometimes more art than science, so don’t be afraid to combine different techniques or even make up your own.

Ideally, the best welds are built up in layers. If you have a large channel to fill, especially if it’s wider than your filament width, let each layer cool before starting the next one. This ensures a deep and secure join.

Welding may leave bumpy trails behind. Post-processing is essential to getting that perfect, seamless joint. So what’s the best way to smooth the bumps out and finish your PLA weld?

• Sandpaper: Weld in layers until the marks go above the joint and then sand the extra off. Like with smoothing any other print, it’s best to start with coarse sandpaper and later switch to a finer one.
• Paint or polish: Painting or putting polish on your print doubly helps to hide any blemishes from welds. With each successive layer, bumps and marks slowly disappear.

To get the best results, you can combine both methods. It’s surprising how stunning the final product can turn out. For more detail on the exact procedure, refer to this article on PLA smoothing. 

And with that, we’ve completed our PLA weld!

As promised, here are some alternatives to PLA welding:

• Superglue is a maker’s best friend, and works great for most cases. However, it can be tricky to apply and often isn’t as effective with PLA compared to ABS.
• Soldering irons or heat guns can be used to melt plastic. Though it works in a pinch, this tends to make very messy joints. Soldering irons especially can burn PLA and leave black marks. Melted PLA also likes to stick to soldering guns, which is pretty unpleasant to clean up.
• 3D pens are super awesome for joining parts together as well, but not very common. If you’ve got one lying around, though, why not?

Feature image source: 3D Printing Nerd / YouTube