Laser-cutting PLA

By Ben Everard. Posted

We looked, last month, at how to recycle PLA from old 3D prints and support materials into sheet material. The resulting sheet is about 1 to 3 mm thick. In the previous issue, we also looked at slump-moulding this to create bowls and, in this article, we’ll start with those same recycled sheets and look at how to laser-cut it.

PLA has some slightly unusual properties that make it great for 3D printing; the main one being that it softens at a very low temperature. When laser-cutting, this means that the bit that the laser hits obviously vaporises (as happens with other laser-cuttable plastics).

However, the heat from this spreads out and softens much more plastic than you would usually expect. It’s quite common to find that the part is still soft when you’ve finished cutting. The make-up of the laser bed is really important. If there’s not enough support, you may find that your parts sag under their own weight once they soften from the heat. You might also find that some of the softened and liquidised PLA sticks to the print bed and can make it tricky to get small parts off. A bit of a bash usually gets things unstuck, but the more delicate the part, the more of a problem this is likely to be. Sharp corners tend to get a bit rounded off as it just melts away – the same goes for thin branches. If you’re particularly good with your laser control software, it might be possible to plan the job so that cuts on either side of a hole or branch are done at different times, to give the parts more time to cool down, but this isn’t something that we’ve tested.

We’ve also had problems where small parts were cut out and then blown slightly by the air-assist, and rewelded themselves to the main piece. Most of the time, we’ve had success prying them out afterwards but, again, the smaller and more delicate the part, the harder it is. On the whole, we’ve had very few problems with parts (or holes) over about 2 mm × 2 mm, and things get a bit tricky once you go smaller than this. That said, with a bit of post-processing, we’ve had some much smaller parts come out fine.

The final part that makes things a bit tricky is that the sheets we’ve made aren’t perfectly uniform, and can vary in width by about 50%. With care, you can minimise this but, unless you upgrade to a purpose-built sheet press, you’re probably not going to be able to eliminate this. Obviously, this means that you need your power and speed settings calibrated for the thickest part of the material. This means you’re overpowering some of the cuts.

There are some potential benefits to how PLA reacts to the heat as well. When cutting finger joints, we found that if we took the parts straight off the print bed and fitted them together, there was enough softness from the heat to both help tight-fitting parts get together, and then weld themselves slightly together, ensuring a really solid part.

Jewellery making

We really like the look of the swirls of colour you get in the recycled PLA sheets, so we tested the process out by making jewellery.

The first problem is that with this method of pressing sheets, you end up with an irregularly shaped sheet. We’d recommend the method mentioned last month – of scooping up the molten plastic, twisting it into a blob, and then re-pressing it. Not only does this mean the colours swirl together much more pleasingly, but it also means that you end up with a circular sheet. Most CAD programs are designed to work with rectangular sheets, but at least a circle is sufficiently regular to be easy to work with. Use a tape measure to find the smallest distance across, then draw a circle of this diameter in Inkscape (or your CAD program of choice), and whatever fits in this circle, should fit on the sheet. You can delete the circle before cutting.

As we’ve mentioned, there are a few aspects that make small parts tricky to cut. We hedged our bets on this by cutting more than we needed. When making earrings, for example, we cut three of each shape and picked our favourite two. This  also helps because the pattern is quite complex, so it can be hard to place designs to get two earrings with equally balanced colours. By cutting three, you have a better chance of finding two that work well together. This might sound wasteful, but don’t forget that any you don’t use can just be put back in the recycling mix for the next sheet.

Once you’ve got your parts off the cutter, the next challenge is the finishing. The laser leaves quite a lot of residue. Some of this is smoke and some of this is molten plastic that’s been blown away from the cut line. This is exacerbated by the fact that PLA can have two very different finishes, depending on how it cools. Sometimes you’ll find it ends up matt, sometimes really shiny. We won’t go into the technical details, but you often end up with a shiny outline next to the cut and a matt finish elsewhere. This can leave it looking a bit untidy.

How big a problem this is depends on exactly how the part has come off and what it’ll be used for.

Let’s examine a couple of examples of how to get parts looking good. The first star pendant design is quite large and has space to show off a lot of the pattern from the plastic. Some negative space adds an opportunity to give it a bit more detail.

The relatively simple design of this means that there aren’t many lasered edges, so not much needs removing. A quick sanding down with 180 grit sandpaper removes almost all the imperfections. There are still some slight blemishes on the final piece if you look closely enough, but it’s pretty hard to make these out.

The second piece is more organic-looking. A lot of details have been cut with the laser, which means that it needs more work. We start with a heavy sand with 60 grit paper. This removes most of the waste material, but leaves it rough and with a lot of burrs in the details that are hard to remove. We go through the grits to 180 grit paper. This removes the worst of it, but leaves it looking very muted. A flame-polish (see box) brings out the colours with minimal bubbling, but some parts warp a little.

Laser-cutting recycled PLA is a great technique that can lead to really interesting-looking designs that are also kind to the environment. However, it’s not a universal technique that can be used to make anything look great. You need to think about your design and take the time to post-process if you really want to end up with something that looks great.

From HackSpace magazine store


Subscribe to our newsletter