Why PETG?

February 5, 2024

I make a lot of parts using 3D modelling and FFF printing with PETG.

Polyethylene terephthalate (PET) is a food grade plastic (yum) and PETG is the same thing with glycol added so it can be printed with Fused Filament Fabrication (FFF¹ ) techniques.

I settled on PETG for pinball parts for a bunch of reasons.

Impact Resistance

Impact resistance is an absolute essential for anything above the playfield. Stuff gets bashed.

Imagine my delight when I read that PETG is know for its impact resistance. I gave one of my samples a solid belting with a hammer. Not a scientific test, but there was good bounce, and no damage. I’m looking forward to doing some playfield tests in the future.

Strength

Strong, stiff, and durable - what everybody wants! In practice, a 12mm square rod, 20cm long, 20% infill, is surprisingly hard to bend.

UV Resistance

PETG is way better than ABS at withstanding UV. I want my parts to last 10 years minimum, so this is a handy attribute.

Recyclable

The product is recyclable, and can be purchased as recycled material. This meets our overarching goal #3, RRR. You can even make your own filament out of old drink bottles - that’s not for me :-)

Chemical Resistance

PETG has very good chemical resistance (but not some solvents). ABS has poor chemical resistance in comparison. Printed parts should be good with playfield polish and paints.

Warp Resistance

In FFF¹ printing, ABS can warp, while PETG loves to stick to itself, and is much more warp resistant.

Painting

While it’s not easy to paint PETG, it can be done with a good plastic primer applied. Rust-Oleum is reported to work well, and since that’s a common paint brand used in pinball machines (and a good bet for restorations and reused cabinets to avoid chemical reactions with old paint), that’s good enough for me.

Sanding

I read it sands better than PLA, that’s good news. I don’t design parts that need to be sanded, but things happen during a print, and some of those things can be fixed with sandpaper. It’s handy to have as an option.

Vapour Smoothing

It can be done - I’ve not experimented with it, and the results are described as less pronounced than for other filament types, due to PETG chemical resistance. As for sanding, it’s handy to have. Some of my playfield designs have nice detail. It may be that vapour smoothing lies in my future.

What PETG is Not Good at

It’s hard to glue, and it’s more difficult to paint than many alternatives.

Problems in the Real World

People report melting tubes in their hot-end - this can happen if you have a plastic liner in the tube that takes the filament to the hot bit - some of the PETG temperatures are hot enough to melt the liner. Check first before you crank up the heat, or find a filament that works at a lower temperature.

Hygroscopicity is a fancy word that means water-suck-up-ness. PETG has big big big water-suck-up-ness. I did a test by leaving some filament out to see how it changes. While PLA would become brittle and snap, my middle-of-the-range PETG looks and feels fine after a few days out in the moist summer air, but for me it underextruded, and hated sticking to itself - so prints became way less detailed, and un-strong. Concerningly - a perfect-looking print of a rack (rack and pinion) was strong to touch, but the teeth just rubbed off. A second print at a higher temperature compensated for the lack of adhesion, but I’m attributing the cause to water absorption.

Some use the term FDM, but that’s trademarked by Stratasys.