Transparency tests on PET

[geert_2 558]

I did a couple of transparency tests on waterclear PET. See the pics.

 

Test specs:

- material: PET, colorless ("waterclear"), brand: ICE (Belgium).

- model: block of 20mm x 10mm x 10mm, with a watermark logo included halfway at 5mm height, saying: "©️GEERT" (Caps-heigth = 3.5mm).

- printing temp: 215°C, unless written otherwise. This is at the lower end of the specified range of 215...250°C for this material.

- top row: all printed at 50mm/s, and 150mm/s travel speed (=the defaults for my UM2).

- bottom row: all printed at 10mm/s, and 20mm/s travel speed, thus ultraslow.

- layer height from left to right: 0.40mm, 0.30mm, 0.20mm, 0.10mm, 0.06mm.

- nozzle: 0.4mm (standard nozzle of my UM2, non-plus).

- infill: always 100%.

- flowrate: always 110%, to really push the material into all corners and close the gaps.

- exceptions to the specs: model at bottom-right (=10mm/s, 0.06mm): printed at 210°C to avoid overheating and discoloring. Models at top-row left (0.40mm and 0.30mm) printed at 225°C, to easier melt the material at that higher flowrate.

- I have not post-processed any of these models (except cutting off the final "take-off string"), thus no grinding, no polishing. So the irregularities on the top and sides do distort the transparency and visibility of the logo.

 

Results:

- The prints are far from transparent, but in some cases the watermark can easily be seen.

- The best results are at the lowest layer heights.

- Slower speeds give clearer models. However, when printing very slow, the model tends to discolor and get yellow-brown. This seems a bit similar to the brown goo that sometimes accumulates under nozzles.

- Transparency (or lack of) gets worse as layer heights increase. You can see there is something inside the "frosted glass", but you can't read the logo.

- At the thickest layer heights, clarity improves again, but then a crystal- or prism-effect begins to occur, with sparkles and rainbow-colors, which drowns the watermark.

- If the model would be only 2 or 3mm thick, it is still transparent enough and the watermark can always be seen, although it gets distorted in the thickest layer models.

- When printing very slow at thicker layers, the model starts to bulge due to over-extrusion.

- At 0.40mm layer-height and 50mm/s the model does not cool enough, stays soft and deforms.

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited January 24, 2019 by geert_2

[geert_2 558]

I polished such a test piece, see the pics below. Both were printed at 0.06mm layer height, 10mm/s. The unpolished one (=same as from previous pics) was printed at 210°C, the new polished one at 200°C, thus far below the recommended range of 215...250°C.

 

Hard to see on photo, but in real life the coolest printed one is less yellow.

 

The difference in transparency is most obvious from the side, which looks dull in the unpolished model, and sparkling in the polished one. The top layer of the polished one is okay, but the bottom contains voids between the first couples of layers, which can't be polished out, unless I would remove all these layers. These voids are clearly visible from the top (the diagonal infill-lines), and they sort of destroy the transparency effect.

 

So, this seems to be about how far I can get with standard PET.

 

Unfortunately, on photo you can't appreciate the 3D-effect of the logo floating halfway the model, which makes it stand out compared to usual 3D-printed models.

 

This concept is good for watermarking your models in STL-files you distribute. If your model contains enough of these watermarks, then plagiarists will have a hard time finding and removing them all. Selecting and deleting watermarks on the surface is one thing, deleting them from deep inside a model is another. In your CAD- and STL-files you can also model them microscopically small, so that they are almost invisible. These won't be sliced in Cura and don't show up in layer view (no need to, since they can't be printed anyway). But they are visible when zooming in a lot, in transparent or X-ray view. So, if you hide microscopic watermarks in corners, you can still easily claim ownership by zooming in in Cura.

 

 

 

 

 

 

[geert_2 558]

When doing these transparency tests, I also noticed that bubbles do not only occur in the material due to moisture. Even when the filament was very dry, I got exploding bubbles when printing fast and hot at 0.3mm and 0.4mm layer heights. The air which is trapped between the extruded sausages heats up and expands. This causes the still molten plastic to erupt like a micro-volcano, leaving a crater afterwards. Some bubbles don't make it to the surface, but they get big enough to destroy transparency, and to cause an irregular surface. It is clear that they are in the seam-lines between sausages, not in the center of the sausages.

 

This is visible under a binocular microscope (which gives good depth of view), but extremely hard to photograph, so no photos yet.

 

So, even though printing hotter makes the filament flow into all corners easier, it worsens this effect. Thick layers cool slower than thin layers, which again gives trapped bubbles more time to expand and to make it to the surface. Maybe that is one of the reasons why thin layers, printed slow and cool give a better result in my tests here?

 

Edited January 29, 2019 by geert_2
corrected typo

[PaulK 14]

10 minutes ago, geert_2 said:

Even when the filament was very dry, I got exploding bubbles when printing fast and hot at 0.3mm and 0.4mm layer heights

 

TGlase (a PETT) instructions for printing clear include turning the nozzle temp up until bubbles appear (and backing off slightly), so it seems to be a common thing for transparency.

[JonGregory 0]

I've been running similar tests - I'll send the results when I get a moment. But I've noticed that quite a few of my prints become opaque as soon as the second layer is laid down - it looks like the first layer is laid down with good transparency (when the temperature is low enough to avoid bubbles) - but the nozzles scores this fresh surface as it lays down the second layer, scratching it and making it opaque.

 

Has anyone else noticed that kind of behaviour and found a fix?

 

Cheers

 

[geert_2 558]

Is it scratching the surface, or is it entrapping air bubbles inbetween both layers, when seen through a microscope? This also makes it opaque like frosted glass. I only have seen the entrapping in my tests: mostly in the seems between the little sausages, sometimes on top of surfaces as the top layer is not perfectly flat.

 

(In case you don't have a microscope, sometimes a high-resolution camera with powerfull macro lens can also show this, better than with the naked eye.)

 

[JonGregory 0]

I don't have a microscope, but I'll try and get a few shots. 

 

What build plate temperature did you set for your PET builds? 

[geert_2 558]

5 hours ago, JonGregory said:

I don't have a microscope, but I'll try and get a few shots. 

 

What build plate temperature did you set for your PET builds? 

 

I think it was 80°C, maybe 85°C, something around that.

 

If you would have binoculars, or a telescope, you could try to hold them upside-down with the ocular very close to the model, close to a bright light. Quality won't be good, and you will get a lot of deformation, but it might be good enough for inspection. A telescope works exactly opposite to a microscope: the lenses are swapped (and in real life of course adapted for minimal distortion). In a microscope the little, most curved lense is close to the object, and the flattest and big lense is the ocular.

 

For people who do lots of 3D-printing and post-processing, and who get a bit older (=eyes deteriorating), it might be a good idea to get an old binocular microscope. This makes post-processing a lot easier. I use it daily. Sometimes universities or research institutes sell their worn-out models when replacing them with new ones equipped with cameras and all sorts of fancy stuff. But for our kind of work, 40 year old models are good enough, provided that the lenses are still good.