Testing colorFabb varioShore TPU - Foaming 3D printing filament

Previously, we've already been taking a look at another of Colorfabbs foaming filaments, and that was Light Weight PLA. By printing that material at temperatures of up to 250°C, you can lower its density by almost 60%, giving it a really nice texture and, more importantly, making it well suitable for things like RC airplanes, as, for example, Eclipson perfectly demonstrates.

Colorfabb uses a similar approach and adds a foaming agent, which is in the most simple case baking soda, to TPU. This foaming agent that is finely distributed all over the material is activated at elevated temperatures and releases a gas, for example, CO2, and makes the material foam up in it's molten state. The higher the temperature, the more gas is released; hence you can use the temperature to adjust the material's density. With Light Weight PLA this is used to make the material itself lighter while still preserving some mechanical properties. VarioShore TPU specifically uses the degeneration of mechanical properties when you lower the density. This results in very soft parts on printers that would otherwise not be able to handle flexible filaments, or even printing flexibles faster, because you generate more material volume in the nozzle, and therefore, the extruder doesn't need to work as hard.

Colorfabb uses a similar approach and adds a foaming agent, which is in the most simple case baking soda, to TPU. This foaming agent that is finely distributed all over the material is activated at elevated temperatures and releases a gas, for example, CO2, and makes the material foam up in it's molten state. The higher the temperature, the more gas is released; hence you can use the temperature to adjust the material's density. With Light Weight PLA this is used to make the material itself lighter while still preserving some mechanical properties. VarioShore TPU specifically uses the degeneration of mechanical properties when you lower the density. This results in very soft parts on printers that would otherwise not be able to handle flexible filaments, or even printing flexibles faster, because you generate more material volume in the nozzle, and therefore, the extruder doesn't need to work as hard.

Colorfabb sells Varioshore TPU on 700g spools, which usually cost around 50 bucks. Currently, the material only comes in natural and black. It doesn't look significantly different than any other TPUs with maybe the minor difference of a slightly rough and matte surface. This appearance is probably from the foaming agent or due to the lower temperatures they use during filament extrusion to not activate the additive while making the filament. I have the feeling that the slightly textured surface makes it less sticky and I didn't even have issues printing it on an Ender-3, for example. Usually, I'd definitely recommend a direct drive extruder for most flexibles, but the Varioshore TPU seems to be usable on Bowden style printers as well, of course, at low speeds.

First, I had to tune the material, and in this case, I had to find the correlation between extrusion temperature and the flow I needed. To investigate that, I printed simple single wall parts. In the first iteration, the material flow for all temperatures was set to 100% in the slicer, which of course lead to thicker walls at temperatures where we have foaming than the set 0.44mm in the slicer. With those measurements, I started a second tuning iteration in which I already lowered the flow for each temperature according to the last wall thickness with the goal to get all walls to the same dimension. Since the expansion coefficient depends not only on temperature but also on the extrusion rate, the second values were still not spot-on. I again calculated new flow values with which I ran a 3rd test interation after which the walls of each temperature were pretty close to 0.44mm. The final extrusion multipliers were 113% for 190°C where no foaming occurs, and 57% for 220°C, where we have the maximum expansion.

Interestingly, at even higher temperatures, the expansion seems to be smaller. I don't have a definitive answer for that, but I assume that at elevated temperatures, the bubbles in the material get too big and partly collapse. Still, at the ideal temperature, this results in an expansion of 100%, so half the weight or double the volume as the base material, which is significant!

Printing quality for normal models is mostly okay, but due to the foaming nature, stringing or material oozing during travel moves is inevitable, which can be a pain to clean up. Bridges also don't look great with my un-tuned profile. Everything else looks really great, though—smooth and matte surfaces with no layer lines and good overhangs. So if you design your part with this filament in mind and keep travel moves to a minimum, you might be able to produce some really nice looking parts!

We've seen that we can foam up the material to twice the volume but does this also mean that the material becomes twice as soft? To test the Shore Hardness variability of Varioshore TPU I reached out to my friends at Mitutoyo and asked them if I could borrow two of their hardness testers, to which they kindly agreed. With such a durometer, you measure the indentation depth in a material created by a standardized indenter at a given force. The hardness of a material is then measured in Shore hardness on a given scale that corresponds to a specific indenter and force. 100 Shore means no penetration of the indenter, 0 Shore means that the whole range of the tip indented the material. The most common harness scales we're usually confronted with in Shore Hardness A and D. D is for plastics and harder rubbers and features quite a sharp tip, whereas Type A is for medium and soft rubbers where a blunt tip is used. There is a range in which both scales overlap, and theoretically, both methods should work.

In order to test the hardness range of Varioshore TPU I printed a bunch of these small plates that are 3mm thick. The ISO standard for harness testing asks for a minimum material thickness of 6 mm, so I always stacked two on top which is specifically allowed.

Mitutoyo Hardmatic shore Harness Testers

Let's start with Shore D, so the scale for the harder materials. You press the durometer on the material, ideally with a specified force, and usually read the measurement after 15s. To get a feeling for that scale, let's start with standard 3D printing materials. PLA was the hardest with 78, then came ASA and Prusas PC-Blend with 73 and PETG was even a bit softer at 70 Shore D. My cutting matte is around 60 Shore D. Than we made quite a big jump with 46 Shore D for Extrudrs medium hardness TPU. Next, we finally get to Colorfabbs Varioshore TPU. Parts printed at 190°C, when no foaming happens, measured in at 35 Shore D. The samples printed at 200°C already were significantly softer at 25 Shore D and we further decrease the hardness at 210°C with 20 Shore D. You can also definitely feel that with your hand when you bend the material. The 190°C sample is still quite firm, whereas the 200°C sample that's already a bit foamed up is way more compliant. Now it gets interesting because at 220°C we've seen the highest degree of foaming. Will this also be the softest sample? I measured 16 Shore D and as expected, the sample printed at 230°C was already a bit harder at 19 Shore D and on a similar level as the 240°C and 250°C samples. The only material I had that was even slightly softer was Diabase X60 TPU, which measured in at around 14 Shore D, which is already interesting. X60 is soft like Spaghetti, and not the one cooked al-dente, and I wasn't able to print it on my Prusa, not even to speak of a Bowden-style printer like the Ender-3. This material requires a special extruder for flexibles like the OmniaDrop of which I have one on my E3D Toolchanger. Though, if we print the Varioshore TPU at the ideal 220°C we get almost the same level of softness but it's way easier to handle! Think about that!

I repeated the same tests on the Shore A range, where the measurements are usually taken instantaneously using the drag indicator. I didn't even bother testing the standard materials because they are all outside the range. Extruder Medium Soft TPU was the hardest and measured 95 Shore A. Then came all of the VarioShore results. They were starting at 92 Shore A for the samples printed at 190°C. The softest piece again was the 220°C one with 65 Shore A. The parts that were printed even hotter measure in at around 70 Shore A. The black spaghetti, Diabase X60, tested, just as advertised at 59 Shore A so again just slightly softer than our 220°C Varioshore TPU. Pretty cool or what are your thoughts?

So, we were able to double the volume of Varioshore TPU by foaming it up, but does that mean our hardness halved? Let's take a look at the Shore A scale which is the one we should use for this material. We've previously learned that the shore harness values correlate with how far the indentor went into the material. If we calculate the indentation depths from the measured hardnesses, we have 0.21mm of indentation for the unfoamed material and 0.87mm at the maximum foamed level. This means that we not only doubled the softness but even more than quadrupled it! Pretty cool and again confirms the scaling laws for foams that say, that foaming level and mechanical property are usually not correlating linearly, but at, for example, half the density, you lose more than half of a mechanical property. More on that in the Light Weight PLA video.

Let's now also quickly take a look at a way we can vary the softness of materials with simple slicer settings. 3D printing allows us to print parts with variable amounts of infill, which results in variable hardness for TPU parts. To quickly investigate that, I printed samples with different degrees of rectilinear infill in Extrudrs Medium TPU and measured the Shore Hardness. As expected, Shore Hardness A and D decrease with lower infill values, especially between 30 and 10% reaching a softness almost comparable to the maximum foamed Varioshore TPU or Diabase X60. Though this method is not always feasible because wall thickness and orientation will effect those values quite a bit. This investigation is by no means complete, and different infill patterns will give you different results, but I think I could show you the general idea.

So what's Colorfabb's Varioshore TPU now good for? For once, I see it as a material that you can use if you want to print something really soft, but your extruder cannot handle spaghetti filament. Also, you can basically double your printing speed for flexibles because at the same feed speed of the extruder, twice the material comes out of the nozzle. Then, the aesthetic with the nice, matte textured surface finish might be a good base for painting because the color can grab into the surface and things like markers don't smear out like on other 3D prints. I can also think of special applications where you could vary printing temperatures within a model to create some pretty exciting part properties, for example for shoe soles or bike grips. Then there is the density of the material. In the foamed up state, it is around 0.5 to 0.6 g/cm³ which make parts printed with this material quite buoyant. Especially interesting if you need to print something 100% dense, because you need to make sure that no water can seep into the infill. With its softness and therefore toughness, low density and closed porosity it might be a great material for parts that need to float and perform a function.

Because many asked after the last video on foaming materials, I quickly wanted to find out if the parts really don’t absorb water and not act like a sponge. For that, I weighted parts at different forming levels before and after I submerged them in water for 14h. Even though the foamed up parts absorbed slightly more water, in this case, 1% of their weight, they were not like a sponge. I suspect that the internal gas bubbles are mostly not connected, which makes it a closed-cell foam. Therefore this material may be a good solution for things that need to float and where you don't have to be afraid that your model absorbs water or leaks into it.

Testing colorFabb varioShore TPU - Foaming 3D printing filament

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