There are many resin printing materials out there marketed as “engineering”, “for functional parts”, “heavy-duty”. Since I got into the resin printing world, I tried a large number of them. However, none of them in my opinion didn’t deliver what was promised. The main challenges are not only low strength but also low impact resistance and most notably insufficient surface properties. Most of the 3D-printing resins out there are easy to scratch and when two surfaces mate, they have relatively high friction, and, most notably, they grind each other and form a white powder.
I was given the opportunity to test a new material – Siraya Tech Fast Mecha that claims to be suitable for articulated functional parts. The marketing is that the material doesn’t grind when two surfaces interact. Is it true? We will find out in this hands-on review. For clarity; I was given a sample of Siraya Tech Fast Mecha for free before it was available to the general public. I wasn’t paid in any other way for this review and all opinions are mine.
Fast Mecha is a composite material, similarly to Sculp Ultra. That means it is a resin with a high amount of filler in it. Filler is a fine-grained powder that forms suspension with the resin. The filler is supposed to improve resin properties in some way. In extreme cases when the amount of filler is large, the resin only serves as glue to the powder.
Fast Mecha comes in a nice snow-white color that is caused by the filler inside. The resin smells just like regular Fast but it is thicker. However, it is much thinner than other resins like Sculpt or Blu. Unlike Blu, when you heat it doesn’t get much thinner – only slightly. Fast Mecha requires practically the same exposure times as Fast – that is about 2.5s on Elegoo Saturn. Compared to Fast it requires bigger rest times (more on that later) and also cleaning is a little harder.
The printed parts are really nice – they are matte, snow-white. The prints have visible layer lines in the Z-direction (pretty much like any other resin), but the surface in the XY direction is really smooth. First, that seemed strange, however, once I dug into the dimensional accuracy of the prints, I came up with a reasonable solution. We will explain this phenomenon in the following section where we discuss dimensional accuracy.
The surface of the prints is nail-scratch resistant. The printed parts have very low surface friction and no matter how you rub them against each other, they don’t grind and nicely slide against each other without any lubrication. I haven’t seen such property in a resin so far. I was pleasantly surprised that the marketed value of “no white residue on mating surfaces” is true. However, since the resin has notable layer lines, you can hear a “hard” or “ceramic-like” sound when sliding two parts across the layer lines. It sounds exactly like when you take a file and rub it against hardened steel. You get very similar results. However, the surface of the resin isn’t particularly hard – the resin is relatively easy to sand, drill, or tap. Especially the threads came out nicely.
When you slide two surfaces against each other under large pressure the surface seems to develop a shiny surface finish. It seems like the individual grains of the filler settles in position and form an even surface. This effect happens relatively quickly. Once the surface is shiny, it doesn’t seem to undergo any other changes in long-time stress.
As one of the first testing pieces, I printed an old design – a worm-gear gearbox. Though the design wasn’t meant to be back drivable and with the standard materials it is not, to my surprise; the design was back drivable. This seems like a neat example of the low surface friction properties.
What To Be Careful About
Since the resin is relatively thick, it is prone to blooming as I have explained in one of my previous posts. For Saturn, it seems that for large surface cross-sections resting for about ~8-10 seconds seems sufficient.
The second problem I found with this resin is dimensional inaccuracy. According to my experiments, the resin doesn’t suffer from shrinkage similarly to other composite resins (i.e., Sculpt Ultra). However, the resin suffers from “overgrowing” in the XY direction. Basically, it overgrows the dimensions by 30-70 µm (no matter what the dimensions are). Based on consultations with Siraya Tech we assume that this is caused by the snow-white filler in the resin that spreads the UV light all around the cured voxel. If it is indeed the underlying mechanics is hard to tell, but it seems a possible explanation. Nevertheless, this dimensional inaccuracy can be compensated by the slicer. Once I introduced these compensations, I was able to get the voxel-round dimensions within 20 µm.
The light bleeding also might be the reason why the surface in the XY direction is so smooth – the resin “anti-aliases” itself. Note that I don’t observe similar behavior with Sculp Ultra which is also a composite resin. The prints with Sculpt ultra are dimensionally spot-on – no shrinkage, no overgrowing.
Since it is a composite resin, you should be careful to properly mix it before every use as the filler might sediment. However, I haven’t experienced much sedimentation of the filler – there is some, but it seems marginal to me, e.g., compared to Sculpt Ultra. The resin forms a surprisingly stable suspension.
Also, the resin seems to have good adhesion to the build plate – so strong, that it often gets some aluminum dirt to the face facing the build plate. See the picture below:
Mechanical Properties & Real Life Tests
Note that the characterization I provide in this section is based on subjective observation and experience; I don’t have a universal testing machine to validate properties of resins rigorously (though, I have recently considered getting one similar to Stefan’s).
The printed parts have roughly the same strength/brittleness as regular Fast. For resin, the material is relatively ductile (and it is not as brittle as the standard Elegoo Standard rubbish resins). It is also much more ductile compared to another composite resin from Siraya Tech – Sculpt Ultra.
To test the mechanical properties of resin under real-life conditions I use my 1:85 compound planetary gearbox. The gearbox has M0.5 teeth (roughly half the size of Lego) and requires quite high precision of the parts to assemble and run smoothly. In my tests, I print several pieces of the gearbox, test how they run, and then measure the torque they are able to transmit before breaking.
The printed parts are really nice. I would say they have the best surface finish from all the resin I tested so far. Judge for yourself in the photos below. The prints were printed with 4× AA.
With the compensations mentioned in the previous section, there are no problems with assembling. Everything fits nicely and the bearings have a good press fit.
Since Fast Mecha seems to have low surface friction I performed all my experiments without lubrication of the gearbox. When you try to spin it, it runs very smoothly. First, I let run the gearbox with 5000 RPM for one hour. The gearbox is running smoothly and quietly. Then I disassembled the gearbox to inspect the damage on the teeth. To my surprise, there were no signs of teeth wear – no white residue, no visible damage on the teeth. The teeth were just a little shinier just like I observed in my other experiments.
Then I moved to the load test. My first experiment failed at 2.4 Nm – which is the same as regular Fast failed. However, after inspecting the failure, the teeth of the gearbox were intact. What was broken seems to be the pins in the planet’s cage that carry planets’ bearings. They are indeed one of the weak points of the design. Since the teeth didn’t fail, I replaced the planet cage with a PU cast one and repeated the experiment.
In this scenario, the gearbox failed at 3.8 Nm. To my surprise, the teeth on the sun and planets weren’t damaged on all test samples. What was damaged was the outer output ring – it broke off in one place. See the pictures below. The material forms glass-like break surfaces with a lot of sharp edges. It also doesn’t yield and fails quickly and unexpectedly.
This type of failure suggests that Fast Mecha has higher strength under compression than under tension. Which is, after all, not a surprise for material with filler. Nevertheless, in the practical test it performed better than Fast and any other commonly available resin I have tested so far.
The up-to-date table of resin comparison is below. Note that this is the most recent version of the table, so there can be new resins since this review was written.
3D-printed Compound Planetary Gearbox
Mixing The Resin
Based on my observations with the resin, there were two questions I wanted to answer – can we mix in any other resin to make Fast Mecha more ductile? If we mix something in, how much will the surface properties suffer? Will it suffer from surface wear?
Therefore, I started to mix in Siraya Tech Tenacious, which is in my experience one of them so far the best resin for increasing ductility. And I have a great experience with adding it to other resins.
I tested various percentages up to 50 % of Tenacious. What goes with the surface properties I can conclude that the friction of mating surfaces increases with the content of Tenacious. Starting with 20 % of Tenacious, the worm-gear design is no longer back drivable. However, even with 50 % of Tenacious, the surfaces don’t seem to suffer from grinding and no white residue is formed.
When it goes to overall practical strength, 25 % increased it the most. In my experiments with the gearbox, the gearbox was able to transmit 4.6 Nm and thus, it is the strongest 3D-printed material I tested. Even the sintered Nylon PA12 from JLC PCB 3D printing service was weaker than this mixture (thought, only slightly). However, the material still fails unexpectedly and doesn’t yield. Also, the mode of failure was interesting. In 2 out of 3 test specimens, the teeth were intact. What was damaged was the connection of the outer teeth ring to the output face. This seems to suggest that the material has a tendency to crack in sharp corners (and also discovered a weak point in my design). Still, pretty impressive. On the other hand, we are only on ⅔ of the strengths that two-part polyurethanes offer.
I am generally impressed by the resin. First of all, I haven’t seen resin with such great surface properties. Even when it is a special engineering resin, it is relatively easy to print. It doesn’t require high exposure times, no extra heating. Just a little higher resting times. What I admire is the visual of the surface finish. I haven’t seen such nice-looking prints in a long time.
Mechanically it is stronger than I expected. However, I wouldn’t suggest applying the resin to scenarios with heavy load as it doesn’t yield and fails unexpectedly. It seems to be more suitable for articulated parts than heavy loads. However, I can imagine printing a bushing from it – the low surface friction is really interesting and the resin seems to be pretty strong under compression.
The resin isn’t cheap – about 75 USD per kilogram. However, for what it offers I consider it a good deal. I have tested other resins (e.g., 4× as expensive Licquerate Strong-X) that were weaker and overall worse-performing than Fast mecha. And I hope to get it cheaper over time if there will be enough demand for it.
What I am excited about the most is not any of the properties of the resins, but the fact that it exists. Having composite resins with superior properties that are easy to print on low-end machines is amazing – and I hope more and more resins like this will come. I hope that we will see Blu Mecha or even new resins that are stronger, yield more but still have a nice low-friction surface. I hope the future is here and such resins will be available soon!
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