Thanks to a tip, we’ve found a rather interesting patent that promises to allow a vast selection of thermoset material compositions to be 3D printed using the common SLS process.
SLS, or Selective Laser Sintering, is one of the original 3D printing processes. It involves blasting a heated flat bed of powder with a laser to cause sintering to occur. This, layer-by-layer, results in a complete 3D object.
Today the SLS process is commonly found in equipment from multiple vendors, ranging from small desktop systems to huge devices capable of 3D printing larger objects.
Thermosets or Thermoplastics?
But there’s one thing they all have in common: they print thermoplastics. Thermoplastics are a plastic that can be reversibly solidified. Heat it and you can reform it into a new shape. Reheat it and you again can reshape its form.
Thermosets are a different kind of plastic where a chemical reaction occurs once to solidify the material, and it cannot be reversed. One you may be familiar with is epoxy, where two chemicals are mixed to cause a final reaction. This is commonly seen in liquid photopolymer resin SLA 3D printing processes.
Thermoplastics have the unfortunate property of being terrible at heat resistance: if you heat them up, they soften. To make an object from a “heat-resistant” thermoplastic you must simply heat it more. Thermosets are far more resistant to heat because reheating does not change their form.
But I’ve never heard of thermosets being used in an SLS machine. Thermosets usually begin life as liquids, which are not the kind of material an SLS machine expects.
Tiger Coatings Thermoset Powder
Now scientists at Tiger Coatings have apparently developed a new material that seems to be usable in SLS systems, yet produces a kind of duroplast material. A duroplast is a thermoset material that leverages fibers, typically cotton, to provide reinforcement in the same way that fiberglass functions.
Tiger Coatings is a large, worldwide company that produces fine finishes for surfaces, such as powder coatings. Apparently they’ve leveraged their expertise in this field to provide a way to make thermoset powder suitable for use in SLS 3D printers, based on known powder coating chemistry of epoxies, carboxylated polyester, and unsaturated polyester.
Thermoset Powder Patent
They’ve even been issued a patent for this process, WO2017046132A1, which is worth a read. It says:
”The present invention provides for the use of a thermosetting polymeric powder composition in a Selective Laser Sintering process to produce a 3D duroplast, wherein the composition comprises at least one curable polymeric binder material and wherein during each pass of the SLS process said polymeric binder material is at least partially cured within the layer thus formed and also at least partially crosslinked with the previous layer.”
“The powder composition used according to the present invention may therefore comprise a curable polymeric binder material (a) and at least one curing agent (b), where (a) and (b) are able to react with each other to form a cured network. A catalyst and/or initiator (for UV-systems) may be added, either instead of or together with the curing agent, to initiate the curing reaction or to accelerate the reaction once started, depending on the specific chemistry of the reaction.”
Basically it seems they have found a way to somehow mix all the parts to make a thermoset into a powder in a stable form. It’s stable until exposed to the laser in the SLS system.
Does this actually work? Here is an image from their patent of a part that’s made by an SLS 3D printer from thermoset powder:
What’s in this powder, exactly? They say:
“The curable polymeric binder material is contained in the thermosetting polymeric powder composition preferably with less than 99 wt-%, more preferably with from 10 to 70 wt-%, particularly preferably with from 20 to 60 wt-%, of the total composition.”
They have a base polymeric material and then add things like catalysts, initiators, absorbers (like carbon black), which are all specific to the particular polymeric base material. I suspect there could be many different combinations available here.
But it’s even more complex than that! The powder can also contain fillers that can provide additional properties to the final print. Here’s a list of some of the possibilities from the patent:
”Carbonate-based mineral fillers, magnesium carbonate, calcium carbonate, barium sulphate, dolomite, kaolin, talc, micro-mica, alumina hydrate, wollastonite, montmorillonite, zeolite, perlite, nano fillers, pigments, such as titanium dioxide, anatase tinanium dioxide, transition metal oxides, graphite, carbon black, silica, alumina, phosphate, borate, silicate and organic fillers, such as polymer powders, like copolymers, elastomers and thermoplastics, used alone or as a mixture of two or more of these materials.”
3D Printing Thermoset Material Mix
In the patent they provide several examples of actual material mixes. This is one:
“The mixture was composed of 600 parts of Uralac® P3490 (DSM), a saturated carboxylated polyester resin, 45 parts of Araldite® PT-910 (Huntsman), 320 parts of Titanium dioxide (Kronos® 2160, Kronos Titan GmbH), 15 parts of Resiflow PV 5 (Worlee-Chemie GmbH), 8 parts of Accelerator DT-3126 (Huntsman) and 7 parts of Benzoin. All components were premixed in a high-speed mixer for 1 min and then extruded in a twin-screw ZSK-18 extruder at a screw speed of 400 rpm with a rear-zone temperature of 80 °C and a front-zone temperature of 90 °C. In an alternative setting of the extruder, a temperature gradient of 40 to 100 °C and a cooling device for the feeding area was used. The compound obtained was then cooled down, granulated and fine ground to obtain a powder having a D50 of less than 80 μηη. The powder can be used in a SLS laser sintering 3D-printing machine.”
Thermoset SLS Benefits
There will be many benefits from this approach. For example, prints will be strong enough to survive sandblasting, making post-processing removal of stray powder far easier.
I believe this to be a very notable innovation in 3D printing, as it could open up SLS machines, or at least those allowing open materials use, to produce thermoset objects that would be far stronger than the thermoplastic objects of today.
Add Tiger Coatings to the list of future major 3D print materials providers.