October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 2 9 enough drying or curing occurs to form a soft solid. Without a yield point, the surface tension rounds off all the fine features of a part during the build time (10 seconds to several minutes). A very high viscosity is not sufficient, except for extrusion through a large nozzle to achieve coarse resolution. Measure- ment of yield behavior is one method to ensure consistency and suitability of the material for 3D printing via robo- casting. The degree of yield strength re- quired can be estimated on the basis of what is needed to resist shape changes due to surface tension and to prevent slumping under gravity. The relation be- tween surface tension and yield is simi- lar to the equation from statics for yield of a cylinder under pressure. The yield strength should be greater than the sur- face tension multiplied by the radius of the cylindrical extruded bead. For a slurry with a surface tension of half that of water (0.04 N/m), this gives a yield point of about 100 Pa at an extruded cylinder diameter of 0.8 mm, typical for our systems. For comparison, the yield point of toothpaste is about 200 Pa. Gravitational stress increases with sample height. With a density of about 1.5-2 g/cc for a typical silica-based slur- ry, this would result in a stress of ap- proximately 150-200 Pa for a 1-cm-high sample. This means that slumping will be a problem for denser ceramics or taller parts unless some partial curing occurs during the building of the part. In principle, a purely viscous ma- terial could be 3D printed. The Deborah number relates the relaxation time of a liquid to the experimental time, with flow dominating at low numbers and elasticity at high numbers. In this case, the experimental timescale would be the curing or hardening time (typical- ly 1 minute to 1 hour). The relaxation time can be taken as the viscosity di- vided by the stress. For a high viscos- ity of 100,000 cP (100.s) and a stress of 100 Pa, the relaxation time is about 1 second. Thus, a purely viscous mate- rial can only be printed if the hardening time is very fast, such as in heat remov- al from a resin, or if the viscosity and ex- trusion forces are very high. base and forced out of a metal die us- ing a large motor mounted on a Strata- sys machine. This worked well but was slow and ponderous. Machines are now available for the filament or cylinder ex- trusion of clay. One would expect that the viscosity would limit the resolution (fineness of the nozzle diameter) and speed, but this will not be a problem for many pottery art applications. The robocasting method relies on a high volume fraction dispersion of particles in water. This slurry is close to the liquid-solid transition of the disper- sion, such that a very small amount of water loss during extrusion causes the slurry to solidify. The aqueous medi- um keeps the slurry viscosity low (a few hundred cps, like thick cream) so that it can be extruded through a syringe needle down to about 0.3 mm. Other volatile solvents can also be used as the fluid medium, provided care is tak- en to control the evaporation rate and prevent cracking during consolidation via drying. The stereolithography approach can also be applied to a slurry of ceram- ic particles in liquid monomer. It is nec- essary to achieve a high enough volume fraction of ceramic (50-60%) with a low enough viscosity, and one would expect curing depth limitations arising from light scattering [5] . The system will work best when the particle-liquid system scatters less due to a better refractive index match, and when the particles do not absorb light. We have recently had some suc- cess with a hybrid system, extruding a slurry of particles in liquid resin through a needle and photopolymerizing this during the building process. A saying from the inkjet printer industry is, “The ink maker is king.” Control of slurry characteristics, especially rheology, is crucial to effective 3D printing as well. RHEOLOGY Two ideal materials for testing ex- trusion systems are toothpaste and sili- cone caulking. Both are Bingham fluids with a distinct yield point, followed by a low viscosity at higher shear stress- es. The yield point causes the extrud- ed material to maintain shape until The development of a slurry with high volume fraction, low viscosity, and a yield point is an exercise in colloid sci- ence and is very much a combination of science and art (i.e., witchcraft). The fluid, powder, dispersant, particle size distribution, and particle shape are all important. Experience suggests that two-phase liquids (structured fluids) tend to have yield points when particles are added to form a suspension. Other 3D printing systems will in- volve different parameter constraints from those outlined here, but the same types of issues apply to all methods. In any 3D printing process, a new material is always a new challenge, so commer- cial systems tend to work with a limited material set. EXTRUSION In fused deposition modeling and in the robocasting system, a stream of fluid is extruded through a fine nozzle and laid down as a bead of pasty sol- id that dries or cures. The first concern in this process is the pressure needed to drive the viscous liquid through the nozzle. Using the Poiseuille equation for a typical nozzle (0.8 mm diameter, 2 mm long), table speed (10 mm/s), and viscosity (100 Pa.s), the pressure is 100 kPa (14 psi). For a Bingham fluid, the yield point would be added to this but is relatively low. For a given table speed, the pressure varies as the square of the nozzle radius and so the resolu- tion for the part is very sensitive to the available pressure. High-pressure sys- tems can be built, but pressures for conventional plastic syringe extruders are limited to 100 psi. A number of more detailed con- siderations affect the extruder design. Some commercial printers use gas/ pressure-driven extrusion. This works well for fluids with well-defined rheol- ogy, but any change in viscosity during building or between parts leads to a change in flow rate and poor results. Because viscosity is sensitive to compo- sition in ceramic slurries, our extrusion systems use stepper or servo-motor driven pistons. This allows the pres- sure in the syringe to rise or fall in order to maintain a constant extrusion rate.