Why does viscosity matter?
Low-viscosity resins can’t meet industrial standards and homologations due to the limited ingredient set available to keep viscosity within the processability range. Therefore, legacy formulations are predominantly formed by monomers and a very small fraction of oligomers, additives, fillers and modifiers can be used since its inclusion spike viscosity out of the processing window. Consequently, low-viscosity resins will inevitably have compromised mechanical, thermal and chemical and performance.
During photopolymerization, monomers and oligomers bond covalently at their terminations, forming a matrix that is crucial for the final material properties. As low-viscosity resins are mostly formed by short-chain monomers, there are many more covalent bonds in the matrix, and it becomes inevitably over-crosslinked, leading to brittle parts that are prone to crack propagation from weakened bonds at functional termination, and inferior elastomeric behavior because of poor tear strength or elongation at break.
Some AM processes rely on heating up the material to lower its viscosity and enable its processing, but this has several drawbacks: The material undergoes curing reactions when exposed to heat, leading to inconsistent properties among layers as the last ones are affected by partial curing during a longer pre-heating period. Moreover, heat increases the VOC emissions, which pose serious health risks and require expensive equipment in the facilities to ensure a safe work environment.
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