In the field of 3D filaments, photosensitive resins are widely used in SLA and DLP technologies due to their high precision and high surface quality. However, the volume shrinkage during the curing process has always been a key factor restricting printing accuracy and finished product quality. The shrinkage rate of photosensitive resins mainly stems from the transformation of intermolecular forces from van der Waals forces to covalent bonds during curing, leading to a significant reduction in intermolecular distance and consequently, volume shrinkage. This shrinkage not only causes dimensional deviations in printed parts but can also induce internal stress concentration, resulting in defects such as warping and cracking. Therefore, reducing the shrinkage rate through formulation optimization is an important direction for improving the performance of photosensitive resins, requiring a comprehensive approach encompassing multiple dimensions, including matrix resin selection, reactive diluent matching, photoinitiator control, filler modification, and process synergy.
As the core component of photosensitive resins, the molecular structure and properties of 3D filaments directly determine the curing shrinkage rate. Epoxy acrylates, due to their rapid curing, high hardness, and low shrinkage rate, have become one of the most commonly used matrix resins for 3D printing photosensitive resins. The epoxy groups and acrylate groups in the molecular chain of photosensitive resins can be cured through both cationic and free radical polymerization mechanisms, forming a network structure with high crosslinking density, thereby effectively inhibiting shrinkage. Furthermore, base resins with excellent weather resistance, such as hydrogenated bisphenol A epoxy resin, can also further optimize shrinkage performance by reducing the content of unsaturated bonds in the molecular chain, thus minimizing volume changes during curing.
Reactive diluents in photosensitive resins not only regulate viscosity but also participate in the curing reaction, affecting the final shrinkage rate. Monofunctional reactive diluents, such as isooctyl acrylate, can reduce system viscosity, but their low crosslinking density after curing may lead to increased shrinkage. Polyfunctional reactive diluents, such as trimethylolpropane triacrylate, can increase the number of crosslinking points, improving the rigidity and dimensional stability of the cured product, thereby offsetting some of the shrinkage effect. Therefore, a reasonable combination of monofunctional and polyfunctional reactive diluents can effectively control shrinkage while ensuring leveling properties.
The type and amount of photoinitiator have a significant impact on the curing rate and shrinkage behavior of photosensitive resins. Free radical photoinitiators, such as acetophenone derivatives, cure quickly but are prone to causing volume shrinkage. Cationic photoinitiators, such as diaryliodonium salts, cure more slowly but have lower shrinkage rates and can be compounded with free radical photoinitiators to form hybrid curing systems, balancing curing efficiency and dimensional stability. Furthermore, optimizing the absorption wavelength of the photoinitiator to match the light source can improve light energy utilization and reduce post-shrinkage caused by incomplete curing.
The introduction of fillers is a direct and effective means of reducing the shrinkage rate of photosensitive resins. Inorganic fillers, such as silica and nano-calcium carbonate, can reduce the amount of resin used through physical filling, while their high modulus properties can suppress volume changes during curing. Organic fillers, such as aldehyde-ketone resins and high molecular weight epoxy resins, have better compatibility with the matrix resin and can improve the toughness and impact resistance of the material while reducing shrinkage. In addition, monomers with expansion properties, such as certain special acrylates, can offset some shrinkage through chemical compensation mechanisms, but their cost and application limitations must be considered.
Formulation optimization also needs to consider process synergistic effects. For example, by adjusting printing parameters such as layer thickness and exposure time, the curing depth can be controlled, reducing interlayer stress accumulation; post-curing treatment can further promote the polymerization of incompletely reacted monomers, reducing residual shrinkage. Furthermore, the introduction of smart materials, such as shape memory polymers, can endow photosensitive resins with self-healing or shape-adjusting functions, indirectly compensating for dimensional deviations caused by shrinkage.
Controlling the shrinkage rate of photosensitive resins is a multidisciplinary problem involving materials science, chemical engineering, and 3D printing processes. Through molecular design of the matrix resin, precise matching of reactive diluents, synergistic regulation of photoinitiators, composite modification of fillers, and optimization of process parameters, the shrinkage rate can be effectively reduced, thereby improving the dimensional accuracy and mechanical properties of 3D printed parts. In the future, with the integration of emerging fields such as bio-based materials, nanotechnology, and smart surface technology, the formulation optimization of photosensitive resins will develop towards a more environmentally friendly, efficient, and intelligent direction, providing stronger material support for the application of 3D printing technology in high-precision manufacturing.
