Post-curing with light and heat is a key step for stereolithography (SLA) printing, especially when using materials with advanced properties.
Every Formlabs resin combines advanced, light-sensitive polymers, uniquely formulated with other active chemical components to deliver distinct functional properties. The Form 2 uses a 405 nm laser to cure the liquid resin, producing a highly-accurate, solid part. Each material’s print settings are designed and refined to print parts successfully at optimal speeds. Additional post-curing further improves the functional properties of the materials.
Curing During and After Printing
Laser-curing during printing brings a printed part to its “green” state. In the green state, some polymer connections are not yet fully reacted. Post-curing exposes parts to light and heat and strengthens crosslinks in the polymer structure, improving the parts’ strength, stiffness, and temperature resistance. Due to the increased number of bonds the material becomes more tightly “packed.” Therefore, post-curing also causes minor shrinkage, and each material’s print settings are designed to account for the expected shrinkage during printing and post-curing.
Determining Post-Cure Settings
To develop the recommended post-curing settings for each resin, Formlabs prints and tests the performance of multiple tensile bars, a standard size and shape sample used to test materials. All sample bars are washed with the same process, then exposed to predetermined temperatures for varying amounts of time.
Following the complete printing, washing, and post-curing process, Formlabs tests each tensile bar according to regulated ASTM standards to determine the modulus of elasticity. The modulus of elasticity helps determine a single recommended time and temperature setting for each Formlabs resin to achieve its optimal properties. Formlabs testing assumes that modulus of elasticity serves as an acceptable indicator of relative change for mechanical properties, because modulus of elasticity approximates the completeness of crosslinking in photopolymer chains. Therefore, Formlabs uses the modulus of elasticity to represent overall completeness of curing.
Biocompatible materials, such as Dental SG and Dental LT Clear resins, are further tested after post-curing to ensure that the printed and post-cured parts meet the safety and accuracy requirements for biocompatibility.
Considerations for Post-Curing
Although using a higher temperature for post-curing results in a faster post-cure, a higher temperature setting may also cause some materials to warp, depending on the part geometry and features. When choosing to modify the recommended post-cure settings, the material must be able to withstand the temperature and maintain a stable material structure.
Formlabs resins are designed for printing and post-curing with 405 nm light. For the best results, use the recommended time and temperature settings tested specifically for use with Form Cure. Formlabs provides full guidance for using Form Cure, but Formlabs can only provide minimal guidance for alternate post-cure systems, which will produce different material properties and may require experimentation.