During the photocuring process of stereolithography (SLA), resin undergoes shrinkage and deformation, which is an inherent part of its curing mechanism. This shrinkage can significantly impact the accuracy of the final prototype, including both its shape and dimensional precision. Below is an in-depth explanation of the resin shrinkage process and its effects during the SLA fabrication.

1. Shrinkage and Deformation During the Curing Process

Resin shrinkage occurs throughout the curing process, with typical volumetric shrinkage around 10% and linear shrinkage around 3%. On a molecular level, the curing of photopolymer resins involves the transformation from short, small molecules to long-chain polymer molecules. This change in molecular structure leads to significant shrinkage.

The shrinkage during the curing process is due to two main factors:

• Curing Shrinkage: As the resin polymerizes, its molecular structure changes, resulting in a decrease in volume.
• Thermal Shrinkage: When the laser scans the liquid resin, it raises the temperature of the material, causing slight thermal expansion or contraction. However, the thermal expansion coefficient for common resins is relatively small, around 10^-5/°C, and the area affected by temperature changes is limited, meaning that thermal shrinkage is generally negligible compared to curing shrinkage.

While thermal shrinkage is minimal, the volumetric shrinkage that occurs during the curing process is a key factor in determining the final precision of the parts produced. This shrinkage is a natural outcome of the polymerization process, where the molecular forces between liquid molecules (van der Waals forces) are replaced by stronger covalent bonds in the solid polymer state, leading to a reduction in volume.

2. Deformation Caused by Resin Shrinkage During Part Formation

During part formation, the resin is solidified layer by layer by the laser scanning process. As each layer is cured, its molecular structure contracts, leading to a shrinkage in volume. This shrinkage can cause noticeable deformations, which are critical to the accuracy of the final part.

One key effect of resin shrinkage is that the cured length of a scanned line will always be shorter than the given length due to the shrinkage inherent to the resin. When longer lines are scanned, the absolute shrinkage becomes more pronounced. This shrinkage causes each layer to pull on the layer beneath it as the resin solidifies, which may result in warping.

For example, if the part includes a cantilever beam (a part with one end unsupported), the beam’s free end will initially float in the liquid resin and will not experience any constraints during curing. Therefore, it does not exhibit warping until it is constrained by the layers built on top. In some cases, the cantilever beam may even experience a slight downward bend during the scanning process. However, as subsequent layers cure and the material shrinks, this bend can result in an upward pulling force on the previous layers, causing noticeable warping.

This warping effect is not limited to parts with cantilever beams. Warping forces can affect any part during the SLA process, leading to deformation, and this is directly tied to the resin’s inherent shrinkage properties and the internal stress distribution throughout the part. The geometry of the part, as well as the resin’s shrinkage rate, will influence how much deformation occurs.

Additionally, surface irregularities such as bumps or depressions may appear due to the action of auxiliary mechanisms (e.g., scraping) or inherent properties of the resin. These imperfections can impact the surface precision of the prototype.

3. Shrinkage and Deformation During Post-Curing

Although the resin undergoes polymerization during the laser scanning process, it is typically only partially cured. Some residual liquid resin remains uncured or incompletely cured within the part. The final strength of the part is achieved during the post-curing stage, where any remaining resin undergoes further polymerization.

During post-curing, the resin continues to shrink as it fully polymerizes, which can lead to additional deformation. Unlike the deformation that occurs during the scanning process, post-curing deformation involves changes in the internal structure of the part as it is subjected to heat and UV radiation.

Characteristics of Post-Curing Shrinkage:

1. Non-Uniform Shrinkage: Post-curing shrinkage is not uniform throughout the part and tends to increase as the length of the scanned lines grows. This is because larger parts contain more uncured resin, which leads to greater shrinkage.
2. Path-Dependent Shrinkage: Post-curing shrinkage can vary significantly depending on the scanning path used during the initial fabrication. This variation is due to the differing quantities and distributions of uncured resin within the part. The choice of scanning path during the fabrication process, therefore, plays a crucial role in minimizing deformation during post-curing.
3. Post-Curing Contributes Significantly to Total Shrinkage: Post-curing shrinkage typically accounts for around 25% to 40% of the total shrinkage that occurs during the entire SLA process. This underscores the importance of the post-curing step in stabilizing the final dimensions of the part and ensuring its dimensional accuracy.

Conclusion

Resin shrinkage is a fundamental aspect of the SLA process that directly impacts the precision and quality of the produced parts. The shrinkage during the curing and post-curing stages must be carefully managed to minimize deformation, such as warping and surface imperfections. Understanding the molecular changes occurring during resin polymerization and controlling the scanning process can help improve part quality and dimensional accuracy. Additionally, post-curing is a crucial step that helps stabilize the final product and enhance its mechanical properties. Proper management of both the curing and post-curing processes is essential for achieving high-quality, accurate prototypes in SLA manufacturing.