The UV curing resin used for SLA is basically the same as the common UV curing prepolymer introduced below. However, since the light source used for SLA is monochromatic light, different from the common UV light, and has higher requirements for curing rate, the UV curing resin used for SLA should generally have the following characteristics.
(1) Low viscosity. UV curing is based on the CAD model, and the resin is superimposed into parts layer by layer. When a layer is completed, because the surface tension of the aldehyde resin is greater than that of the solid resin, it is difficult for the liquid resin to automatically cover the surface of the solidified solid resin. The resin liquid level must be scraped and coated once with the help of an automatic scraper, and the next layer can be processed only after the liquid level is leveled. This requires the resin to have a lower viscosity to ensure its good leveling and easy operation. Now the resin viscosity is generally required to be below 600 cp · s (30 ℃).
(2) The curing shrinkage is small. The distance between liquid resin molecules is the action distance of van der Waals force, which is about 0.3~0.5 nm. After curing, the molecules are cross-linked, forming a network structure. The distance between molecules is converted into covalent bond distance, which is about 0.154 nm. It is obvious that the distance between molecules before and after curing is reduced. The distance of one addition polymerization between molecules will decrease by 0.125 ～ 0.325 nm. Although in the process of chemical change, C=C is transformed into C-C, and the bond length increases slightly, the contribution to the change of intermolecular interaction distance is very small. Therefore, volume shrinkage is inevitable after curing. At the same time, before and after curing, the disorder will become more orderly, and volume shrinkage will also occur. Shrinkage is very unfavorable to the forming model, which will produce internal stress, easily cause deformation, warping, cracking, etc. of the model parts, and seriously affect the accuracy of the parts. Therefore, the development of low shrinkage resin is the main problem SLA resin faces at present.
(3) Fast curing rate. In general, the thickness of each layer is 0.1~0.2 mm for curing layer by layer, and hundreds to thousands of layers are required for curing a part. Therefore, the curing rate is very important if the solid is to be manufactured in a relatively short time. The exposure time of the laser beam to a point is only in the range of microseconds to milliseconds, which is almost equivalent to the excited state life of the photoinitiator used. Low curing rate not only affects the curing effect, but also directly affects the working efficiency of the molding machine, which is difficult to apply to commercial production.
(4) Small swelling. During the molding process, the liquid resin is always covering the solidified part of the workpiece, which can penetrate into the solidified part and cause the solidified resin to swell, causing the part size to increase. The accuracy of the model can only be guaranteed if the resin swelling is small.
(5) High light sensitivity. Since the SLA uses monochromatic light, it requires that the wavelength of the photosensitive resin and the laser must match, that is, the wavelength of the laser should be as close as possible to the large absorption wavelength of the photosensitive resin. At the same time, the absorption wavelength range of the photosensitive resin should be narrow, so as to ensure that the curing can only occur at the point irradiated by the laser, thus improving the manufacturing accuracy of the parts.
(6) High curing degree. It can reduce the shrinkage of the post curing molding model, thereby reducing the post curing deformation.
(7) High wet strength. The higher wet strength can ensure no deformation, expansion and delamination during post curing.