One of the major pain points to using both SLA and DLP printers is that the optical transmission of the PDMS windows change over time. This "clouding" or "fogging" is very clear to the naked eye, as the two photos of a used windows beside a new one show. There are two causes for the clouding. First, the UV absorbing species in the resin soak into the PDMS decreasing its transmission over time. In some resins this appears as a pale white glow over the whole window due to the fluorescence of the UV blocker. This is particularly obvious in the lower angle left photo. In resins like Autodesk Standard Clear, the uptake of resins by the PDMS is small, and the amount of UV blocker absorbed it even smaller. However, this small amount is still sufficient to greatly reduce the optical transmission. This process takes weeks and ultimately reduces the transmission from 90% to ~ 65% even when they are not used for printing.
The second mechanism occurs only in areas used for printing. In this case the surface becomes rough and scatters light. If the same print is performed over and over a very clear image will form on the PDMS. In the photos above this is the more opaque clouding in the upper right of the window.
This Instructable illustrates what happens when the same exact image is printed over 30,000 times (33 inches of 25 micron layers!) on the Ember printer. Throughout the course of this experiment the optical transmission of the window was measured using UV-VIS spectroscopy, and performance of the printer evaluated via a test print. Afterwards, a microscope and FTIR spectrometer were used to examine the window. This experiment demonstrates that it is possible to keep printing long after the PDMS window clouds. The Autodesk Standard Clear was observed to cloud the window at 5000 layers; however, there was no change in printing performance until 10,000 layers. Between 10,000 and 30,000 there was a small loss of performance. Somewhat surprising, the exposure times had to be reduced as the window clouded. A general recommendation would be that the windows need to be replaced when divots and tears to PDMS can be seen with the naked eye.
When PDMS windows are soaked in resin they absorb a small amount of the resin. While this amount is small, 0.7 % wt./w.t for Autodesk Standard clear, the photoinitiator(s) and UV blockers present in the resin can also be taken up by the window. Because these species strongly absorb light, the optical transmission of the window is reduced. This process is not fast, and it requires about a month for the window to reach equilibrium.