UV light’s role in photopolymerization
The polymer is able to harden or cure only by chemical reaction. This reaction is kicked off when special additives within the resin mix, called "photoinitiators," get excited by the UV wavelength of the light source. As they absorb the UV light, the additives produce a chemical that reacts with the other resin compounds and bonds them together. This crosslinking creates the solid we see when the final build is pulled out of the vat.
Photopolymerization itself is nuanced because the results you’ll see depend largely on many factors, including the type of materials to be used and the light source. The wavelength of the light source plays a large role here, as it is the catalyst for that chemical reaction mentioned above. The most common wavelengths used in DLP-based 3D photopolymerization are 365nm, 385nm and 405nm.
But there are other performance criteria that should be understood before choosing a light source, especially a UV DLP projector, as they vary from one system to the other and affect the photopolymerization reaction.
Two of the most critical criteria to consider are energy density and irradiance. Irradiance, or output power, is the original intensity of the light source (in W). It depends largely on the quality of the projector’s optics, which have to be designed to enable maximum light transmission.
Density, while only a consideration for DLP printing options, is the dose or amount of light concentrated on the build area at a time (often measured in mW/cm2). The energy density depends on the irradiance and the image size or resolution of the projector. For any given irradiance, the energy density will increase with the resolution, since the power will be concentrated across a smaller image. With larger images, the energy density per cm2 will decrease.
While most applications would benefit from a higher light intensity to achieve higher throughput, the cure rate of a 3D printing process also depends on the material used. It all comes down to the right energy density and exposure time required to complete the photopolymerization reaction and ensure that your print will have the mechanical properties you want.
In the case of DLP projectors, you’ll also want to make sure that all the pixels in the image will deliver the same amount of energy, so you get a uniform curing over the whole layer. Not all DLP projectors are designed with attention to uniformity, which leads to a concentration of the energy towards the middle of the image, leaving spots towards the edge of the build area undercured.