What is Photopolymerization? Explain the VAT polymerization process with neat sketch.
Photopolymerization is a chemical process where a liquid material, known as a photopolymer or resin, rapidly changes its properties and solidifies when exposed to a specific wavelength of light, typically in the ultraviolet (UV) or visible spectrum. This process is a type of polymerization, which is a reaction where small molecules (monomers and oligomers) link together to form a long chain of larger molecules (polymers).
The key to this reaction is the presence of a photoinitiator in the resin. When the photoinitiator absorbs light of the correct wavelength, it becomes an active species (e.g., a radical) that initiates the polymerization process, causing the resin to "cure" or harden. This reaction is irreversible, meaning the solidified material cannot be turned back into a liquid.
Vat Polymerization Process
Vat polymerization is a category of additive manufacturing processes that uses this photopolymerization principle to build 3D objects. The process is characterized by a vat or tank filled with liquid photopolymer resin. The object is built layer by layer from the bottom up or top down, with each layer being solidified by a light source.
The most common technologies within this category are:
Stereolithography (SLA): Uses a UV laser to trace the cross-section of each layer.
Digital Light Processing (DLP): Uses a digital light projector to flash the image of an entire layer at once.
Liquid Crystal Display (LCD): Uses an array of LEDs with an LCD screen as a mask to cure an entire layer simultaneously.
All of these share the same fundamental methodology, which can be visualized in the following steps
Preparation: The process starts with a vat filled with liquid photopolymer resin. A build platform is positioned at the top, slightly submerged just below the surface of the resin.
First Layer Curing: A light source (e.g., a projector) beneath the transparent window at the bottom of the vat projects the image of the first layer's cross-section. The light cures and hardens the resin that is in contact with the build platform, creating the first solid layer.
Platform Movement: The build platform then raises slightly by a distance equal to the thickness of one layer. This allows fresh liquid resin to flow into the gap between the last-cured layer and the transparent window.
Subsequent Layers: The light source then projects the image for the second layer. The light cures the new resin, causing it to bond to the previous layer.
Repetition: This process of curing a new layer and raising the build platform is repeated hundreds or thousands of times. The object is built upside down, hanging from the build platform.
Post-Processing: Once the printing is complete, the part is removed from the build platform. It is typically a "green" part, meaning it is not fully cured and may be a bit soft. It is then washed to remove any excess uncured resin and placed in a UV curing chamber for a final, thorough cure to achieve its final mechanical properties.
