Vat photopolymerization is another popular 3D printing process. The process is based on the hardening of photopolymers by exposure to ultraviolet radiation. The photopolymer is in the form of a liquid resin filled inside a vat, which is why it was called vat photopolymerization.
There are two common vat photopolymerization techniques –:
1) Stereolithography (SLA)
2) Liquid Interface Continuous Production (CLIP)
Stereolithography
Stereolithography was invented in 1986 by Charles Hull at the time he founded his company 3D Systems. For stereolithography, a vat is filled with liquid photopolymer resin. The 3D model is built on a platform that dips into the resin to the depth of a single layer. The height of a single layer is generally 0.05mm to 0.15mm. An ultraviolet light source that directs radiation into the resin by reflection in motor-controlled mirrors is mounted on the top of the device. The laser light head can move along the horizontal plane, generating layer by layer, hardening the liquid resin by directing the radiation at specific coordinates along the plane. Once the layer is created, the platform is raised and the model is removed from the resin. After creating each layer, the 3D model needs to be removed from the resin for delamination and recoating. It must then be repositioned in the resin to create the next layer. Thus, the platform is again lowered to a depth that all layers constructed so far are immersed in the resin and the resin for creating the next layer is available for UV exposure. In this way, each layer is created and after finishing the final layer the platform is finally removed from the liquid resin and allowed to cool.
During Stereolithography, the 3D model remains floating in the resin, therefore, to keep the model in a fixed orientation, support structures are required. Support structures are attached to the platform just before starting the process. These support structures then kept the created 3D model in a fixed position. The support structures are later manually removed when the model is removed from the resin.
The biggest advantage of Stereolithography is its speed. However, SLA machines, as well as photopolymers, are quite expensive.
Continuous Liquid Interface Production (CLIP)
CLIP is the latest innovation in the Vat photopolymerization technique, which is 100 times faster than the traditional SLA technique. The technique was introduced by a lesser-known company, Carbon3D. In this technique, the laser projector is placed at the bottom and the vat resin filled in a container has an oxygen permeable window between the laser projector and the resin. The oxygen permeable window allows faster continuous construction of the 3D model and is also capable of changing the pulses of ultraviolet radiation. The oxygen permeable window creates a dead zone between the remaining liquid resins other than the resin exposed to radiation. This makes it possible to break the traditional layer-by-layer construction of the SLA technique. Due to the continuous construction process, the 3D model is built faster.
In the next article, the Binder Jetting technique for 3D printing will be discussed.
