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Vat

Polymerization

  • It is a process where light source selectively cures a liquid photopolymer resin in a vat.

  • The two most common forms of Vat Polymerization process are "Stereolithography" (SLA) and "Digital Light Processing" (DLP). The fundamental difference between these two types of 3D Printing technologies is the light source they use to cure the liquid resin. PolyJet system spray photopolymer materials onto a build tray in ultra-thin layers (between 16 and 30 μm) until the part is completed. Each photopolymer layer is cured with UV light after it is jetted, producing fully cured models that can be handled and used immediately, without post-curing. Due to the nonlinear nature of photo excitation, the gel is cured to a solid only in the places where the laser was focused while the remaining gel is then washed away. Feature sizes of under 100 nm are easily produced, as well as complex structures with moving and interlocked parts.

  • SLA printers use a point laser, in contrast to the voxel approach used by a DLP printer.

  • World's first 3D Printing technology - SLA uses two mirrors, known as galvanometers, with one positioned on the X-axis and another on the Y-axis. These mirrors rapidly aim a laser beam across a vat of resin, selectively curing and solidifying a cross-section of the object inside this building platform, producing it up layer by layer.

    • Most SLA printers use a solid-state laser to cure parts. The disadvantage of this using a point laser is that it can take longer to trace the cross-section of an object when compared to DLP.

    • A DLP printer uses a digital light projector to flash a single image of each layer all at once, or multiple flashes for larger parts. Because the projector is a digital screen, the image of each layer is composed of square pixels, resulting in a layer formed from small rectangular blocks called voxels.

  • DLP can achieve faster print times compared to SLA. It's because an entire layer is exposed all at once, rather than tracing the cross-sectional area with the point of laser.

  • Light is projected onto the resin using LED screens or a UV light source that is directed to the build surface by a Digital Micro-mirror Device (DMD). This DMD is an array of micro-mirrors which control where the light is projected and then generate the light-pattern on the build surface.

  • "Masked Stereolithography" (MSLA) uses an LED array as its light source, shining UV light through an LCD screen displaying a single layer slice as a mask - hence the name.

  • Like DLP, the LCD photomask is digitally displayed and composed of square pixels. The pixel size of the LCD photomask defines the granularity of a print. Thus, the X-Y accuracy is fixed and does not depend on how well you can scale the lens, as is the case with DLP. Another difference between DLP and MSLA is that the latter utilises an array of hundreds of individual emitters, rather than a single point emitter light source like a laser diode or DLP bulb.

  • Similar to DLP, MSLA can achieve faster print times compared to SLA in certain conditions. It's because an entire layer is exposed at once, rather than tracing the cross-sectional area with the point of a laser.

  • Due to the low cost of LCD units, MSLA has become go-to technology for the budget desktop resin printer segment.

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  • Materials: Photopolymer resin (Standard, Castable, Transparent, High Temperature)

  • Dimensional Accuracy: ±0.5% 

  • Common Applications: Injection mould-like polymer prototypes, Jewellery, Dental applications, Hearing aids

  • Strengths: Smooth surface finish, Fine feature details

  • Weaknesses: Brittle, Not suitable for mechanical parts

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