Mask-Image-Projection-based Stereolithography
Layer-based additive manufacturing (AM) is a collection of techniques for manufacturing solid objects by sequential delivery of energy and/or material to specified points in space to produce that solid. Product components can have material designs with desired mechanical properties, e.g. both soft and hard materials can be embedded in products such as tooth brushes and remote controllers. Such fabrication capability also opens up exciting new options that were previously impossible.

The AM process, Stereolithography Apparatus (SLA), offers high quality surface finish, dimensional accuracy, and a variety of material options by using a laser and liquid photocurable resin. However, speed limitations in this process exist. To address such limitations, research on the mask-image-projection-based Stereolithography (MIP-SL) process is pursued. Compared to the laser-based SLA, the MIP-SL process can be much faster due to its capability to simultaneously form the shape of a whole layer. Building functional microstructures, especially digital material fabrication, require the development of a general MIP-SL process similar to the polyjet process that can fabricate all combinations of multiple resins.


figure1An illustration of the MIP-SL process is shown in Figure 1. Instead of the laser used in SLA, a Digital Micromirror Device (DMD) is used in the MIP-SL process to dynamically define mask images to be projected on a resin surface area. A DMD is a microelectromechanical system (MEMS) device that enables one to simultaneously control ~1 million small mirrors to turn on or off a pixel each at over 5 KHz.
In the MIP-SL process:

  1. The three-dimensional (3D) CAD model of an object is sliced by a set of horizontal planes.
  2. Each thin slice is then converted into a two-dimensional (2D) mask image.
  3. The planned mask image is then sent to the DMD.
  4. Accordingly the image is projected onto a resin surface such that liquid photocurable resin can be selectively cured to form the layer of the object.
    • By repeating the process, 3D objects can be formed on a layer-by-layer basis.

Previous research on developing multi-material SLA and MIP-SL systems are all based on the top-down projection. As shown in Figure 2, in order to accommodate the part size in the Z direction, a large tank has to be maintained for keeping the resin level.
Due to the deep vat, draining and cleaning the current resin before changing to another resin vat takes a long time and leads to significant material waste. To address the challenges of reduction of material waste and increase in cleaning efficiency during the resin tank switching process, a bottom-up projection in the multi-material MIP-SL process was investigated. An illustration of such a system is shown in Figure 3. 
The light source is projected from the bottom of the transparent vat. Since the current built layer is formed at the bottom of the platform, the container depth is independent of the part height. Thus the liquid in the vats can potentially be as shallow as a layer thickness. When switching resin tanks, only the portion of the built model that contacts the liquid resin needs to be cleaned. Thus the material changeover efforts can be significantly reduced with less material.