DLPPrinter

Basic Idea

On March 12, 2012, Guan wrote me (Peter) to say that some hackers are assembling 3D printers using light modulated by mirrors to polymerize a light-sensitive resin in layers, and inviting me to participate. This is the method (called stereolithography) used by many high-end commercial printers.

DLP Projectors

DLP stands for Digital Light Processing. A DLP chip has an array of mirrors whose angle is programmable. A light illuminates the array, which is approximately a centimeter across. A low-resolution DLP chip might have 800x600 mirrors. Each mirror separately either shines the incoming light through a lens or deflects it to a heat sink. To get color, a typical DLP projector has a rotating wheel with transparent colored segments. The wheel rotates at a high rate of speed. As each colored segment passes in front of the mirror, the program sends the appropriate color layer to the lens. Since the wheel rotates rapidly, the eye sees the merged, full-color image.

DLP Printers

The idea here is to remove the color wheel, or at least the colored segments, so we get full white light through the lens at all times. Since the light has appreciable UV content at the 385nm wavelength used to polymerize most resins, this light can be used for polymerization. A thin layer of resin is exposed to light. The program projects a shape corresponding to the cross-section of the piece being printed at the current Z position. Once exposure has been made, the piece is moved so that a thin layer of liquid resin intervenes between the growing piece and the light beam. The next layer is then exposed, and so on until the entire piece has been printed.

The design we are looking at shines the light through a flat-bottomed vessel from below. A platform holds the growing piece from the top, and is always positioned so that the platform (at the start) or the most recently polymerized (later) of the growing piece is held a small distance from the bottom of the vessel. After the current layer is polymerized, the growing piece is raised to a position where the next layer can be polymerized.

The bottom of the vessel is lined with optically clear PTFE (Teflon) tape, to prevent the layer being polymerized from sticking to the vessel. Instead, we want it to adhere to the growing piece.

UV Cured Resin

We buy it from Bucktown Polymers. The cheapest kind is $40/l, but we bought the low-odor low-VOC version. This cures at 385nm. Bucktown is working on a version that cures at 405nm.

Starting Points

See Lemon Curry for the basic concept and a picture of a working printer. An IRC channel for this technology is #lemoncurry on freenode. Finally, there has been some work in this direction from NYCResistor.

Chronology

22 March 2012

Guan came in with an InFocus LP425Z DLP projector which we purchased on Ebay for $90. We disassembled it, removed the color wheel -- actually, we broke off the colored segments -- and reassembled it. On reassembly, we were able to projet light through the lens, but we did not have an image to project.

29 March 2012

Peter came in with a composite cable and a DVD. We attempted to project an image through the lens to see if that worked, even short of programming. It failed, in that we could only see an undifferentiated rectangle of light, rather than an image of any kind. We were uncertain whether this was because we didn't know how to operate the projector or because we broke it. Discussion on the IRC channel was unfruitful, except to point out that a stripped down kit of the basic DLP parts should become available in about a month. The only thing we could think of to do as the next step was disassemble the projector and see if we could find out what went wrong.

10 April 2012

Possible strategies:

• Go ahead with printer disassembly and see if we get anywhere
• Experiment with polymerization, to get a feeling of time, amount of light needed, etc.
• As a bridge to polymerization, begin to design and build the polymerization vessel and Z-axis assembly
• Work on other things while we
  • Wait for the DLP kit to become available
  • Write up what we've done so far -- something we had neglected

Since I am writing this on April 10, we clearly decided for the last.