Hobbiest scale prototyping?

One of the coolest technologies around are the rapid prototyping technologies: CNCs, Laser scanning, and Stereolithography. I first ran into stereolithography at a UK campus fair soon after I arrived, I forget which one.

In stereolithography a 3D computer model gets formed into a actually plastic part using a process in which heat from a laser polymerizes a liquid. Each pass forms a layer, and layers of under 0.1 mm are added to build the complete part. This technology was developed in the late 1980’s and is now fairly common though very expensive. The machines are $50-150k, and shops will turn out a part for a few hundred dollars. Incredible stuff.

Part made with stereolithography

The University of Kentucky has a Rapid Prototyping shop with a Stereolithography machine on campus.

Two things make this tech expensive, the monomer plastic ($200+ per liter) and the machines. The monomer is magic stuff, 21st century chemistry, and can’t substituted.

Can a machine be made more cheaply? Perhaps. Instead of the UV laser and X/Y scanning optics, how about using a DLP and a UV bulb? The DLP (an array of computer controlled mirrors, the display tech for bright computer projectors) directs the UV light to the correct part of the work area, the monomer polymerizes, the part is lowered, and the cycle repeats.

Could this work? It seems plausible. Best as I can tell, the monomer needs to be heated 20-30°C to polymerize. This is done in ~100μs by the laser, so a UV bulb shining for longer might be able to polymerize the monomer. The UV laser (100mW typically) has an intensity equivalent to 10,000W shining on a small 2cm x 2cm work area, so a 200W or 400W UV bulb (commercially available) shining for 1/10 of a second should in theory work. Losses due to heat dissipation may make a longer light pulse necessary, but it would likely still be under a second if this will work at all. The monomer could be preheated to the highest temperature where polymerization doesn’t occur to help polymerization along.

The result would be a simple and cheap stereolithography device with a small working area (2cmx2cmx15cm)! It could be built for very little ($200 UV bulb, $100 old DLP projector, $200 lenses and misc parts), with the high cost of the plastic monomer the biggest limitation. A DLP chip with 1000×1000 mirrors would have a 0.02 mm resolution. If UV intensity required for polymerization isn’t limiting, at 0.075 mm resolution a standard DLP would give a 7.5cm x 7.5cm working area and allow larger parts to be built.

What is the cost in plastic monomer for the parts? About 15c to 50c for a small Lego size piece, compared to the 5c a block cost for Legos.

 

Update: I had a look around for cheaper & more accessible plastic monomer. Two turned up–rubber stamp photopolymer resin and screen printing photopolymer emulsion.

The rubber stamp photopolymer is inexpensive and would likely UV cure in under a minute (1s or 60s is hard to gauge). Unfortunately, the resin is quite viscous, 60,000 cps, quite thicker than honey. So likely too thick for adding a thin layer at a time.

Screen printing photopolymer emulsion appears more promising. It polyvinyl alcohol or polyvinyl acetate with diazo or SBQ crosslinking initiators. It forms a plastic that is fairly weak and not water resistant but which can be hardened with secondary UV curing and hardening agents. UV polymerization typically takes a few minutes (with a source of 1-2 W/cm2); a strong UV source should reduce the time to under a minute. These photopolymer emulsions typically have viscosities of 3,500 to 12,000 cps, much more suitable!

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>