Senior Member
Joined: 10/01/2016
Location: United KingdomPosts: 214
| Posted: 01:09pm 17 Dec 2020 |
It is a matter of differential surface properties, as you are guessing quite correctly. The window is a stretched film of FEP (Fluorinated Ethylene Propylene Polymer) a.k.a. "Teflon". The platform surface is roughened steel. The resin sticks preferentially to the roughened steel and releases easily from the flexible FEP film. The latter is a consumable item but lasts a good time if it is cleaned properly between runs.
Just as LEDs have mutated from tiny little red things to full-on floodlights and car headlights, the same applies for UV LEDs. Actually, to activate a film of resin that is 25-100 microns thick requires only quite low energy UV. The trick is to keep the exposure dose above the activation threshold where solidification is required but below the activation threshold in the bulk of the resin where some light diffusion in the resin bulk is inevitable.
I had the pleasure of working at university with the one of the inventors of the UV activation system; Jo Jaworzyn who worked at ICI in Macclesfield where the system came from. The original intent of the UV activation system was to overcome the problem of painting ships. Successive layers of paint do not easily adhere to the last coat and hydrolysis and debonding was/is a problem. The original idea was to paint the whole ship hull in one coat and then light-activate the entire surface using an array of UV fluorescent strip lights. A number of problems prevented this application and it went onto the shelf for several years until the dental materials division became aware of it and the first light-activated dental filling materials was produced. The hazards of using a mercury UV lamp in the patient's mouth, albeit via a light guide, was remedied when a new activation chemistry sensitive to blue light led to the material used today.
Onto the reasons that the initial enthusiasm for 3D printers may have waned somewhat. Firstly, the fused deposition systems are basically rather crude. The resolution is poor and the generated surfaces are rough. Builds take hours. 3D design software is challenging. The scope is therefore rather limited. As I have already described, the UV system is just a little bit messy.
I have three laser cutters; the largest can handle an 8 foot x 4 foot sheet of 1/2 inch ply; the smallest allows me to cut acrylics with accuracy in the region of a few microns. I can run up a design on screen and hit, literally, the "print" button and in a few minutes the component is made. Sometimes I may find that a fit of two components is a little loose or too tight; I can wind back the dimensions by, say 50 microns and then hit "print" again. I find that in my model making the lasers are used far far more often than the stereo photolithography system.