Here are a few things to consider in the 3D printer:
I find that making lists like this before every new build really helps me and the people I am working with (it is "we" now) to come up with an effective design that really meets our expectations. This printer will be a prototype, we plan to build a cooler, better more advanced version in a later future. Hopefully to be completed at the end of 2013. And if all works out maybe we can even get to a kickstarter. (Dreaming freely here)
The printer must be,not in any specific order:
The basic operating procedure:
- Open source
- High resolution.
- Compatible with a wide range of materials.
- Easy to use.
This is how a Photo Lithographic 3D printer works. Photo Lithography is very simple, light illuminates the resin
and the resin hardens.
To be more exact a quantity of light falls/shines onto the resin, if the energy quanta of that light is high enough it will induce photo polymerization of the resin. First thing is to decide on a light source:
The key part in this is quantity of light energy
, a therm that comes from the world of radiology.
The dose is divided in to three vectors as you will, namely photon energy, light intensity and duration of illumination, together giving the total energy dose. Usually in the UV curing of materials the dose is only measured for a specific part of the spectrum. The rest of the light will usually be reflected or absorbed and converted in to heat.
Only photons with a high enough energy will take part in the photo polymerization. This means that the resin that you will be using is the determining factor in the part of the light (electromagnetic) spectrum that we are interested in. Most photo curing resins will cure under the influence of UV light. Light with a wavelength of between 365nm and 420nm.
Some resins also allow for curing with longer wavelengths but these are usually rare and expensive. 1) One of the things to consider is that in order to be able to print with a wide range of resins we would like as much UV in our light as possible. I will explain this in depth when designing the basin, mirror and anti stick coating.
The other part is time of illumination and illumination intensity. The illumination intensity, or luminous flux is the amount of Photons per unit of time that is emitted by the light source. The longer you illuminate the resin the deeper the light penetrates and the harder and thicker your printed layer gets. This is a very unique feature of stereo lithography where the illumination time is another factor to consider as this determines the build layer thickness. 2) The light source must be of high intensity so the illumination time can be as short as possible allowing for a faster build.
3) An other thing to consider is that the light source needs to be very controllable in switching from illuminating the resin to not illuminating the resin.
In the principle of photo lithography, what gets illuminated polymerizes and what does not get illuminated stays liquid. This means that our resolution or minimum feature size is determined by the minimum spot size.
3) The third parameter for our light source is that it must have the possibility to illuminate a spot that is as small as possible.
Googleing we found that there are two viable light sources/systems that will meet these demands. A blue/UV laser with nice optics to produce a small spot size and a Galvo Head
or A DLP projector. A Lasers are cool but to achieve a small accurate spot with a galvo system
felt to us as going way over our heads. Since non of us has any experience in setting up a laser, laser optics and a galvo system. And having the guys from Form 1
as an example (patent issues), maybe one day we would like to offer the world a kit too. We decided to go for the DLP projector option. There is a whole world of DLP projectors out there.
A light source passes through a rotating colour wheel and falls on a surface with actuated micro mirrors. These mirrors in synchronization with the colour wheel decide when to either reflect light through the lens or deflect it to somewhere else. Together many micro mirrors form the image.
As from our considerations in the above we can easily state what properties we want our projector to have:
- high UV content (determines if the projector works to cure the resin)
- high light intensity (shorter cure time)
- high contrast ratio (gives a higher resolution with less light contamination)
- high resolution (results in a smaller feature size)
Last but not least we only have 1000euro's to spend on a beamer. So there is a financial limit too. I realize this is not a small budget for a decent projector, but if the project fails I can always watch a movie on it.
In the end we decided to use an Acer 7077365 Acer H6510BD DLP FHD 1080p, with 1920x1080pixels. Which we ordered at a local store
. Having the light source sorted we can now decide how to use our light source in our 3d printer:
Wait who ho ho stop, yes I know we are just designing a 3D printer but lets do a quick google on resins (photo curing resins). We found that these materials aint cheap. So this cancels the top down approach option. In common stereo lithography the light source illuminates a pool of resin from above. As consecutive layers form the build platform sinks down in to the vat of resin. This means your work piece can only be as high as your basin is deep. This also means that no matter what the size of your build, you must always have a full vat of resin. Meaning that if you want your largest object that you can print to be the size of a shoe, you will need a constant volume of about 3L of resin in your tank. At 80 Euros per litre, there are always 240euros sitting in the tank.
To us this is a bit much. So bottom up it will be.
There are two reasonable configurations when considering a bottom up 3D DLP printer. We can either project directly onto our build area or we can use a mirror to have our projector at an angle in respect to our build area. We chose to put our projector at a 90 deg angle and use a single surface mirror to project a crisp image on to our build surface.
This because we are aiming for a true desktop machine, something that really fits on our desktop and is as compact as possible.