I recently changed tack. Instead of pursuing a powder printer design I decided to focus my energy on building a DLP resin printer. Once I had considered what my end goals really were I came to the conclusion that the powder approach would not yield the resolution and quality I would want. Looking over the blogs and forum posts in the 3D printing community I suspect several people have followed a similar line of reasoning. There is also a growing interest in this type of printer amongst the 3D printer community and so I imagine a better chance for support.
This post, and presumably subsequent ones, will attempt to document what I have achieved so far, outline the issues such a printer has, and how I intend to tackle them. As is often the case, everything I write about has already been done before in some way or another, but hopefully this account will add to the shared experience. I have to caveat that my goal here is purely fun and education, I’m not trying to develop a product, or even a guide on how to build your own version of this. It’s all quite hacked together, as will become apparent! Still, I will still make all files and designs available in case they prove to be useful.
I should also say that it’s very early days and whilst I have had a couple of “successful” prints, my definition of success here is purposely quite flexible! There’s a lot to do, not least calibrating for dimensional accuracy, repeatability and robustness.
Here I run through the various parts that make up the printer. Future posts will no doubt go into more detail for each one, but this should give a good idea of what I have done so far.
My initial idea was to build the frame with wood but I soon changed my mind for 20mm aluminium profile for the flexibility it would give. Additionally, I picked up various brackets and small steel sheet parts from the local hardware store and my scrap pile to hold things in place. The remaining parts were printed on the Mendel, with the majority of the design coming ScribbleJ’s Thingiverse entry, but with a few parts modified to fit the aluminium profiles.
I swapped the motor coupling for a thrust bearing design, in order to reduce wobble, and changed the arm that drops into the vat to a modular design using threaded rods and a custom base. The Z axis is so high simply due to the rods I had lying around being that length. The middle sketchup design shows an alternative where the bulk of the Z-axis goes below the vat table, but this introduces more parts and a potential source for more wobble.
I can already foresee that this component will undergo many revisions. The interaction between the vat floor and the layer of resin being activated by the UV light is critical, and subject to much discussion on the yahoo forum. As I will detail below, already my initial version quickly developed problems and has had to be rebuilt. But for reference, in version 1 I used off cut aluminium extrusions for the walls and 2mm thick perspex (acrylic) for the base. On top of the base, secured between it and the walls, is a PTFE film layer (PDF data sheet). The whole thing was made water-tight with standard transparent silicon caulk.
The key component which, to date, has impeded DIY DLP resin printers due to it’s exorbitant cost. However, this is already changing. I bought 1kg of Spot-GP resin from spotamaterials here in Europe for ~€85 (+€35 shipping to Germany). Still way more expensive than materials for FDM printing, but I imagine prices will drop over time as they did with PLA filament.
This is perhaps a tricky component to acquire as there are so many options open and one wants to be sure of getting the right one. After several days of hunting on Ebay, local classifieds and even Amazon, I lucked-out and found an InFocus IN2104 on Ebay which I picked up for €170.
The most important feature is that the projector has to be one with DLP technology! LCD projectors filter the necessary UV light and appear simply not to work. The next key properties I looked for were the wattage of the bulb and the native resolution. My initial plan was to get one with 800×600 resolution, but I changed my criteria to a higher resolution for the simple fact that I could imagine soon wanting higher definition in my prints (it affects the X/Y resolution of course) and would be kicking myself if I had to buy another projector after 6 months. The IN2104 has a resolution of 1024×768.
I considered bulb wattage a more important factor than the lumen rating as it seems the lumen values given by various manufacturers varies considerably, and I got the feeling, after reading many reviews, that the lumen value is not very dependable. Saying that, higher lumen is generally better, but also look at the bulb wattage. The InFocus has a 200W bulb. Most projectors I looked up had values around 180W, particularly Acer, and so the exact value will depend on the brand and model. I set myself the hard criteria of type and resolution, and then was a bit more flexible on bulb wattage, price and availability.
The part which the first layer attaches to and, in my case, rises up from the resin. My initial tests were with wood and then perspex because I had no local source of glass pieces, which seems to be the preferred material. These proved unsuccessful and so I tried a couple of other materials I had lying around. It seems that copper PCB board and stripboard work quite well. With the latter allowing the resin to harden within the holes and potentially provide a better grip – but this is only a hypothesis. I shall no doubt replace this in the future revisions, but it provides a cheap starting point.
The board is simply glued to a small 3D printed piece which allows four PCB stand-offs to be attached. These provide the lift over the rim of the vat. The stand-offs are then screwed to another 3D printed part which in turn goes on the vertical Z-axis rods. Over the stand-offs are stiff springs – these allow each corner to be individually adjusted so that the base is parallel with the vat floor.
Whilst using threaded rods for the vertical Z-axis was handy because I had some around, I think these will soon be replaced with an L-bracket design as there is some flex in the rods. The weight of the model is negligible, but the suction force of the part and the vat floor can be considerable and it is therefore a design goal to reduce flex where it is not wanted.
Lens Throw Reducer
The shortest distance the projector can throw, and be in focus, is approximately 35cm (if I remember correctly). One could possibly open the projector and physically modify the focus mechanism, or lens, in order to reduce the throw (as Mike has done with his B9Creator), but I’m wary of making modifications to the projector itself and so opted to use a cheap magnifying glass to do the job. This choice has obvious disadvantages – optical distortion, inconsistent image placement, etc – but it’s cheap and it works fine during the initial phase.
I expected to have to design a resin delivery system – perhaps something as simple as an upside-down bottle with a tube to the vat floor, with the liquid pressure regulating the delivery – and I may still do this. In the meantime the vat holds enough resin to complete my small test prints, and I load the resin via a large syringe.
I am using a modified version of Printrun, adding the following features:
- Open the presentation window fullscreen in a second monitor.
- Project a calibration grid
- Display the first layer of the model – useful for displaying the first layer longer than the subsequent layers.
- Accept SVG from Slic3r. (Update: To do this I replaced the SVG module with wxpsvg which has a dependency on pyparsing. Install instructions are on the pyparsing site.)
- Accept a set images (bmp, png, jpg) in a zip file (with suffix .3dlp.zip so it’s recognisable but can also be opened in a regular archive program).
Sprinter for it’s ease of hacking.
I’m not 100% sure how necessary post-curing is. The parts appear quite stable directly after coming off the base. A bath in isopropyl alcohol may suffice for post-processing, to remove non-cured resin. However, as I had a UV lamp handy I threw the parts under that for a few minutes.
The following are a few random notes and findings as I got to print #1.
As the resin photo-initiates the reaction is very exothermic. A very quick test with the thermocouple of my multi-meter and a drop of resin on a thin clear plastic sheet in the bright sun produced a temperature change from 23°C to 74°C. This may have been a contributing factor to the demise of my first vat bottom, which I discuss below. 74°C won’t burn too much, but it’s definitely worth considering – particularly if you get some on your skin and then walk into the sunlight! So be careful! I use latex surgery gloves when testing as the resin tends to get everywhere.
I used a RAMPS board and the related reprap firmware largely because it was lying around. There are a few alternatives as well as the option to roll your own of course. Chris Marion has put together a python based Host which works with the Firmata firmware. It’s currently in beta and windows only, but worth considering. Chris’s software uses freesteel to produce png slices – this is also worth playing with, although it’s not open-source.
The initial successful test pieces came out well partly by luck – the model’s design is such that even though the exposure times per layer were slightly too long this only reinforced previous overlapping layers directly above. I attempted a more complex print soon after and the exposure times resulted in a solid block of resin where it should have been a mesh of struts within. A standard technique to bring more control over the exposure times is to add a coloured pigment to the resin (between 0.2% and 2% by weight according to Fernando at spotamaterials). Currently I am testing the resin without a dye until I order some. My very unscientific initial tests seemed to show an exposure time of 1 second will harden ~0.1mm of resin – but I have to caveat that there are many factors which can play a part, not least of which is my my poor process. I shall endeavour to produce more reliable figures in the near future.
The adhesive properties of the resin are not the only force to contend with. The bottom-up approach has the added problem of a vacuum force – just as with a suction cup. There are several ways to overcome this: brute force (not recommended of course), tilt mechanisms, and slide mechanisms. At the moment the printer simply pulls up, and because the parts are quite small, in comparison with the forces involved, the parts are forcefully pulled vertically off the vat floor. This obviously will have to change, and in fact a subsequent test print nicely highlights this interesting failure:
This should be a cube with a sphere coming out of one edge. The sphere is deformed due to the ‘top’ of the cube sticking to the vat floor, from vacuum force, for several slices. Either the vat floor, the Z-axis rods, or the something else, flexed sufficiently to allow the part to remain attached, until the force was overcome, and could continue. One way of resolving this would be to adjust the print cycle so that the Z-axis rises several millimetres at the end of an exposure, and then lowers to the next slice height. However this would likely only defer the problem until later, larger prints, where the vacuum force would be greater. I think I will introduce a tilt mechanism as part of the next iteration.
Soon after my initial test prints the perspex vat bottom developed some cracks in the middle, which developed to the point of failure. Prior to this a deformation had appeared at a spot after I had exposed for several seconds (whilst attempting to get the resin to adhere to the base). The autopsy showed that this deformation was mainly in the perspex, and slightly in the PTFE film.
I had done quite a poor job of positioning the bolt holes, and consequently, as I tightened the bolts, slight cracks appears at the holes. Therefore I think it is safe to assume that the entire vat bottom was under some lateral pressure, which may very well have contributed to the overall failure. I also had chosen to use only 2mm thick perspex which could also play a part.
The next iteration has a 4mm thick base and I took much more care to align the bolt holes. I expect a similar failure with this version too, but will hopefully be able to test some more whilst I design iteration 3.
As an aside – as I was writing this post and gathering links and facts I came across an anti-friction dry PTFE lubricant from WD-40 (PDF Data Sheet) which may be worth investigating. It would certainly simplify vat construction! One could simply use a Pyrex dish and spray some PTFE in before each print run. (Of course that’s a bit glib – I suspect oven dishes won’t have the optical clarity required!)
The blogs and forum are certainly worth a read! My thanks to the authors for the information they have shared within.
This post is turning out to be quite long, so I shall end here and put the remaining stuff into smaller posts in the near future. Hope the information is useful to someone.