Using 3D Warehouse Objects for 3D Printing

With the price of SLA UV LCD Resin printing dropping like a rock, I plunged in with both feet and bought an Elegy Mars printer for well under $400 USD. I also went full hog and got an ultrasonic cleaner and built a timer-controlled UV curing box with turntable. I am a model railroader and wanted to “grow” some 1:48 scaled machine tools for one of my buildings. I found beautifully drawn objects that looked perfect.

They are not. Most of the drawings I’ve found in the warehouse are incapable of being used for 3D printing as they are when downloaded. You can make a very pretty and highly detailed drawing in SketchUp that is entirely unprintable. To do so, you don’t need to know if something is a “solid” by 3D printing’s definition. You also don’t need to be concerned if a face is right-sided or reversed.

When an SU drawing is converted to a Stereo Lithography file format (SLT), anything that is is not solid will not print and any reversed face will be invisible to the file converter, and subsequently to the slicing software that produces the printer’s instruction file. This can be very elusive. The entire part can be correctly faced, but some tiny aspect reversed and the part will fail. I’ve had this happen where a fairly complex lever was printing detached from its hub, but luckily attached at some other point. After very close inspection of the SU file, I found that the surface surrounding the hub was reversed. No hub means no lever!

My first prints were failures. I thought it was how I was orienting the object in the machine. Later I began to evaluate the object’s printability by not only using the various “Solid Inspector” plug-ins available for SU, but viewing the object in the SLT viewer that shows the file when you double click on it in your file list.

You can zoom and rotate the object to examine it very closely to see if anything is open or missing. When you see yourself looking inside the object instead of on a face, you will know that something’s wrong. In your slicing software you can also see any surface that shows up as black hole, meaning there is a missing surface.

All 3D printers work by creating solids. If the object is not solid, meaning that all the edges and surfaces are in full contact with one another, the printer doesn’t know what to do with it and it will fail. Any part of the object that is not solidly connected to a part that in turn is connected to the printer’s base will start to form and then fail. The printer can’t print something that is existing in thin air. Furthermore, a face with no depth is unprintable since such a thing can only exist in a drawing or in math. A surface has length and width, but no depth. Therefore, it does not exist in our world and the printer will not produce it. And one other aspect…. overhangs.

Overhangs are bit more arcane to grasp. Resin printers work by exposing the liquid polymer, layer by layer, to UV light projected through an LCD screen mask that exposes one layer at a time. The base layer attaches to the platen which in turn is connected to the machine’s one moving axis, the vertical Z-axis. When a layer is exposed and therefore turned to a solid, the Z-axis rises a bit (about 5mm) and then returns just one layer higher so the next layer can be exposed. Each time the Z lifts, it must raise the forming object off the teflon window at the reservoir’s bottom to enable fresh resin to fill in behind the object to create the next layer. If the platen rises and the nascent object detaches or was never connected, it will stayed glued to the base film and never form another layer. In other words, the print will fail.

Each tiny detail must be connected in some way to the platen so it can lift from the forming surface. There is an elaborate set of supports created by the slicing software to tie all the parts to the platen as it forms, but occasionally, some part starts forming before the support grows to reach it, and that aspect will fail. It will form, but then being unconnected, will either stick to the bottom or drift away into the soup. In the slicing software you can view the object and view animations of the forming process and see if anything is forming without support and then edit the arrangement.

The last challenge is with the SU artists who, even though they produce beautiful objects, do not understand about grouping. Without object grouping it is very difficult to edit the drawing. I’ve downloaded very complex drawings that are all one group. Every surface and edge was stuck to its neighbor. When you try and move one thing, the drawing turns “Cubist” and looks like it was drawn by Picasso. The instant you make your first 2D shape, group it or make it a component. Don’t start adding shapes to it until you do that. This is SU 101, but many people haven’t learned it.

To make my editing job easier, I’ve changed the reverse face colors (in the face edit in the Styles menu) to a bright red or green. This instantly lets me know how much work I have to do. Here’s an example. This drawing was downloaded as is from the warehouse. When you download it in color it looks terrific. Just put it on the machine and let it rip.

But turn off the textures in View and this is what you see. Everything that is red WILL NOT PRINT.

No only were faces reversed all over the place, but the entire center bolster was unprintable due to not being a solid. Much of it were faces with no depth. It took a lot of editing to redraw it to solid and then thicken the walls so there would be something there when it did print. I’m going to print this object just for fun to see how well it works. It will require about 30 cents worth of resin. The artist did use components to good effect, but the objects within the components were not grouped by geometry and therefore very hard to edit.

After about an hour of cleanup work, here’s what a printable file looks like.

The last point I will make is about resin printer resolution and scale. I’m working in 1:48 which means a foot in the real world is 1/4" in my scale, or… put another way, 1 inch in the real world is 0.021" in 1:48. While the resin printer is capably of reproducing a detail that is three times finer than a human hair, the photopolymer resin will simply disintegrate when you try and handle it. This depends on how the shape is supported. If it’s just a surface detail, e.g. a bolt head, then it can be very small, but if it’s a rod, a protruding detail, or a wall of some kind, then it must be at least one inch in 1:1 to be able to maintain integrity when you attempt to cut away all the supporting structures. My early prints had parts like this that were so thin that they fell apart when you touched them. Even though I thickened some parts of that railroad truck, there are still many that are so fine they the print may still be unsuccessful. You will learn this by experience.

I decided to write all this because with the new pricing of these hi-res 3D systems means many more people are going to have them and try and use them. The printers themselves are ridiculously simple to understand and operate. The challenge shifts from the machine to the quality of the drawings that drive them.