Initial Design Considerations

My design decisions are evaluated with the following values:

• Time Constraints - Will this cause a lapse in progress? Will this prevent me from finishing this summer?
• Budget - within $1000
• Knowledge - Do I know how to do this? Can I learn how within the time constraints?

If multiple options fit the above values, I'll evaluate in the following order:

1. Reliability - Less maintenance, easily replaced parts
2. Print Speed - More time for testing, learning, and use
3. Print Size - I'd like to make laptop cases (about 12 in.)
4. Print Precision - 100 micrometers is enough
5. Print Materials - Familiar materials are easier to use for designs.

SLA or FDM?

3D printers come in 2 varieties: stereolithgraphy (SLA) and fused deposition modeling (FDM). I've chosen to make a SLA 3D printer because the results are smooth with much less mechanical accuracy compared with FDM 3D printers. SLA 3D printers require only 1 servo (z-axis), compared with 4 for FDM 3D printers (x, y, z-axis and filament feeding). The less moving parts, the less that can go wrong, especially from a rigidity standpoint. Also, I can acquire less precise parts quicker and for less money. SLA printers are also much faster, because an entire layer can be printed at a time, instead of single lines each movement.

The trade off compared with FDM printing is the cost, at least in 2015. Resins are about $55 per liter, and projectors cost more than servos. Without major modification, projectors have a do not output enough UV light for print areas greater than about 10-20 cm. Resins don't have the same familiarity as ABS or PLA plastics, so there is some testing needed to find the mechanical limits of your prints. That said, new resins are actively being researched, so resins are likely to get much better and cheaper. FDM printers can print in multiple colors and materials in the same print, which SLA cannot currently do.

Top-Down or Bottom-Up?

SLA 3D printers can print top-down or bottom-up. Bottom-up designs need a clear window to project light into the bottom of the resin vat. The resin layer then sticks to both the object and the window, so the window needs a non-stick layer and a tilt mechanism to allow each layer to be peeled off. Each layer's height is precisely controlled by being between the window and previous layer, with the window helping to flatten the layer. The resin vat only needs enough resin to cover the bottom plus the volume for the object being printed. However, the non-stick layer adds regular maintenance and cost. A tilt mechanism would add complexity.

Top-down designs project light onto the surface of the previous layer from the top of the resin vat, omitting the window. Each layer's height is dependent on the surface tension of the resin, and requires even settling of resin at the surface. For larger objects, a wiper is often used to move the resin into the middle of the previous layer. The vat needs to be filled as deep as the object being printed, which may increase the resin waste. Recent top-down designs have filled the vat with salt water and a floating resin layer, reducing the waste.

I have chosen the top-down design to reduce the cost, maintenance and complexity of the printer. A wiper system is optional, and won't have recurring maintenance costs compared with a non-stick window.