Enough books to fill a bookshelf have been written about Design-For-Manufacture (DFM) parts that will be machined or casted. On the other hand, when it comes to resources on how to DFM for 3D printing, there is a void.
What You Need to Know About PolyJet 3D Printing and Fused Deposition Modeling (FDM)
PolyJet 3D printing performs similarly to inkjet document printing. However, instead of jetting drops of ink onto paper, PolyJet 3D printers jet layers of liquid photopolymer onto a build tray and cure them with UV light. The layers build up one at a time to create a 3D model or prototype. Fully cured models can be handled and used immediately, without additional post-curing.
Fused Deposition Modeling (FDM) Technology is a powerful Stratasys-patented additive manufacturing method. FDM builds concept models, functional prototypes and end-use parts in standard, engineering-grade and high-performance thermoplastics. 3D printers that run on FDM technology build parts layer-by-layer by heating thermoplastic material to a semi-liquid state and extruding it according to computer-controlled paths.
Above Image: An FDM print head laying down a bead of plastic – artist’s rendition
This basic layering interaction is the foundation for most of the FDM design rules for 3D printing success.
For example, if your printer lays down beads with a 0.010” thickness in the Z or vertical direction and the part you want to build needs to be exactly 2.515” high, you’re going to have a problem.
Rule #1: Make sure your part height is a WHOLE multiple of your printer’s Z thickness.
A whole number prevents your print software from having to decide if it should add an extra layer of plastic to reach your height or take one away. You want a human to make that decision, not the software.
Rule #2: Make sure your wall thickness is at least TWO multiples of your printer’s bead thickness.
Even if your 3D printer can theoretically make beads that are 0.005” in the X or horizontal direction, you can’t just slap up a wall that thin and expect it to stand. When printing with Stratasys FDM printers, we’ve found that side walls two beads thick can be successfully made. However, your print will usually be better off doing even greater multiples:
There are many factors you need to consider when planning and designing a part to be 3D printed, and part dimension is just one. Optimizing build times and reducing material use are very important and lead to the second basic principle: building support.
Most commercial 3D printers have two print heads: one for the actual plastic material your part will be made from and one for the support material which will get washed or broken away in post-processing. To decide when to use support material to buttress your actual plastic, the print software uses the 45-degree rule for overhangs:
The black support beads take the same amount of time to lay down as your actual plastic beds, thus adding to your build time and cost. While it’s not always possible to remove overhangs, it can be easier than you think:
Simply reversing or changing the build direction could save a lot of time and support material. Therefore:
Rule #3: Experiment with different build orientations to reduce support usage and maximize part strength.
You don’t have to actually print the part in two directions, as shown above, to get the benefits of less support. Most printing software will automatically calculate the support material required in each instance. All you have to do is set up the print and compare the estimates. One thing print software CAN’T calculate, however, is part strength.
Due to how the FDM process extrudes material, if your part is pulled in the X or Y direction it will be strong (the plastic beads are pulled in tension). But if your part is pulled in the Z direction, it will be weaker (the stress tries to separate the layers, which bond more weakly):
It’s hard to tell how much weaker the part is in the Z direction due to differences in the bonding process between machines and material types. We have found that finished parts can be up to 33% weaker in tension trying to separate the Z layers versus trying to stretch the beads.
Your job doesn’t end after you’re done designing your part. Post-processing your print is just as important to your end-use as the design process.
Want to learn best design techniques for your Stratasys machine? Browse our 3D printer training courses.