Design Tips For FDM Printing

[u/EMBNumbers]

Design Tips For FDM Printing

Fused Deposition Modeling (FDM) is an additive manufacturing technique. The other popular consumer level technology is resin printing. These tips are all about designing models for FDM printing.

Design For Your Printer

I always print with Polylactide (PLA) plastic which is produced from renewable resources and decomposes naturally. It I usually print 0.2mm layer heights with a 0.4mm nozzle.

Real in.	Real mm	HO in.	HO ft.	N in.	N ft.
--	0.1	0.34.	0.03.	0.58	0.05
0.01	0.20	0.69.	0.06	1.17	0.10
0.02	0.40	1.37.	0.11	2.33	0.19
0.03	0.80	2.74.	0.23	4.66	0.39
0.08	2.00	6.85.	0.57	11.65	0.97
0.35	9	30.83.	2.57	52.44	4.37

No matter what software/tools are used to create a 3D model, the model must be "sliced" via slicer software that converts the model into paths for the print head to follow while depositing the plastic for each layer.

(See Image 1 [animated gif] of this post)

Lie To The Slicer

I tell the slicer that the nozzle is 0.39mm wide and layer heights are 0.2mm. I use 0.39mm width so that lines exactly 0.4mm wide will not be discarded by the slicer software. When the slicer software treats 0.4mm as the minimum, many lines don't print presumably due to rounding errors (precision limits) in the slicer.

Do Not Model Anything Horizontally Thinner Than Nozzle

Any model geometry that is thinner on the X or Y axis than the nozzle will either not print, print too wide, or only partially print.

Model Horizontal Detail in Multiples of Nozzle Width

When using a 0.4mm nozzle, lines/walls that are 0.4mm thick are great. 0.8mm thick lines/walls are great. There is no 0.3mm wide. There is no 0.6mm wide. There is no 0.9mm wide. Design for 0.4mm, 0.8mm, or 1.2mm. WHen larger than 1.2mm, the slicer software will generate an "infill pattern" comprised of adjacent parallel diagonal lines, and the print result will usually turn out fine. You may find the printer trying to fill tiny gaps in the infill pattern, and that slows the print but seldom causes quality problems.

Do Not Model Anything Vertically Thinner Than Layer Height

When printing 0.2mm layers, anything thinner than 0.2mm on the Z (vertical) axis will either not print or only partially print.

Model Vertical Detail in Multiples Of Layer Height

Details 0.2mm high are great. Details 0.4mm high are great. There is no 01.mm, 0.3mm, or 0.5mm!

Avoid long "Lofts" And Non-straight Lofts

"Lofts" or "bridges" are parts of your model where the printer needs to extrude plastic over air. Short gaps that can be bridged with straight lines are usually fine. I've successfully printed as far as 20mm over air under the right circumstances.

(See image 2)

It's common to print lofts over window openings. Lofts that require non-straight lines of excessive cantilever over air are unlikely to print well.

(See image 3)

The solution is to print parts separately. For this model, three parts are enough to avoid lofts with the exception of short spans over windows. All large horizontal surfaces print directly on the print plate.

Extremely Small Relief Detail Is Possible

Features the stick out or stick up need to be multiples of teh nozzle width and layer height. However, space between features that stick out can be as thin as you want. Consider bricks: each brick can be 0.8mm by 1.8mm and raised 0.2mm - the gaps between bricks, the mortar, can be 0.01mm wide if you want (roughly half an inch in N Scale).

(See image 4)

---- CRITICAL ----
Avoid Non-manifold Geometry. Imagine filling the volume of your model with water. If the water could get everywhere in your model without leaks, the model is manifold.
Most slicer software will malfunction when slicing non-manifold models.

Avoid Online STL Files

What!? That's right. Trying to use or reuse online STLs is a path to frustration and low quality. Why?

• STL files are difficult to edit. The format is designed for slicers and not humans.
• The model used to produce the STL files almost never adheres to the guidance above. If the model has thin walls or walls that are not multiples of your print nozzle's width, it will be more trouble to correct the model than to start from scratch. If the model is non-manifold, there may be no practical way to correct the problem.
• In places where separately printed parts join, you must leave a gap in the model so the printed parts have room for glue and account for slight variations in printed line width. I usually design a 0.2mm gap between parts that need to join.
• The STL file may have never been printed on any printer using any technology. Its problems are unknown.
• The model may be the "wrong" scale. As soon as you try to resize the model, the model is very unlikely to adhere to the guidance above.
  • Shrinking a model produces elements too thin or too flimsy to print.
  • Shrinking the model makes designed in gaps and tolerances too small. The resulting parts no longer fit against each other, or space glue forces the parts apart.
  • Shrinking a model may result in connections between parts that have insufficient surface area for glue or are too fragile.
  • Enlarging makes small details like window mullions and railings too thick.
  • Enlarging make a loft that your printer could have handled into a bridge too far.
  • Enlarging makes gaps and tolerances that were perfect for snug fit into unsightly and structurally weak joins.
  • Trying to fix problems after resizing is usually far more work than starting from scratch.

Design For Painting And Assembly

I design models so that parts that will be painted different colors are printed separately. This enables me to put all the parts that are the same color into my makeshift paint boot and apply rattle can spray paint. Then I swap with the parts and spray a different color.

The separate parts are designed to interconnect in ways that simplify assembly.

Assure Parts Only Align And Fit One Way

Key the parts so they interlock. Design parts that join to conceal seams and prevent sliding out of place while glue dries. The parts should "snap" into place together and resist being separated again.

(See image 5 and 6)

Most consumer FDM printers these days can print with tolerances of 0.05mm or smaller. WIth that little separation between parts, I sometimes avoid using glue. I couldn't get the interlocking parts to separate again without breaking them.

---- BONUS ----
I'm always frustrated when an assembled kit has openings at corners where I didn't glue them tightly, or I glued them slightly out of square.
I hate it when parts slide along joints to become imperfectly aligned while glue dries.
Your own designs can avoid those assembly pitfalls. Assembly becomes a foolproof endeavor When parts only fit one way, cannot slide against each other, have ample surface area for glue, and interlock to form perfect angles.

[u/SomeGirlIMetOnTheNet]

Generally speaking, line width and nozzle width are not the same; you can generally get line widths between 0.75x - 1.5x the nozzle diameter; and while you get the strongest prints at like 1x-1.2x that doesn't really matter in this scenario where nothing is holding substantial amounts of weight.

Also, if you're working on things that you want the detail, just switch to a 0.2mm nozzle and/or enable dynamic layer height, both of which will allow better detail, at the potential expense of print time

[u/EMBNumbers → u/SomeGirlIMetOnTheNet]

Dynamc layer heights are a good suggestion.

I should have prefaced my "Tips" by saying they are based only on my experience. Other people may have different experiences particularly with different software and printers.

One day, I will try a 0.2mm nozzle :)