FDM Systems

TEAM Lab Equipment: FDM 3D Printing

Basic Overview of FDM Printing:

Materials: Type: PLA (Polylactic Acid) [95% of our prints from this printing technology], ABS Plastic, other thermoplastics Properties: PLA – Plant derived plastic; Biodegradable. more rigid, more accurate than ABS. ABS – Very High Yield Stress, standard engineering plastic.

For more information on the different materials and their properties, please view this excellent guide compiled by "hubs"

Accuracy: Moderate/Good accuracy: 0.1mm minimum layer thickness (0.2mm standard), Overall 0.2-0.4mm accuracy depending on geometry. Smallest free-standing feature approximately 0.4mm Support Structures: Same Material, or overhang structures where possible

Maximum Build Dimensions: 12x12x24 inches (305x305x610 mm) – Raise3D N2 Plus, 12x12x12 inches (305x305x305 mm) – Raise 3D N2, 11.2×6.0×6.1 inches (285x153x155mm) – MakerBot

Build Time: Total: 15 minutes to 24+ Hours (dependent on geometry & size)
*Turnaround time is workload dependent. Expedited parts may come with an additional fee.

Cost: UC: Black or White ABS or PLA: $0.30/Gram + $3/hr of build time + Setup and CleaningOutside UC: 32% Premium (above cost)

Pros: Good impact resistance, good accuracy. Least expensive printing technology.

Cons: Least accurate technique (visible layers), some geometry may not be printable.

In Depth Overview of FDM Printing:

FDM 3D Printing is a technique whereby a layered object is deposited as a continuous, or semicontinuous thermoplastic filament extruded from a hot-nozzle. In other words, the deposition process this machine uses is similar to that of a hot-glue-gun. This is the most common 3D printing technique in existence, as it’s generally inexpensive, and is capable of making durable parts. However, its ability to achieve complex geometry is somewhat limited in comparison to other technologies, and its printing resolution is comparatively very course (with layer thicknesses generally between 100, and 300 microns).

Vs. Reinforced FDM

Reinforced FDM is more-or-less the same as FDM printing, with the addition of continuous bands of fiber filament for increased stiffness and strength.

Vs. SLA

In comparison to SLA printing, FDM parts are quite limited in shape complexity, but are typically stronger and less expensive.

Vs. Polyjet

In comparison to Polyjet printing, FDM parts are far stronger, and far cheaper, but are also significantly less accurate. Again, FDM parts are also far more limited in shape complexity when compared to Polyjet printing.

Aesthetically, these prints will be less “finished” looking than other printing technologies (directly off of the machine). However, with post-processing, they can be finished to the point where they appear indistinguishable from an injection-molded part. See the finishing guide at the end of this document for an example of this process.

Materials:

All materials must be thermoplastics, due to the thermal extrusion process used by the machine.

Common:

PLA – Biodegradable rigid polymer, vast majority of (desktop) FDM prints are made of this polymer due to its mechanical performance, and compliant behavior while printing (holds geometry well, doesn’t shrink or distort as much as ABS). Also available in a variety of colors. Technically, natural-colored PLA (translucent) is biocompatible after printing, but maintaining sterility post-print is not a proven process, and insufficient research has been done on sterilizing prints for clinical applications.

ABS – Used in cases where increased strength is required over PLA. More difficult material to print with; some shapes may not be appropriate for this material due to ABS’s tendency to contract while cooling (causing distortion, and occasionally failed prints). Available in a variety of colors.

Nylon – Higher impact strength than ABS, but less rigid. Also low friction coefficient. Extreme durability (in comparison to PLA and ABS) makes this polymer suitable for gears, and other moving parts.

Less Common:

Flexible PLA blends

Conductive Polymers

Commercially Available Blends of other Polymers (including wood, and metal filled polymers, PET, hybrid mixtures)

Exotic Polymers – the TEAM lab has the ability to generate our own polymers via filament extrusion, derived from pellets. Check with the TEAM lab manager for safety and plausibility.

Techniques:

Designing for FDM Printing:

As FDM prints are generally regarded as low-precision, and prints can vary from batch-to-batch, refinement (multiple design iterations and hand-finishing) are often required to achieve desired results. Also, overhanging features often necessitate the existence support structure (often made of the build material) which must often be mechanically removed. In this case, expect print defects in these locations (rough finish, drooping, etc). Through careful design, often times support structure can be avoided (ideal scenario). Support structures are covered in more detail below.

Hollow Prints - One important consideration for FDM prints – parts derived from this technology are rarely solid; instead, they are generally created in a series of “shells” extending from the outside surface inward, coupled with an “infill” to provide bulk-support. This process is achieved within the printing/slicing software, not within CAD.

Figure 1: Zoomed Depiction of 3D Print Cross-Section

Adding Threads – Threads are generally not directly printed onto parts, but are instead added as a secondary process by way of tapping. Exceptions may occur where two bodies are designed to couple/decouple by way of coarse threads. Other (superior) techniques for adding threads involve using threaded inserts (usually brass), or capturing a nut; Both of these techniques result in much stronger threads than a printed part alone!

Figure 2: Coarse Threads on Two Printed Bodies (via MatterHackers)

Figure 3: Different Techniques for Adding Threads to a Printed Part. Tapping, Threaded Insert, Captured Nut (via Formlabs)

Distortion:

Due to thermal contraction, all FDM prints are prone to distortion from their intended geometry. The following parameters are major factors in this process:

Aspect ratio – As a part becomes long, relative to its width, thermal distortion is more likely (bowing). Occasionally, this distortion can become so bad that prints fail due to the nozzle knocking against the print, causing it to detach from the built platform. It’s generally recommended to limit prints to a 5:1 length to width ratio (or smaller).

Figure 4: Intended Part with Large Aspect Ratio (via Markforged)

Figure 5 Resultant Part is Warped (via Markforged)

Wall Thickness/Shell Count – While thick walls result in stronger parts, too many “shells” can result in print defects that lead to poor surface finish, or warping. Typically, we use 2 shells for light-duty prints, 3 for heavy-duty, and 4 for instances where holes may be tapped (threads added). Note: Actual wall thickness (dimension) is a multiple of the Nozzle diameter (typically 0.4mm), aka 1 “shell” thickness.

Figure 6: Different Shell-Counts (via prusaprinters.org)

Infill – This parameter adjusts how solid the interior of a part is. Low values (10-15%) reduce print time, and decrease weight with minimal impact on strength. High Values (15-25%) increase strength, but increase print time and can cause distortion. Very-High Values (25%+) can cause severe warping.

Figure 7: Infill Percentages (via zx3d.com.au)

Figure 8: Different Infill Patterns, all 30% fill (via Makerbot)

Sharp Corners – Sharp corners (~90 degrees) engaged with the print-surface can (in some cases) cause peeling/warping away from the build platform, where parts have a large footprint. To counteract this, we often recommend adding “Lilly pads” AKA “dog-ears” to the base of prints to encourage good adhesion. These features are then clipped off afterwards with little effect on the finished print. TEAM lab Tools and Techniques – FDM 3D Printing TEAM@UCD BME, Revised JUNE2018

Figure 9: Warped Corner, no Lilly pad/Dog-ear (via Ultimaker)

Figure 10: Sacrificial Anti-Warp Features Added to a 3D Print underside (via Thingiverse)

Accuracy:

As FDM printing is generally considered a low-resolution process, expect to achieve a tolerance of +/- 0.250mm. However, this number is dependent on machine calibration, and part geometry. Again, achieving proper fit may require multiple iterations. Also note, part orientation has a significant impact on dimensional accuracy.

Overhanging features and Support Structure:

As FDM printing relies on adhesion to the build platform, or a previous layer below for the extruded filament to stick, some prints may require support structures where overhangs become too severe. Else, drooping, or failed printing may occur. Please see the figures below for an example of this property:

Preparing Files for FDM Printing:

The team lab requires two sets of files for each 3D printed part – the original parametric model file (Solidworks/Inventor/Fusion360/etc), and a millimeter scale STL file (point-mesh file) Standard Settings – Unless otherwise specified, we will usually print parts at medium resolution (0.2mm layers), with two shells, and 15% infill; Please specify your ideal parameters, if these do not match your needs.