FDM vs PolyJet

What’s the difference between FDM and PolyJet?  Which is best for your design?  What factors in a design make it better suited for one process or the other?  Today, we’ll answer these questions and more.

1. The Difference

FDM stands for fused deposition modeling (a term invented by Stratasys), and works by extruding a molten plastic filament through a heated tip onto a build tray where it solidifies and fuses to plastic already deposited.  The part is built in this fashion layer by layer, with dedicated support materials being placed to allow for overhangs as needed.  This is a familiar process to most hobby 3D printers, which use much the same technology, although the patented heated build chamber and precision control of our Stratasys FDM machines generally make parts much stronger and more accurate than consumer-grade machines.  

FDM printers in use

PolyJet printers also build the part layer by layer, with hydrogel support material placed to allow overhangs.  But instead of melting a solid filament, they deposit a UV-curable liquid resin via a micromachined piezoelectric jet array, similar to an inkjet 2D printer head.  A UV lamp cures each layer as it’s deposited, fusing it to the previous layers with even greater adherence than FDM, resulting in a part that’s almost equally strong in all directions (isotropic).  ​

PolyJet printer curing resin with UV lamps

2. Which is Best?

Of course, it depends!  In general, we recommend FDM for most parts that don’t have fine features (less than 1/16” or 1.5mm) and PolyJet for parts that do.  Bulkier end-use parts that don’t need ultrasmooth surfaces do well with FDM, both due to the material qualities (industrial thermoplastics) and due to the lower cost of materials (not to mention the ability to produce sparse infill, which saved a ton of material in the context of large parts. 

​When it comes to small parts, however, PolyJet is a much better choice.  Material costs become negligible compared to cleaning costs and quality concerns in the land of small pieces.  PolyJet wins on both fronts with forgiving hydrogel support structures and its ultrathin layer precision printing.

We generally recommend PolyJet for parts with small, detailed features

FDM sparse infill

FDM parts are generally more robust, so they often make good functional prototypes and end use parts.  On the other hand, PolyJet is more precise because of its higher resolution, so can make a better prototype for fit testing purposes or parts that require precision dimensions.  

Because of the lower resolution and the nature of the bonds between contours, FDM parts are generally not watertight, so PolyJet can be a better choice for parts that need to contain liquid or form a seal.  FDM parts can be vapor polished to better seal their surfaces (though this is not perfectly watertight either), or can be coated in paint or epoxy for the purpose as needed.  

Large format FDM part. The faceting is due to exporting the STL file at a lower resolution to keep the file size down.

We have capacity for large format FDM parts (F770 and Fortus 900 machines), as well as by printing multi-part assemblies which can be solvent welded together.  Joining PolyJet parts is possible as well, but FDM generally makes the most sense once a single part gets larger than the PolyJet build volume (19.3” x 15.35” x 7.8” or 490 x 390 x 200 mm) due to the factors discussed earlier. 

​Lastly, our PolyJet machines are capable of multi-material prints which allows for unique designs that are not practical with FDM.  A great example is printing rigid and rubberlike materials in a single assembly to prototype or replace a co-molded part.

Multi material PolyJet print. A rigid grey part is co-printed with a rubberlike cap that extends into the body.

3. Design Factors

Some designs are not well suited to either process, due to thin (less than 0.04”/1mm) walls or internal voids.  We’re happy to work with you on getting your design ready for printing, but some basic things you can check beforehand are:

• Make sure your STL is “watertight”.  The 3D printing software can’t handle a mesh whose edges don’t all meet cleanly.  This is usually an artifact of conversion from one filetype to another, and often an easy fix in a software like GrabCAD, but unexpected things can happen if your file is not sealed.
  

Repairing a file in GrabCAD

• ​No internal voids.  Internal cavities will be filled with support that becomes impossible to remove.  PolyJet support can swell over time, especially with humidity, which can distort or even break the part.  Even if a cavity is technically open to the outside, but only via a narrow channel, it can be impossible to fully remove the support, even with soluble support material.  Line of sight access to all parts of the model is ideal for cleaning.

A narrow cavity was left inside this part (pink) when two bodies were combined in CAD. If printed as is, the printer will fill this cavity with support, increasing print time and risking deformation or damage to the part.

• Be aware of your minimum feature size.  For FDM, features smaller than 0.04” (1mm) may not resolve well.  Tiny embossed or debossed text, snap tabs, small holes or pins may be better saved for PolyJet, or ultimately an injection molded part.  Holes and pins can be post-machined in an FDM print. 

We often print very small features like this pin separately and later bond them to the larger part to ensure quality and precision.

This hole was drilled post-printing to ensure roundness and accuracy. Tapped holes are also possible.

PolyJet can resolve small features reliably down to about .02” (0.5mm) but walls thinner than .04” (1mm) can be extremely fragile and may not survive the support removal process.  

These thin snap tabs were printed with PolyJet PP-like material to ensure they’d be flexible enough to withstand use.

The wider fins around this hole held up, but the narrower based ones near the edge snapped off during cleaning.

While 3D printing -- whether with FDM or PolyJet -- enables geometries that would be impossible to make in a subtractive process, care must be taken to ensure that your design will work well in the chosen process.  At POP we always strive to make the best part possible; that process starts with good communication and DFM suggestions that we hope will ensure our clients get what they need, even if it's not exactly what was originally quoted.