What is Variable Width Fill? Plus 4 Ways It's Useful

Often overlooked, the Variable Width Fill is a helpful modelling tool.

Load a model into your F123 Series, Fortus 380mc, Fortus 450mc, or Fortus 900mc printer and open the model settings tab in GrabCAD Print. If you hover your cursor over the information logo next to the Variable Width Fill checkbox you see, "Fill small gaps in the infill using a variable bead width of model material". 

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What does that mean?

In order to show what this feature can do, I drew up this thin wall example model. The model has wall thicknesses ranging from .005" to .100" in 5 thou increments.

Test Model

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When printing thin wall features you might see an air gap in the middle of your tool paths. This air gap is an intentional feature added by your printer slicer to try and protect part dimensions. 3D printers are able to print beads of plastic at small variations in width, but very few use this functionality. Instead, most slicers use a single set bead width for each layer height. If the width of your wall feature is not divisible by the set bead width, you'll see the air gap.    

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Top-down view of a thin wall air gap tool path

This air gap can cause a couple of different problems in our 3D prints. First, it creates weaker parts. For thin walls, like the one above, only a single tool path of material will be deposited on each side of the gap. This reduces over strength and can cause stability issues the taller the wall gets. Second, it further reduces how watertight your FDM 3D prints can actually be. Fully solid parts eliminate more air gaps in FDM prints, which can handle higher pressures without leaking.

Variable Width Fill Off

Below is a top-down sliced view of my example part. This was sliced on the Stratasys F370 with ABS material at a .010" layer height, without variable width fill. 

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Variable Width Fill Off

You'll notice our .005" and .010" walls have disappeared. This is actually your slicer trying to protect your part accuracy by not printing features it deems too small for the selected tool path size. GrabCAD Print does a decent job of automatically filling many of our smaller walls, but a gap does appear at the .045" wall thickness. Solid fill tool-pathing ends around .060" mark and turns back into the typical crosshatch pattern.

Variable Width Fill On

Next is a top-down tool path view of the same part with the variable width fill feature turned on. This was sliced using the same base features. ABS .010" Solid Fill.

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Variable Width Fill On

You can see that none of the walls have an air gap and more walls have a solid fill. All the walls up to .080" mark now have a solid, non-crosshatching giving us better mechanical performance. 


1. Thin Walls

The most common reason I tend to use this feature is to give extra strength to thin walls. As mentioned above, single tool paths or air gaps reduce the structural integrity of features and lead to part failure. By adding the additional variable bead width, I add rigidity to thin walls and have a higher chance for printing success.

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 Thin Wall With & Without, Variable Width Fill 

2. Cylindrical Towers

Fast back and forth print head movements, like those used in crosshatch infill patterns, can cause tall features like cylindrical towers to fall over. Not only does variable width fill give you the additional strength you achieve with thin walls, but it adds additional printing stability for taller thin towers.

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Cylindrical Features With & Without Variable Width Fill

3. Water Tight Parts

Due to the FDM printing process, parts are almost never completely watertight. That being said, you can increase the likelihood of your prints holding water or air by decreasing the air gaps within the part. Eliminating air gaps means less opportunity for water and air to find its way through your part features.

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Cross-Sectional View of Water Tight Cube

4. Threaded Inserts

Whether you plan on using a threaded insert, or just threading a screw directly into a printed part, you want to make sure you are not cutting into the infill. Inserts and threads need material to bite into, without it you could see screws and inserts stripped instantly when put into the part. Variable width fill can ensure cylindrical holes have enough material to bite into threads and prevent the inserts from becoming stripped and spinning freely.

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In Review

Variable Width Fill is an extremely simple to use and powerful tool embedded within GrabCAD Print. It allows users to quickly strengthen thin wall features, with added benefits like better water tightness and easier threaded insert placement. Give it a try on your next thin wall print!