FDM 3D Printing: Top Benefits of Sparse Fill vs. Solid FDM Parts

FDM® (Fused Deposition Modeling) is one of the original 3D printing processes, utilized across industries for production of manufacturing aids, rapid prototyping and production parts. FDM 3D printing technology builds parts with strong, engineering-grade thermoplastics like ABS and Polycarbonate. At Stratasys Direct, FDM industrial 3D printers are a popular choice among aerospace and transportation companies for high-performance applications requiring heat-resistance or impact strength.

One unique function of FDM 3D printers is a build mode that allows users to fill in thicker sections of the part geometry with what’s called “sparse fill.” Sparse fill is when plastic is extruded in a scaffolding-like structure instead of a standard 3D printed layer. This results in sections of a 3D part being nearly hollow, but with the support needed to retain strength and rigidity. Think bridge cables or railing posts adding strength to the overall structure with gaps between.

The fill style chosen for FDM parts affects the interior of parts, but the exterior appears and operates the same as a solid fill part. Sparse-filled parts can be finished similarly to solid-filled parts with post processes like sanding, epoxy, primer and paint.

a sparse fill fdm part

Benefits of Sparse Fill

1. Reduced weight

Sparse fill means less material built into the part, so the weight of the final part is significantly reduced. If you’re designing a part that would benefit from being lighter, sparse fill is an easy way to achieve the reduced weight while maintaining strength.

2. Reduced build time

Because the 3D printer has less plastic to lay down in each layer, a part that utilizes sparse fill takes less time to build. With the speed benefits of 3D printing already, the time saved with sparse fill means getting your parts delivered faster.

3. Reduced part cost

Additionally, the reduction in material used to build spare fill parts and faster print time contributes to a cheaper overall part. At Stratasys Direct, time to build and material costs are the main factors contributing to the cost per part.

a sparse fill fdm part

When to Use Solid Interior Fill

Stratasys Direct recommends customers use solid interior fill when:

1. The part strength is the critical feature

While parts that utilize sparse fill in sections still retain strength, solid fill typically delivers an even stronger part. If you’re functionally testing a prototype that will endure high impact, for example, you should consider filling in thicker sections of a part with standard solid fill.

2. The part is comprised of fine details and thin walls

At Stratasys Direct, thin walls in a design are built solid by default, and the only sections that are sufficient will have sparse fill. This recommendation typically depends on layer height, and we have defined the default thicknesses from 0.2”-0.6”. Thinner thicknesses are possible with help from a Stratasys Direct engineer.

When to Use Sparse Fill

Stratasys Direct recommends customers use sparse fill when:

1. The part can benefit from being lighter

Many companies that utilize 3D printing are looking for lightweight alternatives to designs that would be machined out of metal or heavier materials. 3D printing can offer reduced weight by delivering complex shapes and sparse interiors.

2. The part has a large interior

If your design has interior features that need fill, it’s a good idea to take advantage of sparse fill. Not only will your cost per part be reduced, but your part will be delivered much sooner. Additionally, if you’re designing something large, you may benefit from reducing the weight significantly with less plastic in the interior.

fdm sparse fill fixture on a bmw

BMW Utilizes Sparse Fill in a Testing Fixture

The BMW AG plant in Regensburg, Germany used conventionally machined aluminum fixtures when assembling and testing bumper supports. The metal fixtures were uncomfortable and heavy for workers, who manually held the fixtures for each test and when applying the bumper supports. BMW recognized an opportunity to test out a more optimized fixture using 3D printing.

Switching to FDM sparse fill with ABS-M30 thermoplastic material for their fixture, BMW was able to create more customized fixtures that had previously been too costly to produce. The new 3D printed fixtures are 72% lighter than the previous fixtures. The lightweight fixtures have improved productivity and accuracy thanks to improved ergonomics that are far less taxing on the assembler. By switching to 3D printing, BMW has realized a 58% saving in cost per fixture and a 92% increase in faster lead time.

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