Fused Deposition Modeling or FDM 3D printing is an excellent and popular choice for 3D printing rapid prototypes, jigs, fixtures, tooling, and low volume production parts, due in part to its material choices. FDM allows you to build complex parts with the same tried and tested thermoplastics found in conventional manufacturing.

A lesser known fact is that with FDM technology, you can chose between different interior fills to increase your savings when requesting your next 3D printing job. Depending on the material requested and your prototyping needs, you can print the interior of your part in up to three standard options plus additional custom options. Today we are going to cover the two most popular options:  

  • Solid
  • Sparse

Regardless of the interior fill that you choose, the exterior of your part will always be printed in solid. We have completed jobs in which we printed the same part using both of the above-mentioned interior fills, and the parts looked identical on the outside! There was a noticeable weight difference when they were picked up, which helped to tell them apart. When choosing interior fills, there are a few factors to keep in mind. These are some basic guidelines; each project is different and should be evaluated on an individual basis.

When to choose a solid interior fill:

  • Part strength is a critical feature
  • Part is comprised of fine details & thin walls

Choosing a solid interior fill will produce the strongest part. If part strength is a critical factor for your prototype, then a solid fill is the way to go.  A good example of this is any part created to replace metal tooling, as these parts typically need to withstand high levels of impact and heat. Functional prototypes are another area in which you may not want to sacrifice strength.  If you are going to put your part through the ringer, and the interior needs to resemble the final product as closely as possible, you will want to keep it solid.

Another consideration is the amount of fine details or features that your part has. If your part has thin walls or fine features, you will want a solid fill. With thin walled parts, there typically isn’t enough room for a sparse fill.  Your savings would be marginal and it wouldn’t be worth any loss of strength.

Aerospace 3D printing

Polycarbonate (PC) form tool used in hydroforming machines

Medical Prototype 3D Printed with Ultem 1010 thermoplastic material

Parts that have thin walls are best printed in a solid interior fill

When to choose a sparse fill: 

  • Weight is an issue
  • You have a part with a large interior and strength isn’t a factor

Jigs, fixtures, and trade show parts are three great fits for a sparse interior fill. BMW was able to reduce the weight of one hand-held assembly device by 72 percent by using a sparse fill printing technique. When a worker uses a tool hundreds of times in a shift, the reduced weight can make a big difference. Large trade show parts that are shipped frequently can also benefit from a sparse fill; the reduction in weight can help reduce not only shipping costs, but the strain of setting up the parts for each show.

Perhaps the most popular way to use a sparse interior fill is on a prototype you are printing for concept design or for fit and form testing. In most of these cases, you do not need the maximum strength of the thermoplastic material for your part.  You need to hold the part in your hand, make sure it is the right size, make sure it will fit nicely inside of the final assembly, etc. It is a one and done part before moving onto manufacturing or making additional design changes. If this is the case, consider giving the sparse interior fill a try for your next 3D printed project.

3D printed ergonomic production jig

Hand held device printed in a solid interior fill to reduce the weight by 72%

aerospace 3d printing materials

Aerospace prototype 3D printed using a sparse printing technique

How much will I save with a sparse fill versus a solid?

This is a very popular question and the answer, like so many others, is that it is geometry dependent. If your part has a large solid interior, i.e. a dumbbell, then you would experience far more significant savings by printing in a sparse fill versus a part that is mostly hollow, i.e. a cup. Depending on your part geometry and your prototyping needs, this printing technique can save you a couple of dollars, or it can cut your project cost in half. If you have a part for which you would like a quote, simply upload your STL or native CAD files here and mention in the comments that you would like to try a sparse printing technique.

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