Oct 12, 2016 | By Nick

Image: 3ders.org

We think we know that the infill can have a major impact on your finished model, but a new study has investigated just how the parts you don’t see affect the ones you can. It really doesn’t make as much difference as we think when it comes to tensile strength.

The study, led by Miguel Fernandez-Vicente, Wilson Calle, Santiago Ferrandiz and Andres Conejero, was a collaborative effort between the Universitat Politècnica de Valencia and the Universidad Politecnica Salesiana del Ecuador.

It started with the simple premise that a lot of open-source 3D prints have an infill design that prioritizes saving money and materials. Often the infill is a skeletal or cellular structure that gives the product much needed support and means you can produce thinner walls on a three-dimensional product.

But, of course, the infill itself is a complex equation and you can opt for a much sturdier infill that will give you a stronger product. That will have a significant impact on the print time, weight and cost of each and every product, though.

So many people opt for the lightest possible infill and, in essence, a series of lightweight struts. The study asked the valid questions: Do we have our priorities right when it comes to infill and can we make a big difference with a simple pattern change on the infill?

For this study, the team broke out the trusty i3 RepRap Prusa with a 0.5mm tip and Slic3r software. It used a standard 3mm ABS filament for the study.

The study found that changing the pattern does have an effect on the finished print’s tensile strength, but they managed to produce a change of less than 5%. The traditional honeycomb pattern was the strongest, but with such a small advantage then the team could not rule out the plastic’s deposition as the reason for that.

It is still significant, so companies or individuals that need to maximize the strength of each and every print would do well to read the study and adopt a honeycomb structure, but it isn’t the profound impact they were hoping for. The team found that a rectilinear pattern, such as bricks in a wall, gave the best results overall and a tensile strength of 36.4 Mpa. That is less than 1% less than raw ABS. The airfill and the angle of the raster, or the actual angles of the infill’s bonds, seem to have the greatest impact on the tensile strength and the mechanical behavior of the print.

The mechanical strength is a complex equation and the team tested the ABS filament in a number of ways, analyzing the mesostructured of the material and then the tensile strength of a single raw filament. The polymer chain orientation was also an integral part of the overall mechanical strength and this tied in to a mathematical model based on laminate theory.

This helped the team discover that the voids in a product’s infill do have a profound impact on the mechanical properties and even the angle of the infill structure can change the product from ductile to brittle.

The study highlighted a previous study by Baich and Manogharan, which revealed that solid infill is stronger and costs the same as a double-dense infill. Their work seems to disagree, stating that the relationship between infill density and tensile strength can be fitted in a squared-X model.

If we can truly master infill then we can have a series of custom patterns to work with that a software program could modify for each and every print to optimize the product’s tensile strength, weight and print time.

Our priorities change with each and every product and the infill could, one day, be just as important as the parts of the 3D print you can see. As commercial organizations adopt 3D printing across the board then this will be an increasingly important science.

But for now, if you want the rapid takeaway from this study, go for the rectilinear or honeycomb structure and make sure you don’t skimp on your infill if you need a solid product. If you want to know more, you can read the study in full here.

 

 

Posted in 3D Printing Application

 

 

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Ed Israel wrote at 1/2/2017 3:29:28 AM:

A comment and then a question. We are printing a lot with TPU. We are designing cushioning systems for shoes. If we print (for example) a 50mm x 50mm x 15mm thick cushion and use a round infill pattern it is much more stiff than using a straight (or crisscross pattern). Above you mention that straight is stronger. How does stiffness relate to strength?

Erik Richman wrote at 10/14/2016 12:44:35 AM:

This is about tensile strength not compressive strength for FDM materials. Shape will have very little effect.

Flat Land Larry wrote at 10/13/2016 1:01:01 PM:

But a triangular infill also contains the hexagon pattern.

Flat Land Larry wrote at 10/12/2016 3:55:00 PM:

I dont believe it, triangles are surely the strongest?

Willem staal wrote at 10/12/2016 1:53:15 PM:

The shape of the infill is important. Triangular infills add strenght. but not when they are applied injust two dimensions. honeycomb infills look like beehive patterns and in nature this cell structure proves their strenght. Designers can make use of this shape to add a thirth dimension in their infill subroutines. A interconnected icosahedron shape would be adding even more strenght!



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