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Testing Fracture Resistance in FDM and MJF 3D Printing of Polymers

Researchers from Australia and Germany are exploring more about improving 3D printed parts in the recently published ‘Fracture Resistance Analysis of 3D Printed Polymers.’ Because failure—sometimes ‘catastrophic’—often occurs due to instability and cracking, the authors experimented with samples fabricated via both fused deposition modeling (FDM) and multi-jet fusion (MJF) to understand more about load-carrying capacity.

Overall, FDM 3D printing shows great potential for improving mechanical properties in 3D printing. This is an ongoing topic of study in many research labs today too, spanning many different techniques and use of materials, from studying the effects of color with PLA, to using additives, to examining mechanical properties and biocompatibility. In some cases with FDM 3D printing, however, users may experience issues with composites that have pre-existing cracks—caused during manufacturing, surface defects, or notches that may be growing slowly.

In this study, the research team analyzed fracture of U-notched, 3D printed thermoplastic components:

“As the J-integral failure criterion is one of the most common brittle failure models used in the study of notched specimens, we investigated whether EMC could be combined with the J-integral failure principle to predict the fracture of U-notched 3D-printed specimens subjected to tensile loading,” explained the researchers.

Failure was studied in both FDM and MJF dogbone samples under mode I and mixed mode I/II loading regimes within the framework of combined EMC and J-integral criterions. Materials of choice to be used in the experiment were Nylon 12 filament and PA12 nylon powder.

The samples were 3D printed as follows:

  • 13 mm width
  • 5 mm thickness
  • 50 mm gauge length
  • 100% infill rectangular plates

A schematic of centrally located bean-shaped notch with two U-shaped ends (dimensions in mm).

Twelve nylon samples were created on a Fortus 450mc FDM 3D printer, while the same number of samples were created via MJF on an HP 3D printer.

A dog-bone shaped MJF nylon 3D-printed nylon specimen; before (a), and after (b) tensile test.

A typical tensile stress vs strain curve for the 3D-printed nylon specimens.

The researchers performed tensile tests, evaluating strength and modulus.

Mechanical properties of the 3D-printed nylon specimens.

“The results of tensile tests showed that the average value of the modulus of elasticity of MJF 3D-printed nylon was 780 MPa, whereas the FDM sample had a lower value of 493 MPa. The average value of the percent breaking strain of the FDM and MJF samples were 16 and 13 and the average tensile strength of FDM and MJF samples were 44.8 and 34.9 MPa, respectively,” stated the research team. “Although FDM samples had lower elastic modulus, they exhibited higher tensile strength and percentage elongation compared to MJF, as well as higher modulus of toughness.”

An MJF nylon 3D-printed specimen under tensile test conditions; (a) before (b) after fracture.

During testing, the researchers axially stressed samples with an Instron 300LX machine until they failed via crack growth.

“Irrespective of the notch orientations, all MJF samples exhibited brittle behavior with flat fracture surfaces. Considering the tensile tests, the failure load of MJF 3D-printed nylon was observed to be greater than the FDM samples for β60°.

‘>60°. In addition to the effect of crack angles, it was observed that increase in crack radius was associated with reduced critical load in both types of 3D-printed samples,” stated the researchers.

“Finally, the equivalent material concept (EMC) was combined with the J-integral failure principle to predict the fracture failure of U-notched 3D-printed specimens subjected to tensile loading under mode I and mixed mode I/II loading regimes. The agreement between the experimental and simulation results proved the EMC-J approach to be capable of successfully predicting fracture in the 3D-printed notched ductile material components.”

Close-up photographs FDM and MJF printed nylon specimens after fracture from left to right, respectively, with (a) 60°, (b) 30°, and (c) 0° notch orientations.

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Illustration of strain energy density around the notch border for MJF nylon with a notch tip radius of 2 mm at different notch orientations.

[Source / Images: ‘Fracture Resistance Analysis of 3D Printed Polymers’]

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Crispin Orthotics Purchases HP 4200 from Europac 3D for 3D Printing Orthotic Devices

In 2016, 3D printing, scanning, and inspection business Europac 3D, headquartered in the UK, was named a UK Channel Partner for HP, which means that it’s responsible for the sales and servicing of all of HP’s 3D printing equipment in the country. Recently, Crispin Orthotics contacted Europac 3D in search of a new 3D printing and CAD software package that would be able to keep up with the demand for orthotic devices, while also speeding up production and lowering 3D printing costs, and the company suggested HP’s 4200 Multi Jet Fusion 3D printer for the job.

“The HP Multi Jet Fusion 4200 is able to accurately mass produce bespoke orthotic devices, which in turn can cut costs and speed up production times,” said John Beckett, the Managing Director and Founder of Europac 3D. “Crispin Orthotic’s use of the HP’s Multi Jet Fusion printer is indicative of how additive manufacturing is revolutionising the orthotics industry.”

[Image: Sarah Goehrke for 3DPrint.com]

Crispin is a top HCPC registered clinic specializing in producing and maintaining orthotic devices, and has already worked with HP in the past. By making a further investment in its MJF technology, Crispin is able to produce parts that are strong and flexible enough to be made into orthotics that can endure a human’s everyday movements.

HP’s 4200 MJF can produce parts up to ten times faster, and at a 50% cost-per-part reduction, when compared to other SLS 3D printing systems. This reduced cost and increased speed means that Crispin will be able to 3D print hundreds of personal, customized orthotics in just 12 hours.

Crispin tested out its new HP 3D printer by pairing it with Siemens NX CAD software, featuring topology optimization, so its technicians could add strength to important areas and make designs more lightweight. In addition, the software is also capable of organizing multiple 3D parts so they nest, or perfectly fit, together on the print bed. This capability decreases the number of print jobs, which also lowers cost and increases speed.

“3D scanning and printing has revolutionised the speed and quality of parts we’re able to produce for clients.  Having the ability to create a bespoke devise that is lightweight, durable and accurate to 0.2mm has obvious benefits to the user. The business also benefits from the speed of 3D printing parts as well as cost savings of approximately 40% on each part by removing the need for multiple components in the supply chain and assembly,” said Mark Thaxter, Managing Director of Crispin Orthotics.

“Using 3D scanning and printing also provides greater freedom on the design of products particularly those with complex geometry.  Having the ability to vary the thickness of the device in certain parts also allows us to produce devices not possible with current methods of manufacturing.”

Crispin used its new HP MJF 3D printer, and its Siemens NX CAD, during a recent project. The combined technologies made it possible for the company to create a 3D printed arm orthotic with an integrated elbow joint and an attachment at the end, which makes it possible to pair up prosthetic devices with it. The device was 3D printed in a single part out of a durable but lightweight nylon material.

The sample parts that Crispin 3D printed on its new HP 4200 MJF system passed all of the necessary tests. But even more impressive is that the parts all had homogeneous strength in the three separate build axes, which just goes to show that build orientation does not have any negative impacts on the quality or strength of 3D printed parts.

Discuss this news and other 3D printing topics at 3DPrintBoard.com or share your thoughts in the Facebook comments below.

[Images provided by Europac 3D, unless otherwise noted]