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images (3)With metal printing beginning to reach a high in the 3D industry and titanium as the up and coming trusty material for manufacturing spanning the gamut from bike frames to spinal implants, the topic is popular, to say the least. But researchers from the University of Waterloo in Ontario have put a new spin on the subject regarding 3D printing processes with metal and layer thicknesses in material as relevant to the ever growing industry of orthopedic applications.

On the influence of sintering protocols and layer thickness on the physical and mechanical properties of additive manufactured titanium porous bio-structures,’ authored by Ahmad Basalah, Shahrzad Esmaeili, and Ehsan Toyserkani, outlines their recent study, obviously very important work as it sheds light on how manufacturers can better control the mechanical and physical integrity of 3D printed medical implants. Exactly how they do that can have a direct impact on the condition of the patient after surgery.

As 3D printing and additive manufacturing take a hold in the implant industry, offering the opportunity for customization and personalized care, as well as sometimes greater affordability, the last thing anyone wants to see is issues arising with such new technology, especially in causing a patient to experience pain or discomfort.

The research team, points out however, that aseptic loosening is all too often a concern—where the bond doesn’t hold between an implant and bone—causing patients to be taken back surgery after joint replacements.

The loosening is generally caused by stress shielding, caused lack of lack of bone density—referring back to the insertion of the implant which can essentially sometimes cause the bone to weaken and fall victim to reabsorption. With this in mind, it’s obvious that the creation of a bone implant is a serious process that must involve sophistication and precision.

1-s2.0-S0924013616302631-gr1Popular for many applications due to its strength and reliability, here titanium is used because of its great biocompatibility. As an implant material, it provides higher corrosion resistance than other materials as it offers up a protective oxide layer, which forms on the surface of the implant. To alleviate potential issues with stress shielding, as well as taking advantage of all the other benefits titanium can offer here, the researchers say that the best option is a titanium foam structure, allowing for the proper rigidity and weight.

“Many attempts have been made to mimic cortical bone properties by employing titanium foam,” state the researchers.

Several different studies were performed in consideration to:

  • Particle size
  • Sintering temperature
  • Powder compaction level

The variables were then changed to manipulate the porosity and mechanical properties.

“The results indicated a decrease in porosity associated with a linear increase in Young’s modulus. This porosity was highly affected by varying the powder particle size and level of powder compaction when the same sintering temperature was used,” stated the researchers in their paper. “However, changing the sintering temperature did not cause a noticeable variation in the porosity. It was concluded that the crucial factors affecting porosity are initial powder size and the level of powder compaction.”

In investigating how layer thickness affects powder compaction during 3D printing, as well as how temperature variations affect bonding, the researchers aim to help close any gaps that might occur as implants loosen and cause inflammation and other issues for patients.

“In this paper, we developed a model to predict the density of the printed part, and then we could calculate the stiffness and strength which the printed part can afford,” Ahmad A. Basalah, a PhD from the Department of Mechanical and Mechatronics Engineering, University of Waterloo, and the Department of Mechanical Engineering, University of Umm Al-Qura, told 3DPrint.com. “Consequently, we could optimize the printed part regarding the printed material.”

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For this study, titanium powder with particle size range of 38–45 μm was used to 3D print the samples on the ZPrinter 310 Plus by 3D Systems. The researchers printed four different types of layer thicknesses to examine powder compaction. Sintering temperatures were set at:

  • 800°C
  • 1000°C
  • 1200°C
  • 1400°C

The researchers reported that samples were furnace cooled at the end of the sintering process. Other factors were offered, as follows:

  • Porosity study
  • Compression test
  • Shrinkage measurements
  • Microscopic characterization
  • Statistical analysis

Results from their testing showed that porosity was directly affected as the sintering temperature increased.

“The decrease in the layer thickness causes a significant reduction in the porosity at the highest sintering temperature (1400 °C) and relative reduction in porosity at other sintering temperatures,” stated the researchers.

The average yield strength of the samples increased significantly (p < 0.05) when the sintering temperature was increased, the team reported. They saw ‘significant influence’ at conditions of 1000, 1200, and 1400 °C, but reported that a temperature of 800°C was ‘not significant.’

Increased temperatures led to increased shrinkage, as did increasing powder compaction.

“In addition, the interaction between the sintering temperature and the compaction of powder significantly increases the shrinkage in both directions (p < 0.05) according to a two-way ANOVA statistical test,” stated the researchers.

In conclusion, they found that the compaction of powder. which is one in the same with layer thickness, did affect all the following aspects:

  • Porosity
  • Strength
  • Stiffness
  • Dimensional variation of titanium porous 3D printed structures

 

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Four varying temperatures were used.

In combination with the sintering temperature, the researchers observed the two parameters directly affecting the outcome of 3D printed parts both mechanically and physically. Both powder compaction and temperature variables were able to influence the size of the sinter neck and the ‘volume of voids’ found in their structures.

“In addition, the regression analysis demonstrates a good fit between the porosity model and the experimental data,” stated the researchers in conclusion.

Figure four: Micro-structural arrangements of spherical particles after the sintering process.

Micro-structural arrangements of spherical particles after the sintering process.

The study, with the agreement between the model that they constructed and the results of the experiments, bodes well for the continued use of 3D printing in implants, and those constructed to mimic cortical bone properties. Discuss this project further in the 3D Metal Printing & Layer Thicknesses forum over at 3DPB.com.

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SEM images of various samples sintered at different sintering temperature and printed using two layers thickness, i.e. 62.5 and 175 represent the extreme edges of powder compaction.





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