University of Texas Thesis Improves Tensile Strength of FDM Parts Through Annealing and Pressure

Improving strength in parts is a topic of ongoing study in 3D printing, and thesis student, Rhugdhrivya Rane, at The University of Texas at Arlington, recently tackled the subject further in ‘Enhancing Tensile Strength of FDM parts using Thermal Annealing and Uniaxial Pressure.’ Rane opens by discussing the revolutionary and disruptive qualities of 3D printing but points out that there are obvious challenges in producing parts that are strong enough for many applications.

‘Inherent deficiency of weak tensile strength’ is often caused by weak polymer interfaces, with insufficient mechanical properties found in the z-plane direction.

“This deficiency in mechanical properties is due to the weak Inter-laminar bonding in the adjacent layers of FDM parts leading to an overall reduction in part strength,” states Rane. “Thus, to enhance the use of FDM parts in actual engineering application and not just artistic renderings, the overall anisotropy of the parts has to be reduced while increasing the strength.”

As the researcher investigated thermal annealing, and thermal annealing with unidirectional mechanical pressure in the Z direction, Rane’s overall goal was to figure out how to increase inter-bead bond strength overall, 3D printing a variety of specimens in ABS and testing them in different temperature ranges and pressure gradients. He also studied bond lengths and resulting effects on tensile strength.

FDM 3D printing was chosen as the method for testing due to its increased popularity in the mainstream today, which Rane attributes to simplicity and affordability—along with the use of ABS as a material, due to its potential in so many applications. His research delves further into why there are issues with FDM parts, but also how strongly they are affected by parameters chosen by the user. Rane states that such parameters played a large part in his study, especially as they allowed for comparisons in settings and post printing. Attention was also payed to infill percentages and patterns, as they are closely related to strength.

Different infill patterns used in FDM

Other parameters connected closely to strength include:

• Perimeter shells
• Print orientation
• Layer height
• Flow rate

While there are many benefits in FDM 3D printing, Rane points out that it causes many restrictions too, with one of the main issues being that FDM parts often miss the mark considerably in comparison to those still being produced conventionally—as in injection molding. Deficiencies in FDM 3D printing relate to the ever-challenging issues with porosity, but also more specifically, imperfect weld lines. The researcher states that realistically, techniques in producing FDM parts must be significantly improved before they can be seriously considered as final use parts.

Parts printed using varying numbers of perimeter shells

He further explores bond formation and strength, along with the varying degrees of intimate contact between parts, thermoplastic healing, why polymer chains become disengaged, and how temperature affects viscosity.

Temporal disengagement of polymer chains from the initial tube

Parts were tested using custom specimens.

“The parts were printed using two different sets of print parameters: high and low settings, to investigate the effect of heat treatment on both sets of print parameters. The values of temperature, time and applied pressure during heat treatment were varied to obtain a detailed comparative study and the correlation between the given variables and the increase in ultimate tensile strength.”

“A cross-sectional view of the parts was obtained under a microscope to study the changes in the mesostructure of the parts after the post processing. This provides us with the means to explain the increase in the strength based on visible physical changes in the mesostructured.”

Printing of Dogbone specimens in Z- direction.

Tensile testing of dogbone specimens

Each specimen was printed individually. Rane stated that this allowed for the maximum in interlaminar bonding, along with reducing the thermal gradient. Dogbone specimens were tested with a custom aluminum fixture meant to avoid deformation, along with supplying necessary pressure in the build direction.  Overall, Rane discovered that higher temperatures and longer exposure to heat produced better tensile strength, along with increased ductility.

“Though thermal annealing and uniaxial pressure cause an increase in the strength of the parts, the print parameters play a vital role in determining the initial mechanical properties of the parts. When the parts are fabricated with a higher values of flow rate and extrusion temperature, they exhibit significantly higher mechanical properties as compared to parts printed with substandard setting,” concluded the researchers. “Thus, by controlling the print parameters and using the right values of temperature and pressure we can see substantial increase in strength of FDM parts.”

Trying to improve materials in 3D printing is almost as a vast a topic as that of the innovations being brought forth today. Scientists have delved into tensile strength, along with investigating tensile properties of PLA specimens and other issues like creating new materials for large-scale parts. Find out more about research into textile strength in FDM 3D printing here.

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Print Parameters used to print the dogbone specimens