Plastic pellets for injection molding exist in a much wider variety than 3D printing filaments, thus offering the potential for a much greater range of applications. At the same time pellets are, at the moment, significantly less expensive than filament. There are a small number of pellet-extrusion 3D printers on the market, but the fact that there are not more is somewhat surprising. One team of researchers from Massey University in Auckland, New Zealand is performing its own work to change things.
Previously, the group—led by Khalid Mahmood Arif, Senior Lecturer in Mechatronics and Robotics from the school’s Center for Additive Manufacturing—developed a micropellet extruder for 3D printing. The researchers went further than creating the technology and even characterized for strength, surface quality and consistency of the parts that it made.
The team is continuing its work by developing and characterizing new materials for the 3D printing system. Most recently, Arif, et al. published an article in Materials and Manufacturing Processes discussing a new combination of ABS/HDPE for use in their micropellet extrusion 3D printer, resulting in a printed material that actually grew stronger as it aged, beating out some of the stronger polymers in desktop and industrial FFF 3D printing.
In the paper, titled “Preparation and characterization of thermally stable ABS/HDPE blend for fused filament fabrication”, the authors describe the advantages and drawbacks of both ABS and HDPE. As many of our readers may know, ABS is nearly ubiquitous in desktop 3D printing, for its tensile strength, ability to deform without breaking, ability to repel water and other features that make it suitable for a wide variety of applications. HDPE, on the other hand, is a very difficult material to print and is rarely used.
ABS good qualities degrade as ABS is exposed to temperatures over 40°C. In an attempt to preserve these desirable mechanical properties, the researchers decided to blend the plastic with high-density polyethylene (HDPE), which demonstrates good thermal stability when blended with other polymers. Required in the mix was a polyethylene graft maleic anhydride (PE-g-MAH).
The team made a variety of combinations of the materials, settling on one after others proved unprintable. Arif, et al. also had to modify its micropellet extruder by adding a liquid cooling system, which prevents uncontrolled heating inside of the extruder, resulting in a uniform extrusion temperature. In order to ensure the printability of the final mixture, they had to apply some tape to the printbed.
Once the proper blend was achieved, tensile, compression and flexure samples were printed according to ASTMD and ISO standards. They also tested printability under various temperature regiments, with extrusion temperatures ranging from 185°C to 205°C and bed temperatures ranging from 25°C to 75°C. In addition to physical tests, the authors performed visual and machine analysis of the printed material that included fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis and scanning electron microscopy (SEM).
The results of the study were actually quite striking. According to the authors, the ABS/HPDE blend not only demonstrated good thermal stability, but it actually got stronger with time. Moreover, the authors claim that, based on research regarding other durable 3D printing materials and processes on the market, the flexural strength of their blend might be the strongest. In particular, they said that an aged blend of ABS and HDPE had a greater flexural strength than ABSPlus from Stratasys and polyamide 6 with carbon fiber reinforcement from Markforged.
“As compared with literature, the ultimate compressive strength of 162 MPa achieved for 6 days aged samples is one of the highest in FFF to date, to the best of our knowledge,” the authors wrote.
The authors concluded with these three takeaways:
“The blend has one of the highest mechanical proper- ties among the existing FFF blend materials. The tensile strength is the second highest, flexural strength and compressive strength is the highest among existing literature on blends for FFF.
The aging has significantly increased the mechanical strength (tensile, flexure, and compressive) instead of degradation. This shows the thermal stability at high temperatures.
Thermal aging has improved the cold crystallization as observed in DSC. This heterogeneous crystallization has improved the diffusion among beads during 3D printing that leads to improved properties.”
The research team did not discuss the feasibility of such an ABS/HPDE blend in filament form, nor any plans for commercialization.
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