Screw extrusion-based additive manufacturing of PEEK

Highlights

• A new screw extrusion-based 3D printing system was designed to solve the problems of conventional filament feeding ones.

• High viscosity and melting temperature material e.g., polyether-ether-ketone (PEEK), has been printed with good quality.

• Stable flow ability (<3% variation) with high printing speed (up to 370 mm/s) and reproducible quality was demonstrated.

• 96% of the mechanical strength of the bulk material (PEEK) in additive manufacturing printing was achieved.

• SEM and XRD results revealed the thermal condition was the major factor affecting mechanical strength.

• The post annealing process had no significant effect on the material properties.

Abstract

Polyether-ether-ketone (PEEK) has high mechanical strength, thermal performance, and biocompatibility, and is widely used in biomedical and chemical engineering applications. However, to date there are no guidelines for the building of a 3D printer for highly viscous materials, e.g., PEEK. In this paper, a screw extrusion method was developed to overcome the existing problems of the filament-feeding method. Excellent flow stability (< 3% variation) and high printing speed (up to 370 mm/s) for PEEK printing were achieved. Highly reproducible mechanical tests of the printing products were demonstrated with 96% of the bulk material strength for the first time. Furthermore, an exchangeable printing head was built to cover both line- and plane-printing needs to widen its applications and improve printing surface quality (up to 0.945 nm in Ra). All printed material had a more brittle character in comparison with the bulk material and the post annealing process was found to have no significant effect on the mechanical strength. Additionally, porous artificial intervertebral cages with controllable size and distribution were manufactured to demonstrate potential applications.

Introduction

For the applications in the biomedical field, polyether-ether-ketone (PEEK), a semi-crystalline thermoplastic, is a well-known alternative to metal implants because of its excellent biocompatibility and comparable modulus to human cortical bones, which can potentially reduce the stress shielding effect after implantation [1], [2], [3]. Moreover, it is radio-transparent, which can provide better evaluation of postoperative recovery situations and has been widely used in clinical applications such as for intervertebral cage fusion and craniomaxillofacial reconstruction [4], [5], [6]. Although PEEK possesses several bio-favorable properties, it still suffers from inadequate osseointegration [7], rendering porosity expression in artificial implants necessary to enhance the bio-interaction inside the human body to more closely mimic natural tissue. Meeting this requirement would largely increase the manufacturing complexity, especially in forming pore interconnectivity and would introduce contamination issues [8], [9].

Recently, particulate leaching and selective laser sintering (SLS) have been identified as promising methods to overcome the manufacturing bottlenecks due to their more physical mechanism and flexible platform compared to currently available fabrication processes [10], [11]. However, these techniques still suffer from limited capability of designing microstructure for particulate leaching and expensive apparatus requirements for the SLS method [12]. An alternative, which balances the economic concerns, is fused deposition modeling (FDM), which has been recognized as the most efficient method in additive manufacturing (AM) to construct products for tissue engineering (e.g., scaffolds) due to its wide material compatibility, low-cost apparatus, and great flexibility in terms of structural design [13]. Commercial FDM incorporates a metal (e.g., copper) nozzle in which the melted filament is extruded to construct layer-by-layer structures by the relative motion between the nozzle and substrate.

The filament feeding mechanism is the most commonly used method in FDM additive manufacturing, but it is mainly restricted to the extrusion of low viscosity materials such as poly lactic acid (PLA, 10 Pa·s) and acrylonitrile butadiene styrene (ABS, 140 Pa·s) to avoid buckling and back-flow induced flow instability. The gap between the filament and feeding tubes is the main cause for these problems, especially during filament feeding in the fast printing process [14]. Once the melted material flows back to the lower temperature region, it increases in resistance and begins to solidify, which inhibits feeding efficiency due to the buckling effect of the feeding filament and lowers the printing precision and can even cause clogging.

Despite these challenges, Valentan et al. [15] first demonstrated the feasibility of using FDM for the highly viscous PEEK by a custom-made machine in 2013, although it suffered from quality incompleteness (e.g., warpage, delamination and bubbles) and non-reproducibility (e.g., due to non-uniform filament) of the printed products. In 2015 Vaezi & Yang [16] also used filaments to print PEEK specimens with different porosities (14% and 31%) for mechanical test comparison with the bulk material. The printed specimens had reduced mechanical strength because of air gaps between the infill pattern and entrapped micro-bubbles inside the filaments. Similarly, Wu et al. [17] and Yang et al. [18] investigated the mechanical strength of the FDM products and showed the importance of printing orientation with respect to the load direction and the necessity of thermal management during printing process to improve the mechanical properties of the printed specimens.

In contrast to the filament feeding mechanism, Vaezi & Yang [16] also chose powder as a raw material to avoid the use of filaments and used a pushrod (or syringe) to provide higher pressures to overcome the back flow issue, and successfully extruded melted PEEK through the printing nozzle. A similar design appeared in a previous study by the present author [19] except a PEEK pellet was used as the raw material. However, the pushrod mechanism has the inherent drawbacks of a batch process incapable of continuous printing, and suffers from lower printing quality due to the manufacturing imperfectness of the pushrod mechanism, especially for extruding the highly viscous PEEK material for fast printing.

In addition to printing PEEK material, different methods of generating 3D PEEK composite for various biomedical applications have been developed. For instance, Vaezi et al. [20] used first FDM to print a porous hydroxyapatite (HA) scaffold and then compressed melted PEEK into the HA scaffold to form a PEEK/HA composite to enhance the bioactivity of PEEK material. Similarly, in an attempt to develop smart materials, Yang et al. [21] added carbon fiber (CF) to the melted PEEK in the printing process, forming PEEK/CF and PLA/CF composites, to bend the resulting material by the electrocaloric effect for biomimetic robotic application, e.g., artificial muscle. Based on this trend of composite printing, it is likely that more PEEK composites will be developed combining new 3D printing technology for future applications.

In the following Experimental section, the main facilities and test procedures are described. The design of the screw extrusion mechanism by directly using raw pellets, system stability & operation test for the line- and plane-printing head, tensile and compression tests, determination of crystallinity under various operation conditions with some printed PEEK examples (e.g., an artificial intervertebral cage with pore size of 0.4 mm and 50% porosity and a high aspect ratio of hollowed Taipei 101) are described in the Results and Discussion section.