Improvement of the layer-layer adhesion in FFF 3D printed PEEK/carbon fibre composites

Abstract

Improvement of the mechanical properties of FFF 3D printed CF/PEEK composites was achieved by printing under favorable crystallization conditions. It was possible to improve the layer-layer tensile strength more than fivefold from 6.96 MPa to 36.28 MPa by printing in a specially customised, low cost printer operated with a heated print chamber at 230℃. The influence of the chamber temperature on the mechanical and crystalline structure was investigated to determine the best conditions. A maximum flexural modulus of 14 946 MPa and flexural stress at break of 248.9 MPa were achieved. A proof of concept study involved the 3DP of a CF/PEEK mould tooling insert for injection moulding. This insert replaced a costly traditional metal insert to print short production runs of ABS and HIPS polymers. This work offers a low cost and rapid means to produce effective tooling inserts for the injection moulding industry.

Introduction

The Additive Manufacturing (AM) of thermoplastic polymers is a rapidly growing branch of polymer processing, which, with the assistance of Computer-Aided Design (CAD) software, gives unmatched freedom in product design [1]. Two common methods are used for 3D printing (3DP) of thermoplastic materials and their composites: Selective Laser Sintering (SLS) and Fused Filament Fabrication (FFF), also known as Fused Deposition Modelling (FDM) [2], [3].

A key limitation of 3D printing has been the small number of engineering polymers that can be successfully printed. Limitations include printing machines that are not capable of the high temperatures and accurate temperature control needed for successfully printing such materials and also the poor Z direction strength or layer-to-layer adhesion in printed parts. Current interest from sectors such as aerospace is focused on advanced materials such as the PAEK family (polyaryletherketone) or PEI (Polyetherimide) and their composites. PAEK semicrystalline materials with their high strength to weight ratio are particularly interesting for aerospace as they have the potential to replace heavier materials such as steel or aluminum. Moreover, their self-lubricating properties, wide thermal stability windows as well as recyclability and biocompatibility are also of benefit in the automotive and medical sectors.

SLS has been successfully used for manufacturing a 3D printed PEEK (polyetheretherketone) prosthesis to replace a damaged scapula [4]. SLS was also used to print parts to investigate the mechanical performance of PEEK cranial implants made in different print orientations. There was a difference of over 300% in the failure load values for specimens printed in a horizontal alignment compared with those printed in a vertical alignment thus clearly indicating the high anisotropy of the SLS technique [5], [6].

Anisotropy is also present in FFF 3DP parts due to poor layer-to-layer adhesion. For example, a tensile specimen printed horizontally as a flat tensile bar had a strength of 98.9 MPa whereas when it was printed in the Z direction its tensile strength dropped to 19.6 MPa [7]. The anisotropy in FFF reduces the applicability of the printed parts because the weakest direction determines the performance of the printed part. Thus, techniques to improve the layer-to-layer adhesion of FFF parts require investigation.

In FFF 3DP the mechanical and thermal properties of CF/PEEK composites are affected by the system of coarse pores at the layers' interface, the system of fine pores at the passes' interface and a third type of imperfection is lack of fibre impregnation. The first two are related to the thermally unfavorable conditions during printing at relatively low temperatures (typically at a building plate temperature of 95℃) which can lead to the generation of microcracks releasing the stress caused by shrinking of the material and voids. Whereas lack of impregnation of composites reinforced with continuous fibre can occur due to the short contact period between melted polymer and fibre in the extruder and nozzle during printing [8], [9]. In addition to the imperfections in a 3DP structure due to poor layer-to-layer adhesion or the presence of voids, it is necessary to consider that at a fast cooling rate (e.g. when the deposited filament meets a cooler filament that has already been laid down) the crystalline phase at the interface may not be properly formed. A low degree of crystallinity will reduce the mechanical and thermal performance of the printed PEEK part. Additionally, during printing with fibre based thermoplastic composites the printing head will arrange fibres in the “XY” plane resulting in even higher anisotropy.

PEEK crystallinity can be increased by post-processing of the 3DP object. DSC studies on annealing of PEEK between T_g (143℃) and T_m (343℃) showed that it is possible to tune not only the crystallinity fraction but also the PEEK heat distortion temperature [10], [11]. This points to the potential benefits of printing with a higher chamber temperature in terms of improving mechanical performance of the printed part. It was proven that increasing the nozzle temperature from 380℃ to 420℃ at ambient chamber temperature produced an increase of the part crystallinity from 16% to 20% and increased tensile strength from 49 MPa to 59 MPa [12]. Changing the chamber temperature from 25℃ to 200℃ resulted in an increase in crystallinity from 17% to 31% and tensile strength from 58 MPa to 84 MPa. According to Yang et. al. [12], at 100℃, the crystallinity reaches around 29% and there is no significant increase in the crystallinity when chamber temperature is higher. However, the elastic modulus values at 100℃, 150℃ and 200℃ are respectively 3.3 GPa, 3.9 GPa and 4.2 GPa. This suggests the possibility of tuning the microstructure by varying the amount of energy in the form of the heat delivered to the printed part. It also suggests that not only does the level of crystallinity matter but also the thermal character of the crystalline domains.

A study of the kinetics of PEEK crystallization showed that crystallization half-time depends on temperature and is fastest around 230℃ [11]. At elevated temperatures, crystallization is controlled by nucleation whereas at low temperature it is controlled by a slow diffusion process [10]. It was noted also that PEEK possesses double melting behavior. The primary crystal melting peak has a stable position around 343℃ and is characterized by a strong, thick, well defined lamellar structure whereas a secondary melting peak can appear between T_g and T_m. The melting point of secondary crystallites depends on the temperature of annealing and is attributed to the melting of thinner and weaker lamellae. Upon heating, secondary crystallites are melted into the metastable melt and have the potential of crystallizing into new, thicker lamellar structures only if there enough time to reorganize [7], [13]. A study of the influence of primary and secondary crystalline structure on the thermal and mechanical properties of CF/PEEK is one of the aims of this study.

The mechanical performance of a 3DP object also depends on printing parameters such as infill ratio with properties decreasing as the ratio decreases [6]. The layer thickness and infill architecture also play a significant role [14], [15]. An extensive study of the effect of raster orientations of 0⁰, 90⁰ and alternating 0⁰/90⁰ on the tensile strength of a PEEK composite showed that the highest tensile strength of 73 MPa was achieved for the 0° orientation direction (parallel to applied load). 54 MPa was achieved for the 90° orientation perpendicular to the applied load and 66.5 MPa for the alternating 0°/90° configuration [16]. However, considering all the factors contributing to reduced mechanical performance of 3DP objects, the weakest point of the 3DP object is still the sub-optimal adhesion between the layers.

Our study presents in detail how to improve the layer-layer bond for CF/PEEK (carbon fibre PEEK) composites using a modified, inexpensive, commercially available open-source printer, Ultimaker 2+ (UM2+). To the best of our knowledge this work demonstrates the first successful 3D printing of 150CA30 PEEK mould inserts for injection moulding. This allowed our industry partner to produce prototype injection moulded parts (in the hundreds part number range) without the expense of manufacturing an expensive metal insert.

Mechanical performance of the material and parts was studied via tensile testing and dynamic thermomechanical analysis (DMTA). Morphology and crystallinity were measured using XRD, DSC, CT scanning and SEM. The 3DP process was recorded using a thermal imaging camera which showed that a suitably heated chamber is essential to reduce defects in the printed part, especially in the Z direction. The research provides a better understanding of the 3DP of semicrystalline materials where a well-formed crystalline phase is essential to achieve good thermomechanical properties. Moreover, the results of this work can be applied to other advanced semi-crystalline materials and can also be useful in the optimization of SLS 3DP.