Extrusion Additive Manufacturing Process Research Articles

Complex flow and temperature history during melt extrusion in material extrusion additive manufacturing

3D printing using the materials extrusion additive manufacturing (ME-AM) process is highly nonisothermal. In this process, a solid polymer filament is mechanically drawn into a heated hot end (liquefier) where the polymer is ideally melted to a viscous liquid. This melt is extruded through an orifice using applied pressure of the solid filament that is continuously being drawn into the extruder. The extruded filament melt is deposited to build up the desired part. The poor thermal conductivity of most polymers inevitably leads to temperature gradients, in both the radial and axial directions. Here we quantify the temperature evolution of the polymer filament in axial direction using embedded fine thermocouples as a function of process parameters. Information about the radial gradients is obtained by introducing dye markers within the filament through understanding the flow behavior through the extruder by the deformation of the dye from a linear to pseudo parabolic profile. The polymer is heated above the glass transition temperature for less than 30 s for reasonable print conditions with the center of the filament remaining cooler than the liquefier temperature throughout the process. These process measurements provide critical data to enable improved simulation and modeling of the ME-AM process and the properties of the printed parts.

Fused filament fabrication melting model

This paper presents an analytical melting model inside the nozzle of a fused filament fabrication process. The model presents the limiting case scenario where the maximum melting rate is controlled by the applied force. Here, instead of having a nozzle filled with polymer melt, the melt is reduced to a melt film at the tip of the filament as it is pushed against the exit of the nozzle. The model uses a mode of melting that is governed by melting with pressure flow melt removal. The model includes effects of initial filament temperatures, heater temperature, applied force, nozzle tip angle, capillary diameter and length as well as rheological and thermal properties. The analytical solution is compared to controlled experiments done on a specially designed set-up. Furthermore, the model is used to assess the effect of nozzle tip angle, heater temperature and initial filament temperature on the melting rate within the nozzle. The comparison between the experiments and the model show that assumptions used for the model development are plausible, and that the model can be used to optimize the melting within a material extrusion additive manufacturing process, as well as predicting the performance of new materials.

Torsion analysis of the anisotropic behavior of FDM technology

Several reports have studied the mechanical properties of the material extrusion additive manufacturing process, specifically referred to as fusion deposition modeling (FDM) developed by Stratasys. As the applications for 3D printed parts continue to grow in diversity (e.g., gears, propellers, and bearings), the loading conditions applied to printed parts have become more complex, and the need for thorough characterization is now paramount for increased adoption of 3D printing. To broaden the understanding of torsional properties, this study focused on the shear strength of specimens to observe the impact from additive manufacturing. A full factorial (42) design of experiments was used, considering the orientation and the raster angle as factors. XYZ, YXZ, ZXY, and XZY levels were considered for the orientation parameter, as well as 0°, 45°, 90°, and 45°/45° for the raster angle parameter. Ultimate shear strength, 0.2% yield strength, shear modulus, and fracture strain were used as response variables to identify the most optimal build parameters. Additionally, stress-strain diagrams are presented to contrast elastic and plastic regions with traditional injection molding. Results demonstrated an interaction of factors in all mechanical measured variables whenever an orientation and a raster angle were applied. Compared to injection molding, FDM specimens were similar for all measured torsion variables except for the fracture strain; this led to the conclusion that the FDM process can fabricate components with similar elastic properties but with less ductility than injection molding. The orientation in YXZ with the raster angle at 00 resulted in the most suitable combination identified in the response optimization analysis.

Using multi-axis material extrusion to improve mechanical properties through surface reinforcement

ABSTRACTDue to the layer stacking inherent in traditional three-axis material extrusion (ME) additive manufacturing processes, a part's mechanical strength is limited in the print direction due to weaker interlayer bond strength. Often, this requires compromise in part design through either adding material in critical areas of the part, reducing end-use loads or forgoing ME as a manufacturing option. To address this limitation, the authors propose a multi-axis deposition technique that deposits material along a part's surface to improve mechanical performance. Specifically, the authors employ a custom 6 degree of freedom robotic arm ME system to create a surface reinforcing ‘skin’, similar to composite layup, in a single manufacturing process. In this paper, vertical tensile bars are fabricated through stacked XY layers, followed by depositing material directly onto the printed surface to evaluate the effect of the skinning approach on mechanical properties. Experimental results demonstrate that surface-reinforced interlayer bonds provide increased yield strength.

A hybrid additive manufacturing method for the fabrication of silicone bio-structures: 3D printing optimization and surface characterization

The incremental trend in the practical applications of Additive Manufacturing (AM) brings up the challenge of developing novel methods for the fabrication of new materials. Thermoset polymers such as silicone are considered challenging materials in terms of the AM adoption. Printing silicone at a high speed may revolutionize multiple industries, specifically the medical sector. In this paper, a hybrid system that combines material jetting and material extrusion AM processes will be introduced. This method is not only capable of printing the non-Newtonian viscous silicone, but also increases the fabrication velocity between 10 to 20 times compared to the regular extrusion methods. Statistical optimization methods are employed to explore the optimum range of input parameters with the goal to maximize the 3D printing resolution, and improve the surface quality of 3D printed features. Finally, the working principle of the hybrid manufacturing method will be explained based on the surface characterization results.

Poly(ether ester) Ionomers as Water-Soluble Polymers for Material Extrusion Additive Manufacturing Processes.

Water-soluble polymers as sacrificial supports for additive manufacturing (AM) facilitate complex features in printed objects. Few water-soluble polymers beyond poly(vinyl alcohol) enable material extrusion AM. In this work, charged poly(ether ester)s with tailored rheological and mechanical properties serve as novel materials for extrusion-based AM at low temperatures. Melt transesterification of poly(ethylene glycol) (PEG, 8k) and dimethyl 5-sulfoisophthalate afforded poly(ether ester)s of sufficient molecular weight to impart mechanical integrity. Quantitative ion exchange provided a library of poly(ether ester)s with varying counterions, including both monovalent and divalent cations. Dynamic mechanical and tensile analysis revealed an insignificant difference in mechanical properties for these polymers below the melting temperature, suggesting an insignificant change in final part properties. Rheological analysis, however, revealed the advantageous effect of divalent countercations (Ca2+, Mg2+, and Zn2+) in the melt state and exhibited an increase in viscosity of two orders of magnitude. Furthermore, time-temperature superposition identified an elevation in modulus, melt viscosity, and flow activation energy, suggesting intramolecular interactions between polymer chains and a higher apparent molecular weight. In particular, extrusion of poly(PEG8k-co-CaSIP) revealed vast opportunities for extrusion AM of well-defined parts. The unique melt rheological properties highlighted these poly(ether ester) ionomers as ideal candidates for low-temperature material extrusion additive manufacturing of water-soluble parts.

Quantifying effects of material extrusion additive manufacturing process on mechanical properties of lattice structures using as-fabricated voxel modeling

During additive manufacturing processes, part geometry is approximated because the layer by layer deposition procedure can yield stair-step irregularities between layers. Moreover, since finite-sized filaments are deposited in the material extrusion process, air gaps are generated among the filaments. These lead to geometrical errors in additively manufacturing parts and degradation of the parts’ mechanical properties, such as elastic modulus and strength, based on slicing and material deposition strategies. Geometric errors that arise during the manufacturing procedure have a particularly significant impact on fabricated lattice structures, which consist of a network of small struts, because they have large bounding surfaces that must be approximated during fabrication. In addition, since the struts in lattice structures are generally small, voids among filaments affect the structures’ mechanical properties significantly even if they are small. In order to avoid property degradation it is necessary to consider these phenomena during lattice structure design. In this paper, an as-fabricated modeling approach for a material extrusion process is proposed, for use in modeling and assessing the effects of geometric degradation on additively fabricated lattice structures. The approach implements a voxel based modeling technique to consider stair steps and deposition paths at each layer. Using the proposed method, numerical models for evaluating mechanical properties are generated. Estimated mechanical properties using the as-fabricated voxel modeling approach are compared with experimental results. The effects of the stair step and deposition path phenomena on mechanical properties are quantified and demonstrate good correspondence with experiments, particularly for elastic modulus.

Extrusion-based additive manufacturing of PEEK for biomedical applications

ABSTRACTThere has been a trend in recent years to develop polyetheretherketone (PEEK)-based medical devices due to the excellent cell biocompatibility and desirable mechanical properties of PEEK, which has elastic modulus comparable to cortical bone. Different manufacturing techniques such as injection moulding, particulate leaching, compression moulding, and selective laser sintering (SLS) have been used to produce porous PEEK for biomedical applications. Despite a large number of publications on extrusion-based additive manufacturing (AM) of porous structures using various materials, there have been very few general reports on extrusion AM of low quality small PEEK parts without defects such as warpage and delamination and no further assessment of mechanical properties. Successful low-cost 3D printing of PEEK structures using filament-based extrusion AM process is reported in this paper for the first time. Hot extrusion head design, extrusion temperature, and ambient temperature were identified as important factors need to be taken into consideration for printing PEEK structures without warpage, delamination, and polymer degradation. Compression and tensile tests were conducted to investigate mechanical properties of these new 3D printed PEEK structures. The air gap between infill pattern and entrapped micro-bubbles inside filaments were identified as the main source of mechanical properties reduction. In addition, three-point flexural test was performed on the 3D printed PEEK and compared with flexural specimens printed using other AM materials and techniques.

A review of melt extrusion additive manufacturing processes: II. Materials, dimensional accuracy, and surface roughness

Purpose – The purpose of this paper is to critically review the literature related to dimensional accuracy and surface roughness for fused deposition modeling and similar extrusion-based additive manufacturing or rapid prototyping processes. Design/methodology/approach – A systematic review of the literature was carried out by focusing on the relationship between process and product design parameters and the dimensional and surface properties of finished parts. Methods for evaluating these performance parameters are also reviewed. Findings – Fused deposition modeling® and related processes are the most widely used polymer rapid prototyping processes. For many applications, resolution, dimensional accuracy and surface roughness are among the most important properties in final parts. The influence of feedstock properties and system design on dimensional accuracy and resolution is reviewed. Thermal warping and shrinkage are often major sources of dimensional error in finished parts. This phenomenon is explored along with various approaches for evaluating dimensional accuracy. Product design parameters, in particular, slice height, strongly impact surface roughness. A geometric model for surface roughness is also reviewed. Originality/value – This represents the first review of extrusion AM processes focusing on dimensional accuracy and surface roughness. Understanding and improving relationships between materials, design parameters and the ultimate properties of finished parts will be key to improving extrusion AM processes and expanding their applications.

Modeling and characterization of fused deposition modeling tooling for vacuum assisted resin transfer molding process

The material extrusion additive manufacturing process, i.e., fused deposition modeling (FDM), as opposed to traditional subtractive manufacturing, offers a superior way of manufacturing tooling components in terms of great design flexibility, rapid tooling development, material requirement reduction and significant cost savings. However, it is always challenging to design a tool structure with minimized material and labor cost while maintaining satisfactory tooling performance. In the current study, a comprehensive finite element model was developed for ULTEM 9085 FDM tools subjected to applied pressure and elevated temperature for vacuum assisted resin transfer molding (VARTM) process. Both solid-build and sparse-build tools were studied. Material properties of the tools were obtained from solid coupon testing at elevated temperatures. The thermo-mechanical behavior of tools during the VARTM process was investigated using the finite element model. The ULTEM tools were manufactured using Stratasys Fortus 400mc FDM machine. Thermal cycling of the tools was performed at elevated temperatures (180°F and 250°F). Dimensional analysis and surface roughness of the tools were evaluated after thermal cycling. This study on the performance of FDM tooling for VARTM composite manufacturing process can be extended to other composite manufacturing processes.

Effective mechanical properties of lattice material fabricated by material extrusion additive manufacturing

In this paper, a two-step homogenization method is proposed and implemented for evaluating effective mechanical properties of lattice structured material fabricated by the material extrusion additive manufacturing process. In order to consider the characteristics of the additive manufacturing process in estimation procedures, the levels of scale for homogenization are divided into three stages – the levels of layer deposition, structural element, and lattice structure. The method consists of two transformations among stages. In the first step, the transformation between layer deposition and structural element levels is proposed to find the geometrical and material effective properties of structural elements in the lattice structure. In the second step, the method to estimate effective mechanical properties of lattice material is presented, which uses a unit cell and is based on the discretized homogenization method for periodic structure. The method is implemented for cubic lattice structure and compared to experimental results for validation purposes.

A review of melt extrusion additive manufacturing processes: I. Process design and modeling

Purpose– The purpose of this paper is to systematically and critically review the literature related to process design and modeling of fused deposition modeling (FDM) and similar extrusion-based additive manufacturing (AM) or rapid prototyping processes.Design/methodology/approach– A systematic review of the literature focusing on process design and mathematical process modeling was carried out.Findings– FDM and similar processes are among the most widely used rapid prototyping processes with growing application in finished part manufacturing. Key elements of the typical processes, including the material feed mechanism, liquefier and print nozzle; the build surface and environment; and approaches to part finishing are described. Approaches to estimating the motor torque and power required to achieve a desired filament feed rate are presented. Models of required heat flux, shear on the melt and pressure drop in the liquefier are reviewed. On leaving the print nozzle, die swelling and bead cooling are considered. Approaches to modeling the spread of a deposited road of material and the bonding of polymer roads to one another are also reviewed.Originality/value– To date, no other systematic review of process design and modeling research related to melt extrusion AM has been published. Understanding and improving process models will be key to improving system process controls, as well as enabling the development of advanced engineering material feedstocks for FDM processes.