Full Length ArticleProcess-structure-property effects on ABS bond strength in fused filament fabrication
Introduction
Additive manufacturing is an emerging manufacturing technique in which objects are fabricated in a layer-wise manner. Material extrusion (ME), also known as fused filament fabrication (FFF), is an additive manufacturing strategy whereby polymer filament is fed through a heated liquefier, is extruded through a nozzle, and is deposited on a build surface or previously printed layers where it quickly cools. Structural integrity of AM parts is derived from bonding between adjacent and stacked extruded roads. Bonding forms via a polymer coalescence mechanism and is a function of thermal history at road interfaces. The cooling profile at road interfaces defines a time window over which molecular diffusion and randomization of the polymer chains at the interface can occur and is limited at the low end by the polymer glass transition temperature at which polymer dynamics drop by an order of magnitude. Additionally, the quality of the bond depends on the neck growth which is formed between the adjoining rasters. Increasing interfacial contact area between road interfaces offers greater potential for strong bond formation as long as thermal conditions drive polymer coalescence. Analysis that combines thermal history at road interfaces with printed part mesostructure is critical as it relates to measured mechanical properties and elucidates relationships between the ME process, printed structure, and structural performance. In this work, tensile strength of bonds between adjacent roads and between layers in printed parts is measured while considering two factors: cooling profiles at layer interfaces and road-to-road contact. These factors are varied by manipulating print parameters in accordance with a design of experiment. Tensile strength results are analyzed as a function of road-to-road contact normalized by the road dimensions.
Print parameters available for user control have a large impact on the mechanical performance of printed parts. While available parameters depend on the slicing software used and machine limitations, some common parameters include: layer height, infill orientation, infill density, and extruder temperature (TE). Several studies have utilized a design of experiment (DoE) approach to analyze the effects of print parameters on mechanical properties of parts printed with acrylonitrile-butadiene-styrene (ABS) thermoplastic polymer. Tensile properties have been measured in response to build orientation, infill angle, road width, air gap between adjacent roads [1], extruder temperature, polymer colorants [2], unidirectional infill angle [3], [4] and angle-ply laminates [5], [6], [7], [8]. Flexure properties [9], [10] and compression properties [11] have also been studied with respect to similar print parameters. Other properties that have been considered include dimensional accuracy [12], [13] and elastic flexibility [14]. In general, these studies found infill angle and air gap to have the largest influence on mechanical properties with infills aligned along the loading direction and small air gaps resulting in higher performance. Smaller layer heights were also found to slightly improve mechanical performance in most cases. Extruder temperature was generally not observed to have a large impact on mechanical performance. All of these studies varied print parameters and measured changes to some performance metric. However, underlying relationships between print parameters and resulting mesostructure of printed parts and their impact on mechanical performance have not been explored in detail.
In the ME process, extruded roads of material partially coalesce with adjacent and underlying roads. Road-to-road coalescence is the mechanism that drives bond formation between roads. Formation of strong bonds is critical to the structural performance of printed parts. Bond formation involves neck growth, molecular diffusion between road interfaces, and polymer chain entanglement which are strong functions of temperature [15]. Polymer coalescence has been studied in literature [16], [17], [18], [19], [20], [21], [22] and is typically expressed as a function of the material properties surface tension and viscosity. For the ME process, the thermal history at road interfaces influences the dynamics of molecular diffusion and entanglement and defines the processing window over which physical bonds can form. Thermal history of printed roads in the ME process have been analyzed using thermocouples [9], fiber Bragg gratings [23], and IR cameras [24], [25], [26], [27], [28], but little work has been done to couple to printed part mechanical properties.
In this work, a DoE was applied to select print parameters to observe the effects on extrudate cooling profiles, road-to-road contact lengths, and transverse tensile strength. To reduce the number of parameters in the DoE to a tractable amount, the four most significant parameters were varied: extruder temperature, print speed, layer height, and build orientation. Extruder temperature and print speed were selected as means of varying thermal history while layer height was selected to manipulate mesostructure. Build orientation was probed by fabricating tensile specimens in two orientations shown in Fig. 1a with infill patterns arranged perpendicular to the loading direction such that either road-to-road bonds (Fig. 1b) or interlayer bonds (Fig. 1c) carried applied load. Other print parameters not included in this DoE, such as nozzle size, feeder system, infill pattern, and build plate temperature, are expected to have minor effects on thermal history of interlayer bonds and mechanical performance. Nozzle size is expected to affect the number of roads in a layer and thus may impact road-to-road contact length and interlayer strength to a small degree. Changing feeder systems is expected to have a negligible effect on mechanical performance provided the target volumetric flow rate out of the extrusion nozzle is satisfied. Infill pattern of specimens printed in the XY orientation as shown in Fig. 1a will have a large effect on mechanical properties. However, in the ZX orientation, infill pattern will affect the number of roads in a layer, but the effect on interlayer bond strength is expected to be minor. Cooling of interlayer bonds was followed with an IR camera during the printing process. Printed mesostructures were analyzed by optical microscopy and normalized contact lengths were measured. Tensile specimens were printed in the build orientation described because road-to-road and interlayer bonding are identified as performance limiting. Therefore, tensile specimens were loaded to assess the performance limiting strengths. A tensile test was seen as the most direct way of probing road-to-road and interlayer bond strength and was therefore selected over other test methods, such as bending, which introduce other modes such as compression and shear. Measured tensile strengths were examined relative to normalized contact lengths.
Section snippets
Definitions of mesostructure parameters
Specimens were printed in two orientations to measure road-to-road and interlayer bond strengths. Bond strength between adjacent roads within the same layer aligned in the axial direction can be assessed with tensile coupons printed flat (XY orientation). Vertically printed (ZX orientation) tensile coupons have bonds between layers oriented in the axial direction (Fig. 1a). Mesostructures are described by measured parameters shown in Fig. 1b and 1c where Lcx and Lcz are the contact lengths over
Rationale for print parameter selection
Increasing the extruder temperature was expected to increase both the initial polymer diffusion rate as well as the amount of time the polymer was above its glass transition temperature (Tg). Expanding the time window for diffusion of polymer chains across road boundaries was expected to increase polymer coalescence, entanglement, and therefore bond strength. Print speed was expected to affect the thermal history of bondlines. As bond strength is derived from polymer diffusion which is a
Conclusions
Relationships between print parameters, thermal history between roads, mesostructure, and tensile strength were studied with DoE methodology. Print parameters selected for manipulation were extruder temperature, print speed, layer height, and print orientation. Increasing print speed was found to negatively affect the tensile strength and contact length for both print orientations. Extruder temperature played a more minor role than print speed, but increasing temperature led to increased
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgements
The authors would like to acknowledge Ronald Trejo and Dayle Pearson for their assistance in performing mechanical testing and digital image correlation. Richard Reibel and Sathish Shamachary are acknowledged for lending the use of the IR camera.
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