Cold-drawing Behavior Research Articles

The influence of load direction, microstructure, raster orientation on the quasi-static response of fused deposition modeling ABS

PurposeThe continual growth of additive manufacturing has increased tremendously because of its versatility, flexibility and high customization of geometric structures. However, design hurdles are presented in understanding the relationship between the fabrication process and materials microstructure as it relates to the mechanical performance. The purpose of this paper is to investigate the role of build architecture and microstructure and the effects of load direction on the static response and mechanical properties of acrylonitrile butadiene styrene (ABS) specimens obtained via the fused deposition modeling (FDM) processing technique.Design/methodology/approachAmong additive manufacturing processes, FDM is a prolific technology for manufacturing ABS. The blend of ABS combines strength, rigidity and toughness, all of which are desirable for the production of structural materials in rapid manufacturing applications. However, reported literature has varied widely on the mechanical performance due to the proprietary nature of the ABS material ratio, ultimately creating a design hurdle. While prior experimental studies have studied the mechanical response via uniaxial tension testing, this study has aimed to understand the mechanical response of ABS from the materials’ microstructural point of view. First, ABS specimen was fabricated via FDM using a defined build architecture. Next, the specimens were mechanically tested until failure. Then finally, the failure structures were microstructurally investigated. In this paper, the effects of microstructural evolution on the static mechanical response of various build architecture of ABS aimed at FDM manufacturing technique was analyzed.FindingsThe results show that the rastering orientation of 0/90 exhibited the highest tensile strength followed by fracture at its maximum load. However, the “45” bead direction of the ABS fibers displayed a cold-drawing behavior before rupture. The morphology analyses before and after tensile failure were characterized by a scanning electron microscopy (SEM) which highlighted the effects of bead geometry (layers) and areas of stress concentration such as interstitial voids in the material during build, ultimately compromising the structural integrity of the specimens.Research limitations/implicationsThe ability to control the constituents and microstructure of a material during fabrication is significant to improving and predicting the mechanical performance of structural additive manufacturing components. In this report, the effects of microstructure on the mechanical performance of FDM-fabricated ABS materials was discussed. Further investigations are planned in understanding the effects of ambient environmental conditions (such as moisture) on the ABS material pre- and post-fabrication.Originality/valueThe study provides valuable experimental data for the purpose of understanding the inter-dependency between build parameters and microstructure as it relates to the specimens exemplified strength. The results highlighted in this study are fundamental to the development of optimal design of strength and complex ultra-lightweight structure efficiency.

In-situ SAXS study of the plastic deformation behavior of polylactide upon cold-drawing

The cold drawing behavior of PLA has been investigated with special attention given to the identification of the elementary plastic deformation mechanisms involved. A combination of SAXS and AFM studies proved to be the most adapted and complementary approaches for this purpose, providing relevant information at a pertinent mesoscopic scale. Regarding the mechanical response, the present work shows that a brittle to ductile transition occurs when the draw temperature is increased and/or when the stretching speed is decreased. Morphological studies reveal that the brittle behavior of PLA is correlated with the occurrence of crazing, whereas in the case of a ductile, several plastic deformation mechanisms are involved and interact in a rather unusual way. In particular, the formation of “shear band crazes” was identified and a tentative explanation of its origin is proposed. In addition this study shows shear banding, activated at high temperature, plays a dual role. While shear bands promote the formation of crazes, they also stabilize them during their growth. Finally some correlations between chain dynamics and the observed mechanical behavior of PLA are also discussed.

Elasticity at large deformations and high strain rates in injection molded polypropylene

AbstractThe deformation behavior of isotactic polypropylene (PP) as a function of strain rate was investigated at 50°C in uniaxial tension. Injection molded dogbone specimens were tested at high strain rates, ε = 10−1 – 102 s−1, and the local deformation in the neck was studied using fast tensile videometry. A strong elastic recoil was observed after fracture in this strain rate range with local elastic strains as high as ϵe = 2.0 – 3.2. The recoil is very fast and takes place within 1 ms. The elastic fraction of the strain at break was found to increase with the local strain rate. The elasticity further depends on strain and temperature. The elastic deformation behavior is part of the known transition from ductile cold drawing behavior to brittle fracture that occurs with strain rate or temperature. The elasticity in PP is thought to be due to a decrease in crystallinity, resulting in a discontinuous crystalline structure comparable to that of thermoplastic elastomers.

Time resolved simultaneous small- and wide-angle X-ray scattering during polyethylene deformation—II. Cold drawing of linear polyethylene

The cold drawing behaviour of a range of unoriented linear polyethylenes was investigated using the technique of simultaneous small- and wide-angle X-ray scattering during deformation. The influences of molecular weight and thermal history were studied. The role of both variables was found to be highly important since by altering the percentage crystallinity they influenced the relative amounts of inter- and intra-lamellar deformation. The micromechanical deformation processes governing the different regions of the load-extension curve were found to be the same as for cold-drawn ethylene-α-olefin copolymers, demonstrating the fundamental similarity between all types of unoriented polyethylene PE.

Time resolved simultaneous small- and wide-angle X-ray scattering during polyethylene deformation: 1. Cold drawing of ethylene-α-olefin copolymers

The cold drawing behaviour of a range of polyethylene copolymers was investigated using the technique of simultaneous small- and wide-angle X-ray scattering (SAXS and WAXS, respectively) during deformation. The influence of branch amount, branch length and lamellar population were studied. Branch length was found to be unimportant. Increasing branch content lowered the percentage crystallinity which resulted in an increase in the importance of interlamellar deformation mechanisms. Double yield points were observed in some samples. Molecular and lamellar deformation was totally reversible up to the first yield point, and was apportioned to the activation of fine chain slip. After the second yield point, for which coarse chain slip leading to lamellar fragmentation was held responsible, deformation was irreversible.

Opto-Mechanical properties of fibres. 7: Relation between cold Drawing behaviour of nylon 6 fibres from an Egyptian manufacturer and their optical properties

Optical properties change with strain produced by different stresses. Undrawn Nylon 6 fibres from an Egyptian manufacturer have been measured dynamically by interferometry. Both two- and multiple-beam interferometric techniques were used to determine refractive indices and birefringence of the investigated Nylon 6 samples with strain produced by different stresses. Using a micro-strain device attached to a microscope stage and through the application of the appropriate mathematical equations, the refractive indices and the birefringence values were determined as a function of the draw ratios. The resulting data were utilized to calculate the polarizability per unit volume, Poisson's ratio, the strain optical coefficient, and several other parameters and constants. Relationships between the various optical parameters and the draw ratios are plotted and the effect of draw ratio on the refractive index profile is studied. Microinterferograms are given for illustration.

Relations between cold drawing behaviour and optomechanical properties of vestan (polyester) fibres

A modified stress-strain device is used to investigate the dynamical behaviour of optomechanical properties. The optical properties and strain produced in vestan fibres by different stresses have been measured at room temperature interferometrically. It has been found that the relation between strain and birefringence is linear up to strain of 12%. For greater strain the rate of change of birefringence with strain is cut off due to breaking. An empirical formula is suggested to represent the variation of the cross-sectional area of vestan fibres with draw ratio and the constants of this formula are determined. An expression has also been suggested for the birefringence related to the strain. The strain optical coefficient is determined. Poisson's ratio, Young's modulus, elastic shear modulus and the compressibility are calculated over different strain values. Microinterferograms are given for illustration.

Gel-spun polyethylene fibres

In addition to spinning temperature, spinline stretching and spinning speed, the properties of gel-spun polyethylene fibres hot-drawn to the maximum draw ratio also depend on the polymer concentration, molecular weight and molecular weight distribution. Reducing the polymer concentration reduces the number of entanglements, and fibres with better properties are obtained. However, a minimum number of entanglements is necessary to ensure sufficient coherence of the entanglement network and avoid premature breakage of the spinline. Therefore, an optimal concentration exists which is shown to shift to a lower value for polyethylene with a smaller molecular weight distribution. Fibres with a tensile strength exceeding 6 GPa and a modulus of about 160 GPa can be prepared as long as spinline stretching is avoided. A smaller molecular weight distribution enhances the deteriorating effect of spinline stretching. The difference in morphologies for as-spun fibres prepared from different gel compositions and under different spinning conditions also strongly affects the cold-drawing behaviour of the extracted as-spun fibres.

Mechanism of cold drawing in melt‐spun poly(ethylene terephthalate) fibers

AbstractAn investigation has been made into the mechanism of cold drawing in melt‐spun poly(ethylene terephthalate) (PET) fibers. An analysis of the cold‐drawing behavior using wide‐angle x‐ray diffraction, orientation measurements, calorimetric and mechanical techniques was performed. The evidence suggests that the cold‐drawing process involes stress‐enhanced crystallization which occurs in conjuction with incresing orientation of the crystalline and amorphous regions. A degradation in the fiber properties after cold drawing was observed for fibers spun below 1,000 m/min while fibers spun above 1,000 m/min exhibited an improvement in fiber properties with cold drawing. This behavior was explained by the existence of two distinct irreversible deformation micromechanisms for fibers spun below and above 1,000 m/min.

Tensile drawing behaviour of poly(ethylene terephthalate)

A detailed study has been undertaken of the drawing behaviour of poly(ethylene terephthalate) over the temperature range 20° to 80°C. Cold drawing behaviour was observed at the lower temperatures and homogeneous deformation at 80°C. Samples were also subjected to two-stage drawing: homogeneous draw at 80°C followed by cold drawing at 20°C. In all cases the geometry of the deformation was monitored by measuring the changes in macroscopic dimensions. In addition, measurements were made of the final birefringence, and the shrinkage force developed when the drawn samples were heated to a temperature above T g . The results are discussed in terms of continuum models for the deformation of polymers. In particular, the relevance of a simple molecular network model is considered. It is shown that many of the observations are consistent with the deformation of a molecular network, although our understanding of the molecular processes involved in cold drawing is still incomplete.

Effect of molecular weight on the morphology and drawing behaviour of melt crystallized linear polyethylene

The effect of molecular weight on the cold drawing behaviour of melt crystallized linear polyethylene has been studied. It is shown that the draw ratio achieved under comparable conditions rises with decreasing M ̄ w , very high draw ratios (∼36) being possible for optimum morphology of the undrawn polymer. The yield behaviour was also examined, and it is shown that the yield stress is affected in a complex fashion by both crystallization conditions and molecular weight. These results are discussed in terms of the crystallization and morphology of the melt crystallized polymer.

Preparation of ultra-high modulus linear polyethylenes; effect of molecular weight and molecular weight distribution on drawing behaviour and mechanical properties

A systematic investigation of the effect of molecular weight and molecular weight distribution on the cold drawing behaviour of linear polyethylene has been undertaken. In the molecular weight range studied, the natural draw ratio was very sensitive to the morphology of the initial material; spectacular effects on the natural draw ratio were observed provided that an optimum initial morphology was achieved. These effects can be related to both molecular weight and molecular weight distribution. The extensional modulus and melting behaviour of the drawn material was also examined. To a first approximation the extensional modulus related to the natural draw ratio, and at very high draw ratios (∼30) extremely high extensional moduli (∼700kbar) were obtained. The structure and properties of the drawn material did, however, also depend on the molecular weight and molecular weight distribution. In particular, when certain molecular weight requirements were satisfied, the oriented samples showed the presence of extended chain material. It does, however, appear that although differences in molecular weight and molecular weight distribution give rise to differences in extensional moduli, the presence of extended chain crystallization per se is not a necessary requirement for the production of high modulus material.

Properties of Ultra-high Modulus Linear Polyethylenes

We have recently been studying the effect of molecular weight and molecular weight distribution on the cold drawing behaviour of linear polyethylene. Some of the results seem to be sufficiently unusual to warrant reporting at this stage.

The cold drawing of polyethylene terephthalate

The cold-drawing behaviour of polyethylene terephthalate has been studied over a wide range of test conditions and polymer constitution. In addition to measurements at different temperatures and strain rates, the influence of pre-orientation, crystallinity and chemical cross-linking was investigated. Although crystallinity and cross-linking altered to some extent the details of the drawing process, the overall pattern of behaviour remained substantially unchanged despite rather severe changes in the microstructure of the polymer. Pre-orientation affected the yield stress and produced marked changes in the natural draw ratio. Previous studies by Pinnock and Ward have shown that pre-orientation is introduced during extrusion in a manner similar to stretching a rubber network and that the degree of stretch can be determined by the subsequent high-temperature shrinkage of the pre-oriented fibre. Measurements of the natural draw ratio and shrinkage of a series of pre-oriented fibres, described here, showed that the natural draw ratio may be regarded as equivalent to stretching a network to a limiting total extension. This limiting extension appeared to be a constant independent of the division of extension between the extrusion and drawing processes, and could provide a geometrical explanation for the large influence of pre-orientation on the natural draw ratio. An alternative view is that the degree of strain hardening achieved for a given elongation during the cold-drawing process will be affected by pre-orientation, and that the natural draw ratio could be very sensitive in this respect. Even with this explanation, the change in strain hardening may be associated with the limiting extension of a network. Measurement of the birefringences and moduli of the cold-drawn fibres showed that the development of optical and mechanical anisotropy could be fitted to a good approxi mation by an aggregate theory. This theory assumes that the drawn fibre consists of an aggregate of optically and mechanically anisotropic units which are aligned by the drawing process.

The Extension of Unoriented Nylon 66 Filaments: Part II: Phenomenological Relationships

Previous experimental investigations of cold drawing have employed two different techniques, (i) a study of load-elongation curves obtained on a constant-rate-of-elongation apparatus, and (ii) the operation of a two-roller drawing machine. In the first case, the length of yarn is fixed and the travel of the neck is followed along this length. In the second case, the neck is fixed in space by adjusting the speed and draw ratio applied to the traversing fiber. A third experimental method is to apply a constant load to an undrawn fiber and measure the increase in length with time (creep). In this experiment, cold drawing is observed by a marked yield region on the logarithmic time scale. Prior to the yield point, there is a region of uniform extension or pre-yield creep. Following the yield point, there is also a region of uniform extension or post-yield creep. These creep curves can be used to predict cold drawing behavior on both a two-roller drawing machine and a constant-rate-of-elongation apparatus. In particular, the location of the neck between feed and draw rolls, the drawing tension, and the speed dependence can be understood. These concepts are illustrated using results on nylon 66 monofilaments.