Hydrostatic extrusion behaviour of high density polyethylene

The solid state processing of a high density polyethylene by hydrostatic extrusion at room temperature has been investigated. The extrusion pressure for a given extrusion ratio is found to depend on the pressure fluid used, the die angle and the velocity of extrusion. The strain hardening behaviour of the resulting extrudates is found to be independent of the extrusion ratio. An effective flow stress equation which accounts for the dependence of the flow stress of high density polyethylene on strain, strain rate and hydrostatic pressure is developed and used in an analysis of the extrusion process based on the upper bound approach. The calculated values of the extrusion pressure as a function of the various extrusion variables are in reasonable agreement with the experimental results.     

Ultra high molecular weight polyethylene deformation and fracture behaviour as a function of high strain rate and triaxial state of stress

The dynamic response and fracture characteristics of Ultra High Molecular Weight Polyethylene (UHMWPE) were investigated both experimentally and numerically. The strain rate sensitivity of the material was studied by carrying out tensile tests on smooth cylindrical specimens over a range of high strain rate conditions using the purpose built `flying wedge' testing machine at separation velocities up to 9 m/s. The effect of the initial stress triaxiality conditions on the material's ductility at different strain rates was studied using pre-notched cylindrical specimens with different notch radii. The true stress-strain results indicated that the tested material is highly sensitive to strain rate changes. Post-fracture geometric measurements of the fractured specimens indicated that the ductility of UHMWPE is strongly dependent on both the initial stress triaxiality conditions and the strain rate. Numerical simulations of the quasi-static and high strain rate tests were used to predict, for different notch radii, variation of the centre-most element radial strain and stress triaxiality factor with the average radial strain. Based on the combined numerical and experimental results, a simple relation for the ductile fracture of UHMWPE was derived as a function of stress triaxiality and strain rate.     

The hydrostatic extrusion of linear polyethylene at high temperatures and high pressures

An account is given of a series of hydrostatic extrusion experiments which has been carried out on linear polyethylene over a wide range of extrusion temperatures and environmental pressures. The results of pressure-to-pressure hot extrusions, which have been performed on melt crystallised polyethylene and also on pressure crystallised extended chain polyethylene, are discussed, with particular reference to the physical properties of the extruded products.     

The fatigue behaviour of notched polyethylene as a function ofR

The fatigue behaviour of linear polyethylene with a shallow 0.4mm notch was investigated at 0.5 Hz under plane strain conditions with both square-wave and sine-wave loading. The maximum stress was constant at 10 MPa andR was varied from +1 to –1.5. The rate of damage was monitored by measuring the crack opening displacement,. The rate of damage during most of the lifetime could be represented by the equation d/dN= whereN is the number of cycles, exhibited a minimum with respect toR. The time to failure varied inversely with and exhibited a maximum atR 0.5 for a square-wave loading and 0.1 for the sine-wave. Generally, the square-wave produces faster damage than the sine-wave. The results are quantitatively explained in terms of two mechanisms; (1) the time under load, which disen tangles the fibrils and (2) the unloading which bends and crunches the fibrils in the damaged zone. A quantitative model was obtained for describing the dependence of onR.     

Performance of metal meshes as a function of incidence angle

An inductive mesh was measured for transmission as a function of frequency, incidence angle, and polarization. The experimental data agree well with Chen's waveguide theory for meshes as long as an adequate number of modes are included in the calculations. To simplify calculations of mesh transmission a lumped circuit model has been developed to fit Chen's theory. This model predicts transmission for the two major polarizations and a variety of angles to within 1%.     

Evolution of morphology in high molecular weight polyethylene during die drawing

Deformed high molecular weight polyethylene (HMWPE) rod, formed by die drawing at 115C, was cleaved longitudinally at liquid nitrogen temperature and the cleaved surface was etched by the permanganic etching technique, A series of etched surfaces of HMWPE sections of variable draw ratio (1–13) was analysed by scanning electron microscopy (SEM), The evolution of crystalline structure in HMWPE during die drawing was observed directly. In undrawn HMWPE, the spherulites were made up of sheaf-like lamellae and scattered within an amorphous phase. During die drawing, first, microscopically inhomogeneous deformation occurred and the spherulites aligned along the drawing direction; then at a draw ratio of about 7, local melting occurred, the spherulites disintegrated and the sheaf-like lamellae oriented, followed by strain-induced recrystallization and the growth of the lamellae; finally, at a draw ratio of about 12, plastic deformation of the lamellae occurred and microfibrils were formed by drawing the lamellae.     

Crystalline structure and morphology of a hindered phenol in a chlorinated polyethylene matrix

The crystallization behavior of tetrakis [methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyloxy] methane (AO-60) in a chlorinated polyethylene (CPE) matrix was investigated. When the AO-60 content is 15 wt%, CPE/AO-60 is an incompatible system. The crystals on the surface of samples changed sequentially from a polyhedron shape to a platelet shape and then to a needle shape as the annealing temperature was reduced. The annealed samples showed two melting peaks. The high temperature peak is due to the melting of the AO-60 crystals observed. In contrast, in the case of a compatible CPE/AO-60 (5 wt%) system, AO-60 crystals also were observed but there was no change in the shape of crystals. The crystal growth of AO-60 within a CPE matrix is thought to depend on the compatibility of CPE/AO-60.     

Hydrostatic extrusion of composites based on linear polyethylene

Blends of linear polyethylene with carbon black (1 and 10 wt%) and two low-molecular weight substances, camphor (10 wt%) and dimethylerephthalate (DMTP) (10 wt%), have been hydrostatically extruded under different conditions. Both camphor and DMTP were partly compatible with the polyethylene. Compared to an unfilled linear polyethylene, hydrostatically extruded under similar conditions, an increase in tensile strength is noted in some cases. This increase in strength may be accompanied by a decrease in stiffness. The composites containing the compatible substances could be processed at a significantly lower pressure level compared to the unfilled polyethylene.     

Shrinkage as a measure of the deformation efficiency of ultra-oriented high density polyethylene

A simple test is described that allows the evaluation of the molecular extension in solid state extruded high density polyethylene at maximum extrusion draw ratio 36. The samples, which were prepared by shaving the extrudates to films of average thickness 0.4 mm, were melted very rapidly and shrunk at 160° C. Experimental evidence shows that at high heating rates (800° C min⁻¹) all the molecular extension is recovered elastically. The variation of shrinkage with molecular weight indicates a difference in the molecular extension produced during the extrusion, and a simple relationship between molecular draw ratio and modulus is considered.     

Dependencies of grain refinement on processing route and die angle in equal channel angular extrusion of bcc materials

The efficiency of grain refinement in equal channel angular extrusion of body-centered cubic (bcc) materials is investigated based on slip activities from crystal plasticity simulations, which account for both the macroscopic and crystallographic features of deformation. It is shown that the characteristics of slip activities, especially the relative contributions of slip systems newly activated or reversed at the transitions between successive passes, vary significantly with the processing routes (A, B and C) and die angles ( = 90° and 120°). The simulations assuming {1 1 0}111 slip suggest that routes B and A lead to the most significant contributions of newly activated slip systems and hence are most efficient for grain refinement with  = 90° and 120°, respectively. Further incorporation of {1 1 2}111 slip systems leads to the highest efficiency by route B for both die angles. These predictions are in partial agreement with experimental observations in the literature. Comparison of these results with those of face-centered cubic materials reveals the relevance of crystal structure and deformation mechanism during grain refinement.     

连续挤压铜扁排的阻流角设计及物理场分布

基于Deform-3D平台,建立了Conform连续挤压生产大宽厚比铜扁排有限元模型,研究了连续挤压过程中阻流角变化对模具出口处金属流动及相应应力场分布的影响规律,并结合方差分析法获得了模具出口处的流速均方差,观察模具阻流角对扁排成型的影响.研究表明:模具阻流角从3°~15°变化时,随着阻流角的增加,金属流速均方差先增大后减小,其阻流角区域等效应力值以及模具入口处沿挤压方向应力也逐渐增加;在定径带长度一定以及模具载荷允许的情况下,选择3°、5°、15°的阻流角可使模具出口处金属流速相对稳定,扁排成形性较好;当模具容易损坏时,选择3°和5°阻流角既可降低模具载荷又可获得板形均匀的扁排.     

Micromechanics of fracture in extrusion die punch

Die punches which fractured during extrusion of steel billets were investigated to assess the possible causes for premature failure. Visual examination and hardness measurements were carried out to support the evidence for failure. Detailed metallurgical investigation (optical and electron microscopy) of fractured die punches demonstrated the presence of cleavage and/or quasi cleavage fracture features typical of a brittle mode of fracture. It was observed that there was a need to evaluate the fracture toughness property of the material and the judgement based solely on hardness values seemed inadequate.     

Direct ram extrusion of polyethylene; a correlation between chain-folding and tensile modulus

The paper concerns the production of highly oriented polyethylene by means of pressing the softened but essentially still solid material through a reducing capillary between 90 and 120° C. The objectives were to explore this route towards the achievement of high tensile modulus and to assess its practicability, both of which are presented in the paper. In addition, however, information was obtained relevant to deformation processes and to the relation between tensile properties and microstructure in general. Thus the induced deformation in the solid state and its dependence on processing variables was found to be interpretable in terms of Eyrings theory of viscosity with simple shear as an important element. Both changes in the fold length and fold content could be followed most informatively by low frequency Raman spectroscopy and it was observed, in the course of the present solid-state deformation, that the chains partly refolded to a fold length appropriate to the processing temperature, and partly unfolded. The former is in accord with earlier analogous observations on cold-drawing [13], the latter, as assessed through the reduction of fold content, correlated in an apparently uniquely defined way with the measured tensile modulus where the lowering of the fold content corresponds to an increase in modulus. Thus, this Raman spectroscopically determined fold content represents a promising new parameter in the correlation of structure with properties, it is applied further in the present paper for assessing modulus variation across the width of the extrudate. Other observations on the extrudate include a quantitative analogy with wood, the fibrous structure element being the common denominator.     

Flow analysis of a power-law fluid confined in an extrusion die

The one-dimensional equations in cylindrical coordinates governing flow in an arbitrary cross-sectional shape of a cavity and the slot are derived by accounting of the order of magnitude of terms by using scaling arguments and asymptotic techniques. The derived equation is based on an average momentum and mass balance within the cavity. The one-dimensional equations governing flow in a single-cavity die which can be used to predict the geometry of the cavity and the film thickness deviations for given geometry and operating points and thus yield a design strategy for extrusion die optimized for specific applications. The derived one-dimensional governing equations with the exception of coordinates system for extrusion dies are found to be identical to that of Leonard [Polym. Eng. Sci. 25 (9) (1985) 570] and Weinstein and Ruschak [AIChE J. 42 (9) (1996) 2401] who used the shape factor in the different manner and can be reduced to Miller’s [Ind. Eng. Chem. Fundam. 11 (4) (1972) 524] simple model under the certain restrictions.     

多级拉伸挤出对纳米有机蒙脱土/高密度聚乙烯复合材料形态结构及性能的影响

采用多级拉伸挤出工艺和传统挤出工艺制备了纳米有机蒙脱土/高密度聚乙烯（OMMT/HDPE）复合材料,利用广角X射线衍射（WAXD）、TEM、DSC和小角X射线散射（SAXS）等手段对比研究了两种不同工艺制备OMMT/HDPE复合材料的形态结构和性能。结果表明：多级拉伸挤出作用能够细化OMMT的颗粒尺寸,促进OMMT在HDPE基体中的分散;随多级拉伸挤出次数的增加,OMMT的分散性越好,OMMT颗粒的长径比增加,从而使OMMT/HDPE复合材料的初始成核速率和结晶速率明显提高,诱导复合材料形成取向串晶（Shish-kebab）结构,显著提高OMMT/HDPE复合材料的拉伸强度;当OMMT∶HDPE分别为1∶100和5∶100时,多级拉伸挤出OMMT/HDPE复合材料的拉伸强度相对于传统挤出OMMT/HDPE复合材料的拉伸强度分别提高了50%和356%。     

Failure analysis of a hot extrusion die used for Al-Li alloy processing

Al-Li alloys being developed as lighter, substitutes for conventional high strength Al alloys are to be processed by routine methods. During extrusion of a 8090 Al-Li alloy, the extrusion die container failed causing some alarm. This failed die container was analysed to examine if the failure was caused by interaction of Li diffusing out of Al-Li alloy with the carbides of die steel. The evidence, although not conclusive, is sufficient to exercise caution during such processing.     

Characterization of piezoelectric films of foamed polyethylene obtained by extrusion

Foamed polyethylene (MDPE) films obtained by extrusion have been characterized by X-ray diffraction, thermogravimetric analysis and tensile tests. The piezoelectric properties have been determined by calculation of piezoelectric constant d₃₃, thermally stimulated depolarized current method and calculation of activation energy depolarization of electrets. It has been found that the crystallinity and internal morphology, including cellular structure, affect the possibility of an effective polarization in order to obtain a piezoelectric material from foamed MDPE. The conditions of processing have a great influence on the piezoelectric properties. The value of piezoelectric constant attains ~18 pC/N and ~75 pC/N for low stress range and diminishes for higher stresses, temperature T_m?~?72 °C and ~82 °C, approximate activation energy attains 2.67 and 3.28 eV, for compacted and non-compacted film, respectively. Because of the simple and cheap method of foaming PE films of piezoelectric properties it is worthy to take care regarding their application for pressure sensors.