High-pressure injection molding (nominal pressure 500 MPa) is known to substantially improve the mechanical properties of high-density polyethylene of a high molecular weight (HMWPE). This work shows that if the mold is equipped with an exit cavity, the tensile modulus and strength of HMWPE-bars molded is further improved at high pressure levels. The maximum values of the stiffness and strength (thin bars, 1 mm) obtained with the exit chamber is about 12 GPa and 260 MPa, respectively. The improvement due to the exit cavity is of the order of 30 percent for the tensile strength for thicknesses lower than 4mm, while the modulus increases about 1 to 1.5 GPa for bars with thicknesses between 1 and 6 mm. The orientation of the melt during the filling of the mold was also found to have an influence on the mechanical properties of the HMWPE bars. 相似文献
AbstractA key factor which limits the production speed of the polymer die drawing process is the premature fracture of the material on exit from the die. In this paper, the growth of damage in the material during the die drawing process has been studied using a combination of thermoplastic finite element analysis and structural characterisation by means of scanning electron microscopy and small angle X-ray scattering for the specific case of die drawing of polyoxymethylene. It is demonstrated that special profiled dies offer a more beneficial strain rate distribution than the conventional conical dies and allow higher production speeds to be obtained. Voids grow in the material as a result of the tensile stresses pertaining near the die exit and then, crazes appear from within the material at a critical stress level leading ultimately to final fracture. The results suggest that although the crazes initiate at a critical stress, the extent of crazing at the maximum draw ratio obtained (~13) is independent of the type of die and hence the stress level. Fracture of the drawn product occurs at different stresses for different die profiles but always at the maximum draw ratio of 13, suggesting that this relates to the limiting extensibility of a molecular network. 相似文献
The properties of ultra-oriented polyoxymethylene tubes produced by drawing under microwave heating have been assessed by mechanical testing, optical microscopy, scanning electron microscopy, X-ray analysis, birefringence and differential scanning calorimetry. The highest Young's modulus of 58 GPa was obtained at room temperature (77 GPa at ?150°C) at a draw ratio of 33. The maximum tensile strength was 1.7 GPa at a draw ratio of 26. The nonuniformity of Young's modulus in a radial direction has been compared with the nonuniformity of the birefringence and heat of fusion. 相似文献
Solid‐state extrusion of poly (L ‐lactic acid) (PLLA) through rectangular die was performed to produce high flexural strength plates that can be used as internal fixation devices. A single‐angle wedge‐shaped rectangular die was utilized having the die exit dimension of 4 mm × 1 mm. Billets were machined out from vacuum compression molded PLLA having different crystallinities to have various thicknesses and thus various imposed draw ratios. Solid‐state extrusion of billets was performed at various drawing rates at 130 °C, above glass transition and below melting temperature. Extruded plates had the width and thickness smaller than the die due to the further drawing outside the die. The decrease in width was larger than the decrease in thickness, and this became more prominent with increasing draw ratio and drawing rate, resulting in a significantly smaller aspect ratio. Contributions of die drawing and further drawing outside the die were estimated from the extruded plate dimensions, by which the drawing rate effect was attributed to the further drawing outside the die. As actual draw ratio increased, crystallinity, melting temperature, crystalline orientation factor, and birefringence increased. Throughout the whole process the decrease in molecular weight was largely suppressed to be about 10%. As billet crystallinity, draw ratio, and drawing rate increased, both flexural strength and flexural modulus increased up to the maxima of 202 MPa and 9.7 GPa, respectively. This enhancement in mechanical properties was correlated with structural developments.
Changes in flexural strength of solid‐state extruded PLLA plates as a function of draw ratio (the effect of drawing rate is co‐plotted by hollow symbols at corresponding draw ratio.). 相似文献
Fibers (strands) with various draw ratios were spun from the liquid crystalline state of a pure aromatic liquid crystalline copoly(ester amide) and the melts of its blend with polycarbonate. Scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS), and differential scanning calorimetry (DSC) were employed to investigate the structure and properties of the resulting fibers. Mechanical properties of the fibers were also evaluated. It was found that both the crystallite size and heat of fusion of the liquid crystalline polymer (LCP) increase steadily with draw ratio. However, the crystal-nematic transition temperature of the LCP is virtually unaffected by drawing. Moreover, heat of fusion of LCP is much smaller than that of isotropic condensation polymers despite the presence of very sharp diffraction peaks in WAXS measurements. These results are ascribed to the (semi)rigid rod nature of the LCP chains and the persistence of an ordered structure in the LCP melt, i.e., entropy effect. It was further observed that tensile modulus and tensile strength along fiber axis rise with draw ratio for the composite fibers. The elastic modulus of the composite fibers were found to be as high as 19 GPa and tensile strength reached 146 MPa with draw ratios below 40 and an LCP content of 30 wt%. Compared with the thermoplastic matrix, the elastic modulus and tensile strength of the in-situ composite have increased by 7.3 times and 1.4 times, respectively, with the addition of only 30 wt% LCP. This improvement in mechanical properties is attributed to fibrillation of the LCP phase in the blend and the increasing orientation of the LCP chains along the fiber axis during drawing. 相似文献
A tensile drawing process under dielectric heating has been developed for polyoxymethylene. The influence of ambient temperature, electric field strength, and strain rate on the maximum draw ratio and the tensile modulus has been examined. Tubes possessing tensile moduli up to 63GPa were produced by the new drawing technique. It is speculated that the achievement of such ultrahigh moduli is due to the fact that the stress by drawing is used effectively to orient the molecular chains in the noncrystalline regions and at defect regions within the crystal lamellae. This is because these regions are heated to higher temperatures than the crystalline regions during dielectric heating. 相似文献
Orientation of initially amorphous poly(ethylene terephthalate) films and sheets was carried out by means of tensile drawing in a tensile tester, roll-drawing using a series of four rolling stations, and by die-drawing. The drawing temperature was 80 and 90°C and drawing rate ranged from 2 to 20 cm/min in the different processes. Crystallinity was observed to increase with draw ratio for all these processes. The onset of crystallinity development depends on the drawing rate. The glass transition temperature was essentially constant and crystallization temperature decreased with increasing draw ratio. The trans conformers content was observed to increase with draw ratio at the expense of the gauche conformers for the three processes. The orientation of the trans conformers increases readily from the beginning of draw and saturates rapidly. The orientation of the gauche conformers was negligible. Some differences are observed for the behavior of the 1020 and 730 cm?1 benzene ring bands, which may be due to differences in the benzene ring configuration at the surface as a result of different deformation mechanisms for the die and roll-drawing. However, further investigations to elucidate this hypothesis are needed. The mechanical properties obtained in the longitudinal direction increased for all the processes. In the transverse direction, the roll and die-drawing processes induced a decrease in modulus and strength with increasing draw ratio, similar to that observed for uniaxial orientation. This indicates that these processes are mainly uniaxial, despite the plane strain nature of the deformation. 相似文献
Initially amorphous and semicrystalline films of poly(ethylene 2,6-naphthalate) with different molecular weights were drawn by two-stage drawing, that is, coextrusion at low temperatures (25–160°C) followed by tensile drawing at high temperatures (200–245°C). Both films could be drawn up to a draw ratio of 8–10 by this method under controlled conditions. The tensile modulus and strength of drawn samples were greatly affected by the draw temperature for the first stage, predrawn morphology, and molecular weight. The remarkable effects of these variables on the tensile properties are closely related to the difference in the resultant amorphous chain orientation of the samples, reflecting the disentanglements and chain slippage during drawing, and the dissipation of chain orientation after processing. 相似文献
The crystalline state deformation of high density polyethylene has been examined at an extrusion draw ratio of 30 over a range of temperatures and pressures. The experiments involve combined pushing (extrusion) and pulling through a conical die. The pressure dependence of the extrusion rate through conical dies is given by a logarithmic relation and the temperature dependence by an activation energy of ~95 kcal/mole. An equation established for the total applied force linearly relates the pulling and extrusion pressure components and represents a force balance at the die entrance and exit. Steady-state extrusion, with or without pulling, was feasible in a pressure range beyond which fractures occurred owing to strain rate and shear or tensile failure. Under some circumstances the extrusion rate was increased by ten times. The mechanical properties and mode of deformation were not affected by pull load and fibers with a tensile modulus of 55 GPa were produced at T < 110°C. 相似文献
The roller drawing of polyoxymethylene (POM) sheets was carried out in the temperature range of 140–157°C. The mechanical properties, the molecular orientation, and the microstructure of the roller-drawn POM sheets were investigated by means of tensile test, dynamic viscoelasticity, wide-angle X-ray diffraction, small-angle X-ray scattering, visible dichroic spectrum, electron microscopy, and so on. The Young's modulus and the tensile strength increased with increasing draw ratio up to draw ratio, λ of 14–15. The improvement of the mechanical properties is concerned with structural changes, such as the increase in orientation function in the crystalline and amorphous regions and the formation of taut tie molecules and crystalline bridges in the intercrystallite and interfibrillar regions. In the higher draw ratio range (λ > 15), the increase in Young's modulus and tensile strength was restricted by the formation of interfibrillar microvoids. 相似文献
The continuous production of transparent high strength ultra‐drawn high‐density polyethylene films or tapes is explored using a cast film extrusion and solid‐state drawing line. Two methodologies are explored to achieve such high strength transparent polyethylene films; i) the use of suitable additives like 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐ditertpentylphenol (BZT) and ii) solid‐state drawing at an optimal temperature of 105 °C (without additives). Both methodologies result in highly oriented films of high transparency (≈91%) in the far field. Maximum attainable modulus (≈33 GPa) and tensile strength (≈900 MPa) of both types of solid‐state drawn films are similar and are an order of magnitude higher than traditional transparent plastics such as polycarbonate (PC) and poly(methyl methacrylate). Special emphasis is devoted to the effect of draw down and pre‐orientation in the as‐extruded films prior to solid‐state drawing. It is shown that pre‐orientation is beneficial in improving mechanical properties of the films at equal draw ratios. However, pre‐orientation lowers the maximum attainable draw ratio and as such the ultimate modulus and tensile strength of the films. Potential applications of these high strength transparent flexible films lie in composite laminates, automotive or aircraft glazing, high impact windows, safety glass, and displays. 相似文献