Die drawing technology of high molecular weight polyethylene

A comprehensive investigation of the die drawing technology of a high molecular weight polythylene (HMWPE) rod has been carried out. The effect of draw temperature, draw speed, nominal draw ratio, and exit diameter of the dies has been studied. The oriented HMWPE products were characterized mainly by the determination of the three-point bend modulus and the tensile strength. The tensile strength and the modulus of the drawn HMWPE rod could reach 700 MPa and 18 GPa, respectively. In addition, it was found that forced cooling at the die exit was essential when drawing billets with large section areas. © 1993 John Wiley & Sons, Inc.     

The effct of asymmetries of die exit geometry on extrudate swell and melt fracture

Experimental investigations were performed to see how the die exit geometry and the extrusion velocity influence on extrudate swell and melt fracture for several polymer melts [low-density polyethylene, styrene-butadiene rubber (SBR) and SBR/HAF (carbon black) compound]. Four different types of die exit geometry were considered; 0° (symmetric. usual capillary die), and 30°, 45° and 60° (asymmetric dies) were chosen for the die exit angle. Extrudate diameters were measured without draw-down under isothermal condition. Polymer melts were extruded into an oil that has the same density and temperature as those of the extrudate. Extrudate swells from dies with different diameters were correlated with volumetric flow rates. It was observed that the extrudate swell increases with increasing volumetric flow rate and exhibits through a minimum value at about 45° die exit angle. As to the fracture phenomena, it was observed that the critical shear for the onset of melt fracture increases with the increasing die exit angle up to 45°. However, for 60° die exit angle, the onset of melt fracture is again similar to that of 0° exit angle.     

Zur spannungsrißbildung von hochpolymeren

Crazing is a phenomenon, which is often observed in polymers, when mechanical stresses are applied. In this paper we report about investigations of crazing of polymethylmethacrylate as an example of an amorphous polymeric material. We deal with parameters which influence the formation of crazes. A method is described which allows one to avoid crazing, thus an improvement of the material concerning the formation of crazes is achieved. We further attempt to interprete the formation of crazes. The anology between crazing and fracture and flow behavior (cold drawing) of polymers is stressed. We suppose that the primary molecular processes underlying these phenomena are basically the same, and that they consist of an orientation of the macromecules in the direction of the applied stress.     

Fluid flow stability analysis of multilayer fiber drawing

We here investigate drawing of multi-layered Newtonian and non-Newtonian fluid fibers, drawn under isothermal and non-isothermal conditions. We first develop one-dimensional equations governing mass, momentum, and energy balances and solve them numerically to obtain steady state draw root shape, velocity, and temperature profiles. These solutions are then used to perform linear stability analysis. For the case of isothermal draw, the system displays an oscillatory instability when the draw ratio (ratio of cross-sectional areas of fiber at the entrance and exit of the drawing) is higher than a critical draw ratio (highest stable draw ratio) of about 20.21. Investigation of stability behavior under non-isothermal draw conditions is performed by considering radiative heating and convective cooling. Employing only radiative heating enhances the critical draw ratio, and simultaneous heating and convective cooling increase the critical draw ratio even further. For the case of simultaneous heating and cooling, with increasing convective cooling strength, the critical draw ratio first increases, reaches a maximum, and then gradually decreases. However, with only convective cooling, the critical draw ratio decreases with an increase in convective cooling strength. We also find that the stabilizing effect of a non-isothermal operation can be enhanced by considering fluids with higher viscosity sensitivity to temperature, increasing the maximum temperature, and for sharper attenuation of the fiber cross-sectional area with length. For the case of isothermal drawing of non-Newtonian fluid fibers, the system has a higher critical draw ratio for shear thickening fluids (power-law exponent, n>1). In contrast, the use of a shear thinning fluid (n<1) reduces the critical draw ratio. Consideration of a non-isothermal operation of non-Newtonian fluid fibers reveals that the critical draw ratio is primarily determined by the non-Newtonian behavior rather than the non-isothermal drawing.     

Fracture behavior of die‐drawn toughened polypropylenes

The Leeds die‐drawing process has been used to make oriented sheets of toughened polypropylenes. Die‐drawn oriented sheets were produced by drawing at 110°C to draw ratios of 4, 6, and 10. Comparative measurements have been undertaken of the plane stress fracture toughness at room temperature using the essential work of fracture method for isotropic and oriented polypropylene homopolymer and the two polypropylene blends containing 10 and 25% of a polyethylene‐based elastomer. In the isotropic state, the blend containing 25% elastomer exhibited higher fracture toughness than the homopolymer and the 10% blend. The oriented sheets were tested both parallel (cracks perpendicular to the draw direction) and perpendicular (cracks parallel to the draw direction). For the latter case of cracks parallel to the draw direction, the fracture toughness of all the materials decreased with increasing draw ratio and up to a draw ratio of 4 the 25% blend exhibited higher fracture toughness than the other two materials. At higher draw ratios, however, the unfilled polypropylene was tougher than the blends. When tested parallel to the draw direction, all three materials failed with the cracks growing slowly initially followed by sudden rupture. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1336–1345, 2003     

Some rheological aspects of fracture in thermoforming

This paper focuses on several aspects of drawability, including the interactions between material parameters, operating temperatures, and frictional properties of the material. The deep draw process for a molten plastic sheet can be described by a simplified model using the characteristics of the normal stress as a lumped parameter. A rapid evaluation of the draw ratio and mold closing speed can be obtained by systematically drawing a series of three draw ratios. The fracture mode should then be examined to determine the appropriateness of molding temperatures. Therefore the maximum attainable draw ratio can be calculated from the elongational viscosity data. To support the analysis, data is provided on the formation of a cup with polystyrene sheets and using Maxwell extensional model as an example.     

Analytical and experimental study of the die drawing of circular rods through conical dies

An analysis is presented of the process of die drawing isotropic polymers, in the form of circular rods, to produce highly oriented materials with enhanced mechanical properties. The stresses in a small element of material undergoing the drawing process have been analyzed using a force-equilibrium approach and the initial yield and flow stresses in the material have been predicted from the von-Mises yield criterion. The stress-strain-strain rate characteristics of the polymer used in the analysis were deduced from uniaxial tensile test data obtained at the same temperature at which the die drawing occurred. Experimental results are presented of the stress in polypropylene GSE-108 rods when die drawn at 90°C in a purpose-built die drawing facility. Novel techniques were used to determine the stress and strain distributions along the die. A comparison of the experimental results and the analytical predictions shows good agreement.     

Experimental study on cold press workability of aluminum-polyethylene sandwich laminates

The cold press workability of aluminum-polyethylene sandwich laminates is experimentally investigated by a deep drawing process with a conical die. “Planium” of 2 mm thickness, in which the core layer, high density polyethylene, is sandwich-laminated between two aluminum sheets, is used as a test specimen. Soybean oil is employed as a lubricant. The dimensions of the drawing die set are determined for a circular, blank so that the tensile fracture of the drawn.cup occurs at the top corner of the punch. The limiting draw ratio (LDR) is experimentally explored by using many blanks of various initial diameters. In general the maximum punch load in successful drawing increases linearly with increasing draw ratio. Because of the sandwich structure, the fracture of the drawn cup occurs twice. The initial fracture (LDR_s) corresponds to a fine bending fracture at the aluminum surface. The second fracture (LDR_B) is the complete fracture of the whole laminate. The effect of punch corner radius, working velocity and thickness fraction of aluminum and polyethylene on LDR_S and LDR_B are studied. The strain distributions of deep drawn cup in three orthogonal directions are analyzed experimentally.     

Finite element analysis of wire coating

A general-purpose finite element program has been used to simulate the flow of polymers through wire-coating dies. The analysis includes Newtonian and power-law fluids. The effect of normal stresses was examined through a simple viscoelastic constitutive equation, Nonisothermal wire coating was studied to obtain the temperature field within the melt. The effect of a slip condition at the solid boundaries was also examined. The determination of the coating melt free surface was carried out through an iterative procedure. The finite element solution provides details about the existence and extent of recirculation regions, about hot spots due to viscous dissipation, and also captures the stress singularities present at the impact of the melt with the wire and at the exit from the die. Pressure distribution, maximum temperature rise, haul-off wire tension, maximum wire tension, and stresses at the wire surface and die wall are also presented.     

Flaring dies to suppress die drool

Die lip build‐up is the unwanted material accumulation on extrusion die lips. Here, flared dies are shown experimentally to suppress die lip build‐up. A semiempirical method for flared die design is also provided. Nonlinear viscoelastic constitutive equations are used to calculate the wall shear stress and first normal stress difference in flared dies. By incorporating melt memory, a promising design method for die flaring is presented. The stress history upstream of the die exit governs the die design. The upstream gap is selected to maximize undershoot of the first normal stress difference N₁ at the die wall caused by flaring. The flare length, on the other hand, is selected to ensure a steady N₁ at the die lips.     

New criterion for craze initiation

Existing criteria for craze initiation are reviewed, and their limitations are discussed. The most obvious problem is that they are formulated simply in terms of principal stresses, making no provision for the known effects of small inclusions and surface imperfections. To solve this problem, a new criterion is proposed, which is based on linear elastic fracture mechanics. Craze initiation is treated as a frustrated fracture process rather than a yield mechanism. Calculations show that the strain energy release rate, G_I(nasc), required to generate a typical 20 nm thick nascent craze, is less than 1 J m⁻². This explains why flaws less than 1 μm in length are capable of nucleating crazes at stresses of 20-30 MPa. Subsequent craze propagation is dependent upon two flow rates, one relating to fibril drawing at the craze wall and the other to shear yielding at the craze tip. Under biaxial stress, the second principal stress σ₂ affects craze tip shear yielding but not fibril drawing. This model is used in conjunction with the von Mises yield criterion to derive a new expression for the crazing stress σ₁(craze), which provides a good fit to data on visible crazes obtained by Sternstein, Ongchin and Myers in biaxial tests on cast PMMA [Sternstein SS, Ongchin L, Silverman A. Appl Polym Symp 1968;7:175; Sternstein SS, Ongchin L. Polym Prepr Am Chem Soc Div Polym Chem 1969;10:1117; Sternstein SS, Myers FA. J Macromol Sci Phys 1973;B8:539].     

Improvement of Flexural Strengths of Poly(L‐lactic acid) by Solid‐State Extrusion, 2. Extrusion Through Rectangular Die

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.).     

Effect of flow history on poly(vinylidine fluoride) crystalline phase transformation

This study was devoted to the effect of extensional flow during film extrusion on the formation of the β‐crystalline phase and on the piezoelectric properties of the extruded poly(vinylidine fluoride) (PVDF) films after cold drawing. The PVDF films were extruded at different draw ratios with two different dies, a conventional slit die and a two‐channel die, of which the latter was capable of applying high extensional flow to the PVDF melt. The PVDF films prepared with the two‐channel die were drawn at different temperatures, strain rates, and strains. The optimum stretching conditions for the achievement of the maximum β‐phase content were determined as follows: temperature = 90°C, strain = 500%, and strain rate = 0.083 s^?1. The samples prepared from the dies were then drawn under optimum stretching conditions, and their β‐phase content and piezoelectric strain coefficient (d₃₃) values were compared at equal draw ratios. Measured by the Fourier transform infrared technique, a maximum of 82% β‐phase content was obtained for the samples prepared with the two‐channel die, which was 7% higher than that of the samples prepared by the slit die. The d₃₃ value of the two‐channel die was 35 pC/N, which was also 5 pC/N higher than that of the samples prepared with the slit die. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

The onset of wall slip and sharkskin melt fracture in capillary flow

The capillary die flow of high density and linear low density polyethylenes is simulated under slip conditions to investigate the origin of sharkskin melt fracture. As suggested in the literature, it is shown that sharkskin originates at the exit of the die and is due to the acceleration (high stretching rate) of the melt as it exits the die. It is also shown that both adhesion and slip promoters eliminate surface defects by decreasing the stretching rate of the polymer melt at the exit region of the die. The effect of length-to-diameter ratio of the die on the sharkskin melt fracture is also examined. It is found that sharkskin is more pronounced in short dies which is in accord with experimental observations. Finally, it is suggested that applied pressure at the capillary exit suppresses surface defects.     

Effects of zone drawing on the structure of metallocene polyethylene

The influence of zone drawing on bulk properties and structure of metallocene polyethylene (m‐PE) is reported. Two different m‐PE materials were subjected to tensile stresses above the yield point by zone drawing in the temperature range from 50 to 100°C. Drawn materials were characterized by using small‐ and wide‐angle X‐ray scattering (SAXS, WAXS), molecular retraction, and small‐angle light scattering (SALS). Structural changes were studied as a function of drawing temperature, engineering stress, and draw ratio. WAXS showed strong crystalline orientation in drawn samples, and only the orthorhombic crystal modification was observed. SAXS showed lamellar orientation in drawn samples. At low drawing temperatures of 50 or 60°C, draw ratio increased as a step function of stress. There is a stress barrier, which must be exceeded before high‐draw ratios can be achieved at these temperatures. At drawing temperatures of 70°C or above, the barrier stress is low enough that draw ratio increases nearly linearly as a function of stress. Below the stress barrier, spherulitic structure is observed by small‐angle light scattering (SALS). Elongation occurs via deformation of the interspherulitic amorphous phase. Molecular retraction was low for these samples, indicating mostly plastic deformation of the amorphous material. Above the stress barrier, SALS showed that spherulites are destroyed. Elongation occurs via deformation of the intraspherulitic amorphous phase. Molecular retraction for these samples was high, indicating elastic deformation of the amorphous material. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3492–3504, 2001     

The effects of stress cracking on the fracture toughness of polycarbonate

The growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase in k_IC with time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.     

A comparative study of fracture behavior between carbon black/poly(ethylene terephthalate) and multiwalled carbon nanotube/poly(ethylene terephthalate) composite films

Fracture behavior of amorphous poly(ethylene terephthalate) (PET) films added multiwalled carbon nanotube (MWCNT) has been compared with that of the PET films added with carbon black (CB) to elucidate the effects of the large aspect ratio of MWCNT. Fracture toughness has been evaluated using the essential work of fracture tests. Evolution of the crazes has been analyzed by conducting time‐resolved small‐angle X‐ray scattering measurements during tensile deformation of the films at room temperature using synchrotron radiation. CB and MWCNT increased the fracture toughness of the PET film by increasing the plastic work of fracture. This resulted from the effects of the fillers to prevent the localization of deformation upon the crazes formed at earlier stages of tensile deformation and to retard the growth of the fibrils in the crazes to a critical length. The CB particles provided a number of sites where the crazes were preferably formed due to stress concentration. In the case of MWCNT, on the other hand, the widening of the crazes formed at earlier stages was suppressed due to the bridging effect arising from the large aspect ratio of MWCNT. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Structural order in oriented PVC mouldings

The structural changes occurring upon drawing and annealing of compression and injection mouldings of commercial poly(vinyl chloride) were studied by wide-angle X-ray diffraction. Low temperature drawing appears to lead to a reduction in 3-dimensional order and an increase in oriented 2-dimensional order. The degree of order of drawn and annealed PVC depends on draw ratio, annealing temperature and the restraint during annealing. The maximum in 2-dimensional order occurs on annealing at 110°C. Tensile yield stress is significantly increased by the drawing process and it was shown that the anisotropy of this mechanical property decreased upon annealing. This could not be explained by the reduction in amorphous orientation alone. Electron microscopy of the fracture surfaces shows a structure which appears to be related to the drawing and annealing process.     

Melt fracture,melt viscosities,and die swell of polypropylene resin in capillary flow

The melt fracture, shear viscosity, extensional viscosity, and die swell of a polypropylene resin were studied using a capillary rheometer and dies with a 0.05‐cm diameter and length/radius ratios of 10, 40, and 60. A temperature of 190°C and shear rates between 1 and 5000 s^?1 were used. A modified Bagley plot was used with consideration of pressure effects on both the melt viscosity and end effect. The shear viscosity was calculated from the true wall shear stress. When the true wall shear stress increased, the end effect increased and showed critical stresses at around 0.1 and 0.17 MPa. The extensional viscosity was calculated from the end effect and it showed a decreasing trend when the strain rate increased. Both the shear and extensional viscosities correlated well with another polypropylene reported previously. The die swell was higher for shorter dies and increased when shear stress increased. When the shear rates increased, the extrudate changed from smooth to gross melt fracture with regular patterns (spurt) and then turned into an irregular shape. In the regular stage the wavelength of the extrudates increased when the shear rate increased. The frequency of melt fracture was almost independent of the shear rate, but it decreased slightly when the die length increased. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1587–1594, 2003     

Craze failure by midrib creep

It has been known for some time that crazes thicken during growth mainly by drawing in fresh material from the craze-bulk interface, keeping the average craze fibril draw ratio approximately constant. Since Creep effects contribute only negligibly to craze growth rates these effects have generally been considered unimportant regarding craze breakdown. However, it is also known that the first stage of fracture is failure of the craze midrib, which is a highly drawn, very thin region down the middle of a craze. Because of the very low thickness of the midrib it has little influence on craze behavior, and information on midrib behavior is difficult to obtain. It is the purpose of this paper to attempt to rationalize what information is available.