摘要:

AbstractIsothermal experiments on gas‐assisted displacement of viscoplastic liquids in tubes show that a liquid coating remains on the tube wall. The thickness of this coating approaches 0.35 of the tube radius at high gas penetration rates, the asymptotic limit previously observed for Newtonian liquids. At low gas penetration rates, the viscoplastic coating is much thinner than its Newtonian counterpart. During the displacement process, the gas front moves faster than the liquid front and, prior to blowout, it rapidly accelerates as the amount of liquid downstream of the gas is depleted by the liquid coating. Based on these observations, a simple isothermal model is developed to describe the gas‐liquid dynamics. This model provides an insight into the gas‐assisted injection molding process in which the injection of molten plastic into a mold is assisted by a pressurized gas. In particular, the results show that the wall thickness around the hollow cores in gas‐assisted parts is set during processing by the solid skin and a thick molten layer. The thickness of the molten layer depends on the gas penetration rate and the viscous behavior of the molten plastic. By coupling the isothermal model with a one‐dimensional heat‐transfer analysis, the gas penetration rate is shown to be several orders of magnitude higher than the plastic freezing rate, so that most of the heat transfer between the melt and the gas occurs after the mol

ISSN:0032-3888    

DOI:10.1002/pen.760351102

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractPresence of weldlines introduces an element of uncertainty to the performance of injection molded parts. Weldlines are particularly problematic in reinforced plastics because, unlike molecular orientation in neat polymers, the flow induced fiber orientation does not relax. This paper deals with the structure and mechanical behavior of weldlines in glass fiber reinforced nylon 66, a plastic known for excellent fiber‐matrix adhesion. Two molds were used to generate weldlines: a double gated tensile sample shaped cavity in which the weldline is formed by a head‐on collision of melt fronts flowing in opposite directions and a film gated rectangular plaque with a circular insert in which the weldline formation behind the insert is followed by additional flow. In both cases the weldline zone is several millimetres wide: in the plane where the melts fronts have met fibers are oriented parallel to this plane (random‐in‐plane in the double‐gated cavity and unidirectional in the cavity with insert). The transition zone between the weldline plane and the rest of the sample is characterized by an increased presence of microvoids. Weldline tensile depends little on the fiber concentration and on the sample shape or thickness: values close to the matrix strength are found: in samples without weldlines strength increases with the fiber content. However, in instrumented impact penetration test during which the material is subjected to multiaxial loading, the weldline effect appears n

ISSN:0032-3888    

DOI:10.1002/pen.760351103

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractWide angle X‐ray diffraction pole figure, small angle X‐ray scattering (SAXS), and refractive index techniques have been employed to investigate structure and deformation mechanisms in unidirectionally cold‐rolled ultrahigh molecular weight polypropylene (UHMWPP) films. All reciprocal lattice vectors (plane normals) of (110), (040), and (130) were found to populate in the film normal (thickness) direction, suggesting that more than one orientation process must be involved in the deformation. SAXS studies on the cold‐rolled UHMWPP reveal an oriented lamellar structure with its long axis perpendicular to the rolled direction, however, the lamellae are somewhat tilted. Refractive index and tensile measurements were undertaken to determine the optical and mechanical anisotropy of the cold‐rolled UHMWPP films. A similar study was undertaken of unidirectionally rolled conventional polypropylene (PP) for c

ISSN:0032-3888    

DOI:10.1002/pen.760351104

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractThe mechanisms of plastic deformation in cold‐rolled ultrahigh molecular weight polypropylene (UHMWPP) have been explored by means of wide angle X‐ray (WAXD) pole figures. The melt crystallized UHMWPP slabs were rolled unidirectionally in a two‐roll mill at 408 K as well as in the cross (transverse) direction. In the unidirectional rolling, all reciprocal lattice vectors of (110), (040), and (130) were found to orient preferentially in the film normal (thickness) direction. The cross‐rolling of UHMWPP shows the preferential orientation of the above plane normals in the film thickness direction, but the distribution of poles broadens toward the transverse direction. Such orientation behavior is very different from that of conventional cold‐rolled polypropylene. Various orientation mechanisms involving the orientation of lamellae in the film plane, twinning of the (110) plane and slippage mechanisms have been taken into consideration to account for the observed pole figures. A theoretical simulation has been carried out based on an orientation distribution function approach by introducing a slippage angle along the maximum shear stress in conjunction with intralamellar slip around the transverse direction. This model simulation conforms closely with the experimental WAXD pole figures of the (110), (040), and (13

ISSN:0032-3888    

DOI:10.1002/pen.760351105

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractA computer simulation, based on a thermoviscous model of cooling and gravity induced flow (sag) in plastic pipe manufacture, which has previously been described in detail and validated against experimental results, is here used to investigate the effectiveness of die mandrel offsetting and pipe rotation in combatting sag. SDR11 MDPE pipes of 315, 560, 800, and 1000 mm diameter are considered. Results are examined in terms of a waste percentage, representing polymer used in excess of that corresponding to the minimum wall thickness. With a concentric mandrel, waste is predicted to be 4.3, 14.0, 17.9, and 18.7% for the four sizes, respectively. Optimum offsets and minimum waste percentages are found to be: 2 mm, 0.8%; 15 mm, 4.6%; 42 m, 9.8%; 63 mm, 12.7%, respectively. For the largest pipe this corresponds to a wall thickness variation of 19.6 mm. Simulations of rotation of the 800 mm pipe show waste falling rapidly to 5% at 1 revolution/4800s, then a slower decrease, reaching 1.6% at 1 revolution/600s. The simulation is equally applicable to all grades and sizes of pipe, and provides a valuable computer aid for the design and operation of pipe cooling lines.

ISSN:0032-3888    

DOI:10.1002/pen.760351106

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractThe mixing performance of an experimental device—the enhanced mixing simulator (EMS)—is studied in this paper. In this device, the Couette flow is perturbed laterally, so that uniform and quick mixing can be achieved. Numerical simulation of the mixing in EMS flows is performed, which shows good agreement with the experimental results. Spline interpolation and advanced integration techniques are used in particle tracking, so that accurate results are obtained at low costs. Poincaré sections are used to study the long‐term mixing behavior. The uniformity of the simulated mixture is evaluated using statistical approaches—the variation coefficient and the Kolmogorov test. The Kolmogorov test is adequately implemented for our purpose and incorporated with efficient sorting algorithms. The mixing rate is determined from the time evolution of uniformity during mixing. It is also shown that although the Lyapunov exponent can identify chaos, it does not measure the mixing rate. Optimal design of the EMS is d

ISSN:0032-3888    

DOI:10.1002/pen.760351107

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractHigh‐speed melt spinning of poly(ethylene terephthalate) (PET), poly(1,4‐cyclohexylene dimethylene terephthalate) (PCT) and ET/CT copolysters were performed and the structure and properties of as‐spun fibers were investigated. Molecular orientation increased with an increase in the take‐up velocity, and orientation‐induced crystallization of PET and PCT started at ∼4.5 and 3.5 km/min. With the addition of CT component to PET, and ET to PCT, the onset of crystallization shifted to higher velocities. In as‐spun fibers of the copolymer of ET/CT = 66/34 mol%, which is known as an amorphous copolyester, the crystallization was not observed even at the attained highest take‐up velocity, 6 km/min. On the other hand, crystallization of these fibers occured on annealing suggesting that although the incorporation of copolymer component suppresses the crystallization, the molecular orientation enhances the crystallization of copolymer systems as in the cases

ISSN:0032-3888    

DOI:10.1002/pen.760351108

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

摘要:

AbstractIt is well known that the injection molding process has a major impact on the properties of injection‐molded polymers. In this study, the effect of the holding pressure on the orientation in injection‐molded discs of polypropylene has been examined. The orientation has been determined by IR‐dichroism and the orientation factors calculated according to Herman's orientation function, the average factor as well as the crystalline and the amorphous. The distribution of the orientation in the length as well as in the depth direction has also been exmined in order to understand how the flow has propagated in the cavity during both the filling and the packing phases. The results show that the holding pressure clearly affected the orientation, but that it was an unevenly distributed effect over the

ISSN:0032-3888    

DOI:10.1002/pen.760351109

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY

ISSN:0032-3888    

DOI:10.1002/pen.760351101

出版商:Society of Plastics Engineers    

年代:1995

数据来源: WILEY