水辅注射成型聚丙烯制品的取向结晶

应用小角X射线散射研究了水辅注射成型(WAIM)等规聚丙烯制品中串晶(shish-kebab)结构的形成、分布以及片晶取向行为.结果表明:根据取向度不同,WAIM制品沿壁厚方向可明显分为表层、芯层和水道层.表层和水道层均有shish-kebab结构生成,且在表层生成的数量比水道层多,而芯层则没有这种结构.这种shish...     

Some aspects of orientation in injection molded objects

The birefringences of injection molded plates and the birefringence during steady, isothermal shear flow were compared for some amorphous polymers. The materials studied were a polystyrene, a “toughened” polystyrene and an acrylonitrile-butadiene-styrene copolymer. The birefringence of the plates, notably the maximum value for the average over the thickness was found to be related to the shear stress at the cavity wall that had occurred during the mold filling process. This relationship was independent of temperature. To a good approximation, it was also the same as the relationship between the flow birefringence and the shear stress at the wall in isothermal channel flow. It thus appears that the anisotropy of injection molded objects is dominated by the shear stresses during the mold filling process regardless of the temperature and of the macroscopic rate of deformation.     

A unified K-BKZ model for residual stress analysis of injection molded three-dimensional thin shapes

The flow-induced and thermally induced residual stresses during injection molding of a thin part with complex geometries are predicted. The injection molding precess was considered to consist of a filling and a post-filling stage (packing coupled with cooling). Additionally, the analysis were applied to successive stages of the process. The model takes into account the viscoelasticity of the molding polymer, which has been neglected in most previous works, because of the complexity of its inclusion. A unified K-BKZ viscoelastic constitutive model, capable of modeling both the fluid-rubbery state and the glass state of amorphous polymers, was employed for simulating this problem. For the flow-induced residual stress predictions of the filling stage, a quasi-steady state approximation was employed for each element of the part, for the calculation of stress profile and subsequent stress relaxation after cessation of flowf. Stress calculations were provided for the thermally induced residual stress predictions of the post-filling stage. These explicit calculations led to the results of pressure and temperature distributions of the part during the post-filling stage into the viscoelastic constitutive model. Additionally, the pressure and asymmetric temeprature profiles of the post-filling stage were based on finite element packing analysis coupled with a boundary element cooling analysis of the molding process. Finally, the total residual stress in the part was obtained via superposition of the flow-induced and thermally induced residual stresses. An example is provided to demonstrate the entire concept. The results indicate that thermally induced residual stress is higher than the flow-induced residual stress by one to two orders of magnitude.     

Effects of stress concentration and residual stress on vibration fatigue behavior of socket weld

The vibration fatigue behavior in socket weld of 304L stainless steel was investigated using experimental and numerical. The vibration fatigue test results indicate that original socket weld at a higher stress tend to occur toe failures while for the case of lower stress failures tend to originate at the root. In addition, the socket welded with groove can improve the fatigue property of weld root, but it has a bad influence on the weld toe. From the simulation results, the groove can decrease the stress concentration factor of weld root, and improve the fatigue property of root; whereas the groove can also increase the tensile residual stress of toe, resulting in having a detrimental effect on fatigue property of toe.     

The effect of orientation on the mechanical properties of injection molded polystyrene

The deformation and fracture behavior of injection molded plaques have been determined, and the results interpreted in terms of the effect of molecular orientation on the crazing and shear yielding behavior. The molecular orientation was characterized by optical birefringence. A range of injection molding conditions and two mold thicknesses were Used and this resulted in a large variation in the molecular orientation, particularly through the sheet thickness. Tensile tests were made on samples cut at different angles to the injection molding direction. The moldings are considered to consist of a composite of layers of material with different orientation, and the properties of the samples cut from the molding are analyzed in terms of the properties of each layer. Results from material oriented unidirectionally by hot drawing have been used to predict the composite properties, and good agreement has been obtained.     

Effects of flow induced orientation of ferromagnetic particles on relative magnetic permeability of injection molded composites

Composite samples consisting of ferromagnetic asymmetric particles incorporated into a polyolefin binder were injection molded using custom designed molds which produced preferential fiber orientations. The relative magnetic permeability values of the composites were measured as a function of the filler volume fraction, injection rate, gate diameter, temperature, aspect ratio of the fibers, and fiber orientation. Fiber orientation was affected by the molding conditions and controlled the relative magnetic permeability of the composites. The degree of fiber orientation was significantly affected by the size of the opening (gate) to the mold, or by the mold geometry going from an edge-gated cylindrical to a center-gated disk cavity. Relative permeability values of the composites were observed to increase when the fiber orientation and the applied field were parallel to one another. For instance, highly aligned composite samples exhibited up to 30% greater relative permeability values compared to those samples which exhibit fiber orientation distributions approaching a random distribution. To our knowledge this is the first study that provides data linking the fiber orientation distribution functions of ferromagnetic asymmetric particles to the relative magnetic permeability values of injection molded composites.     

Residual stress,aging, and fatigue fracture in injection molded glassy polymers I. Polystyrene

Fatigue tests have been conducted on polystyrene bars molded at different pressures and having different post-molding thermal histories. Fatigue crack propagation rates were not sensitive to molding or thermal history but in unnotched bars initiation seemed to happen more quickly in bars aged for long periods at room temperature or annealed at elevated temperature. Fractographic studies showed that initiation occurred at favored sites and that the skin/core morphology affected crack growth. Prolonged storage of polystyrene bars at ?85°C promoted significant physical property changes (stiffness and density). These changes appeared to be at least partly reversible with rapid recovery occurring within the first 24 h after restoring to room temperature.     

Prediction of fiber orientation in injection molded parts of short-fiber-reinforced thermoplastics

Fiber orientation induced by injection mold filling of short-fiber-reinforced thermoplastics (FRTP) causes anisotropy in material properties and warps molded parts. Predicting fiber orientation is important for part and mold design to produce sound molded parts. A numerical scheme is presented to predict fiber orientation in three-dimensional thin-walled molded parts of FRTP. Folgar and Tucker's orientation equation is used to represent planar orientation behavior of rigid cylindrical fibers in concentrated suspensions. The equation is solved about a distribution function of fiber orientation by using a finite difference method with input of velocity data from a mold filling analysis. The mold filling is assumed to be nonisothermal Hele-Shaw flow of a non-Newtonian fluid and analyzed by using a finite element method. To define a degree of fiber orientation, an orientation parameter is calculated from the distribution function against a typical orientation angle. Computed orientation parameters were compared with measured thermal expansion coefficients for molded square plates of glass-fiber-reinforced polypropylene. A good correlation was found.     

Crystal distribution and molecule orientation of micro injection molded polypropylene microstructured parts

Polypropylene (PP) microstructured part which comprises micro columns array and a macroscopical base plate was manufactured by micro injection molding. The morphology distribution in micro columns is quite different from that of the base plate. This article investigates the crystal distribution and molecule orientation of the microstructured part by X‐ray diffraction. The hardness of shear zone of micro columns was evaluated by Nano Indenter. Test results show that both micro columns and macroscopical base plate contain α and β phase. However, the relative proportion of β phase in micro columns is markedly higher than that of the base plate. β phase distributes only in the shear zone of the microstructured part. So, the mechanical properties of micro columns must differ from that of the base plate. In addition, the orientation of Ø100 μm micro columns is slight, which indicates that the mechanical anisotropy of micro columns induced by orientation could be ignored. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Prediction of residual stress and viscoelastic deformation of film insert molded parts

Film insert molding (FIM) has been modeled numerically to predict residual stress and viscoelastic deformation of the part. Nonisothermal three dimensional flow analysis for filling, packing, and cooling stages was carried out by using a commercial software. It was assumed that the inserted film was solid throughout the entire molding procedure although remelting could occur at the interface with the substrate. The flow analysis results, e.g., temperature, stress, and density distribution in the substrate domain, were transported to a finite element stress analysis program for viscoelastic stress analysis. Deflection of the FIM part was obtained as soon as the part was ejected from the mold by assuming isotropic elastic material. The residual stress distribution in the FIM part was acquired by removing the constraints along the boundary of the molded part. Viscoelastic deformation of the FIM part was predicted by performing viscoelastic stress analysis in order to understand long term behavior of the FIM part when exposed to room temperature. Durability of automotive and electronic parts produced by the film injection molding can be predicted by the procedure adopted in this study. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Effects of melt temperature and hold pressure on the tensile and fatigue properties of an injection molded talc‐filled polypropylene

This article examines the effects of melt temperature and hold pressure on the static tensile and fatigue behavior of an injection‐molded 40 wt% talc‐filled polypropylene. Injection molding caused anisotropy in the material. Both yield strength and fatigue strength were higher in the flow direction. The presence of weld line caused a large reduction in yield strength and fatigue strength. For specimens in the flow direction, both yield strength and fatigue strength increased with increasing hold pressure, but they were relatively insensitive to melt temperature. For specimens normal to the flow direction, both yield strength and fatigue strength increased with increasing hold pressure and decreased with increasing melt temperature. For specimens containing a weld line, the yield strength and fatigue strength increased with increasing hold pressure as well as increasing melt temperature. The observed differences in properties are explained in terms of the skin‐core morphology, which was influenced by both melt temperature and hold pressure. POLYM. ENG. SCI., 45:755–763, 2005. © 2005 Society of Plastics Engineers     

Amplification effect of platelet type nanoparticles on the orientation behavior of injection molded nylon 6 composites

The effect of platelet type nanoparticles and processing conditions; mold temperature and injection speed, on the development of local microstructure in injection molded nylon 6 parts was investigated. The molded parts exhibit two crystal forms (α and γ) of nylon 6 in varying proportions from skin to core. The γ crystals preferentially grow near the surface regions and α crystal fraction increases with distance from the surface in all molded parts. However, the spatial variation of crystal phases across the thickness in nanocomposites differs from that of unfilled nylon 6. Nanoplatelets induce high levels of orientation of the polymer matrix throughout the thickness of the molded part even at high mold temperatures where nonisothermal effects are highly suppressed and confined to very close proximity of surfaces. These high chain orientation levels observed in nanoparticle filled systems is a result of the shear amplification effect that occurs in small spaces between adjacent nanoparticles of differing velocity. The local preferential crystalline orientation of nylon 6 resin and nanoparticles across the thickness of the molded parts are investigated using a series of structure characterization techniques including microbeam wide angle X-ray, SAXS and TEM.     

Effects of the binder compositions on the homogeneity of ceramic injection molded compacts

In this study, the effects of different binder compositions on the homogeneity of injection molded ceramic compacts are discussed from the standpoint of the rheological properties of the feedstocks, density, density deviation, and flexural strength of the as-prepared compacts, and the microstructures of the as-prepared compacts, compacts after solvent debinding, and compacts after thermal debinding. Also the pore size distribution of the compact after thermal debinding was characterized to examine the distribution of the binders in the as-prepare compacts. The feedstock with 30% macromolecular binders attains the minimum power-law index n value at 160 °C, hence it is the most appropriate feedstock for injection molding. The optimal composition is feedstock B30, which is the most homogeneous at an injection temperature of 160 °C. The macromolecular binder-to-PW weight ratio of 3:7 allows best mixing of feedstock and more homogeneous binder distribution. B30 has the highest density, the lowest density deviation and the lowest deviations in flexural strength of the as-prepared compact. Compacts prepared with B30 have the most homogenous pore size distribution after thermal debinding, as determined from the sharp, narrow peaks on the pore size distribution curves.     

Localized effects of dynamic melt manipulation on flow induced orientation and mechanical performance of injection molded products

This study investigates the effects that dynamic melt manipulation based injection molding has on the locally induced molecular orientation and tensile strength of injection molded polystyrene. Melt manipulation refers to a process where the polymer melt is manipulated during molding beyond the extent normally encountered in conventional injection molding. The specific melt manipulation process investigated in this article is vibration assisted injection molding, where a conventional injection molding machine is augmented by oscillating the injection screw (in the axial direction) during the injection and packing phases of the molding cycle. The localized final molecular orientation and morphology that results dictates the resultant product response, and typically improved mechanical properties are observed. Specimens with molecular orientation distributed more uniformly along the gage length typically exhibited higher tensile strength than samples with a gradient of orientation along the gage length. Smaller test specimens machined along the gage length of larger molded specimens showed dramatic tensile strength increase in the regions of higher melt manipulation, further supporting the promise of this novel processing methodology. POLYM. ENG. SCI., 47:1912–1919, 2007. © 2007 Society of Plastics Engineers     

Flow‐induced fiber orientation in injection molded fit fiber reinforced polypropylene

A through experimental study of flax fiber orientation in a plate processed by injection molding is presented. The state of orientation is described by oriental tensors and partly by frequency distribution diagrams. Composite stiffness is predieted by use of a modified classical laminate theory including unidirectional mels and orientation averaging. Comparison of the measured and calculated modulus in tension shows good agreement.     

On the effect of the fiber orientation on the flexural stiffness of injection molded short fiber reinforced polycarbonate plates

The through-thickness fiber orientation distribution of injection molded polycarbonate plates was experimentally determined by light reflection microscopy and manual digitization of polished cross sections. Fiber length distribution was determined by pyrolysis tests followed by image analysis. A statistical analysis was done to determine the confidence limits of the fiber orientation results. The fiber orientation distribution was described by using second-order orientation tensors. The through-thickness stiffness variations were determined by the orientation averaging approach. This layer stiffness distribution was used to simulate the behavior of beams subjected to three point bending with a FEM Ansys model. The results were compared with experimentally determined flexural stiffness both in the flow direction and in the transverse flow direction. The effect of flow rate and melt-temperature on stiffness and fiber orientation is discussed.     

Spatial distribution of molecular orientation in injection molded iPP: influence of processing conditions

In this paper, the influence of processing conditions on the spatial distribution of the molecular orientation was determined within the depth of the thickness of injection molded isotactic polypropylene (iPP) plates. Small 35 μm-thick slices were microtomed from the surface to the core of 1 and 3 mm-thick plates. The orientation functions along the three crystallographic axes were determined on the slices from IR dichroism measurements and WAXS pole figures. It was found that the orientation of the amorphous phase was low and the crystalline orientation had a maximum in the shearing layer, which was solidified during the filling stage. The plate thickness seemed to govern the global level of orientation, while the injection speed determined the thickness of the shearing layer without changing the maximum of orientation. Changing the mold temperature from 20 to 40 °C did not modify the molecular orientation. A specific bimodal crystalline orientation was found in the shearing layer. This crystalline structure continued in the post-filling layer, but the local symmetry axes tilted towards the core.     

Electron spin resonance investigation of filler orientation in compression and injection molded polypropylene based composites

Summary Electron Spin Resonance (ESR) spectroscopy was applied for monitoring the orientation and distribution of filler particles in polymer composites by measuring the magnetic anisotropy of naturally occuring Mn(II) centers in CaCO₃ and in talc. The amplitude ratio of characteristic ESR bands gives the order parameter. The orientation of the particles changes as a function of composition, dependends on processing technology, the type of molding (injection vs compression molding) and has a specific spatial distribution in the cross-section of the injection molded specimen. Correlation is found between average orientation of anisotropic particles and the mechanical properties of various composites.     

Modeling and simulation of thermally induced stress and warpage in injection molded thermoplastics

Thermally induced stress and the relevant warpage caused by inappropriate mold design and processing conditions are problems that confound the overall success of injection molding. A visco-elastic phase transformation model, using a standard linear solid for the solidified polymer and a viscous fluid model for the polymer melt, of 2-D finite element scheme with 8 noded overlay isoparametric elements was used to simulate and predict the residual stress and warpage within injection molded articles as induced during the cooling stage of the injection molding cycle. Computed results are in good agreement with published experimental data. The approach proposed here is to examine and simulate the injection molding solidification process with the intent of understanding and resolving more inclusive and realistic problems.