The Microcellular Injection Molding of Low-Density Polyethylene (LDPE) Composites

This paper reports the study of microcellular injection molding of low-density polyethylene- (LDPE) based composites. The effects of adding nanoclays and polymer additives in LDPE as well as rheological property of materials on the cell morphology, mechanical properties and surface properties of microcellular injection molded LDPE based composites are presented. For the microcellular injection molding process, when 3 wt% of nanoclays are added into LDPE-based polymers, the cell morphology can be significantly improved due to the nucleating effects resulting from the broad interface areas between polymer and nanoclays. Also, the addition of low melt flow LDPE into high melt flow LDPE could achieve smaller and denser bubbles in the polymer matrix than neat high melt flow LDPE.     

Injection molding of polypropylene discs. I: Effect of holding pressure on orientation distribution

It 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 disc.     

Dynamics of semi-flexible and breakable fibers under Poiseuille flow

In the processing of fiber-reinforced polymer composites, especially in injection molding, the fiber's orientation, length, and distribution vary depending on the location of the channel flow and its properties, which affects the performance of final products. To investigate the intricate behavior of fiber suspensions under Poiseuille flow, we used the hybrid simulation approach, multiparticle collision dynamics–molecular dynamics (MPC-MD), which takes hydrodynamic interactions and fiber properties (strength, flexibility) into account. For non-breakable and rodlike fibers, fibers align well along the flow direction while showing more alignment near the wall. As fiber becomes breakable and/or flexible, the length and orientation of fibers strongly depend on their properties. The interesting phenomenon is specifically seen for breakable and semiflexible fibers, where the orientation of the fiber exhibits non-monotonic behavior depending on the flow rate. This complex behavior highlights the importance of comprehending the dynamics of many types of fibers and necessitates research into the best conditions for injection molding.     

Investigation of melt manipulation phenomena during injection molding via in situ birefringence observation

This study investigates the effects of melt manipulation on the development of molecular orientation during injection molding processing. Vibration‐assisted injection molding (VAIM), a particular method of melt manipulation, is a variation of conventional injection molding in which oscillatory energy is imparted to the polymer melt by vibrating the injection screw axially during the injection and packing stages of the molding cycle. Previous studies have shown that this process positively affects the tensile strength of polystyrene parts, but that the magnitude of the increase is dependent upon the processing parameters. Observation of birefringence patterns in VAIM processed samples show a significant impact on molecular orientation. A specially designed mold and associated image capture system has been developed and is used in this study to record the birefringence patterns of the polymer melt within the cavity during processing. Observation of birefringence shows that orientation develops primarily during post‐vibration packing of the part and not during the vibration phase as previously thought. The observed effects of process parameters such as melt temperature, packing pressure, and vibration duration are discussed. POLYM. ENG. SCI. 46:1691–1697, 2006. © 2006 Society of Plastics Engineers     

Mathematical modeling and computer simulations of the flow,nematic phase orientation,and heat transfer in thermotropic liquid crystalline polymers

This study includes a mathematical analysis and computer simulations of the injection molding of thermotropic liquid crystalline polymers (LCPs). The particular area of interest is the modeling and numerical evaluation of the orientation of the rigid rods in the nematic phase of LCPs during the filling stage of injection molding. The model is based on a continuum mechanics approach in order to derive the orientation distribution function of rigid rods for different orientation angles. Numerical distributions of average orientation factors are also derived. Computer simulations have been conducted in case studies that investigate the effects of changes in the processing conditions.     

短纤维—热塑性聚氨酯弹性体注射充模过程的研究进展

介绍了短纤维－热塑性聚氨酯弹性体（ＳＦ－ＴＰＵ）注射充模过程国内外的研究进展,ＳＦ－ＴＰＵ注射充模过程与通常高聚物有许多相似之处,更有其不同特点。国内外的研究者们对其注射充填过程中的传热、流动、纤维取向与加工条件、模腔几何参数之间的关系进行了研究,但纤维与流体的相互作用、前锋流对纤维取向的影响等,还须进一步研究。     

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.     

Combined effect of shear and nucleating agent on the multilayered structure of injection‐molded bar of isotactic polypropylene

Molten polymers are usually exposed to varying levels of shear flow and temperature gradient in most processing operations. Many studies have revealed that the crystallization and morphology are significantly affected under shear. A so‐called “skin‐core” structure is usually formed in injection‐molded semicrystalline polymers such as isotactic polypropylene (iPP) or polyethylene (PE). In addition, the presence of nucleating agent has great effect on the multilayered structure formed during injection molding. To further understand the morphological development in injection‐molded products with nucleating agent, iPP with and without dibenzylidene sorbitol (DBS) were molded via both dynamic packing injection molding (DPIM) and conventional injection molding. The structure of these injection‐molded bars was investigated layer by layer via SEM, DSC, and 2 days‐WAXD. The results indicated that the addition of DBS had similar effect on the crystal size and its distribution as shear, although the later decreased the crystal size more obviously. The combination of shear and DBS lead to the formation of smaller spherulites with more uniform size distribution in the injection‐molded bars of iPP. A high value of c‐axis orientation degree in the whole range from the skin to the area near the core center was obtained in the samples molded via DPIM with or without DBS, while in samples obtained via conventional injection molding, the orientation degree decreased gradually from the skin to the core and the decreasing trend became more obvious as the concentration of DBS increased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Molecular orientation distributions during injection molding of liquid crystalline polymers: Ex situ investigation of partially filled moldings

The development of molecular orientation in thermotropic liquid crystalline polymers (TLCPs) during injection molding has been investigated using two‐dimensional wide‐angle X‐ray scattering coordinated with numerical computations employing the Larson–Doi polydomain model. Orientation distributions were measured in “short shot” moldings to characterize structural evolution prior to completion of mold filling, in both thin and thick rectangular plaques. Distinct orientation patterns are observed near the filling front. In particular, strong extension at the melt front results in nearly transverse molecular alignment. Far away from the flow front shear competes with extension to produce complex spatial distributions of orientation. The relative influence of shear is stronger in the thin plaque, producing orientation along the filling direction. Exploiting an analogy between the Larson–Doi model and a fiber orientation model, we test the ability of process simulation tools to predict TLCP orientation distributions during molding. Substantial discrepancies between model predictions and experimental measurements are found near the flow front in partially filled short shots, attributed to the limits of the Hele–Shaw approximation used in the computations. Much of the flow front effect is however “washed out” by subsequent shear flow as mold filling progresses, leading to improved agreement between experiment and corresponding numerical predictions. POLYM. ENG. SCI.,, 2011. © 2011 Society of Plastics Engineers     

Rheology studies of foam flow during injection mold filling

This work studies the flow behavior of a developing two‐phase gas‐polymer suspension during injection into the instrumented mold cavity of an injection molding machine. In the experiments, blowing agent type and concentration were varied along with processing conditions, to generate controlled cell structures in two different polymers, low density polyethylene and thermoplastic polyolefin. Experimental results indicate that the rheological properties of two phase gas‐polymer suspensions were sensitive to shear rate, blowing agent concentration, melt temperature, and mold temperature. The viscosity of all gas‐polymer suspensions revealed a reduction compared with neat polymer melt in the presence of gas bubbles, because of the reduced volume fraction of polymer matrix. A two‐phase rheological model has been used for fitting with our experimental results for estimating the shear viscosity of two‐phase flow in the mold cavity of the injection molding machine. POLYM. ENG. SCI., 47:522–529, 2007. © 2007 Society of Plastics Engineers.     

Effect of viscous dissipation on the temperature of the polymer during injection molding filling

During injection molding, viscous dissipation changes the temperature distribution by playing the role of an energy source, which affects heat transfer rates. Understanding the effect of the viscous dissipation assists the designing of the cooling system in injection molding process. In this article, the effect of the viscous dissipation on the temperature distribution throughout a rectangular channel for different polymers at different inlet velocities and temperatures is studied. A cross type rheological model depending on the temperature and pressure is assumed for polymer materials polystyrene (PS) and polypropylene (PP). The evolution of the flow velocity inside the channel is presented. The quantity of heat added due to viscous dissipation to the polymer is also calculated up to different positions through the channel. A numerical finite volume code for the simulation of polymer melt flow in a channel is used and a validation of this numerical code is presented. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Orientation in injection molded polystyrene

The ultimate properties of injection-molded thermoplastics articles are controlled to a large extent by flow and heat transfer phenomena that take place during the injection-molding process. In fact, the thermo-mechanical history of the melt during the molding process leads to a non-uniform distribution of many of the critical properties of the molding. Birefringence has been employed as an indirect measure of the distribution of frozen stresses or strains in amorphous polymers. The present study employs birefringence to study the development of frozen stresses in injection-molded polystyrene. In general, orientation in the flow direction is much greater than the orientation in the transverse direction of the moldings. In the vicinity, of the gate, where mold filling is characterized by spreading radial flow of the melt, the hoop stresses (planar deformation) at the melt front give rise to high orientation in the transverse direction. It appears that relaxation phenomena are not very important during the filling stage; however, they become more, important in the packing and pressure holding stages. With the aid of the appropriate rheo-optical relationship, it is shown that the distribution of frozen-in orientation in injection-molded polystyrene may be estimated on the basis of data relating to pressure variations during the filling stage.     

Structural gradients developed in injection-molded syndiotactic polystyrene (sPS)

In this experimental research, we investigated the influence of processing history on the development of structural gradients in injection-molded syndiotactic polystyrene (sPS). The structures formed during injection molding of this slow-to-crystallize polymer are explained by the complex interplay between the mechanical history of the sample and the cooling conditions dictated by such variables as mold temperature, cavity geometry, and injection speed. When crystallized under shear, sPS exhibits unusually high preferential chain orientation along the flow direction; this orientation gradually decreases toward the core. The mechanical properties are related to the degree of orientation and crystallinity. © 1996 John Wiley & Sons, Inc.     

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     

Mechanical and morphological anisotropy in injection molding of thermotropic liquid crystalline copolyesters

Relationships among mechanical properties, degree of molecular orientation and molding conditions are investigated in injection molded plaques fabricated from a 4,4′-dihydroxy-α-methylstilbene (DHαMS)-based thermotropic liquid crystalline copolyester. Wide-angle X-ray scattering (WAXS) patterns reveal bimodal orientation states at most locations in the plaques. One population aligns roughly along the anticipated flow direction while a separate population is generated as a result of transverse stretching associated with diverging streamlines during mold filling. Micro-tensile bars are cut from the plaques both parallel and perpendicular to the filling direction to assess anisotropy in properties. Enhanced molecular orientation and properties ‘in-shear’ are observed for thinner plaques fabricated at relatively low mold temperatures and melt temperatures slightly above the nominal melting point of the polymer. Injection fill speed is not found to have a significant effect on anisotropy in tensile strength/stiffness. Mechanical properties such as tensile modulus and fracture stress are found to obey a ‘universal’ correlation with X-ray measurements of molecular orientation projected onto the axis of the testing specimens. These results suggest that even in the presence of complex, spatially heterogeneous orientation states, simple average measures of orientation can provide a robust means of anticipating macroscopic properties.     

Flow visualization for injection molding of polyethylene and polystyrene melts

Flow patterns have been observed during the injection molding of rheologically characterized low-density polyethylene and polystyrene melts under various molding conditions. Some studies of high-density polyethylene were also carried out. Various mold designs were included in the study and the flow patterns investigated under both isothermal and cold cavity wall conditions. In addition to investigating injection molding of single polymer melts, flow patterns in the sandwich molding of polyethylene and polystyrene were studied.     

Transient shear flow properties of carbon fiber–filled liquid crystalline polymer

Unsteady flow of filled polymer systems can cause some difficulties in their processing, such as injection or extrusion molding, in which flow in the molten state is involved. In order to determine the optimum conditions and thus improve the performance of the end products, it is necessary to understand the transient/unsteady flow behavior of these systems. In a previous work, we found remarkable non‐Newtonian properties and shear thickening flow behavior of liquid crystalline polymers (LCP) and their filled systems. This article deals with the dependence of transient flow properties, including shear thickening, of pure and carbon fiber‐filled LCP upon the shear rate and shear strain history; the effect of the filler content is also discussed. The results indicate that the abnormal flow behavior of the tested materials may be caused by gradual disintegration of the domain structure formed in LCP (materials exhibit apparent yield stress) and then continual orientation of molecules under shear flow. The shear thickening behavior of the materials seems to disappear with increasing fiber content. This behavior suggests that it is necessary to measure the flow properties under a sufficient ramp point delay time and equilibration time in order to obtain reliable data under stable conditions. POLYM. COMPOS., 26:470–476, 2005. © 2005 Society of Plastics Engineers.     

Measurement of stress birefringence patterns in molten polymers flowing through geometrically complex channels

Measurements were taken of stress birefringence patterns in molten polymers flowing through geometrically complex channels. Six different flow channels were constructed for experiment, some representing the flow geometries of spinnerettes encountered in fiber spinning, and others representing mold cavities encountered in injection molding. All the flow channels had two glass windows, which permitted one to take photographs of the flow birefringence patterns of molten polymers with the aid of a polariscope. Quantitative information on the stress distributions in a flow channel was obtained, with the aid of the stress-optical laws, from the pictures taken of both isochromatic and isoclinic fringe patterns. The significance of flow birefringence measurement is discussed from the standpoint of die design for extrusion operation and mold design for injection molding operation.     

The specification of orientation and its development in polymer processing

A critical review of the specification of orientation and its development in polymer-processing operations is presented. Orientation may in general be specified by orientation distribution functions, but is most conveniently expressed in terms of orientation factors which are second moments of the distribution. The Hermans orientation factor represents polymer-chain orientation for systems with fiber symmetry (uniaxial orientation) and the Hermans-Stein orientation factors express uniaxial orientation for each of the crystallographic axes of crystalline polymers. Biaxial orientation is, however, developed in tubular film extrusion, blowmolding and, indeed, all processing operations other than fiber formation. Orientation factors developed previously by the authors express biaxial orientation in terms of the angles between the machine and transverse directions and the polymer chain axis or crystallographic axes. In flowing polymer melts, the Rheo-Optical Law, which relates birefringence and stress, represents a relationship between polymer-chain orientation and stress. In vitrified polymeric glasses (e.g. polystyrene), the orientation factors are related linearly to the stress field at vitrification. This has been shown experimentally for melt spinning and tubular film extrusion. The results of studies of blowmolding and injection molding are consistent with this. The crystalline orientation factors have also been found to be determined by the stress field at solidification in melt spinning and tubular film extrusion.