Y. C. Lam

Analysis of thermal residual stress in plastic injection molding

Abstract The characteristics of plastic injection molding were considered using the theory of thermo-viscoelasticity by considering the effect of packing bulk strain during the cooling stage of the injection molding process. The mathematical model governing the development of residual stress in the amorphous polymer has been established. The residual stress model is solved with the finite difference method. Due to the rapid changes at the initial stage, a quasi-numerical procedure is adopted for evaluating the integral terms arising from the thermal residual stress model during the cooling stage. The results are discussed with an illustrative example.

Gas-assisted injection molding: the effects of process variables and gas channel geometry

Abstract Polystyrene parts with different rib geometries but having the same aspect ratio were molded using gas-assisted injection molding (GAIM). The process variables that have an influence on the gas bubble distribution, residual wall thickness (RWT), fingering formation and mechanical properties were explored. The test results revealed that there is an inherent relationship between gas fingering and gas bubble penetration that has consequences on part strength. Shot size and delay time are the most dominant factors affecting the gas penetration, fingering formation, RWT and mechanical properties of the GAIM parts. The effects of rib geometries are also discussed. Computer simulation of the GAIM process was carried out using Moldflow, a commercial software. The outcomes predicted by simulation are compared with the experimental results. Based on the results, some useful design guidelines are suggested.

A semi-empirical approach for modeling charged soft microgel particles

We describe a unified semi-empirical approach for predicting the viscosity of dilute and concentrated hard and soft sphere systems. A variable specific volume, k was introduced to convert the mass concentration to effective volume fraction. With increasing particle concentration, the concentration of free counter-ions in the solution can be large enough to induce an osmotic de-swelling of soft particles, resulting in the particle shrinkage. The viscosity data for four different microgel systems at different neutralization degree showed excellent agreement with the modified Krieger–Dougherty model.

Interfacial slip between polymer melts studied by confocal microscopy and rheological measurements

Interfacial slip between high density polyethylene (HDPE) and polystyrene (PS) melts was studied. The HDPE/PS layered structure was subjected to steady shear in a Cambridge shearing system. Through thickness melt velocities of the HDPE/PS layered structure were measured in situ by a confocal microscope. Velocity profiles of both HDPE and PS layers were obtained. Velocity discontinuity was observed at the HDPE/PS interface from the velocity profiles and indicated interfacial slip. The slip velocity obtained from velocity discontinuity is in good agreement with that deduced by independent rheological measurements.

Effect of shear heating during injection molding on the morphology of PC/LCP blends

Fiber relaxation of liquid crystalline polymer (LCP) in the mold during injection molding was investigated. A blend of LCP and polycarbonate was used. The LCP used, namely LC5000, is a thermotropic LCP consisting of 80% and 20% of hydroxybenzoic acid and ethylene terephthalate, respectively. The filling of the mold and the temperature profile of the melt in the mold, after the mold has been completely filled, were computed using the finite element/finite difference method (FE/FDM). The morphology of the fibers was greatly influenced by the temperature of the different layers in the sample. This was confirmed by scanning electron microscopy (SEM) examination of the injection-molded specimen. When shear heating caused the temperature of the melt to increase above 280 °C, relaxation of the fibers was rapid. This resulted in a final morphology where the LCP existed in short fibers or ellipsoids. It was concluded that the high shear rate, which is needed for fiber deformation, must be accompanied by fast cooling to minimize the effects of shear heating, so that the fibers formed could be retained.

Simulation of Particle Migration of Powder-Resin System in Injection Molding

Powder injection molding is an important processing method for producing precision metallic or ceramic parts. Experience, intuition and trial-and-error have been the practice for the design and process optimization of such molding operations. However, this practice is becoming increasingly inefficient and impractical for the molding of larger, more complicated and more costly parts. In this investigation, a numerical method for simulating the mold-filling phase of powder injection molding was developed. The flow was modelled using the Hele-Shaw approach coupled with particle diffusion transport equation for the calculation of powder concentration distribution. The viscosity of the feedstock was evaluated using a power-law type rheological model to account for the viscosity dependency on shear rate and powder concentration. A numerical example is presented and discussed to demonstrate the capabilities and limitations of the simulation algorithm, which has the potential as an analytical tool for the mold designer. The variation of powder density distribution can be predicted, which is ignored by the existing simulation packages. Preliminary simulation indicated that powder concentration variation could be significant. Non-isothermal analysis indicated that most of the key parameters for filling process would change due to a change in powder concentration distribution.

Inverse computational method for constitutive parameters obtained from torsion, plane-strain and axisymmetric compression tests

In this work the authors seek to clarify two different requirements for torsion and compression tests in order to solve the problem of identifying constitutive parameters using an inverse computational method. Considerations are restricted in the present research to the proper use of available experimental data in conjunction with an existing constitutive model. For ease of description, a simple power-law constitutive equation has been chosen that can illustrate the different requirements in an elegant manner. First, it will be shown that the determination of a unique set of constitutive parameters during the experimental–computational inverse solution of the torsion test requires either sets of load–deformation and load–deformation-rate data, or a number of load–deformation measurements each with a different constant twist rate. These data are used to define the required cost function. It will also be shown that for the torsion test, the strain hardening index can be found accurately using a single set of load–deformation data. A new cost function based on load–deformation-rate data will be introduced for the torsion test for determining accurately the strain rate index. The load–deformation-based cost function will be used subsequently in conjunction with the new cost function for proper determination of all parameters during the inverse solution of the torsion test. It will be shown further that for the case of plane strain and axisymmetric compression tests, one set of load–deformation data alone suffices to identify the constitutive parameters uniquely.

Influence of the molecular weight of ethylene vinyl acetate copolymers on the flow and mechanical properties of uncompatibilized polystyrene/ethylene–vinyl acetate copolymer blends

Rubber-toughened polystyrene (PS) has been extensively studied and is a well-established material. However, the use of thermoplastic elastomers to toughen PS is new and not well understood. In this study, three types of ethylene vinyl acetate (EVA) copolymers with the same vinyl acetate (VA) content (27.2–28.8 wt %) but with different melt flow indexes (MFI; g (10 min)−1) of 365–440 (Elvax 210), 38.0–48.0 (Elvax 240) and 2.6–3.4 (Elvax 265) were used as impact modifiers for PS. The uncompatibilized blend systems at different compositions were prepared using a twin-screw extruder and injection moulding to produce the required test pieces. The viscosity of the dispersed phase (EVA) has a significant effect on the mechanical properties of the blends. Rheological studies show that uncompatibilized PS/EVA265 blends exhibit some degree of compatibility when the amount of EVA265 added is below 30 wt %. These results indicate that EVA265 with the lowest melt flow index or highest molecular weight is the most effective impact modifier for PS. The mechanism for such behaviour is still unclear. © 2001 Society of Chemical Industry

Three-Dimensional FE–NCV Modeling of Thermoplastic Composites Pultrusion

A three-dimensional (3D) model for the simulation of the thermoplastic pultrusion process is developed. The simulation procedure and algorithms are based on the Finite Element–Nodal Control Volume (FE–NCV) approach. For illustration purposes, APC-2 CF–PEEK is used as a candidate material along with multiheater steel die assemblies. Heat transfer in the die and the composite is modeled using FEs whereas NCVs are used to model crystallization kinetics and heat of reaction for PEEK. The developed simulation procedure is validated using available data. Subsequently, versatility of the model for simulating various types of die assembly and transient heat transfer effects on crystallization is demonstrated.

DEMOLDING OF HIGH ASPECT RATIO POLYMERIC MICRO-PATTERNING

UV embossing for polymeric micro-patterning thin film is an emerging replication technique. This paper investigates UV curable multifunctional acrylates pre-polymer resin patterned by a micro-structured mold and subsequently cured by UV irradiation. To further enhance this duplication method for high aspect ratio production, demolding must be reliable and repeatable without damage to the embossing or mold. Previously, it has been reported that UV embossed patterns for aspect ratios as high as 14 have been achieved experimentally. Finite element analyses for patterns with aspect ratios of 5 using parallel demolding between two parallel plates have also been reported. However, the parallel demolding method may not be suitable for large area patterns as forces generated were high. As such, an alternative demolding method, namely peel demolding, for micro-patterns with an aspect ratio of 14 was investigated and key parameters identified. The parameters governing the demolding process were the peel angle, the pre-crack condition, shrinkage, interface fracture toughness, tensile strength and modulus of polymer. A pre-crack between the polymer and mold was introduced before peel demolding. Numerical analyses in terms of Cohesive Zone Modeling (CZM) were used to simulate the demolding process. Shrinkage caused by UV exposure was represented by thermal strain effects and the fully cured polymer was peeled off using displacement control. The ultimate tensile strength (U.T.S) of the cured polymer was used as a failure criterion. The stresses involved were crucial for determining clean demolding. As peeling progressed, stresses experienced in the polymer matrix increased rapidly in the region ahead of the crack with little or no stress at the cracked region. When stresses experienced by the polymer were below the U.T.S, demolding was deemed to be successful.