Guiwei Dong

Formation mechanism and structural characteristics of unfoamed skin layer in microcellular injection-molded parts

The microcellular injection-molded part usually consists of a foamed core region and two unfoamed skin layers on the cross section. This paper investigated the formation process, formation mechanism and structural characteristics of the unfoamed skin layers in microcellular injection-molded part. It is found that the unfoamed skin layers are formed in two processes namely “during filling” process and “after filling” process. The shear flow and the fountain flow behaviors of the melt in the filling stage are the main controlling factors on the formation of the unfoamed skin layer in “during filling” process, and the cooling solidification of the melt in cooling stage is the fundamental reason for the formation of the unfoamed skin layer in “after filling” process. Further studies found that the unfoamed skin layer in microcellular injection-molded part has two distinct regions, the outer region is a thin frozen layer which contains deformed and broken cells, and the inner region is a relatively thick solid-like layer which has no visible cells in. The unfoamed skin layer has a minimum thickness in the gate location. The whole thickness of the unfoamed skin layer is decreased with the increase of injection speed and mold temperature, but is slightly affected by melt temperature.

Bubble morphological evolution and surface defect formation mechanism in the microcellular foam injection molding process

The filling stage of the Microcellular Foam Injection Molding (MFIM) process is a three phase flow process of polymer melt, super critical fluid (SCF) and air. It not only includes the nucleation and growth of spherical bubbles, but also the morphological evolution of the bubbles such as deformation, bursting, and vanishing. There are usually silver marks, spiral lines, pits and other defects on the product surface. In order to effectively control the surface quality, it is significant to reveal the morphological evolution law of bubbles and the formation mechanism of surface defects in the filling stage of MFIM. This paper established an incompressible, non-isothermal, and unsteady three-dimensional mathematical model of multiphase flow. A new setting method of the boundary conditions with the exhaust function on the mold cavity walls was proposed. The problem of temperature solution divergence on the interface between the two phases with a high viscosity ratio was solved through the coupling algorithm of energy equation and PIMPLE loop. The tracking accuracy of micron grade bubbles interface in macroscopic scale flow field was improved though the Adaptive Meshing Refining (AMR) technique. Based on the abovementioned mathematical model, the influence law of the temperature field and velocity field on the bubble morphological evolution in the thickness cross-section of the injection flow field was obtained. The deformation, bursting and vanishing process of bubbles with different initial sizes and locations in the shear and fountain flow field was predicted. Combined with a short shot experiment, the formation mechanism of pits, silver marks and collapses on the product surface manufactured by MFIM was also revealed.

Numerical investigation of the crystallization and orientation behavior in polymer processing with a two-phase model

Abstract The crystallization and orientation behavior of a polymeric material can significantly influence the performance of products in practical processing. In this study, the variations in morphology that occur during solidification in polymer processing are mathematically modeled using a two-phase model. The amorphous phase is approximated as a finite extensible nonlinear elastic dumbbell with a Peterlin closure approximation (FENE-P) fluid, and the semi-crystalline phase is modeled as rigid rods oriented within the flow field. The crystallization and orientation behavior are numerically investigated using the penalty finite element–finite difference method with a decoupled algorithm. The evolution of the crystallization process is described by Schneiders equation, which differentiates between the effects of thermal and flow states. The hybrid closure approximation is adopted for the calculation of the three-dimensional orientation tensor. The discrete elastic viscous split stress (DEVSS) algorithm, which incorporates the streamline upwind scheme, is introduced to improve calculation stability. The variations in morphology during polymer processing are successfully predicted using the proposed mathematical model and numerical method. The influence of processing conditions on the crystallization and orientation behavior is further discussed.

Lightweight, thermally insulating, and low dielectric microcellular high-impact polystyrene (HIPS) foams fabricated by high-pressure foam injection molding with mold opening

In this study, we used a high-pressure foam injection molding process to fabricate microcellular high-impact polystyrene (HIPS) foams with a tailored cellular structure. The process is cost-effective, highly efficient and flexible, and can be easily scaled up to complex components. The cellular structure of HIPS foam can be tuned over a wide range by manipulating packing time, cooling time, mold temperature, and mold-opening distance. Microcellular HIPS foam with a weight reduction of up to 60% was prepared, which possesses a low thermal conductivity of 60 mW m−1 K−1 and an ultra-low dielectric constant of 1.25. Both the thermal conductivity and the dielectric constant can be tailored by regulating the expansion ratio of HIPS foam. Mathematical models based on the mixing rule were developed to clarify the dependence of thermal conductivity and dielectric constant on the cellular structure of the foam. The outstanding thermally and electrically insulating properties of HIPS foams come from a large amount of air in the microcellular structure. These lightweight, thermally insulating, and ultralow dielectric microcellular HIPS foams hold great promise as an ultra-efficient insulating material for future use in many applications such as microelectronics and microelectromechanical systems (MEMSs).

Study on reducing sink mark depth of a microcellular injection molded part with many reinforcing ribs

As a novel injection molding process, microcellular injection molding process has the characteristics of saving material, decreasing warpage and surface sink mark, improving dimensional accuracy, etc. But for the plastic part with thick reinforcing ribs, if selection of process parameters are not reasonable, foaming quality of melt will be affected and obvious sink mark defects will appear on the surface of plastic part. This paper selected a medical appliance shell with many reinforcing ribs as research object. Simulation experiments of microcellular injection molding process were performed by using orthogonal experiment method. The influence of different process parameters, such as mold cavity surface temperature, melt temperature, injection rate, cooling time, weight reduction ratio and supercritical fluid level, on the surface sink mark of microcellular injection molding part was studied by using signal-to-noise ratio analysis and analysis of variance . The results showed that mold cavity surface temperature was the most important influence factor on surface sink mark depth of microcellular injection molding part, followed by weight reduction ratio, cooling time, supercritical fluid level, injection rate and melt temperature. Meanwhile, the optimal combination of process parameters was obtained for minimizing sink mark depth of microcellular injection molding part. The average surface sink mark depth of microcellular injection molding part molded by using the optimized process parameters was only 2.62 µm, compared to 4.87 µm of average surface sink mark depth of microcellular injection molding part molded by using the process parameters before optimization, the average sink mark depth of microcellular injection molding part was reduced by 46.2%. Finally, the forming mechanism of sink mark of microcellular injection molding part at locations of reinforcing ribs was discussed, and the influence mechanism of different process parameters on surface sink mark defects of microcellular injection molding part was also analyzed.

Finite element simulation of three-dimensional viscoelastic planar contraction flow with multi-mode FENE-P constitutive model

AbstractViscoelasticity is a characteristic of many complex fluids like polymer melts, petroleum, blood, etc. The investigation of viscoelastic flow mechanism has practical significance in both scientific and engineering field. Owing to strongly nonlinear, numerical method becomes a practical way to solve viscoelastic flow problem. In the study, the mathematical model of three-dimensional flow of viscoelastic fluids is established. The planar contraction flow as a benchmark problem for the numerical investigation of viscoelastic flow is solved by using the penalty finite element method with a decoupled algorithm. The multi-mode finitely extensible nonlinear elastic dumbbell with a Peterlin closure approximation (FENE-P) constitutive model is used to describe the viscoelastic rheological properties. The discrete elastic viscous split stress formulation in cooperating with the inconsistent streamline upwind scheme is employed to improve the computation stability. The numerical methods proposed in the study can be well used to predict complex flow patterns of viscoelastic fluids.

Numerical investigation of viscoelastic flow induced crystallization in polymer processing

The investigation of viscoelastic flow induced crystallization is of great engineering significance in polymer processing like extrusion, injection and blow molding. In the study, the behavior of viscoelastic flow induced crystallization of semi-crystalline polymers is investigated by using finite element-finite difference method. The Schneiders approach is introduced to describe the evolution of crystallization kinetic process. The numerical model of three-dimensional flow induced crystallization of polymer melts obeying Phan-Thien and Tanner constitutive model is established. A penalty method is introduced to solve the nonlinear governing equations with a decoupled algorithm. The effect of flow state on the crystallization behavior is investigated. The crystalline distribution within the flow channel is obtained based on the proposed mathematical model and numerical method.