Short-shot Experiments Research Articles

Numerical and dimensional analysis for the jet buckling of highly viscous fluid

Jet buckling of highly viscous fluid is a common phenomenon both in nature and industry. In order to effectively understand this phenomenon, experiments, numerical simulation and dimensional analysis are employed with the jet buckling of rubber melt and in injection molding. To begin with the construction of two high-resolution discretization schemes for convection terms, a three dimensional algorithm for liquid jet buckling phenomenon is established based on two phase flow model and discretization via finite volume method. The developed algorithm shows its good accuracy and stability for jet buckling simulation. The numerical results are in good agreement with experimental results obtained by camera directly and short shot experiments. It is found shear rate and gravity have an effect on jet buckling. Compared to non-buckling, buckling of liquid occurs at high shear rate. Direction of gravity influences the length of jet and the pattern of buckling. Furthermore, dimensional analysis of buckling is carried out based on experimental and numerical results. Results reveal that the influence of gravity on buckling is greater when gravity is vertical than parallel to velocity. Dimensional analysis illustrates that shear rate has a greater effect on buckling frequency than hydraulic radius of inlet, length of cavity, gravitational force, viscous force and kinematic viscosity. The buckling frequency is approximately proportional to the square of the shear rate.

Experimental and numerical investigation of jetting phenomenon in injection molding

Jetting phenomenon is an injection error without thoroughly understanding by far. In this study, short shot experiments and numerical simulations were performed to investigate the trigger factor inducing jetting phenomenon in injection molding. Polycarbonate was prepared for short shot experiments that carried out in a typical end-gated rectangular mold under different injection speed, mold temperature and melt temperature. It was found that repeatability of jetting was poor and with higher melt temperature came more frequency of folding. Then, three repeatable evolutions of jetting were successfully simulated. Through analyzing the quantities during the initial stage of jetting and comparing with those in simple filling pattern, it has been numerically revealed that velocity kept constant before melt impinged on the wall opposite to gate. The pressure did not increase with the increase in melt flow length and was extremely small, which turned out jetting was not a pressure-driven flow any more. Shear rate in the initial stage of jetting was less than 10 s−1, while shear rate in front of the pillar can be as low as 10−4 s−1, leading that viscosity approached zero-shear-rate viscosity and melt can be seen as a Newtonian fluid. Due to without wetting mold wall enough, friction between mold wall and melt was weak as well as the internal friction among layers of melt, which resulted in melt moving without deformation. Based on the results, a reasonable mechanism was proposed that jetting occurred for lack of shear rate. Jetting could be effectively depressed by changing the magnitude or direction of velocity, which provides a theoretical guidance for mold design and products.

Filling of high aspect ratio micro features of a microfluidic flow cytometer chip using micro injection moulding

Micro injection moulding has been demonstrated as one of the most efficient mass production technologies for manufacturing polymeric microfluidic devices, which have been widely used in life sciences, environmental and analytical fields and agro-food industries. However, the filling of micro features for typical microfluidic devices is complicated and not yet fully understood, which consequently restricts the chip development. In the present work, a microfluidic flow cytometer chip with essential high aspect ratio micro features was used as a typical model to study their filling process. Short-shot experiments and single factor experiments were performed to examine the filling progress of such features during the injection and packing stages of the micro injection moulding process. The influence of process parameters such as shot size, packing pressure, packing time and mould temperature were systematically monitored, characterised and correlated with 3D measurements and real response of the machine such as screw velocity and screw position. A combined melt flow and creep deformation model was proposed to explain the complex influence of process on replication. An approach of over-shot micro injection moulding was proposed and was shown to be effective at improving the replication quality of high aspect ratio micro features.

Model and simulation for melt flow in micro-injection molding based on the PTT model

Unsteady viscoelastic flows were studied using the finite element method in this work. The Phan-Thien–Tanner (PTT) model was used to represent the rheological behavior of viscoelastic fluids. To effectively describe the microscale effects, the slip boundary condition and surface tension were added to the mathematical model for melt flow in micro-injection molding. The new variational equation of pressure, including the viscoelastic parameters and slip boundary condition, was generalized using integration by parts. A computer code based on the finite element method and finite difference method was developed to solve the melt flow problem. Numerical simulation revealed that the melt viscoelasticity plays an important role in the prediction of melt pressure, temperature at the gate and the succeeding melt front advancement in the cavity. Using the viscoelastic model one can also control the rapid increase in simulated pressure, temperature, and reduce the filling difference among different cavities. The short shot experiments of micro-motor shaft showed that the predicted melt front from the viscoelastic model is in fair agreement with the corresponding experimental results.

Three-Dimensional Modelling to Study the Effect of Die-Stacking Shape on Mould Filling During Encapsulation of Microelectronic Chips

Multistacked-chip scale package (S-CSP) is a new technology that provides high density electronic package. A fully 3-D numerical model is developed to simulate mould filling behavior in the epoxy moulding compound (EMC) encapsulation of multi-S-CSP. Four different shapes of chip arrangement namely uniform, rotated, z-staggered-Type A and z-staggered-Type B, have been tested. The EMC is treated as a generalized Newtonian fluid (GNF). The developed methodology combines the Kawamura and Kuwahara technique-based finite difference method (FDM) and the robustness of volume-tracking (VOF) method to solve the two-phase flow field around the complex arrangement of microchips in a cavity. The Castro-Macosko rheology model with Arrhenius temperature dependence is adopted in the viscosity model. Short-shot experiments are conducted to investigate the filling patterns at several time intervals. The results show that the rotated shape die-arrangement gives minimum filling time and better mould filling yield. The close agreement between the experimental and simulation results illustrates the applicability of the proposed numerical model.

Characterization of the microinjection molding process

AbstractThe commonly used plunger injection system in the microinjection molding (μIM) process has separate screw melting, metering, and injection units. As a result, extra operating parameters and complexity are introduced, in comparison with conventional injection molding. In this study, a μIM machine was used to obtain micromoldings of polyoxymethylene, high‐density polyethylene, and polycarbonate. A data acquisition system was developed to record traces of data regarding the evolution of process variables with time. Cavity filling was followed, at the millisecond time scale, using short‐shot experiments and traces of injection pressure, runner pressure, and plunger position. Six characteristic process parameters (CPPs) were defined to characterize both the cavity filling and packing stages. The method of design of experiments was used to investigate the effects of machine settings on the CPPs. Metering size, which was optimized for each set of machine variables, was also used as a CPP. Injection speed was the most significant factor affecting plunger velocity and injection pressure during cavity filling, while the effects of mold and melt temperature varied with the material and machine settings. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

Improving Flow Balance during Filling a Multi-cavity Mold with Modified Runner Systems

Abstract Multi-cavity injection molding has been regarded as low cost mass-production method for producing plastic products such as optical elements. However, the flow imbalance during filling a multi-cavity mold using symmetrical runner system is a serious problem. This paper is devoted to improving the flow balance by modifying the runner system design. For this purpose, four eight-cavity molds with four modified symmetrical runner systems are designed, constructed and tested. They are tapering H-type runners, pinned H-type runners, curved A runners, and curved B runners. The flow imbalance in each runner system is observed and measured with the aid of short-shot experiments. The experimental results indicate that the tapering H-type runner system can improve flow balance. The pinned H-type runner system fails to improve flow balance. The curved A and B runner system can reverse the imbalance trend, but the stability of the curved systems is poor at high melt temperature.

Shrinkage Analysis on Convex Shellby Injection Molding

Abstract Injection molding of a convex shell has induced divergent flow front directions and therefore the shrinkage of a convex shell is difficult to controll. In some optical applications, such as ophthalmic devices of soft contact lens, that can be cast molded with a set of two convex shells made by injection molding there is a demand in maintaining dimensional stability. This research investigates the optimization of injection molding parameters for the minimum shrinkage of a convex shell of polypropylene (PP). The MPI 5.0 software is used first to simulate the filling stage of the convex shell and find the feasible operational ranges of parameters. Experiments were performed by an electric injection machine. Short shot experiments have been tested to compare with results of mold flow simulation. Taguchi's method is then used to find the optimal parameters for the minimum shrinkage of convex shell. Four parameters or factors, including mold temperature, injection temperature, holding pressure, and cooling time are considered in this study. An ANOVA table has been obtained for checking the significance of parameters. Results of simulation and experiments have been compared and the holding or packing pressure is found as the most significant parameter for the minimum shrinkage of a convex shell. Cooling time and injection temperature have been found as the second and third most significant parameters in this study. The optimal parameters have been established and then verified by the optimal injection molding experiment and the minimum shrinkage of convex shell was obtained as 72 μm.

Study on Flow Imbalance during Filling a Multi-Cavity Mold Using a H-type Runners

This study reports the experimental investigation on the flow imbalance phenomena during filling symmetrical multi-cavity in injection molding of optic elements. There are two aspects in this study: First, the flow imbalance in the eight-cavity mold with normal H-type symmetric runners were observed and measured with aid of short-shot experiments. Second, the effects of three processing conditions including injection speed, melt temperature and mold temperature on flow imbalance were investigated. Flow imbalances are consistently observed and quantitatively measured. The results have shown that proper injection speed, high melt and high mold temperature can reduce the flow imbalance.

Three-Dimensional Modeling of Mold Filling in Microelectronics Encapsulation Process

In this paper, a fully three-dimensional (3-D) numerical model is developed to simulate the mold-filling behavior in the plastic encapsulation of microelectronics. The conventional Hele-Shaw approximation is inadequate to analyze such a complex process owing to the 3-D nature inherent in the molding compound flow between leadframe and mold cavity. The developed methodology combines the efficiency of SIMPLE-based finite volume method (FVM) and the robustness of VOF volume-tracking method to solve the two-phase flow field in complex mold geometry. An efficient method for automatic generation of prismatic mesh for plastic packages is also presented. The molding process of a TSOP II 54L LOC package is studied. Short-shot experiments are conducted to investigate the filling patterns at several different flow times. The close agreements between experimental data and simulated results demonstrate the applicability of the present computational model for practical plastic encapsulation simulations.

Injection molding of ribbed plastic plates with a superplastic Zn–22% Al sheet

AbstractThis article presents the development of a hybrid process combining the forming of a superplastic zinc–aluminum sheet with injection molding. The product is a plastic part covered with a superplastic zinc–aluminum layer used mainly for electromagnetic interference (EMI) shielding. This hybrid process involves not only molding a plastic plate, but also forming a superplastic zinc–aluminum sheet with features such as ribs. This research observes the process of filling the cavity and forming the superplastic sheet with the aid of short‐shot experiments. By understanding the molding and forming process, guidelines for process control will be derived. This research further investigates the moldability of SIM systems. To make satisfactory parts, sheet thickness, melt temperature, injection parameters, and other process parameters determine the “moldability” of a specific system. The first part of the moldability study is to identify the relative importance of these process parameters. Results show that melt temperature and injection pressure are the two most critical parameters. The second part of this moldability study is to develop the moldability diagrams based on the molding area on the plane of these two key parameters. Moldability diagrams are employed to evaluate the effects of sheet thicknesses and melt inlets. It is found that a thick sheet, high melt temperature, and an inlet providing short flow length improve moldability. © 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 216–225, 2001

Flow Characteristics of Sheet Molding Compound in Panels with Integrated Ribs

A series of experiments was conducted using two different formulations of sheet molding compound (SMC) materials in an instrumented ribbed mold with outside dimensions of 750 x 600 x 410 mm. The experiments showed that the flow of the material into the ribs (side flow) takes place long before the completion of the flow parallel to the flat surfaces of the mold (radial flow). This observation is in contrast to the general belief that the side flow starts after the completion of the radial flow. Also, the analysis of the data from the thermocouples showed that the rib filling takes place through a "bridging and looping" mechanism as well as "concurrent flow." The above findings were verified using short-shot moldings under various conditions. In these short-shot experiments the plaques were solidified at different stages of the flow and, hence, provided informative evidence with regard to the nature of the flow during the compression molding of SMC. The data from the pressure transducers showed that while the radial flow is in progress, the internal pressure in the material increases to a level high enough to cause the side flow into the ribs. It was then concluded that if the pressure in the vicinity of the radial flow front is artificially increased it will promote the "concurrent flow." This type of flow is im portant because our previous studies have shown that it would lead to a reduction in rib readout.