Hidetoshi Yokoi

Visual analysis of the flow behavior of core material in a fork portion of plastic sandwich injection molding

The flow behavior of core material in the fork portion of a plastic sandwich molding was studied by using a dynamic visualization technique. High viscosity resin GPPS685 and low viscosity resin GPPS679 were used in the experiments. It was found that in the fork portion, although the molding conditions have almost no effect on the flow pattern of the skin material, they influenced the flow pattern of the core material: (1) Mold temperature slightly influences the flow patterns of the core material. (2) The flow properties of resins also affect the flow patterns of core material. An imbalanced design causes more problems than in ordinary injection molding. Low viscosity material yielded a well balanced filling more easily than high viscosity material. (3) The injection rate strongly affects the flow behavior of the core material in a complicated manner.

Influences of molding conditions on die-pad behavior in IC encapsulation process analyzed by Hall element method

It is important to analyze the dynamic die-pad behavior during the integrated circuit (IC) encapsulation process in order to reduce such defects caused by the die-pad movement as exposure of the IC chip on the package surface or breakage of bonding wires. In this study, experiments were carried out to investigate the influences of transfer time, internal pressure after the filling process, polyimide tapes pasted on lead frames, and viscosity of the resin on the die-pad behavior using the Hall element method developed in the previous study. The results showed that 1) the die-pad moved more when the transfer time was longer in this mold, 2) the internal pressure after the filling process had little influence on the die-pad behavior, 3) polyimide tapes restrained the die-pad movement along the thickness direction, and 4) the die-pad displaced larger with higher-viscosity resin.

Process along thickness direction

This paper describes a visualization mold with a glass-inserted structure which enables observation of the dynamic melt behavior inside the cavity along the thickness direction. The melt front profile and the behavior of the melt front surface were observed using a high speed video system, and analyzed with an image processor. Several experiments revealed that the melt advanced slipping on the cavity surface or lead frame. It was also found that the surface of the melt front moved from the cavity surface to the lead frame generally due to the difference in the flow resistance between the cavity surface and lead frame. The surface velocity of the melt front was related to the position of the melt front, that is, the surface velocity of the preceding melt front was slower than that of the following one. It suggests that the melt flow is affected by the melt in the opposite cavity beyond the lead frame.

Effects of Cavity Conditions on Transcription Molding of Microscale Prism Patterns Using Ultra-High-Speed Injection Molding

Abstract Ultra-high-speed injection molding has been reported to be a very effective method for improving transcription molding. In this study, by using stampers with V-grooves having pitches of 25 μm and 100 μm, we investigated the effects of cavity conditions, including cavity thickness, groove layout, and groove pitch size, on transcription molding. These experiments were also conducted under various molding conditions such as injection rates, melt and mold temperatures, and holding pressure, etc. As a result, it was found that the groove layout of a stamper remarkably affects transcription fidelity and these effects do not disappear when the molding is performed with a cavity-vacuum process. This phenomenon was explained using the well-known molecular orientation by the extensional flow during injection molding. When transcription molding was performed with cavities of different thicknesses (0.5 mm, 1.0 mm, and 2.0 mm), the effect of holding pressure on transcription was inversely related to the cavity wall thickness. This result indicates that, in the molding with a thin cavity wall, the transcription mainly completed during the filling stage. In addition to the shortening of cavity filling time and decrease in viscosity by shear heating, an accelerated increase in cavity pressure during the cavity filling stage was also found to be important for explaining the improvement in transcription with ultra-high-speed injection molding.

Visualization Analysis of a Multilayer Foam Development Process in Microcellular Injection Molding

Abstract In this study, cross-sectional analyses were performed on microcellular injection-molded high-impact polystyrene products. The results confirm that the following five types of layers were formed: Skin layers I (the silver streak layer) and II (a nonfoamed layer), Core layers I (cell diameter, d > 150 μm), II (d < 50 μm), and III (d > 100 μm). As the maximum in-mold pressure (Pmax) was increased from 5 to 30 MPa, the thickness of Skin layer II remained nearly constant. However, the foam types in the core layers changed from I and II to II and III or III only, resulting in an increase in cell diameter and a decrease in cell density. The process of cellular structure formation was observed using a glass-inserted mold, which revealed that this process consists of a flow (with a burst of cells at the melt front and the subsequent flow of the melt containing the cells), an end of the filling (involving elastic compression or the dissolution and disappearance of cells formed in the flow stage), and a cooling (new cell generation and growth and cooling solidification). Based on these cross-sectional observations, in concert with melt-pressure measurements and visualizations, we developed a model describing the formation process of Skin layer II and the core layers including a new concept that considers the melt pressure inside the cavity. The following layers are incorporated into the model: Skin layer II: A nonfoamed layer is formed in the area of the melt front where gases diffuse out from within the melt during the filling stage, and this nonfoamed layer moves to from melt front to the surface of the product due to fountain flow. Core layers I and II: A multilayer is formed containing a distribution of cells preserved from the flow stage due to the low compression forces, Core layer III: cells are dissolved in the melt due to strong compression forces at the end of the filling stage and then reform and grow in the cooling stage.

Development of measurement system for die-pad behavior in IC encapsulation process using Hall elements

This paper describes a new method of measuring the die-pad behavior in the integrated circuit (IC) encapsulation process using Hall elements and a permanent magnet. The method enables us to measure the die-pad behavior along both the height and rotational directions during the melt filling and curing process. The measurement results were compared with that of the direct observation of the die-pad behavior using a high speed video camera and the static observation of the cross section of molding products, and the results obtained by Hall elements showed more or less good agreement with those of the other two methods. The experiments were carried out under three kinds of transfer times. The results showed that the die-pad moved more when the transfer time was longer, which suggested that the transfer time affected the die-pad behavior.

Visualization analysis of injection molding phenomena in hot-runner system

Various unsolved defects are known to occur in hot-runner molds, such as stagnation of resins in the manifold channel, asymmetrical cavity filling behavior, flow marks, gate marks, etc. We have been clarifying the causes of various molding defects specifically using visualization technologies inside mold cavity and manifold. This report introduces and discusses the following topics, particularly focusing on clarifying the behavior of resins; (1) Comparison of the imbalanced cavity filling phenomena in between side-fed and center-fed types of valve-gate hot-runner systems, (2) Visualization analysis of the melt flow behaviors and stagnation phenomena inside hot-runner manifolds, (3) Behavior of each stagnant melt around the valve-pin and inner hot-nozzle surfaces, (4) Void generation phenomena remaining on the surface of the valve-pin tip, (5) Melt temperature distribution on the cavity surface during the filling process, (6) Melt temperature profile inside the flowing melt injected from the valve gate, etc.

Visualization Analysis of Resin Flow Behavior around a Flow Front Using a Rotary Runner Exchange System

Abstract In plastic injection molding, external defects such as flow marks are serious problems. These external defects are known to occur during cavity filling processes inside the mold, and are closely related to resin flow behavior in the flow front area. In this study, the authors propose a new method for visualizing in-mold resin flow behavior using a glass-inserted mold and rotary runner exchange system capable of instantaneously switching two types of melt at the runner just before the gate. This method is capable not only of dynamic visualization, but also static visualization by the observation of cross-sections of a sample, because colored resins switched multiple times at the gate spread in layers inside the sample and freeze. Using this method and general purpose polystyrene, visualization analysis of the flow behavior of inner layer resins near the flow front in simple rectangular cavities was carried out. As a result, it was found that resins reaching the flow front are not exposed at the molded product surface immediately after the fountain-flow process, but remain along the circumference of the flow front for a long period of time, and then stretch out into a thin film-like shape to form the surface of the molded product. In addition, the inner layer resins near the flow front form a converging flow just before reaching the flow front and then form a fountain flow. It was also confirmed that this is a general phenomenon in the fountain-flow process during injection molding regardless of whether the resin is crystalline or noncrystalline. Based on these results, we built a model on the flow of inner layer resins near the flow front of general resins.

A Study on Influence of Resin Temperature on Filling Balance of Multi-Cavity Molds

Filling imbalance for PP, long GF-PP, elastomer PC and GF-PC in a multi-cavity mold with a geometrically balanced runner is investigated by using the visualization glass-inserted injection mold and infrared thermometers, through visualizing the flow behavior of the melt in the runner and cavity and measuring the melt temperature near the cavity entrance simultaneously. In addition, velocity distribution in the secondary runner for PP and elastomer is also determined by use of marker tracers in order to further investigate the mechanism of filling balance. The results show that the degree of filling imbalance depends on the velocity difference of the upper and lower melt and its development in the secondary runner during the filling process. The melt viscosity difference in the primary runner dominate the melt velocity difference in the secondary runner., and the viscosity difference depends on the temperature difference and viscosity sensitivity to temperature and shear rate. The development of the velocity difference of the upper and lower melt in the secondary runner depends on the melt viscoelastic property and the runner wall condition.