Minoru Shimbo

Mechanism of Strength Improvement of Foamed Plastics Having Fine Cell

The influence of cell size on tensile fracture strength of foamed plastics is examined in this study. Foamed specimens having various cell sizes with the same foaming magnification are molded. The tensile fracture strength of foamed plastics is measured and the relationship between the cell size and tensile fracture strength is discussed. It is found that the tensile fracture strength of foamed polyethyleneterephthalate and polypropylene which are crystalline resins increases with decrease in cell size more than the strength presumed from the foam magnification. The decrease in cell size results in an increase in cell density. An increase in cell surface area causes molecular orientation thereby effecting strength improvement.

Residual-stress analysis of an epoxy plate subjected to rapid cooling on both surfaces

This paper presents theoretical and experimental methods of finding the residual stress in an epoxy plate subjected to rapid cooling on both surfaces. The theoretical residual-stress distributions in a plate are calculated by using the fundamental equations based on the linear viscoelastic theory. The specimens in the experiment are subjected to rapid cooling. The residual stresses are measured by the layer-removal method. The theoretical and experimental results are compared and discussed.

Viscoelastic analysis of residual stress in quenched thermosetting resin beams

The residual stress generated by the molding process of thermosetting resins exerts serious influences upon their mechanical properties. This residual stress is generally classified by two groups: one produced by shrinkage in the curing reaction of monomers, the other produced by the nonuniformity of the temperature distribution in the cooling process.This paper is concerned with the theoretical and experimental analysis of the generation of residual stress of the latter type, using examples of rectangular beams of thermosetting resins quenched on both the upper and lower surfaces.First, a viscoelastic model is applied to make a qualitative prediction of the residual stress in quenched beams.Second, using linear-viscoelastic theory, fundamental equations are derived for the residual stress in a viscoelastic rectangular beam, where an unsteady and nonuniform temperature distribution is assumed in the direction of depth. The theoretical values of the residual stress in rectangular beams are calculated under various quenching conditions for two resins having different viscoelastic characteristics, i.e., epoxy and unsaturated polyester.The theoretical residual-stress distributions agree fairly well with the residual stress measured experimentally at every quenching condition for both resins. The qualitative prediction that the residual stress in quenched beams is compressive in the vicinity of the upper and lower surfaces and is tensile in the inner parts is confirmed. The relaxation modulus of epoxy resin changes more greatly with time and temperature than that of unsaturated polyester resin. The theoretical and experimental analysis shows that the residual stress for the former resin is larger than that for the latter. Therefore, it is concluded that the generation of residual stress is more significant where the relaxation modulus of resin changes greatly with time and temperature.

Effect of Key Process Variables on Microstructure of Injection Molded Microcellular Polystyrene Foams

The effect of control factors as key process variables on injection molds of microcellular plastics were investigated experimentally in this paper. The continuous foam processing system was constructed using screw preplasticating type injection mold machine, which is capable of supplying blowing agent, plasticity, kneading and keeping of retention time for gas/polymer solution. The propriety of foam processing system constructed in this study was verified. Control factors that affect cell nucleation and growth by injection molding were classified and some of them were considered in this study. In order to investigate cell morphology and cell structure, microcellular polystyrene samples were produced under various ranges in control factors by using the above processing system. In particular, the effect of injection velocity, mold temperature, retention time, plunger stroke and dwelling pressure on cell morphology of microcellular polystyrene were examined and discussed. It was found that cell size decreases and cell density increases with an increase in injection velocity. Cell size decreases about linearly with an increase in injection velocity and becomes about half size for five times of velocity. An increase rate of cell density is about ten times to five times of injection velocity.

Residual stresses and warp generated by one-sided quench of an epoxy-resin beam

This paper discusses the warp and accompanying residual stress in a rectangular epoxy beam produced by water cooling its lower surface. First, the theoretical values of this warp and residual stress are obtained by the linear-viscoelastic theory. The specimen is then subjected to quenching. The variations in the warp are observed. After quenching, the residual stress is measured by a layer-removal method. The experimental and theoretical results are then compared and discussed.

Effect of foaming temperature control on cell size and cell density of microcellular plastics

In this study, noticing foaming temperature as a factor, which induces thermodynamic instability for cell nucleation of Microcellular plastics, the effect of control method of foaming temperature on cell size and cell density - that is number per unit volume of foamed plastics - were investigated. Generally, foaming by using batch process is carried out as follows. First, blowing agent is soaked into plastics until saturation under high pressure and soaking temperature. After plastics were saturated with blowing agent, pressure is released rapidly and then temperature is raised to foaming temperature and cells are nucleated and grown. Finally, rapid cooling controls cell growth. In this case, two methods can be considered for the control of foaming temperature. One is the elevated temperature method in which temperature is raised to foaming temperature and cells are grown after decompression in the foaming process. The other is the constant temperature method in which the temperature is already kept at foaming temperature before decompression. That is, it is the method of performing soaking and foaming at the same temperature. Polymethylmethacrylate (PMMA) resins were foamed under foaming conditions which the same foaming magnification is produced by both methods and cell size and cell density of foamed PMMA were investigated. As results, in case of production of the foamed plastics having the same foam magnification, it turned out that cell density of foamed plastics becomes large and average cell size becomes small but the maximum cell size becomes large by the elevated temperature method. On the other hand, although the maximum cell size becomes small, average cell size becomes large by the constant temperature method.

Experiments on Solid-state Cell Nucleation in Polystyrene and Polycarbonate as a Function of Foaming Temperature and Foaming Time

In this paper, the equivalency between the foaming time and foaming temperature of the cell density, the number of cells per unit volume remaining in the foamed plastics, will be discussed. The foaming was carried out by the following method. The blowing agent was soaked into the solid resin at high pressure under temperature lower than the glass transition temperature of the resin. After the blowing agent reached its saturation state, cell nucleation and cell growth were induced by heating various foaming temperatures higher than glass transition temperature and various foaming times after decompression. Finally, cell growth was halted by cooling. Concretely, using a device that can accurately control temperature and the decompression rate, polystyrene (PS) and polycarbonate (PC) resins which are amorphous resins were foamed under various the foaming temperatures and the foaming times by the above-mentioned method. The following results were obtained. (1) Cell density of foamed PS and PC plastics shows time and temperature dependence as the cell density becomes large when foaming temperature is low and foaming time is short. (2) Equivalency is maintained between the foaming time and foaming temperature dependence of the cell density of PS and PC foamed plastics, and they can be expressed with one master curve respectively. That is, the time-temperature equivalent law is held about cell density. (3) These results seem to become one technique to select the foaming condition of foamed plastics having required cell densities.

Correlation of decompression time and foaming temperature on the cell density of foamed polystyrene

In this paper, the correlation between the foaming temperature and the decompression rate (decompression time) of the cell density that is the number of cells per unit volume remaining in the foamed plastic will be discussed. Foaming was carried out by the following method: the blowing agent was soaked into the resin as a solid state at high pressure under temperatures higher than the glass transition temperature of the resin. After the foaming agent reached its saturation state, cell nucleation and cell growth were accelerated by decompression. Finally, cell growth was halted by cooling. A device that can accurately control temperature and the decompression rate was designed, produced and verified for accuracy prior to this investigation. The polystyrene (PS) specimens were foamed under various foaming temperatures and the decompression rates using the above-mentioned method. The following results were obtained: 1. Cell density of foamed polystyrene shows time and temperature dependence as follows. The cell density increases when the decompression rate is quick, i.e. the decompression time is shortened under the condition of low foaming temperature, and cell density decreases when the decompression rate is slow, i.e. decompression time is lengthened under the condition of high foaming temperature, 2. Correlation is maintained between the temperature dependence and time dependence of the cell density of foamed PS, and it can be expressed by one master curve, 3. Based on this correlation, it is possible to predict the required foaming conditions of plastics having arbitrary cell densities.

Effect of Saturation Pressure on Equivalency of Decompression Time and Foaming Temperature on Cell Density of Foamed Polystyrene

In this paper, the effect of saturation pressure on the time-temperature equivalent law of the decompression rate (decompression time) and foaming temperature of the cell density, the number of cells per unit volume remaining in foamed plastic was discussed. The foaming was carried out in the method described be by using batch foaming process. The blowing agent was soaked into the resin as a solid state at various high saturation pressures under temperatures higher than the glass transition temperature of the resin. After foaming agent reached its saturation state, cell nucleation and cell growth were accelerated by decompression. Finally, cell growth was halted by cooling. The polystyrene (PS) specimens were foamed under the various saturation pressures, foaming temperatures and decompression rates. The following results were obtained. (1) Cell density of foamed PS shows time and temperature dependence as follows. The cell density increases when the decompression rate is quick, i.e. the decompression time is shortened at the condition of low foaming temperature, and cell density decreases when the decompression rate is slow, i.e. decompression time is lengthened at the condition of high foaming temperature under various saturation pressures. (2) The time-temperature equivalent law is maintained between the time dependence and temperature dependence of the cell density of foamed PS, and it can expressed with the same time-temperature shift factor if the decompression rate is the same even if saturation pressure changes.

The Foaming Process and the Damping Capacity and Sound Insulation of Bell Structures

The forming process and the characteristics of bell structures produced using batch-type foaming with a supercritical-fluid blowing agent were examined. Various forming methods were tried for the production of bell structures. Impact and acoustic tests of the structures were conducted, followed by an evaluation of shock absorption and sound insulation. Bell structures, including various kinds of ball, were successfully produced by means of batch foaming. It was verified that these bell structures exhibit the required shock absorption and sound insulation performance.