The sandwich injection molding technique can be used to wide ranges of engineering applications. A simultaneous composite injection molding (SCI. molding) process is a new process of molding composite parts in which two different melts are injected into one mold cavity simultaneously through two channels. The products with special characteristics can be obtained by the sandwich injection molding process.
In this study, the sandwich injection moldings which consisted of polyamide 66 and the fiber reinforced polyamides 66 were molded by using SCI. molding process. The bending properties of the sandwich injection moldings were evaluated by comparing with those of the moldings with single materials. Consequently, the bending properties of the sandwich moldings were more than 80% those of the single moldings in spite of lower fiber contents of the sandwich moldings. It was also clear that the adhesive properties of the interface greatly contributed to the bending properties, in particular, bending strength. The sandwich molding with short glass fiber which had more superior interfacial adhesion, exhibited more superior bending properties as compared with those with the long glass and carbon fibers. Moreover, the bending properties obtained by the composite beam theory were compared with the experimental result. This calculation method using the sandwich beam theory was valid to predict the bending properties of the sandwich moldings.

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Sandwich injection moldings having different structures in the skin and core layers are produced by injecting different resins from two injection cylinders sequentially.
In this study, the internal structure of the sandwich injection moldings was quantitatively investigated. The materials used were polycarbonate (PC), acryronitrylbutadiene-styrene terpolymer (ABS) and PC/ABS blend. The sandwich injection moldings were subjected to chemical treatment in a chromic acid mixture, which etched the ABS component. In this way, the internal structure of sandwich injection moldings could be observed. From these observations, the degree of deformation in ABS parts was quantitatively evaluated. Ordinary injection moldings with a single injection unit and compressive moldings were also investigated to compare with sandwich injection moldings. These evaluation showed that the internal structure of moldings depend on processing methods. The most interesting observation is that the melt flow of the core layer in sandwich injection moldings is “the plug flow”.

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The vibration damping properties of sandwich-injection moldings are investigated. The sandwich plates used in this work are made of polystyrene (the skin layer) and thermoplastic elastomer (the core) formed by the foam process. The vibration tests have been conducted to obtain the damping factor, by applying an impulse with a hammer on the central portion of the plate under the free boundary condition. A method of finite-element analysis was used to analyze the damping factors of the sandwich-injection moldings. After its validity has been confirmed by experiments, the influences of the size and position of the core upon the damping factors are quantitatively discussed.

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Injection molding of thermoplastic composites, poly (ethylene terephthalate) (PET)/polyethylene (PE) alloy and polycarbonate (PC)/poly (acrylonitrile-butadiene-styrene) (ABS) alloy, was examined with a novel mixing nozzle which was expected to have the capability of plasticizing and mixing these materials and making useful polymer alloys conveniently. The effects of the mixing nozzle were investigated by observing morphology of molded samples using a scanning electron microscope (SEM) and by measuring their mechanical properties, for example, tensile strength and Izod impact strength. Mixing of dry-blended PET/PE, which is an immiscible alloy, was promoted by the high shear effect of the mixing nozzle which gave a homogeneous and fine dispersed particles. The tensile strength of the alloy made by the mixing nozzle was almost the same as that made by a conventional open nozzle. However, elongation at break of the samples molded by the mixing nozzle was several times higher than that by the open nozzle. Though the shear effect of the mixing nozzle was also high in the case of PC/ABS, the narrow and complicated flow path in the mixing nozzle resulted in large pressure loss in the dwell process and the dispersed ABS phase in the alloy was deformed and different from that molded by the open nozzle.

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The effects of ultrasonic vibration on injection molding were investigated. An ultrasonic injection molding (UIM) machine in which the mold resonated under ultrasonic vibration was developed. The resonance in the mold occurred at n wavelengths, where n=0.5N and N is a positive integer. A transducer was placed on the movable mold through a directional converter of vibration. The fluidity of the molding materials and the molecular orientation of products were evaluated by the UIM machine. This evaluation allowed the improvement of the apparent fluidity of the molding materials compared to the case without ultrasonic vibration. Consequently, the molecular orientation caused by the flow was decreased. Wall slip was observed between the resin and wall surface with ultrasonic vibration in flow visualization experiments. It was found that the fluidity was improved by the action of sound pressure and the reduction of apparent friction between the wall surface in a cavity and the molding material by ultrasonic waves.

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