The growing range of applications for ABS resins in recent years has led to stricter requirements for dimensional accuracy, making it desirable to reduce their thermal expansion coefficient. However ABS resins containing rubber for improved impact resistance have a larger thermal expansion coefficient because their BR component undergoes large expansion. Since the shear stress distribution in the thickness direction of injection-molded products gives rise to a material property distribution, it can be assumed that the rubber morphology also changes in the thickness direction. Accordingly, it is thought that it should be possible to reduce the thermal expansion coefficient by making clear how it relates to the rubber morphology and the molding conditions.
In the present study, a styrene resin containing rubber was used to investigate the effect of the resin composition and rubber morphology on the thermal expansion coefficient. Additionally, a mechanism for reducing the thermal expansion coefficient in the resin flow direction was also examined. The results obtained made the following points clear.
(1) Rubber deformation occurs only in the skin layer near the mold. The thermal expansion coefficient is reduced only in the skin layer where large rubber deformation takes place.
(2) In a styrene resin containing rubber, increasing the proportion of the deformed rubber layer increases the degree of rubber deformation (i. e., inhibits the relaxation of rubber deformation), thus making it possible to reduce the thermal expansion coefficient.

抄録全体を表示

Metal Injection Molding (MIM) is a combined technology of powder metallurgy and injection molding of plastics. This technology enables the formation of more flexible geometries and extends the variety of material choices. In the MIM process, metal powder is mixed with a plastic binder and wax, kneaded, formed into a shape, and then debound and sintered to obtain metal parts.
Since parts are formed by injection molding and then metallurgically solidified in a series of processes, the initial properties of green compacts (the molded parts are described as “green compacts”), which depend on the injection molding conditions such as the flow behavior for a particular gate and cavity configuration, influence the properties of the final products throughout the debinding and sintering stages.
This study aims to investigate the densification of green compacts under various conditions of injection molding, and analyze the parameters which govern the density of green compacts.
For the analysis, a new parameter, i. e. “Densification Time” was introduced, representing time duration of injection molding necessary for the mold pressure to reach the maximum value. The effects of molding conditions on the sintered density and dimensional change were investigated experimentally.
The results showed that the density of green compacts increased as the packing pressure and the mold pressure increased. The densification time during the MIM process was observed to be significantly shorter than during the plastics process (PP), about one tenth the time. This difference between MIM and PP was considered to be due to the rapid temperature drop of the MIM material. Properties of sintered parts varied with changing molding conditions. Therefore, it is necessary to optimize the molding conditions by using the mold pressure and densification time, which can be measured during the molding process.

抄録全体を表示