Many small injection-molded auto parts are made of fiber-reinforced plastic for the purpose of giving them added strength. Demands for vehicle weight reductions in recent years with the aim of decreasing vehicle fuel consumption have made it desirable to reduce the thickness of these small parts as well, so as to lighten their weight and lower the cost. However, thin-injection-molded parts made of fiber-reinforced plastic are more prone to warping and other molding defects compared with ordinary thin moldings of around 3mm thickness. This tendency stems from their anisotropic material properties and large material property distribution in the thickness direction owing to higher resin flow velocities and the influence of the fibers.
In the present study, using thin plates molded from fiber-reinforced plastic with different skin thicknesses, measurements were made of the material properties of each layer and the mechanism producing anisotropy was investigated. The results made the following points clear.
(1) A correlation between fiber orientation and thermal expansion coefficient ratio can be obtained for the core layer of 3mm injection molded samples.
(2) This relationship between the fiber orientation and the thermal expansion coefficient ratio changes in the skin layer of these 3mm injection molded plates.
(3) For thinner injection molded plates, fiber breakage occurs at all locations in each layer.
(4) Fiber breakage can affect the fiber orientation ratio and the thermal expansion coefficient ratio in these thin injection molded plates.
Fiber breakage depends on the size of the shear force.

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Using a partial thin-wall cavity with an ultra-thin-wall (thickness: 0.05mm) section at the center of the cavity, investigations were conducted to clarify the effects of changes in rise time of the screw injection speed on the fillability of ultra-thin-wall sections and the thickness distribution. Using a high-response infrared thermometer (response time: 8μs, 95%), the dependence of changes in the temperature of the flow-front area on the rise time was clarified through polypropylene molding experiments. The following results were obtained.
Improvements in the rise time was confirmed to be an effective means for enhancing the fillability of ultra-thin-wall areas and reducing the thickness of the molded product. By using a high response thermometer, the characteristic resin temperature changes in the flow-front area during ultra-high-speed filling were successfully extracted in detail for the first time.
For set rise times of 0ms and 30ms (actual measured values were 11.2ms and 27.5ms, respectively) the temperature difference in the flow-front area was about 25°C.
This temperature increase in the flow-front area markedly reduced viscosity and allowed filling of the ultra-thin-wall section in a short period of time, and also served to reduce the thickness by the reduction of in-cavity pressure.

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