Plastic is cheap and has outstanding material properties for its density. Furthermore, it is easy to manufacture, so it is easy to use for the production of various components, which is why it is widely used in industry. Moreover, research on plastics is ongoing and their material properties have been enhanced to allow their use in areas that plastics could not be utilized otherwise because of their poor properties.
One current research direction is focused on enhancing the material properties of plastics to make them suitable for new applications, which has resulted in the development of fiber reinforced plastic. Fiber reinforced plastic is produced by reinforcing a plastic with glass fibers, carbon fibers, natural fibers, or similar material inside of the plastic. Injection products made of fiber reinforced polymers have especially higher mechanical strength and heat-resistance compared to the existing resins. Moreover, as injection molding can be applied to other materials similar to the existing resins without additional processes, fiber reinforced plastics have high applicability and are used widely in the industry. Injection molding is especially popular in industries such as automobile parts and electronics manufacturing, where parts with higher material properties are increasingly required. Fiber reinforced plastic injection products are being used in more areas [1
The problem with fiber reinforced plastic products is the warpage that is generated after product blowdown from the mold during the injection molding process. In particular, the fiber reinforced plastic’s fiber orientation intensifies this warpage [3
]. In this work, to overcome the warpage of fiber reinforced plastic, the microcellular foaming process (MCP) was used.
The microcellular foaming process is a plastic foaming process, in which micropores are formed inside the plastic. The MCP results in smaller cell sizes than the existing foaming processes, as well as a high cell density. Through the use of the MCP, the plastic is lightened by 10%~50 %, and its material properties are enhanced [8
]. In addition, by applying the MCP, process cycle time can be decreased by decreasing the hold and packing time, and with the decrease in viscosity, the process temperature could be decreased. As a result, the application of the MCP decreases the amount of plastics used and it could be said to be an environment-friendly process because the amount of energy used in the process can be decreased [14
]. In this study, the MCP was applied to address the issue of warpage of GFRP. Polyproplyene was selected as the base material and glass fiber reinforced specimens were prepared. We confirmed that the warpage was exacerbated when glass fiber was added and experiments were conducted in an attempt to improve the warpage of the specimen by utilizing the MCP. In a previous investigation on warpage, Baek and Lee [16
] studied the warpage characteristics of film insert molded parts according to the injection molding process conditions. Their research investigated warpage characteristics according to process conditions such as temperature, speed, and pressure, and it corresponds to insert injection process without the MCP. Kramschuster and Cavitt [17
] studied the shrinkage and warpage characteristics of the injection process with the MCP. They confirmed the reduction of shrinkage and warpage in the MCP via a quantitative investigation. Although the supercritical fluid (SCF) content and the injection speed were determined to have the greatest influence on warpage and shrinkage, neither the relationship between shrinkage and warpage nor the cause of the decrease was addressed. Jonathan and Park also studied GFRP using the MCP [18
]. They investigated warpage and shrinkage according to process conditions such as injection speed, gas contents, and shot size. In addition, they examined shrinkage characteristics according to cell density. However, they were only able to confirm that shrinkage improves as the cell density increases. Sadabadi and Ghasemi [19
] studied fiber orientation according to the process conditions in short glass fiber polystyrenes and investigated the change of fiber orientation according to the process conditions. In order to verify the compatibility of GFRP with the MCP, physical and thermal properties were measured. Bledzki and Kirschlig [20
] examined the physical properties of polycarbonate when the MCP was utilized, based on the process conditions. Lin and Zhang [21
] studied the physical properties of foams with short fibers and rubbers. As previously indicated, existing studies are focused on the minimization of warpage via the control of process conditions. However, there is a lack of research on the analysis of the underlying principle or cause of this phenomenon. In this study, we focused on identifying and verifying the cause of warpage reduction when the MCP is utilized as opposed to the reduction of warpage based on changes in the process conditions. Moreover, the correlation between warpage and shrinkage is not considered as a separate phenomenon. As a result, it was confirmed that the shrinkage difference according to direction is closely correlated with warpage. It was confirmed that the MCP minimizes warpage as a result of a reduction of the shrinkage difference. We also identified two causes of shrinkage difference reduction that are related to the decrease in the absolute value of the shrinkage due to cell growth and the randomization of the fiber orientation.