## 1 Introduction  
The last few decades have seen the development of a wide variety of elastomers (rubber materials) for application to diverse fields. Fluororubber (a fluorocarbon elastomer), a special type of elastomer with excellent thermal and chemical resistance, is used in packing missiles and rockets. Silicones are used as materials for medical devices and electrical insulation owing to their non-toxic, weather-resistant, heat-resistant, and cold-resistant properties [1; 2; 3]. Ethylene propylene diene methylene (EPDM) rubber exhibits excellent ozone resistance but poor oil resistance, and it cannot be used in applications wherein it comes into contact with oil [4; 5]. A grommet is one of many products made using EPDM rubber, and it is one of the main parts of automobiles that fixes and protects the wire harness from the outside to the inside of the vehicle [6]. A grommet is manufactured via injection molding of EPDM rubber, and the mounting force is changed according to the mechanical properties of EPDM [7; 8]. The mounting force can be divided into the insertion force generated when the grommet is mounted on the car panel and the detachment force generated when the grommet is released. In the automobile industry, the insertion force is set to 98 N or less for ease of operation and efficiency, and the release force is set to 69 N or more to prevent the grommet from getting displaced. Various studies on grommets are underway. [PERSON] et al. reported that the mounting force was changed according to the grommet shape, and derived the shape of the grommet required to improve the mounting performance [9]. [PERSON] et al.  
reported that nonlinear material constants were derived from the nonlinear material properties of the grommet, and nonlinear analysis was carried out to analyze the behavior [10]. In this study, the effect of the vulcanization accelerator on the EPDM was investigated and a grommet was manufactured to compare the insertion and detachment forces.  
Here, we remark that the mechanical properties of rubber are considerably influenced by the choice of the vulcanization accelerator, and the vulcanization characteristics vary widely depending on the type and mixing ratio of the accelerator [11]. Moreover, natural rubber and synthetic rubber exhibit different reactions to the same accelerator, which may act as an ultra-rapid accelerator for the former and a retardant for the latter [12; 13]. While several studies have been carried out to examine the properties and effects of general accelerators [14], very few studies have focused on deriving or estimating the optimum accelerator mixing ratio via efficient and systematic experiments for realizing desirable, application-specific mechanical properties of rubbers [15; 16].  
In our study, the blending ratio of the EPDM and the vulcanization accelerator was optimized to improve the mounting ability of the grommet. We experimentally analyzed the changes in the properties of the EPDM rubber depending on the vulcanization accelerator and derived the optimum mixing ratio. Using the mixture design strategy, we derived the optimum accelerator mixing ratio. Experiments were conducted to validate the formulation derived from the mixture design strategy [17; 18; 19; 20]. The grommet was manufactured based on the derived blending ratio and the mounting performance was improved compared to the existing grommet.