Process Optimization of Injection Molding for High-Precision Cosmetic Dispenser SealCaps: A Process Capability (Cp, Cpk) Analysis

The dimensional stability of the SealCap, a critical polymer component in cosmetic dispenser pumps, is crucial for ensuring consistent discharge volume and preventing functional defects. This study aimed to optimize the injection molding process for high-precision SealCaps by analyzing changes in machine type and processing conditions. We conducted a comparative analysis between a large 64-cavity high-speed machine and a small 12-cavity precision machine. For the precision machine, three conditions were tested by varying the holding pressure time (3.0–3.5 s), resulting in cycle times of 7.2–7.7 s. The dimensional stability of the SealCap's outer diameter and the discharge volume of assembled pumps were evaluated using process capability indices (Cp and Cpk). The results demonstrated that the small precision machine under condition C (7.7 s cycle time) achieved the highest process capability, with a dimensional Cpk of 1.91, significantly outperforming the large high-speed machine (Cpk = 0.53). Similarly, the discharge volume Cpk was highest under condition C at 1.31, indicating superior functional consistency. These findings indicate that a longer cycle time enhances process stability and reduces inter-cavity variation, leading to improved dimensional quality. Therefore, condition C was identified as the optimal manufacturing process, confirming that utilizing a precision injection molding machine with optimized parameters is a highly effective strategy for producing high-quality, high-precision polymer components.