Recently, consumers' desire for quality has also been increasing as consumers' level of consumption and their eyes have increased. Accordingly, the quality department of finished cars requires strong quality of moving parts including trunk lid.In order to prevent sectional impaction of square pipe during bending molding of Gooseneck Arm, which is the main member of trunk lid hinge arm assay, it is in production using mandrill technique.The purpose of this study is to improve the mandrill to improve the lack of straightness due to the ingress of the top of the square pipe of the trunk lid hinge of the automobile parts and the resulting fault problem.To prevent upper sinking, a mandrill with a slide plate of flap structure was made and the product was produced using it.It was also compared with the existing mandrill.The main purpose of straight ahead is to measure dimensions with three-dimensional scanners and software, and the straight-ahead average between the holes in the three existing R segments and the variable section is 1.27mm, and the straight-ahead average of the three improvements is 13.99mm.This resulted in a higher result than the original target of 5mm requirements.The operational power and durability of the improvement products were also tested, and the bracket operation force of 130 ± 40 N․cm and the link actuation force of 70 ± 30 N․cm were satisfied, and the result was satisfied by measuring the amount of bush wear after 30,000 opening and closing measurements.Environmental tests that tested the actuation force after 4 hours of SOAKING at -30°C and 80°C were also satisfied with the target development value.The performance of this study is to improve the reliability by satisfying the assessment results of straight ahead, working force, durability and environment first, and to improve the occurrence of the problem of two to three mm injury in case the straightness section of trunk lid hinge arm, which is aimed at securing straight ahead section.