Abstract
In response to ever-growing environmental concerns, Mitsubishi Motors Corporation has successfully developed the world's first thermoplastic-elastomer-olefin (TEO) weatherstrips, which are easily recyclable and help improve the production environment, and has already started to equip its commercial cars with them. The TEO weatherstrips which had the same geometry as that of conventional ethylene-propylene-diene rubber (EPDM) weatherstrips developed appearance problems in the foamed sections such as the lip section which hides the ends of the interior trims of riding compartments. The new TEO weatherstrips could therefore be used in only certain car models.
The fault is kinking in the lip section of TEO weatherstrips fitted onto a corner section, and we discovered that the appearance problems were mainly caused by: 1) yield of TEO material; 2) rigidity of weatherstrips in the lip section; and 3) uneven fitting of weatherstrips in assembly lines. We therefore devised countermeasures for each cause. For the first cause, yield of TEO material, we determined the optimum TEO foaming ratio for weatherstrips from the relationship between foaming ratio and yield strain of the material.
Secondly, we modified the cross-sectional geometry of the TEO weatherstrips to conform to different material characteristics, thus solving the rigidity problem in the lip section. We first studied buckling, which is the immediate cause of the appearance problems. Eulers formula shows that buckling load is proportional to rigidity (compressive modulus of elasticity x geometrical moment of inertia). Our research revealed that solid TEO and EPDM had similar compressive modulus of elasticity, but TEO foam had a smaller elasticity than EPDM foam. We therefore increased the thickness in the lip section of TEO weatherstrips to give the same rigidity as EPDM weatherstrips, and verified this solution by bench tests. We then developed a method for setting the cross-sectional geometry of weatherstrips after establishing the correlation between radius and lip section thickness.
Thirdly, we established a countermeasure against appearance problems due to uneven fitting of weatherstrips in assembly lines. It was found that weatherstrips with bending load exceeding a certain limit tend to cause uneven fitting. We modified the geometry of the insert-metal to improve the fitting characteristics through computer-aided engineering analysis on bending load of the insert-metal, as this is one of the major components of weatherstrip bending load.
Consequently, TEO weatherstrips can now be used in all car models, offering better recyclability, approximately 5% weight reduction, and cost reduction.