When replacing a metal engine part with plastic, it is necessary to regard vibration damping as one of the important factors in terms of noise reduction as well as strength and heat resistance as being characteristics of the material.
Plastics are far better for vibration damping than steel or aluminum, but this property is reduced by the addition of glassfiber-reinforced or high heat-resisting resins.
We have taken notice of SMC (Sheet Molding Compound) which has the excellent strength and heat resistance properties and studied it in order to increase its vibration damping property.
Since organic polymers show the peak value for vibration damping performance in the vicinity of the glass transition temperature (Tg), we studied a method to shift the Tg near the operating temperature region in order to improve the vibration damping property. The shifting of the Tg is conducted by a combination of three resins: vinyl ester resin having a high Tg, unsaturated polyester resin having a low Tg and polyisocyanate resin which is used as a thickening agent. Thus, it became possible to develop a new type SMC with a greater vibration damping property making it more effective in reducing engine noise than conventional SMC.
After applying the new SMC to a rockercover, it was confirmed that it is capable of reducing the noise by about 1.0 dB or more as compared with one made of aluminum.