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Precision Dispensing of Liquid Silicone Rubber for Automotive Gasketing
Published January 01, 1991 by Society of Automotive Engineers of Korea in South Korea
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Silicone rubber is firmly established as a premier material for automotive gasketing, especially for demanding power train applications. Historically, silicone rubber has been used for automotive gasketing in two forms, molded gaskets from high consistency silicone rubber (HCSR), and formed-in-place (FIP) gaskets from RTV silicone sealant.
HCSR is molded using compression or injection molding techniques and the resulting gaskets have become the standard of quality over the past decade. Molded silicone rubber gaskets offer high value to the OEM by reducing warrantee costs and improving customer satisfaction; however, constant pressure exists to lower the cost of silicone gaskets. FIP gaskets are dispensed using relatively simple, automated dispensing equipment. FIP gaskets are very cost effective; however, the success of this technique depends on adhesion of the silicone sealant to both substrates. In addition, silicone sealants typically lack the physical property profile and chemical resistance of HCSR.
Liquid Silicone rubber (LSR) has been in existence since the mid seventies. LSR is chemically very similar to HCSR, with lower viscosity base polymers used to make the compound pumpable. LSRs have typically been used in injection molding and material coating processes. Though similar to HCSR, LSRs have typically lacked certain properties, namely compression set resistance and chemical resistance, that are required to make quality compression seals that do not degrade in the presence of automotive fluids. This has changed in the past two years, as significant advances have been made in LSR technology that allow compounds to be formulated that have virtually the same property profile as high quality HCSR.
These new LSRs could be used to injection mold gaskets; however, the modest processing advan tages would not justify the 50-300 percent cost premium of LSRs over HCSRs.
In order to achieve cost reductions in silicone gaskets with LSRs, the pumpability of these materials must be utilized. A gasketing method called cured-in-place (CIP) or dispense-in-place (DIP) gasketing has emerged in the past few years as a way of reducing the cost of silicone gaskets. This method involves dispensing a bead of LSR directly onto a component and then passing the component through an oven to vulcanize the LSR, forming a gasket that seals through compression. The LSRs used in this process are formulated so that they attach themselves with an adhesive bond to the component during the vulcanization process. A cost savings is realized through the elimination of tooling required in a molding process and by eliminating the need to manually assemble the gasket to the component.
A gain in assembly reliability is realized by having the gasket positively located on the cover.
Unlike FIP gasketing, where the silicone bead is crushed before curing, precise dispensing is required for CIP gaskets. Precise dispensing is required because as compression seals, CIP gaskets are engineered for a certain range of compression. The size and shape of the silicone bead must be held within certain tolerances, and the two ends of the bead must be brought together without excessive deformity. Accomplishing these tasks requires sophisticated dispensing equipment that is well tuned to the particular LSR material.
This paper will discuss the necessary equipment for dispensing CIP silicone gaskets and various techniques for gasket bead refinement. The results of designed experimentation dealing with dispensing variables will be presented. Other equipment necessary to complete the CIP silicone gasket manufacturing process will also be discussed.
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