Normally when we hear the term gasket, we think of flat die cut gaskets. This type of gasket is the most economical choice for large volume gaskets. However, with low cost comes other problems when trying to seal for long term use. Using circular cross-sections, molded gaskets can provide a more robust seal design, preventing leakage over the life of the products being sealed
Higher Closure Force
Most gasket applications are “sandwiched” between two mating surfaces. The amount of force needed to compress the rubber material to seal is considered the closure force. One factor that influences this is durometer or hardness of the rubber used, as shown in our article on compression forces.
Gasket design also has a big impact on closure force. Rubber can often be considered an incompressible fluid. Thus, the forces on a solid rubber will exponentially increase as the rubber is compressed. For flat gaskets, they can be considered a solid column when looking at compression forces. With a solid column, the compressive force increases quickly. Using a rounded cross-section allows lower closing force due to the amount of compression needed to deform the rubber into a square cross section or solid column. With a properly designed seal groove, this small deflection is all that is needed to seal properly, since rubber is allowed to deform with the pressure of the fluid being sealed.
Better Tolerances with Molded Shapes
Most die cut gaskets are cut from an extruded or calendered sheet stock. Due to the way it’s processed, the tolerance on the height of the sheet stock can have high variance not found when molding. This is because the height is controlled by the cut cavity of a precision mold. This has been known to cause seal leakage when mating surfaces come out of tolerance, allowing the total tolerance stack up of the seal design to reduce compression of the gasket, ultimately leading to leaks.
Apple rubber also sees an overcompensation for the compression force when die cut gaskets are processed this way. This causes wrappage of the mating components, creating leak paths. This also over compresses the rubber compound causing premature failure of long term compression.
Better Material Choices for Molded Shapes
Because of the way sheet stock is made, the normal stock is not cured optimally compared to a molded shape. This can cause a higher compression set, which reduces the life cycle of the gasket. The amount of time a molded shaped is left in the mold is determined on the T90 of the rubber compound. More time can be added based on the thickness of the gasket. This assures the rubber compound is fully cured and the final gasket will have optimal physical properties.
There is also limited availability with sheet stock material options. Users may have to sacrifice performance based on material availability. Molded gaskets can be made with any type of rubber since uncured rubber is actually cured in the mold and not die cut from a stock sheet material. This also allows for a more robust seal, because the material being used will be chosen based on actual seal use and not which standard sheet stock is available.
Die cut gaskets result in a lot of scrap from the excess material around the die form. The larger the gasket typically leads to greater amounts of waste. Since rubber is a thermoset, it can’t be reprocessed and must be thrown away. Molded gaskets only use the amount of rubber needed to fill the cavity of the mold, plus a little extra for overflow. New processes like Rubber Injection Cold Pot rubber molding can reduce even more material waste. Throughout high volume running and multi-year production, this can greatly reduce rubber scrap and limit the amount of cured rubber going to a landfill.
When comparing a die cut gasket to a molded shape based strictly on cost alone, a cut gasket will normally win. If you look at all of the design factors that go into a molded shape, molded gaskets outperform die cut gaskets. Molded shapes can give the seal design more effective sealing, which can reduce quality returns, loss of reputation and make products last longer. Contact an Apple Rubber Engineer to help design a new gasket or look at a molded shape to replace a current die cut gasket.