By Richard O. Angus Jr.
Loctite Corp.
Rocky Hill, Ct.
Edited by Sherri Koucky
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Robotic dispensing of CIP gasket formulation by moving the dispense head. |
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Gaskets that are heat cured are strong compression components that resist degrading in hot lubricating fluids, coolants, and fuels. |
Typical automotive gasketing applications include water-pump housings, oil pans, rocker-arm covers, and intake manifolds. Less-hostile gasket environments include lamp housings, electrical boxes, and air-filter housings. Here gaskets prevent intrusion into components of water and solid contaminants.
Conventional automotive compression gaskets are preformed items made primarily of rubber, although cork, paper, foam, metal, or other materials are also candidates. Foam is preferred for applications requiring high compression of the gasket or where the mating surfaces have high dimensional tolerances. Foam gaskets are also useful for reducing vibration and noise.
Conventional gaskets are either molded or cut from sheets to fit between two mating surfaces. Some applications glue the gasket to one substrate, the better to survive many disassembly/reassembly cycles, or temporarily bond it for easy removal during servicing.
The problem with conventional gasketing is that it implies a need for an inventory of preformed gaskets. Multiple gaskets, of course, require multiple inventories. And, while it's possible to automate gasket positioning, most still go on by hand.
CURED-IN-PLACE GASKETING
There is no such thing as an inventory of CIP gaskets. This is one reason they are fast replacing preformed gaskets. CIP versions are typically more precisely formed and molded to the part than conventional gaskets. No matter what their shape and size, they all come from the same drum of silicone formulation.
CIP materials may be dispensed via automated methods, permitting easy process changes during production. Parts sealed with CIP gaskets are easy to service and cure rapidly on the component. CIP materials that replace hand-assembled preformed gaskets also eliminate adhesives formerly needed to hold gaskets in place before assembly.
CIP gaskets strongly adhere to numerous substrates, including metals and many engineered plastics. They can also be designed with relatively low adhesion where they'll be replaced after long service use.
Cured-in-place gaskets begin as viscous pastes dispensed in a precise pattern and cured onto the component. Service conditions determine what CIP materials to use. Silicones resist a wide range of underhood fluids such as crankcase and gear oils, automatic transmission fluid, coolants including glycol water solutions, and new organic coolants. Silicones offer the best service temperature range of rubber products, from about –40 to 150°C for extended periods. They also resist weathering and degradation in most automotive fluids. These properties make them excellent candidates for sealing most of the automotive drivetrain. Good, all-purpose formulations are available for most automotive applications. Suppliers can formulate specialized materials for specific requirements.
Silicone-rubber CIP-gasketing pastes consist of linear silicone polymers and crosslinking agents. Crosslinking takes place via various chemical reactions which produce many different silicone formulations having their own performance qualities. Silicone formulations range from soft and squishy to hard and durable. If standard, dense rubber gaskets are too hard for a planned use, foam silicone gasketing materials may be a possibility. Air incorporated into this formulation attenuates sound, cuts vibration, and makes the gasket highly compressible.
Silicone-rubber materials commonly solidify using one of three cure methods: roomtemperature vulcanization (RTV), heat cure, or light cure. While all three silicone-cure chemistries can work in CIP gasketing, rapidcuring formulations are preferred for highspeed production. Light and heat-curing silicones are the most widely used formulations. Heat-cure chemistries deliver tough, strong compression gaskets and resist degradation in hot lubricating fluids, coolants, and fuels. However, they need a bit longer to cure than light-curing silicones. Ultraviolet light-curing materials cure rapidly to depths of about 6 mm, but are frequently more expensive than heat-cure materials. Both curing techniques are commonly used in conveyordriven continuous-cure systems. Regardless of the chemistry used, CIP-gasketing materials adhere to the component with varying bond strengths, based on the formulation and substrate.
RTV silicones develop green strength within a few hours of application and fullrated properties after 7 to 14 days at 50% relative humidity and 70°F. Heat-curing silicones cure completely after a few minutes at 200°C or a few hours at 150°C. Light-curing silicones typically are cured by irradiation with 365 nm light at about 150 mW/cm2 for about 30 sec.
GASKET MATERIAL DESIGN
Retention of sealing force is fundamental to silicone-rubber compression gasketing. A gasket seals only if the silicone rubber and mating substrates maintain enough sealing force. During initial assembly, the rubber pushes back against flanges squeezed together with an equal and opposite force.
Inevitable degradation of the rubber gasket reduces sealing force, eventually causing a seal failure. The best strategy is to choose a material likely to hold up over the life of the assembly or service period. For optimal retention of sealing force, gasketing material should fully cure before completing the assembly.
Rubber that has become permanently deformed under load is said to have taken a compression set. This quality is the basis for a common laboratory test (ASTM D395). A much more comprehensive test, Compression Stress Relaxation (CSR ASTM D6147) measures the sealing force (expressed as a percent of time/sealing force) as a function of time exposed to the test or service conditions. It is the preferred method for evaluating the long-term performance of rubber under compressive load.
Within a few hours under load, most seal materials become nearly permanently deformed and lose about half the time/sealing force. The rate-of-loss of the remaining sealing force is much more gradual. Test conditions are designed to be more aggressive than those in actual service. Chemists estimate the correlation between accelerated aging and real-life conditions during development of gasketing material.
FUNDAMENTAL SEAL DESIGN
Flat-surface CIP gasketing can directly replace molded gaskets without modifying the design when the sealing is low stress. To properly seal more rigorous applications, it's best to plan the seal design and material specifications together at the outset.
The compressed dimensions of the gasket will greatly affect the design of the groove into which the gasketing material will be applied. The volume of the gasket at operating temperature must be less than or equal to the volume of the groove for proper flange positioning. Designers often build in standoffs to limit gasket compression.
As service fluids can swell rubber, the seal designer can either choose to head off swelling by completely enclosing the seal or provide space into which the rubber can swell. Because pressure determines the sideways force on the seal, high-pressure applications usually require substantial grooves.
APPLICATION STRATEGIES
Silicone-gasketing formulations apply as a series of beads that form a pattern of uniform height. There is usually some sag, so the width of the bead will exceed the height. Tailored formulations will handle specific applications and provide aspect ratios (height to width) ranging between 0.5 and 0.9.
When CIP-gasketing materials get applied via a robot arm, assemblers must decide whether to move the dispense head or the component onto which the gasket is applied. As large components are difficult to move, most manufacturers elect to rack the parts on a conveyor, pass the components under a mobile dispense head, then convey them through the cure chamber. Small components that can be easily manipulated can be picked from a rack, positioned under a stationary dispense head, then placed onto dunnage for curing.
Dispense rate, nozzle position, and nozzle trace rate all control bead height. In the simplest case, all controls are held constant to generate a straight line. Maintaining a constant nozzle trace rate generates both straight lines and wide radiuses. For small radiuses, reduce trace rate. To minimize production cycle time, vary the nozzle trace rate with the geometry of the pattern. To maintain dimensional tolerances, manufacturers should closely control volume dispensed as a function of nozzle travel. Today's equipment and software can satisfactorily control trace rate, nozzle position, and speed within the small tolerances required for CIP gaskets.
Smooth knit lines produce close height tolerance, which can be illustrated with a straight-line region of a simple racetrack pattern. Dispense of the bead begins on the straight region and finishes when the dispensed material completes the racetrack pattern and meets the material at the starting point.
Approaching the finish point, the gasketing material must knit into the starting portion of the adhesive, maintaining height tolerance of the bead without introducing bubbles or leaving a void that could be a leak path in the seal. The pattern created looks a bit like a knot in a string, except that the height of the material in the knot area is uniform and equal to the height of the rest of the bead.
The nozzle realizes this configuration by beginning a little to one side of the straight line and a little lower than it will be during the bulk of dispensing. The dispense rate ramps from zero to the desired rate in the transition zone, and the nozzle raises to the full height and swings from one side to the middle of the line. As the nozzle approaches the finish point, it veers to the opposite side of the bead and descends to the start height as the dispense rate ramps down to zero.
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CSR performance of a typical silicone subjected to aggressive hightemperature fluidimmersion accelerated aging conditions. |
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When designing grooves into which gasketing material will be applied, compressed dimensions of the gaskets will greatly affect the design of the groove. Gasket compression is frequently limited by designing standoffs into the configuration. |