Steven Hughes
Applications Engineer
Stockwell Rubber Co. Inc.
Philadelphia, Pa.
Synthetic rubber gaskets and cushioning pads are readily die or water-jet cut. |
Notches should be designed into the gasket if hole(s) must be near the edge. |
Thermoset molded elastomers do not lend themselves to the same level of tolerancing of rigid machined materials. It's handy to use tolerance designations from the Rubber Manufacturers Association (RMA) for those parts. Dimensional tolerance designations are referred to as RMA A2 "Precision" and RMA A3 "Commercial." |
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Compared to die-cut gaskets, those that are water-jet cut have less concavity. This is true even for cellular materials up to 2-in. thick. The compartment-housing gasket is an ideal gasket design for water-jet cutting. The holes are small and placed close to the edge of the gasket. |
Geometrically, gasket design is a simple task. But for a gasket to perform properly and stand the test of time, material selection is key, even more so than the geometry of the part itself. Here's a list of material properties that should be taken into account when designing gaskets and cushioning pads. There are also a few die-cutting tips that will help ensure gaskets meet spec:
THE RIGHT MATERIAL
Durometer is the hardness of a material. The Shore A scale is typically used for solid elastomer compounds. Materials with a very low number are jelly soft while those with high values are hard like the sole of a shoe.
Force deflection is the "firmness" of a material. It represents the amount of force (in psi) needed to compress a square-inch section of material 25%. Force deflection instead of Durometer rating is generally used for specifying sponge materials. Typically a "soft" sponge has a deflection force of between 2 and 7 psi; "medium" ranges from 6 to 14 psi; and a 12 to 20-psi deflection force gives a "firm" rating.
Compression set is a percentage that a material does not rebound in relation to its original thickness, after being compressed for a predetermined time and temperature. This is a critical property for gasketing applications.
"Final gap" is the space remaining between two mating parts after being fastened together. The final-gap dimension is key for determining gasket thickness and the amount of compression that will need to be applied for an effective seal.
Temperature range that the gasket will operate in is also important. Extreme temperatures will require a higher-performance material such as silicone.
Chemical resistance of gasketing material is key to ensuring that there's no degradation and premature failure from exposure to chemicals, fluids, or fuels.
Flame-retardant optimization of the gasket material is needed for some industries and applications. The foremost standard for flame resistance is Underwriters Laboratories' UL94 flammability ratings.
Adhesive placement on nonsymmetrical gaskets must be called out on drawings.
Tolerance standards used for metal fabrication cannot be used when specifying tolerances in elastomers. The Rubber Manufacturers Association (RMA), www.rma.org, has published tolerance standards. The tightest tolerance gaskets come from water-jet cutting compared to conventional die cutting.
Width of a gasket wall, as a -general rule, should be larger than its thickness to avoid a "rolling" condition during assembly and to ensure a proper seal.
Load retention is similar to compression set, but relates to a material's ability to "rebound" over a long time period. It is based on long-term load testing in contrast to compression set which is based on short-term evaluations.
Special performance properties of elastomers including thermal and electrical conductivity can help solve design challenges.
RULES OF THUMB FOR DIE-CUTTING GASKETS
All die-cut parts will exhibit some concavity. This occurs at cut edges and where holes are cut through, as a result of the compression of the material during the cutting action. Additionally, thickness and the hardness of the material as well as the condition of the diecutting tool will contribute to its severity. In contrast, water-jet-cut gasket features won't show the same concavity as their die-cut counterparts.
Feature(s) placement in the gasket design will determine the feasibility of successfully making a steel rule die to cut the part. Here are a few rules of thumb for making sure a gasket can pass muster:
- The "flange width" is the distance from the edge of a hole to the cut edge or from the edge of a hole to another hole. It should be greater than 0.10 in. to accommodate the punch-tube diameter(s) and steel-rule thickness.
- The flange width should be greater than the material thickness. When flange widths are narrower than the material thickness the flanges tend to collapse during assembly and don't provide the same performance.
- If a hole(s) must reside close to the cut edge, designing notch(es) may be needed. If used, the minimum slot width should be greater than 0.09 in.
MAKE CONTACT:
Stockwell Rubber Co.,
(800) 523-0123,
www.stockwellrubber.com