Proper gasket design can simplify manufacturing and extend product life

Don’t let gaskets be a design afterthought.
Engineers often wait until the tail end of the design cycle to think about gasket requirements, but the small components are critical to ensuring a product’s performance and life expectancy. Gaskets protect equipment and assemblies from the environment, wash downs, dust, shock and vibration, and electrostatic discharge and electromagnetic interference.

Engineers need to think about gaskets early in the design process to save time and money. One way to make sure this happens is to develop a gasket strategy that steps through the four requirement areas: function, materials, mechanical, and manufacturing.

Define the function
The first step in gasket design is defining what the gasket will be expected to do. Because a gasket’s primary task is to protect components, it must withstand its operating environment. Is it a dust gasket or meant to seal out wind-driven rain? Does it need to resist harsh chemicals or raging flames?

The gasket may also have to contend with electromagnetic interference (EMI) and electrostatic discharge (ESD). Or it may have to conduct heat or insulate against heat or electricity.

The regulatory environment is another consideration. Gaskets can help designs meet industry requirements like those from the National Electrical Manufacturers Assoc. (NEMA), International Electrotechnical Commission (IEC), Underwriters’ Laboratory (UL), the U.S. military, ASTM International, and the U. S. Food and Drug Administration (FDA).

Select materials
With hundreds of materials and material combinations to choose from, picking the most cost-effective one that meets your application’s requirements can be challenging. Properties you should look at include polymer cellularity, durometer hardness, and compression-force deflection.

Cellularity refers to whether a polymer has cells that are open or closed to each other or whether the material is solid. Open-cell foams tend to absorb fluids, although some of them resist absorption when compressed over 30%. Closed-cell foams don’t absorb water and neither do solid materials. Solid gaskets need more force to compress than cellular materials do, however.

Durometer hardness is usually measured on the Shore A scale for solid materials. Shore A 10 to 20 is considered soft, about the hardness of a bicycle handle grip. Shore A 30 to 40 is medium, like a rubber band. And Shore A 60 to 70 is as hard as a car tire. Gasket materials can go up to 90 Shore A.

Compression force deflection (CFD) is the force required to compress 1 in.2 of material 25%, per ASTM D 1056. It is usually used to describe the firmness of foams and sponges. Materials range from very soft Grade 0 that takes less that 2 psi to compress to firm Grade 4 materials that don’t compress until 16 to 30 psi has been applied.

Of course, materials also need to withstand the environment in which they’ll be working. For example, enclosures that will be outside need to withstand rain, extreme temperatures, and UV light from the sun to prevent electronics failure and unplanned maintenance. Gaskets commonly fail due to environmental exposure through deterioration, stiffening, and compression set — inability to rebound after being compressed.

For intense UV exposure, designers often use gaskets made of silicone or ethylene propylenediene ASTM D1418 Class M (EPDM) rubber. Silicone also resists temperatures from –100 to 500°F. For gaskets that will be exposed to rain or high-pressure wash down, solids, closed-cell sponges or compressed open-cell products work well.
Gasket manufacturers are good resources to tap when help selecting materials for particular applications is needed. And they can provide material data sheets and sample swatches.

Facing mechanical factors
Mechanical-design factors such as housing rigidity, gasket compression limits, gasket thickness, and fastener pattern should also be taken into account.

Housing rigidity is most important in parts made from engineered plastic or thin metal. Both can bow or flex under high loads leading to gaps and other closure problems.

Using soft cellular materials like silicone, neoprene, or EPDM sponges, or silicone or urethane foams minimizes compression rebound forces. Low-durometer solid silicone with Shore A hardnesses of 10 or 20 can also reduce compression force. Heavy-duty applications often use heavy-steel flange gaskets made from solid rubber, fabric-reinforced rubber, or composites.

Gasket compression limits are important because more tightening does not always mean a better seal. In fact, overtightening fasteners is a common cause of gasket failure. Applying too much fastener load can burst cells in closed-cell foams and crush or tear gaskets. Choose a gasket that can withstand compression forces that are reasonably foreseeable in its application. And adding compression stops to seals that are frequently overtightened can extend gasket life.

Gasket thickness requirements depend on sealing-face tolerances and fastener spacing. Two machined-flat mating surfaces seal with thin gaskets because the gaskets only make up for small tolerances. In contrast, sheet-metal parts have looser tolerances, and thicker gaskets can better absorb tolerance stack-up between mating surfaces.

When fasteners are spaced far apart, mating surfaces, even those made of rigid materials, tend to bow. A common example of this is an enclosure door with a hinge on one side and a latch on the other. When gaskets are unevenly compressed, they can also move when hit with debris or spray. Use thicker and softer gaskets to take up misalignment and provide adequate closure.

Managing manufacturability
During gasket design, don’t forget to consider how the gaskets will be made and how they’ll fit into assembly. Common gasket-manufacturing techniques include high-speed die cutting, waterjet cutting, roll slitting (for strip gaskets), injection molding (for liquid silicone rubber), and material lamination.

Production quantities affect which manufacturing techniques make sense. For example, for orders up to 500 pieces, gaskets or pads may be waterjet cut because there’s no tooling and the process uses material efficiently.

But once orders reach the thousands, the higher production rates of die cutting, molding, and slitting are preferred. The line between high and low-volume production depends on geometry and material type.
Adhesive backings laminated to gaskets during manufacturing can simplify gasket installation and bring down cost. Kiss cutting, a commonly used die-cutting variation, uses the die to cut the gasket and adhesive, but leaves the release liner intact. Workers then wind up the parts and liner on a roll for easy installation.

Tolerance call-outs in gasket drawings differ from those for metal components. Soft materials compress and move slightly during cutting, but the degree to which these occur depends on the material, its thickness, and the manufacturing process.

For example, cutting surfaces compress soft foams before they break the foams’ surface tension. The result is a concave edge, the degree of which depends on material firmness and thickness.

Generally, thicker materials require looser tolerances. And waterjet-cut parts have tighter tolerances than die-cut parts.

The Rubber Manufacturers Assoc. publishes a tolerance table, but most fabricators cut parts to tighter tolerances and may even offer their own tolerance tables.

Custom parts may not represent too much of a price increase over stock parts, depending on the manufacturer and process. For low-volume parts, a dxf-format CAD file is all that’s needed to start waterjet cutting with no setup or tooling charges. Even steel-ruled cutting dies used to cut the majority of gaskets and pads cost $200 to $400.

Manufacturers will often work with engineers to minimize development and manufacturing costs. Some will waterjet samples at low cost to help prove out design concepts.

© 2010 Penton Media, Inc.