Soft gasket materials like sponge rubber are tricky to cut accurately because they tend to distort. We deal with this by offering waterjet cutting. It’s a fast and efficient process that’s ideal for cutting large, complex shapes from thinner material. Here’s why to ask for it.

What is Waterjet Cutting?

The process uses a thin jet of fast-moving water to slice through material. The machines look like laser cutters with a gantry that moves the cutting head over a sheet of material. A pump pressurizes the water to around 90,000 psi, forcing it through an orifice 0.015” diameter. This stream of water moves faster than sound, which lets it wear away material it flows against.

For faster cutting an abrasive material is fed into the stream. Abrasive waterjet cutting can cut metals as well as non-metals.

Advantages for Gasket Materials

• Applies little force to the material being cut, which minimizes distortion
• Accuracy of +/-0.007”
• Kerf (the width of the material cut away) less than 0.050”
• Tight nesting is possible to improve material utilization
• Cut holes with diameters as small as 20% of the sheet thickness
• Cut stacks of sheet material: Maximum cut thickness is around 6”, although this is material dependent

In comparison with laser cutting, two advantages are:

• No heat affected zone on material edges
• High cutting speeds on nonmetallic materials: Rubber sheet ¾” thick can be cut at up to 90’/minute.

Limitations

As the jet moves through the material it deflects rearwards from the direction of travel and can waver slightly from side to side. This results in striations along the sides of thicker material and requires lower speeds when moving into corners.

On thick material jet divergence creates a slight taper from top to bottom. This is handled by angling the waterjet away from the edge of the piece being cut, so putting all the taper in the offcut “lace”.

Gaskets Cut by Water

Hennig Gasket and Seals can waterjet gaskets up to 6’x8’ and 5’x13’. If you’re interested in waterjet-cut gasket material, contact us for a quote.

Compressibility goes to heart of what gaskets do. Here we’ll explain what gasket compressibility is and why it’s such an important property of gasket materials.

Compress to Seal

Gaskets are used to seal mating services, particularly between flanges. They take up imperfections in the two surfaces that would otherwise let the fluid being sealed leak out, and accommodate any changes in the gap as might be caused by temperature changes or vibration.

Gasket material works by deforming against the harder flange to take up even microscopic deviations in the surface. Compressibility is a measure of how readily the material does this.

Recovery From Compression

Gasket compressibility goes hand-in-hand with recovery. This is the degree to which the material springs back to its original thickness after being compressed.

Recovery matters because every joint is somewhat dynamic. Whether it’s opened and closed repeatedly, like an access door or panel, or just subject to varying temperatures, loads and vibrations, the effective gap being sealed will keep changing size. To maintain a seal the gasket material must expand and compress with these changes.

Closely related, compression set is when a material doesn’t spring back to its original thickness as the clamping load is removed.

Compressibility Measurement and Specification

The ASTM F36 test provides a standardized way of measuring gasket compressibility. The data needs some interpretation because the test conditions almost certainly don’t replicate your application, but it lets you compare materials.

Compressibility is stated as a percentage of the original thickness. Recovery is given as a percentage of the compression.

Material Selection Considerations

In general, materials that compress readily under light loads lack strength. Open cell foam is a prime example. You wouldn’t use this to seal a pressurized gas or liquid as it would almost certainly fail.

EPDM, neoprene and cork are examples of materials with good compressibility. Higher strength materials tend to have lower compressibility.

Gasket material manufacturers publish ASTM F36 test data, but for advice on a specific application, consult a gasket specialist, like those at Hennig Gasket & Seals.

Two styles of gasket are used for pipe flanges: full face gaskets and ring gaskets. Full face gaskets need holes for the flange bolts to pass through. When you order this type of gasket the manufacturer needs to know the diameter of the bolt circle. Here we’ll explain what that is and how to measure it. First though, an explanation of the two gasket styles.

Gasket Styles

A full face gasket has roughly the same outside diameter as the flanges being joined. The inside diameter is of course equal to the bore of the pipe. Flange bolt holes in the gasket simplify assembly as the bolts hold the gasket in place and stop it intruding into the flow. The downside is that all the bolts must come out when installing a new gasket.

In contrast, a ring face gasket sits inside the bolts and can be replaced without completely disassembling the joint. They also have a smaller outside diameter, which saves material. They are used principally on raised face flanges where the sealing surfaces are inside the bolt hole diameter.

A full face gasket can be used with raised face flange. Likewise, ring gaskets can be used on flat face flanges, providing the impact of the reduced gasket width is considered.

Bolt Circle Diameter Measurement

The bolt circle is defined as a circle running through the centers of all the bolt holes. As these points are in space they can’t be measured directly. However, there is way.

The bolt holes will all be the same diameter and arranged in diametrically opposite pairs. Looking at the flange end-on, measure from the left edge of the leftmost hole to the left edge of the hole diametrically opposite. (Or measure from right edge to right edge.)

To eliminate any measurement error, repeat on a number of other hole pairs and average the result. This is the number you’ll give your gasket manufacturer, along with inner and outer diameters, number of bolt holes, and the bolt hole diameter. With this information they’ll cut the gasket to the size you need.

Gaskets in flanged joints fail in two ways. Either they allow a very slow leak or they blow out. A blowout can be quite spectacular, but also dangerous and expensive. As it’s something to be avoided, here’s an explanation of what causes it and some ways of making sure it doesn’t happen.

Recognizing a Blowout

A blowout occurs when the gasket material fails catastrophically. This can happen when the gasket is resisting significant internal load. A steam system might be one example but it could be any application where the media being sealed against is under pressure. When this pressure exceeds the strength of the gasket the material is likely to fail fast, forming a hole that releases the media.

In a steam system, this results in a dangerously hot jet. In other situations, it could be acid or a flammable fluid that escapes.

Blowout Prevention

One factor is the tensile strength of the gasket material, but a flange system is more complex than that. The gasket is held in place by the clamping force. This creates friction between the flange surfaces and the gasket, and that resists movement.

Analysis of the forces in a flanged joint shows that clamping force is a bigger factor in blowout prevention than material tensile strength. (The full analysis is available on the Fluid Sealing website.) Thus anything that reduces clamping force can cause a blowout. Here are six tips to prevent that happening.

1. Follow best practices when bolting flanged joints together. Use the recommended tightening pattern.
2. Don’t overtighten. This makes the flanges rotate, compressing the gasket at its outer edge while there’s little or no contact on the inside. It can even crush the material, guaranteeing a failure.
3. Minimize vibration through the joint.
4. Choose gasket materials with low creep. (Silicone and nitrile rubber are particularly good.)
5. Consider the effect of temperature. High temperatures will lengthen bolts, (which reduces clamping load,) and let gasket material creep. Conical spring washers may help.
6. Use the thinnest gasket possible that handles unevenness in the flange faces.

Industrial enclosures often have to meet NEMA and IP standards for ingress protection. These aren’t the only standards designers should consider though. There’s also UL50 and UL50E. Established by Underwriters Laboratories, these were developed to ensure that electrical enclosures would be safe. Safety depends at least in part on the gasket, so there are UL standards for gasket materials too.

The UL difference

Achieving NEMA or IP certification entails passing some stringent tests. However, these tests only verify dust and water resistance. They don’t explicitly test the gasket material. UL testing does.

Gasket material is tested because environment and use affect how long it lasts. Some gaskets are exposed to UV outdoors. Others, especially if near high voltage equipment, can be attacked by ozone. Some enclosure gaskets will endure periodic recompression when the door is repeatedly opened and closed. Other are in continuous compression, as might be the case around an electronics assembly.

These variables place different demands on the gasket material. Many gaskets might hold up to NEMA and IP testing but the material could degrade slowly and reduce the protection the enclosure provides.

Gasket Material Certification

To address the impact of use and environment on enclosure performance, UL developed tests for gasket material. These are defined in UL157, “Standard for Gaskets and Seals”. This covers both foam and solid elastomeric and composite gasket materials.

Materials passing UL157 can be considered as “UL Recognized Components”. Using components certified this way in an end product simplifies the process of getting UL approval. (Other UL standards may also be applicable.)

Buy UL-Rated Gasket Material

When designing equipment, using UL-rated materials will save time and money in testing. And later, when gaskets are being replaced, it might also be prudent to stick with UL157 materials. These range from neoprene and EPDM foams to silicon and even cork. (But note that adding a pressure sensitive adhesive nullifies any prior UL rating.)

Finding the right UL-rated gasket material for a specific application can be difficult. If unsure what to use, consult a material specialist at Hennig Gasket.