A bulb trim seal is a rubber gasket that provides sealing and insulation for doors or hatches with radiused corners. This type of industrial rubber product is sometimes called a trim seal or a bulb seal. No matter what you call them, bulb trim seals consist of a compressible bulb and a solid retainer section.

For engineers and designers, it’s important to understand how a bulb seal works. When the door or hatch is closed, the bulb compresses to form a seal between the door or hatch and the jam. The trim installs over the edge of the door or hatch. It provides a finished appearance and edge protection.  

Of course, it’s also important to know how to select and install the right bulb trim for your application. That’s why this guide from Elasto Proxy covers materials and measurements for the bulb section, how to specify the trim section, and best practices for selecting and installing trim seals.  

Bulb Trim Seal Basics: The Bulb  

The bulb portion of a bulb trim seal is made of a sponge rubber that provides environmental sealing and resists compression set – the permanent, unrecoverable deformation of a rubber seal after it’s been compressed. Different compounds are available, but EPDM and TPEs are often used.

EDPM sponge rubber remains flexible at low temperatures and provides excellent resistance to water, weather, sunlight, ozone, and aging. Thermoplastic elastomers (TPEs) are weather-resistant and provide greater impact strength. TPEs are more expensive than EPDM, but they’re also recyclable.

These bulb materials usually have a medium-density hardness or durometer. Generally, the shape of the bulb is round. If you need a bulb with an extended lip, then choose a bulb trim with a teardrop-shaped bulb instead. Trim seals can also have bulbs that resemble a half-moon with a flat bottom.

Bulb location varies. Depending on how your door or hatch swings open, you may need a bulb that’s atop the trim. A trim seal with top bulb is sometimes called a vertical bulb trim seal. You can also choose bulb trim seals where the bulb is on the side of the trim. There are many different options.   

Choosing the correct bulb size is critical. First, find the average jam dimension. You can place some modeling clay in each corner and then close the door to compress the clay. Next, insert calipers into the clay and measure each corner. Now divide this number by four to get the average jam dimension.

To account for bulb compression, add 25% to 50% to the average jam dimension. Most bulb trim seals are designed to compress by this amount. Remember, too, that over-compressing the bulb won’t create a better seal. In fact, over-compression can reduce seal life and cause compression set.

Bulb Trim Seal Basics: The Trim

The trim portion of the seal is made of a solid rubber like EPDM or TPE, or a plastic like PVC. For greater impact resistance, you may need a trim durometer that’s harder than the bulb durometer. Rubber trim is often smooth and black, but you can choose trim with a textured finish or in a custom color instead.   

For additional gripping, your trim section can have interior tongues. For superior impact resistance, choose trim with steel wire or segmented steel cores. You can cut this type of trim with a hand tool, but abrasive water jet cutting provides better quality cuts with reduced labor costs and less material waste.  

In terms of dimensions, specify both the edge thickness and leg length. The edge thickness needs to match the thickness of the door’s edge, so don’t add a percentage to the trim as you did with the bulb. Leg length is about covering the flange. If the flange is shorter, choose a trim with shorter legs.   

Because trim seals are used with rounded corners, you’ll need to specify the minimum edge radius. But don’t use these rubber gaskets with 90-degree corners. If you do, they’ll kink and leak. Finally, when you press the trim or retainer section into place, avoid buckling in the corners that can cause leaking.

How to Get Help with Bulb Trim Seals

Elasto Proxy can help you with bulb trim seal selection and installation. We can also source coils of rubber and cut them to size with our water jet cutting equipment. For additional assistance with bulb trim selection and installation, contact us.  

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