Elasto Proxy’s bulb trim guide explains how to select, use, and install rubber products that are also known as trim seals.

Bulb trim provides sealing and insulation for doors with rounded, or radiused, corners. This type of industrial rubber product consists of a sponge rubber bulb and a solid rubber trim, or retainer. Each section has a different durometer, or hardness, as measured on the Shore A scale. Bulb trim are co-extruded rather than molded, and they’re sold in coils, as cut lengths, or as finished gaskets.    

How Bulb Trim Works

The trim section of the seal installs over the edge of the door and provides a finished appearance and edge protection. The bulb fits between the door and the door jamb. When the door is closed, the bulb compresses to form a seal. When the door is opened, the bulb returns to its original dimensions – unless permanent deformation (compression set) has occurred.

Rubber Bulb Basics

Bulb Material

EPDM and TPE are common choices for the bulb material. EPDM is recommended if you need a non-recyclable product that remains flexible at low temperatures and resists water, weather, sunlight, ozone and aging. TPE provides these same material properties but is recyclable and comes custom in colors. TPE is more expensive, however.

Bulb Durometer and Cellular Structure

The bulb is made of medium-durometer sponge rubber. This durometer needs to support compression without permanent deformation. Sponge rubber has a cellular structure with either open, interconnected cells (open cells) or closed, balloon-like cells (closed cells). Open cells permit the passage of air, water, and other chemicals when the bulb is not compressed. Closed cells do not.

Bulb Shape and Location

Generally, the bulb is round; however, you can also choose a seal with a bulb that is teardrop-shaped (extended lip) or that looks like a half-moon with a flat bottom (D-shape). The location of the bulb also varies, so make sure to account for how your door opens. You can choose a trim seal where the bulb is on top (vertical bulb trim) or on either side of the trim. 

bulb trim
Extended lip (top), side bulb (left), top bulb (right)

Bulb Size and Average Jam Dimension

Choose a bulb size that fills the gap and supports compression. First, find the average jam dimension. 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. Then add 25% to 50% to  the average jam dimension to account for compression.

Rubber Trim Basics

Trim Material and Durometer

The retainer, or trim, section of bulb trim is made of solid rubber instead of sponge rubber. Choices include EPDM,  TPE, or PVC – a  plastic material. Typically, the durometer of the retainer is greater (harder) than that of the bulb. That’s because the trim section doesn’t need to support compression. Instead, it needs to provide impact resistance. Typically, the trim also has a smooth finish.

Gripping and Reinforcement

The retainer attaches to an edge, but interior tongues can provide additional gripping. For superior impact resistance, choose a trim section with either steel wire or segmented steel cores. Remember, however, that metal-reinforced trim is difficult to cut with a hand tool. Although you can buy coils of bulb trim from Elasto Proxy, you’ll save time and money with our cut lengths or finished gaskets.  

trim seal
This trim section is reinforced with steel wire for impact resistance.

Edge Thickness and Leg Length 

When selecting bulb trim, specify the edge thickness and the leg length. The edge thickness needs to match the thickness of the door’s edge. Therefore, do not add a percentage to the trim as you did with the bulb. Leg length is about covering the flange. If the flange is short, choose a trim with short legs. If the flange is tall, choose a trim with long legs instead.  

trim seals
The leg length for the trim is 14mm.

Minimum Edge Radius and Installation

Because trim seals are used with rounded corners, you’ll need to specify the minimum edge radius. Importantly, 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. By selecting bulb trim properly and installing it the right way, you can enjoy sealing success.

Bulb Trim from Elasto Proxy

Elasto Proxy sells standard bulb trim in coils that we keep in our warehouses. We can also special-order trim seals for you. We’re not just a distributor, however. We’re also a manufacturer. So, consider combining your order with our added-valued services. Buy our cut lengths to eliminate in-house cutting, and order finished gaskets that arrive ready to install.

Leave a Reply