Can you use a different bulb trim size than what’s listed on your part drawing? The short answer is that it depends on the gap you need to fill, the bulb’s compression set, your part tolerances, and the channel size of the trim section. The long answer involves understanding what these terms mean, why they matter, and where the data you’ll need to consider comes from. Then, if you’re looking for a bulb trim distributor or rubber fabricator, you may need a partner with equipment that can stretch channels that are too tight.  

In this article from Elasto Proxy, you’ll learn how to determine whether you can use a different bulb trim size. Making this determination isn’t difficult, but you do need to make the right choice in order to avoid problems like doors that won’t shut, window seals that leak, or trim seals that fall off. If you’re a manufacturer with questions about bulb trim selection, usage, installation, or replacement, you can also contact Elasto Proxy to discuss your application.  

Note: See our Bulb Trim Guide for more information about these industrial rubber products.

Gap Size

Gap size is the amount of space between the two surfaces that you’re trying to seal. For example, let’s say you’re sealing a door. The gap is the amount of space between the door and the jamb. With a bulb trim seal, the gap is what the bulb (not the trim) needs to fill. To get the best possible measurement, you’ll need to record the gap size for each of the corners, add these numbers together, and then divide that sum by the number of corners. This gives you the average gap size and accounts for any differences between corners. To calculate a corner’s gap size, you can use modeling clay and calipers.

Compression Set

If you’re trying to fill a gap that’s 1” (2.54 cm), a bulb that’s less than 1” is too small. Yet, the bulb isn’t supposed to fill the gap exactly. Instead, it needs to be larger than the gap that requires filling. So, the questions become “Why?” and “How much?” The answers involve compression set, which measures the ability of a material to return to its original thickness. Here, it helps to remember the door in our gap size example. When the door is closed, the bulb compresses to completely fill the 1” (2.54 cm) gap between the door and the jam. When the door is opened, the bulb is supposed to return to its original dimensions so that it can be compressed again when the door is closed again.

Yet sometimes a rubber bulb doesn’t bounce back. That’s because it’s reached what’s known as compression set failure. When that happens, the bulb can no longer provide proper sealing. The way to avoid this problem is to identify the compression set percentage for the bulb trim size you want to use. Generally, the ideal percentage is 35% to 40%. So, if you’re trying to fill a gap that’s 1” (2.54 cm), choose a bulb that’s larger than this amount and that can compress by 35% to 40%. If the bulb compresses too little, your door won’t close. If the bulb compresses too much, you risk compression set failure.

Rubber Part Tolerances

Determining whether you can use a different bulb trim size also involves part tolerances, allowable variations in the physical dimensions between parts. If you’re an engineer who’s used to working with metal, it’s important to understand that rubber part tolerances are far less precise. That’s because rubber has different material properties and uses different production processes. In order to use a different bulb trim size then, your tolerances need to be right for rubber. Again, remember that the bulb section of the seal isn’t supposed to fit the gap precisely but to compress by a certain percentage.

Channel Size

The trim, or retainer, of a bulb trim seal needs to fit snugly over a flange or surface such as a door jamb. Otherwise, the trim may “pop” loose when the door is opened. Although rubber stretches more easily than metal, some bulb trim seals are harder than others. So, if you need to increase the size of the channel to fit a larger flange, you may not be able to stretch the rubber by hand. You could use hand tools instead, but you risk damaging the rubber. When you work with Elasto Proxy, however, we can use our specialized equipment to expand the channel to fit the flange.

Can You Use a Different Bulb Trim Size?

After reading this article from Elasto Proxy, we hope you can determine whether it’s fine to use a bulb trim size that’s different from what’s listed on your part drawing. If you have questions about your sealing application or are looking for a distributor and fabricator with large product selection, contact us.     

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