Designing an effective seal is critical when you need to contain lubricant or fluid, prevent leaks or keep personnel safe. Seal selection can be a time-consuming process for engineers. Design criteria such as temperature, pressure and chemical interaction must be considered in addition to manufacturing, assembly and operating costs. When these factors are weighed, an inflatable bladder-type seal may be a better choice when compared to a standard compression seal for many enclosures, structures and processing equipment (Figure 1).

Figure 1: Example of inflatable seal profiles. Source: Pawling Engineered ProductsStandard elastomeric seals operate under the principle of compression. The compression of the seal against a surface creates a barrier that contains or prevents the ingress of fluids, gases or other contaminants. For these types of seals to be effective, they sometimes need to be compressed using very high forces. In contrast, inflatable seals are energized with compressed air to provide a positive seal and are more forgiving of irregular or misaligned mating surfaces, overcoming the effects of settling and warpage in the equipment over time. This cuts manufacturing costs and enhances sealing integrity. Inflatable seals are also easier to use and require less force while still providing an effective seal. The operator simply moves mating parts into place and then inflates the seal.

For these reasons, inflatable seals are increasingly found in processing equipment, pharmaceutical isolation chambers, semiconductor chip fab machines and even on hatches on large mega-yachts. They are core components in dry bulk processing and handling equipment and in pharmaceutical and food processing applications. Some of the fastest-growing industrial areas for inflatable seals are found in ultraclean industrial environments such as chip processing and pharmaceutical processing equipment (Figure 2).

Figure 2: Tablet coater door sealing application. Source: Pawling Engineered ProductsIn addition to keeping gases, liquids and contaminants in or out, inflatable seals also act as thermal seals to help keep hot things hot or cold things cold. Simply, inflatable seals save space, weight, maintenance and cost compared to other, more complicated designs.

Another unique application for inflatable “bladders” is as a clamp or actuator, especially when a soft touch is required. Examples of clamp actuator applications include round bar stock and tubing clamps during production, glass container or other fragile product handling in food and pharmaceutical applications, and as an inflatable conveyor brake which uses friction to provide a soft stop to the conveyor belt.

Seal selection design process

When selecting an inflatable seal, the first question the design engineer must address is the preferred direction of seal expansion. Inflatable seals can be configured for either inward or outward radial or axial expansion (Figure 3). Note that not all seal cross-sections are optimized for functioning in all directions, so it is important to consult with the manufacturer to ensure compatibility. Inward expansion can cause wrinkling in corners, requiring generous radii for optimal operation.

Figure 3: Expansion direction options for circular seal configurations. Left to right: inward expansion, outward expansion and axial expansion. Source: Pawling Engineered Products

Configuration

Configuration of the seal assembly is another important criterion. The basic configurations are circular, rectangular, U-shaped and straight. Each configuration can be used for inward, outward or axial expansion of the bladder. When combined with the direction of seal expansion, the most common configurations can be represented as follows.

• Circular: A round-shaped seal formed as a continuous loop (e.g., inward, outward or axial expansion) (Figure 4).
• Rectangular: A rectangular continuous loop with preformed corners (e.g., inward, outward or axial expansion) (Figure 4).
• U-shaped: A U-shaped seal with preformed corners and specially sealed ends. Ends are vulcanized in the deflated condition and do not expand (e.g., inward, outward or axial expansion) (Figure 4).
• Straight: Specially sealed ends; can be used as seals or clamps. Ends are vulcanized in the deflated condition and do not expand (Figure 6).

Figure 4: Rectangular, circular and U-shaped configurations. Source: Pawling Engineered ProductsDepending on the configuration, some seals may require specially formed corners or generous radii to operate properly.

Profiles

The next step in the design selection process is to select the seal profile. Although many cross-sections are available, most inflatable seal applications can be satisfied with one of six basic designs (Figure 5).

Figure 5: Six most common seal cross sections. Source: Pawling Engineered ProductsFor applications such as ultraclean rooms for the food and beverage industry, where cleanliness is of paramount importance, seals with smoother surfaces and fewer crevices to trap contamination allow for ease of cleaning.

The footed seal is the most commonly used shape and can be easily fixed in place using retainers (see “Seal retention” below). The high durability and pressure capacity of the footed seal make it ideal for many applications.

The footless seal is similar to the footed seal and has the same performance characteristics. Instead of a mounting foot, the footless seal is held in place by friction. Footless seals should not be retained by an adhesive since the base changes shape and does not remain flat after inflation. They also are not recommended for axial-expanding applications.

The convoluted snap and channel seal cross-sections have a high actuation travel to width ratio for filling large gaps. One limitation of this cross-section is reduced durability due to its thinner wall. Unlike the footed or footless seals, these profiles collapse into themselves when deflated and are typically reinforced with fabric.

The channel seal is designed for mounting in a groove or metal channel. The channel sidewalls help support the seal and keep it in place. Channel seals are held in place by friction or adhesive and are commonly used as clamps to hold and lift large cylindrical pieces (i.e., large sections of a satellite) in high pressure, low gap applications.

The high pressure, low gap channel cross-section is similar to the footed snap seal but is more durable and capable of handling higher pressures. The straight sides are easy to clean, making it an ideal choice for food and pharmaceutical processing equipment.

Additional design considerations to be addressed are air connection, corner style, seal retention, materials and reinforcing fabrics, corners, end treatment and joining of long extruded seals.

Air connection

Figure 6: Air connection in base of straight section. Source: Pawling Engineered ProductsMost inflatable seals require one air connection, which can be built into the base or the end, depending on design requirements (Figure 6). The bladder is sealed off and pressurized when inflated, like a car tire. Additional air connections may be required if inflating with a fluid, for larger seals, or for faster inflation rates.

Seal retention

Many options are available to retain inflatable seals mechanically, including clips, adhesive, extrusions and grooves.

• Stainless steel clips are an easy and common method for curved or rectangular surfaces.
• Adhesives are an acceptable mounting option for many cross-sections.
• Aluminum extrusions are a good mounting option for many straight sections (Figure 7).

For seals retained in grooves, size the grooves to the width of the seal plus the specified tolerance. This sizing method creates enough tension or compression forces to hold the seal in place and is recommended for circular, radial expansion in either inward or outward configurations, especially for smaller diameters.

Figure 7: Retention options for a footed seal in a straight configuration. Source: Pawling Engineered Products

Material

Inflatable seals can be extruded or molded in all the principal elastomers. Ethylene propylene diene monomer (EPDM) is the most commonly used elastomer for its combination of performance and price characteristics. Seals can be manufactured in a wide range of sizes, from 1 in to 60 ft in diameter. Molded seals can be manufactured in lengths up to 120 ft and, unlike extruded profiles, can be fabric reinforced.

Chemical resistance is an important factor for all seal material selection, and inflatable seals are no different. Many options in addition to material selection can be used to meet special requirements. For example, FKM over EPDM enhances chemical resistance and mesh over EPDM creates a conductive seal suitable for EMI isolation on a machine cabinet or entire shielded room. For extreme cold environments, a heating element used in conjunction with a silicone seal keeps it conformal to -60° F.

Special issues

Design engineers should be aware of the following special issues that commonly arise.

1. Cutting corners

Many applications require inflatable seals to operate around corners. Unless addressed, the seal may not rise to its full travel height at these points unless the bend radius is generous. For outward radial seal applications, corner radii of four to eight times the relaxed height of the seal is recommended. For inward radial seal applications, the radii should be at least eight times the relaxed height. The expanded seal height may still be lower than in straight sections.

Another corner issue designers may encounter is when the seal must wrap around a sharp corner in a pre-existing opening. Fillet pads can be purchased to create a sealing curve and can be mounted to either the seal or the opening.

2. Small diameter seals

Occasionally the outside diameter or inside diameter of the surface to be sealed is too small for a standard cross-section, especially in gripping applications. This occurs at approximately two inches in diameter. If this issue occurs, small molded inflatable bladders can solve the problem.

3. End treatments

Straight seals require a solid section lower than the active length at each end to seal them and provide an anchoring surface. Several clamping options are available to anchor them in place.

4. The “dip”

Areas on inflatable seals, especially ends, corners and intermediate joints, may not rise to the full height of their active section depending on the profile (and material) used. To accommodate this, the designer should create a 10% to 15% margin of safety in the specified inflated height to compensate.

5. Available air supply

Some engineers may have concerns about inflatable seals due to air supply issues. This is not a problem in most industrial or bulk transport applications as compressed air is readily available. Applications such as yachts do not need a steady supply of compressed air, as they require infrequent seal cycling and are primarily used to seal against the elements. If needed, they can be inflated by a foot pump or air compressor.

Specifications checklist

The best way to design an inflatable seal is to define the requirements by filling out a specifications checklist and consulting a reputable seal fabricator early in the process. Being prepared by reviewing the key specifications listed below will help speed up the process when working with an inflatable seal application engineer.

• Sealing against air, water, gas, other
• Pressure differential across seal
• Service temperature range
• Other environmental factors — UV, particulate, ozone
• Duty cycle
• Frequency of inflation
• Duration of inflation
• Applicable codes or standards
• The direction of expansion: radial inward, radial outward or face/axial
• Inflated height
• Overall dimensions for active length
• Corner radii if applicable
• Maximum allowable deflated profile

Pawling Engineered Products

Pawling Engineered Products is a leading global supplier of specialty rubber products, including custom inflatable seals and clamps, custom compression seals, gaskets and tubing, along with other highly engineered elastomeric fabrications.

Pawling’s application engineers are equipped to tackle customer’s tough application challenges and can devise innovative solutions, from practical to highly complex and for low-volume as well as high-volume jobs.

Inflatable seals

An inflatable seal is a rubber seal that inflates, rather than compresses, to create an air or watertight barrier. When compared to elastomeric compression seals, inflatable rubber seals are more forgiving of irregular or misaligned surfaces, which boosts sealing integrity. They are also easier to use because they require less force to ensure a good seal - just move the mating parts into place and inflate the seal to close the gap. This makes them the perfect solution for unique or difficult sealing applications.

Inflatable clamps

Figure 8: Inflatable bladder actuator in a conveyor brake application. Source: Pawling Engineered ProductsInflatable clamps use uniform controlled pressure to hold pieces in place during bonding, laminating, machining and cutting applications. Inflatable bladders offer superior performance, especially when so-called softness is required.

In addition to Pneuma-Seal and Q Inflatable Seals, Pawling’s Pneuma-Cel fabric-reinforced EPDM inflatable bladder clamps are ideal for unique or difficult clamping applications. Pneuma-Cel clamps are available in six standard sizes as well as custom sizes.

Inflatable actuators

Instead of making the seal, inflatable actuators push a secondary compression seal (or fixture) into place to create a uniform seal. A typical application involves an inflatable actuator pushing a harder material (e.g., PTFE) into a rotating piece of equipment to form a seal. Air drains from the actuator when power is lost.

Pneuma-Seal and Q Inflatable Seals can be used in inflatable actuator applications.

Conclusion

Inflatable seals are an ideal option when an engineer needs to reduce the cost of a design involving elastomeric seals or improve the long-term integrity of a seal. Applications with imperfect mating surfaces or irregular shapes can be easily handled using an inflatable bladder. Even if it seems counterintuitive, the inflatable seal may result in superior performance to conventional seals at a lower cost.

Pawling Engineered Products has been a leader in specialty rubber products for over 50 years and has the capabilities and the expertise to help design, select and manufacture an inflatable seal for the most demanding applications.

Contact Pawling Engineered Products today.