With the increase of computing and monitoring capabilities, sensors are finding new and diverse applications. Along with these new applications come the need to miniaturize the sensor and accompanying components while maintaining a robust design that will perform well in a variety of environments.
There are a number of ways to successfully seal the environment from delicate electronic and optical devices. Plastic or metal components may be welded. Other applications may use potting, adhesives or dispensed RTV sealants. All of these processes can provide a hermetic seal. Some are messy and time-consuming, however, and most do not allow the unit to be opened or removed.
A well-designed elastomeric seal will provide the necessary sealing performance and reusability. Here are five considerations to help make your sensor sealing application as durable as possible.
1. Chemical compatibility/temperature
Compatibility of the seal material and application chemicals is a primary concern. It’s important to identify the chemical(s) to be used in the application. Regardless of all other critical design factors, if the basic composition of the O-Ring material is not compatible with its chemical environment, the O-Ring will eventually fail. A primary step in O-Ring selection, therefore, is to match the application’s chemicals with the O-Ring material that offers the best chemical resistance.
In order to do this, it’s important to select a seal material that will exceed the operating temperatures of the application. It’s important to determine the temperature at the seal location, and not assume it’s the same as the heat or cold source a short distance away.
Ozone is a gas composed of three atoms of oxygen. It benefits us by shielding us from the sun’s harmful ultraviolet rays in the upper atmosphere but can deteriorate rubber at ground level. Ozone at ground level can be formed by reactions of oxides, volatile organic compounds and electrical motors. This highly reactive gas attacks unsaturated or double and triple bonds found in rubber polymers.
This attack can cause rubber to show cracks in the surface. With continuous exposure, it can also cause the part to fail. This failure mode is also duplicated by long-term exposure of UV light to the same type of polymers. Materials like unprotected nitrile, natural rubber, SBR and Butyl rubber show poor resistant to this. Silicone, EPDM, HNBR and FKM material have high resistant to ozone and UV.
Rubber compounds are a matrix of polymer, fillers, oils, protectants and curatives. Some of these are not reactive to the polymer, so they can easily be released under extreme conditions. Typically, under vacuum or high-temperature applications, low molecular weight oil or unreacted curatives can outgas from the rubber and deposit on electronics or critical connectors.
A standard screening test is ASTM E 595, with limits of less than 1 percent for total mass loss (TML) and less than 0.10 percent on the collected volatile condensable material (CVCM). Materials that pass this test are typically considered low outgassing materials.
Sulfur and sulfur donor curatives are used to cure many rubber compounds. Rubber compounds cured with sulfur can emit sulfurous emissions when exposed to heat and can cause corrosion to silver or copper used in electronics. This can cause tarnishing, which effects critical performance of electronic instruments. Use of peroxide cured rubber helps eliminate this.
Designers are being asked to fit more and more components into a given device. As a result, the individual components need to be as small as possible without compromising performance. The O-Ring cross-section is the primary feature to consider because this is the section that is squeezed and creates a barrier between two differential pressures.
The tolerance stack of the mating components and seal will determine the minimum O-Ring cross-section. The cross-section must be large enough to absorb the total tolerance stack of the assembly while being within the typical recommended range of 10 to 40 percent for static, and 10 to 30 percent for dynamic applications.
Here are recommended minimums when it comes to O-Ring seals used with tight tolerance precision hardware components:
- A Ø1.0mm (.039”) cross-section O-Ring is typically the minimum seal that can be applied to dynamic radial applications.
- And a Ø0.50mm (.020”) cross-section is typically the minimum seal that can be used in static axial applications.
*Note, these are general limitations and each application should be reviewed individually.
5. Design for assembly
There are three primary O-Ring configurations: axial face, radial rod and radial piston. Here are some items to consider for each when designing with assembly in mind.
- Axial face seal: Does the seal’s assembly or function require the seal to be retained? For example, a small battery cover may need to be removed periodically. If this is the case, design the seal and cover so the seal does not fall out when opened by an operator in the field. Face seal designs with a non-circular groove would make assembly easier if the seal is a net molded shape that dropped right in rather than trying to deform a circular O-Ring to fit the groove geometry.
- Rod seal: This configuration has a female gland to retain the O-Ring. It may be difficult to machine the gland within the bore and to install and visually see if the O-Ring is located correctly. This is especially true with small bore diameters. If the bore is too small for the O-Ring to be folded into, then design a two-piece gland so the O-Ring can be inserted and the other gland wall installed. Just be sure the rod component does not have sharp edges or threads that may cut and damage the seal.
- Piston seal: This configuration has the gland located directly on the OD of the male component. Installation and visual inspection can be easily done. Cover threads or other sharp features if the seal needs to be installed over them. Break all sharp edges, especially the lead-in for the bore. A lead-in angle of 15° to 20° is recommended – this will gently squeeze the seal during assembly.
Why Apple Rubber?
If you require a unique sealing solution, we have the resources to explore and develop new designs, materials and processes. Have more questions about O-Ring seals in sensor applications? Ask the experts. We would love to work with you. For additional information about our products or capabilities, visit us online at applerubber.com.