ASTM F36-15(2021) Standard Test Method for Compressibility and Recovery of Gasket Materials

In the “ASTM F36-15 (2021) Standard test method for compressibility and recovery of gasket materials” developed by the American Society for Testing and Materials (ASTM), it is stated that the short-term compressibility and performance of sheet gasket materials, molded-in-place gaskets and, in some cases, gaskets cut from sheets at room temperature. A test method for determining recovery is described.

This test method is not intended as a test for compressibility under prolonged application of stress, commonly referred to as creep, or conversely for recovery following such prolonged application of stress, commonly referred to as compression set. It is also not designed for use in tests outside room temperature. If desired, a resilience property can also be calculated from the test data (the amount recovered is expressed as a percentage of the compressed thickness).

Briefly, this testing method is performed with both sample and apparatus at the required temperature and compressibility and recovery are calculated.

Gasket compression promotes housing sealing or contributes to gasket failure. This is because rubber seals are durable, but only to a certain extent. Compressing a rubber gasket within permissible limits ensures a reliable seal. However, if the seal is over-tightened, the rubber will not rebound when the compressive stresses are removed. This creates a gap between the rubber gasket and the surface of the housing. Gaps cause leaks, and leaky seals will not support larger product designs.

Rubber seals are compressed or squeezed to a certain percentage of their original size. A gasket that is compressed too much or too little will not provide a proper seal. The ideal compression percentage is 40, with a maximum of 50 and a minimum of 10 to 15.

If the gasket is compressed more than 50 percent, it may not return to its original size. For example, when a lid is closed, the gasket is compressed and fills the space between the lid and the base material. When the lid is opened, the seal must return to its original size so that it can be compressed again when the lid is closed. However, if the gasket is over-tightened, the rubber will not rebound and cannot close the gap. If the problem is that the gasket compression percentage is less than 10 percent, there may be no contact between the two surfaces being attempted to seal, which will not keep air, dust, or water out.

In plastic and metal enclosures, excessive compression can occur when gaskets are installed between metal parts held together by fasteners. If the bolts or screws are overtightened, the seal may be permanently deformed. In other words, the rubber reaches the compression set. When bolts or screws become loose, gaps form between the metal parts and the gasket material.

Applications where temperature changes and vibrations occur are particularly susceptible to fastener loosening. For example, mobile equipment and generator set enclosures may be exposed to hot and cold temperatures that cause the metal to expand and contract. It is also exposed to vibrations from diesel engines. Under these dynamic conditions, even properly torqued fasteners can become loose.

Material loosening also brings seal-related difficulties. In the case seal, a compressed rubber pushes back against the metal parts, filling small gaps that would otherwise form between metal surfaces. However, within an hour of installing the gasket, this reciprocal force may be only 75 percent of the original force. There may be no case gasket failure, but leakage may occur.

Compression-related problems are not limited to flat seals with fasteners, either. Rubber profiles attached to metal surfaces with tape or adhesives are also exposed to excessive pressure. Examples include head gaskets on military vehicles and dishwasher gaskets on appliances. In the second example, the use of hot water and detergent can affect the material properties of the rubber seal and reduce its performance.

Meanwhile, in the “ASTM D395-03 Standard test methods for rubber property - Compression set” standard, three different test methods are defined for the compression setting in rubber materials.

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