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Methods of Failures


Extrusion

When extrusion or nibbling occurs, the seal develops ragged edges, which can leave it looking tattered. This generally appears on the low-pressure side. In general, extrusion failures are caused by one of more of the following factors:
  • Excessive clearances
  • Excessive pressure
  • Low modulus or hardness elastomer
  • Excessive gland fill
  • Irregular clearance gaps
  • Sharp gland edges
  • Improper sizing
Solutions to prevent extrusion include:
  • Decreasing clearance
  • Use a higher-modulus or harder elastomer
  • Properly designing the gland
  • Using polymer back-up rings

Over-Compression

Over-compression causes mating surfaces to appear flat. It may also cause the flattened surfaces of the seal to develop circumferential splits. In general, over-compression failures are caused by one of more of the following factors:
  • Improper design (failure to account for thermal expansion)
  • Chemical volume changes
  • Excessive compression

One solution to prevent over-compression is to consider the material's response to chemical and thermal environments during the gland design process.

Abrasion

Abrasion generally happens in dynamic seals, subjected to reciprocating, oscillating or rotary motion. This failure causes laceration and breaking on the O-ring's surface that experience excessive wear. Abrasion is caused by one or more of the following:
  • Rough sealing surfaces
  • Excessive temperature
  • Process environment containing abrasive particles
  • Dynamic motion
  • Poor elastomer surface finish
Solutions to prevent abrasion include:
  • Use recommended gland surface finishes
  • Consider internally lubed elastomers
  • Eliminate abrasive components

Compression Set

Compression set causes an O-ring's cross section to lose its circular or O-shape and resemble a flat sided oval. The flat side corresponds to the mating seal surfaces. Contributing factors may include one or more of the following:
  • Excessive compression
  • Excessive temperature
  • Incompletely cured elastomer
  • Elastomer with high compression set
  • Excessive volume swell in chemical
Solutions to prevent compression set include:
  • Low compression set elastomer
  • Proper gland design for the specific elastomer
  • Confirm material compatibility

Outgassing or Extraction

This failure is difficult to detect as the O-ring typically shows no visible change. In some instances, it may exhibit a decrease in cross-sectional size. In general, outgassing or extractions are caused by one of more of the following factors:
  • Improper or improperly cured elastomer
  • High vacuum levels
  • Low hardness or plasticized elastomer
Solutions to prevent outgassing include:
  • Avoid plasticized elastomers
  • Ensure all seals are properly post-cured to minimize outgassing

Installation Damage

Damage that occurs to the O-ring during the installation process presents as small cuts, nicks, and gashes to the surface. Installation damage can be caused by one of more of the following factors:
  • Sharp edges on the glands or components
  • Improper sizing of elastomer
  • Low-modulus or low-hardness elastomer
  • Elastomer surface contamination
Solutions to prevent installation damage include:
  • Remove all sharp edges
  • Proper gland design
  • Proper elastomer sizing
  • Use a higher-modulus or harder elastomer

Chemical Degradation

When chemical degradation occurs, the seal may exhibit signs including blisters, cracking, voids, or discoloration. In some cases, the degradation is observable only by measurement of physical properties. In general, chemical degradation is caused by:
  • Incompatibility with the chemical and or thermal environment

To prevent this failure from happening, use a more chemical-resistant elastomer.

Thermal Degradation

Thermal degradation may cause radial cracks to appear on the O-rings. They will be located on the highest temperature surfaces. In addition, certain elastomers may exhibit signs of softening or shiny surface as a result of excessive temperatures. In general, thermal degradation are caused by one of more of the following factors:
  • Elastomer thermal properties
  • Excessive temperature excursions or cycling
Solutions to prevent thermal degradation include:
  • Selecting an elastomer with improved thermal stability
  • Evaluation of the possibility of cooling sealing surfaces

Spiral Failure

When a spiral failure occurs, a spiral pattern or mark appears on the circumference of the O-ring. Spiral failures are typically caused by one or more of these factors:
  • Difficult or tight installation (static)
  • Slow reciprocating speed
  • Low-modulus or durometer elastomer
  • Irregular O-ring surface finish, including excessive parting line
  • Excessive gland width
  • Irregular or rough gland surface finish
  • Inadequate lubrication
Solutions to prevent spiral failures include:
  • Correct installation procedures
  • Use a higher-modulus elastomers
  • Internally lubed elastomers
  • Proper gland design
  • Gland surface finish of 8-16 micro-inch RMS
  • Possibly use polymer back-up rings

Plasma Degradation

When plasma degradation occurs, there is erosion on the O-ring where it was exposed to the plasma. This type of failure generally happens in electrical applications. In some cases, this appears as discolouration or powdered residue on the surface. Contributing factors may include:
  • Chemical reactivity of the plasma
  • Ion bombardment (sputtering)
  • Electron bombardment (heating)
  • Improper gland design
  • Incompatible seal material
Solutions to prevent plasma degradation include:
  • Plasma-compatible elastomer and compound
  • Minimize exposed area
  • Examine gland design

Contamination

When this failure occurs, the foreign materials or contaminants are visible on the surface of the O-ring. In general, contamination is caused by:

  • Process environment deposition
  • Reactions or degradation of the elastomer
  • Non-semiconductor-grade elastomer

To prevent this failure from happening specify contamination level including manufacturing and packaging of the seals.

Rapid Gas Decompression

O-rings that have undergone rapid gas decompression (RGD) exhibit blisters, pits, or pockets on their surface. RGD occurs when there is absorption of gas at high pressure and the subsequent rapid decrease in pressure. The absorbed gas blisters and ruptures the elastomer surface as the pressure is rapidly removed. In general, rapid gas decompression is caused by:
  • Rapid pressure changes
  • Employing elastomer materials that are susceptible to gas permeation
  • Low-modulus or low-hardness elastomer
Solutions to prevent rapid gas decompression include:
  • Use RGD certified elastomer compounds, tested to industry standards
  • Slow the release of the system pressure
  • Increase the gland fill while being careful not to exceed 90%
  • Choose a higher modulus or harder elastomer



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