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The quality of the surface of an O-ring has a great influence over its sealing and contamination performance. Several industry standards exist for the identification of surface quality. Visual inspection standards are written for imperfection detection with the unaided eye. Magnification is used for reference purposes only. 

The Rubber Manufacturers Association (RMA) OR-1 O-ring Inspection Guide— Surface Imperfections Control is intended to provide manufacturers and users with seals that have a surface quality adequate for intended use. The Military Standard MIL STD 413 C is the most common surface standard for O-rings and contains standard imperfection classifications. 

Imperfections are typically classified into eight (8) categories. In addition, different surface quality specifications may be necessary for non-O- ring parts. Common specifications for non-standard parts include surface rough-ness (measured by profilometer), external and internal voids and color. A similar standard for surface quality is included in DIN 3771 (German) standards.

Backrind

A longitudinal recess oriented circumferentially and located only at the parting lines. Edges are irregular and can be present on the full circumference on the OD and/or ID within the specified limits

AS 568 O-ring W (inches) O-ring W (mm) Measure MIL STD (inches) 413 C (mm) -000 < 0.100 < 2.54 Depth  0.000 0.00 Width 0.000 0.00 -100 0.100-0.134 2.54-3.42 Depth  0.003 0.08 Width 0.005 0.13 -200 0.135-0.204 3.43-5.20 Depth  0.004 0.10 Width 0.006 0.15 -300 0.205-0.268 5.21-6.83 Depth  0.004 0.10 Width 0.006 0.15 -400 > .268 > 6.84 Depth  0.005 0.13 Width 0.010 0.25

Parting Line Indentation

A shallow recess located on the parting line at the ID of OD.

AS 568 O-ring W (inches) O-ring W (mm) Measure MIL STD (inches) 413 C (mm) -000 < 0.100 < 2.54 Depth  0.003 0.08 Width 0.010 0.25 -100 0.100-0.134 2.54-3.42 Depth  0.003 0.08 Width 0.015 0.38 -200 0.135-0.204 3.43-5.20 Depth  0.004 0.10 Width 0.020 0.51 -300 0.205-0.268 5.21-6.83 Depth  0.005 0.13 Width 0.025 0.63 -400 > .268 > 6.84 Depth  0.006 0.15 Width 0.030 0.76

Foreign Material

Any contamination or depression formed by its removal.

AS 568 O-ring W (inches) O-ring W (mm) Measure MIL STD (inches) 413 C (mm) -000 < 0.100 < 2.54 Depth  0.000 0.00 Width 0.000 0.00 -100 0.100-0.134 2.54-3.42 Depth  0.003 0.08 Width 0.005 0.13 -200 0.135-0.204 3.43-5.20 Depth  0.004 0.10 Width 0.007 0.18 -300 0.205-0.268 5.21-6.83 Depth  0.005 0.13 Width 0.010 0.25 -400 > .268 > 6.84 Depth  0.006 0.15 Width 0.015 0.38

Parting Line Projection and Excessive Flash

A continuous ridge of material on the parting line at the OD and/or ID within specified limits.

AS 568 O-ring W (inches) O-ring W (mm) Measure MIL STD (inches) 413 C (mm) -000 < 0.100 < 2.54 Height  0.003 0.08 -100 0.100-0.134 2.54-3.42 Height  0.003 0.08 -200 0.135-0.204 3.43-5.20 Height  0.004 0.10 -300 0.205-0.268 5.21-6.83 Height  0.005 0.13 -400 > .268 > 6.84 Height  0.006 0.15

Non-Fill

A randomly spaced indentation having coarser than the O-ring surface.

AS 568 O-ring W (inches) O-ring W (mm) Measure MIL STD (inches) 413 C (mm) -000 < 0.100 < 2.54 Depth  0.000 0.00 Width 0.000 0.00 -100 0.100-0.134 2.54-3.42 Depth  0.002 0.05 Width 0.010 0.25 -200 0.135-0.204 3.43-5.20 Depth  0.003 0.08 Width 0.015 0.38 -300 0.205-0.268 5.21-6.83 Depth  0.003 0.08 Width 0.025 0.63 -400 > .268 > 6.84 Depth  0.003 0.08 Width 0.040 1.02

Mold Deposit Indentations

Surface indentations caused by hardened deposits on the mold cavity.

AS 568 O-ring W (inches) O-ring W (mm) Measure MIL STD (inches) 413 C (mm) -000 < 0.100 < 2.54 Depth  0.003 0.08 Width 0.010 0.25 -100 0.100-0.134 2.54-3.42 Depth  0.003 0.08 Width 0.015 0.38 -200 0.135-0.204 3.43-5.20 Depth  0.004 0.10 Width 0.020 0.51 -300 0.205-0.268 5.21-6.83 Depth  0.004 0.10 Width 0.025 0.63 -400 > .268 > 6.84 Depth  0.005 0.13 Width 0.030 0.76

Flow Marks

Thread-like recesses, usually curved, caused by an incomplete flow and curing of the elastomer.

AS 568 O-ring W (inches) O-ring W (mm) Measure MIL STD (inches) 413 C (mm) -000 < 0.100 < 2.54 Depth  0.002 0.05 Width 0.060 1.52 -100 0.100-0.134 2.54-3.42 Depth  0.002 0.05 Width 0.060 1.52 -200 0.135-0.204 3.43-5.20 Depth  0.002 0.05 Width 0.180 4.57 -300 0.205-0.268 5.21-6.83 Depth  0.002 0.05 Width 0.180 4.57 -400 > .268 > 6.84 Depth  0.002 0.05 Width 0.180 4.57

Off-Register and/or Mismatch

Misaligned (off-register) or mis-sized (mismatch) cross-sectional halves.

AS 568 O-ring W (inches) O-ring W (mm) Measure MIL STD (inches) 413 C (mm) -000 < 0.100 < 2.54   0.003 0.08 -100 0.100-0.134 2.54-3.42   0.004 0.10 -200 0.135-0.204 3.43-5.20   0.005 0.1 -300 0.205-0.268 5.21-6.83   0.006 0.15 -400 > .268 > 6.84   0.006 0.15