I have had many discussions with customers as to the value of using an ASTM elastomer compound description on their prints to define a specific application or elastomer requirement versus listing an approved Parker compound number.
Specifying a compound using the ASTM callout is a good start - it clearly defines what you want, it sets a minimum benchmark and it is easy for competitive vendors to understand what you are asking for. The ASTM standards also set specific test parameters which make it easier to do an "apples to apples" comparison between two compounds. However, over time here is what my customers have learned:
1) The ASTM standards are very general; so when my customer defined a specific FKM they needed using an ASTM callout, they received a compliant material that just barely met the ASTM specifications but did not meet their actual operating requirements. The supplier provided my customer with their lowest cost material. The quality of the material was poor and inconsistent, but it met the ASTM criteria they requested. This customer saw a 15% increase in assemblies requiring rework plus the number of warranty claims rose due to seal failures. The twenty cents per seal my customer saved for their $48.00 application was offset by the cost of increased product failures which also resulted in unhappy customers.
2) The ASTM standard does not specifically list what actual chemicals the seal has to be compatible with as well as the operating conditions. ASTM tests compatibility based on Standardized Testing Fluids which are Oils, Fuels and Service Liquids. ASTM uses standard oils which are defined by IRM 901 and 903. Again, the ASTM standards are excellent for comparing compounds, but most people do not have their seals operating in the ASTM reference oils and many sealing applications are exposed to multiple fluids.
3) Most of the engineers or purchasing people who reviewed or utilized an older drawing had no idea why the original engineer chose the compound or why they used the ASTM callout specified. I typically find that most companies do not know exactly what the ASTM standard is calling out.
So what is the best way to define and specify an elastomer? Most companies go through a technical process to specify, test and confirm that an elastomer is a correct choice for their application. All of the elastomers that were tested and approved for the application should be clearly listed on the drawing. In addition, the drawing should clearly state that the approved materials listed were tested to confirm their suitability for the application. All substitutes or new elastomers must be tested and approved by engineering prior to use.
If you have questions regarding the suitability of an elastomer for your application, consult and work with your Parker Applications Engineer. We offer a plethora of compounds to suit your application needs. Ask our applications engineers and chemists for guidance; their vast seal design experience spans multiple industries and applications to solve your sealing challenges.
Fred Fisher, technical sales engineer, Parker Hannifin Engineered Materials Group