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CC Critical Characteristics

These critical parameters are relevant for safety or legal requirements. They can also refer to stages in the production process. Critical parameters must be included in the control plan.
 

Coatings

There are two distinct coating systems: short-term and long-term coatings. As a rule, short-term coatings are used to facilitate assembly and minimise any friction-based resistances arising. Use is often made here of talcum, Molykote lubricants or silicone oil.

Long-term or permanent coatings are in most cases found in applications aimed at reducing friction permanently. One example of this is the plug-fit connector which is subject to frequent movement over its service life and in which the O-ring is designed to support a slight degree of compression at all times. In this case, a PTFE lacquer coating is normally applied, with the advantage that this allows for the use of colour in the design. However, plasma treatment can also reduce friction through artificial ageing of the surface.
 

Compression set ASTM D 395 B

Compression set test to identify the recovery properties of elastomer materials after an extended period under pressure tension. The test measures the residual deformation of the specimen after it has been exposed to a pressure tension for a specified period of time at a specified temperature. The thickness of the original specimen is measured at the outset. The specimen is then inserted between the spacers in the compression device. The specimen is compressed by 25% of its original height. The spacers allow for exact measurement of the compression force applied. After assembly, the entire device is placed in the heating cabinet at a specified temperature and for a specified time. Once the specimen has been removed from the heating cabinet, the specimen is taken out of the device and set aside to cool for 30 minutes at ambient temperature, before being measured.

DVR= [(to - ti) / (to - tn)] * 100

to = original specimen thickness; ti = specimen thickness as tested after recovery; tn = thickness under 25% compression
 

Critical dimensions

The critical dimensions in a technical drawing indicate those dimensions which are critical for the application. Those dimensions which must essentially be maintained within the required tolerance range are marked to draw attention to their criticality.