Visual Inspection

The Search for Manufacturing Faults

Rubber is a material which is difficult to process because it does not fluidise even at high processing temperatures and because its viscosity can increase considerably during the forming process due to a starting cross linking reaction.

An industrial production of O-rings is under high cost pressure with the result that at curing, i.e. at the forming process of O-rings, manufactures push the limits of feasibility as far as possible. It is thereby approved that a certain amount of O-rings does not fully comply with the requirement of the users regarding form and surface quality. That is why the final inspection is an important part of the production of O-rings because during this inspection the good parts have to be separated from the rejects. In order to do this, different methods are applied starting with random sample tests, over line or table inspections, to automatic inspection machines.

Therefore, a distinctive feature between a good and a bad O-ring supplier is what kind of failed parts there are and how frequent they are when they reach the end user. Figures 1 and 2 (Bilder?) show two manufacturing faults with especially high failure characteristic.

As effective countermeasure it is recommended to perform regular random sample tests regarding form and surface failures within the frame of incoming goods inspection and testing.

For O-rings the limits for allowable tolerances are found in DIN 3771/part 4 respectively in ISO 3601/part 3. For moulded parts part-relevant limits or tolerance samples have to be defined.