Inspection Standards for Rubber Parts

1. Visual inspection

The first moulding of each shift is visually inspected during vulcanisation, while finished goods are inspected 100% visually. Visual inspection is normally carried out by eye. Surface finish on the working surface is judged against the finish specified on the mould drawing.

2. Dimensional inspection

During vulcanisation the first moulding is checked; finished goods are generally sampled. However, for parts with tight dimensional requirements, parts requiring machining, and parts that must be stabilised in the working medium, the principal dimensions should in principle be inspected 100%.

Vulcanised and deflashed parts must settle for 6 hours before dimensional inspection. Rubber parts deform elastically, so an unsuitable measuring method or instrument will affect accuracy. Trial mould approval and trial production generally use universal gauges and instruments, while volume production uses dedicated gauges.

3. Performance testing

1. Shore A hardness test: the specimen should be at least 6 mm thick, 15 mm wide and 40 mm long. If the thickness is insufficient, identical specimens may be stacked — no more than four layers, with parallel top and bottom faces.
2. Oil swelling test: immerse the specimen in oil, acid, alkali or another liquid medium at a specified temperature for a set time, and measure the change in dimensions, volume or weight.
3. Rubber-to-metal bond strength test: a sectioning method is normally used, prying the rubber off the metal part with a steel needle. If the rubber tears away leaving an even bond trace on the metal surface, the bond is acceptable. For each shift, 1–3 parts are usually taken at first-mould inspection and again at finished-goods acceptance for bond strength testing.