When one of our suppliers mixes or buys a batch of rubber, a batch number is automatically assigned. But before it can be molded into usable parts (such as shaft seals), the batch must be tested to ensure that it is a “good batch,” i.e. its physical properties meet specifications. Batch testing is vital in ensuring consistency among finished parts.

To test a batch of rubber’s physical properties, a sample of the material is molded into 6" x 6" x .070" slabs. These slabs are then cut into the various shapes needed to test for hardness, tensile strength, modulus, elongation, and compression set. All of these tests are described in the discussion on physical properties.

Specific gravity is also often measured, though more as a check on compounding consistency than as a physical test. Per ASTM D 792, measurement of specific gravity (or relative density) compares the weight of a molded sample to the weight of an equal volume of water. Specific gravity (S.G.) is noted without units. If a material is twice as heavy as water, its specific gravity is 2. Using the specific gravities of previously-molded compounds for comparison (e.g. ethylene propylene might have a specific gravity of 0.86, or less than that of water), a manufacturer can see if a sample is consistent with prior batches.

If the tested physical properties of a batch of rubber meet all specifications, the batch is approved for production of shaft seals or other articles. If the properties are not satisfactory, the batch must either be reworked (broken down and reformulated) or scrapped. Scrapping an entire batch of rubber and starting over can be very costly and is thus a last resort. But even if the compound’s physical properties are acceptable, it must still meet processing requirements in order to be ready for use in a specific molding facility.

VISCOMETERS
RHEOMETERS
RUBBER PROCESS ANALYZERS
DETERMINING CURE TIMES