Common Defects in Vulcanizates

Most of the defects in the finished products are attributed to improper mixing of the compound. Assuming that proper mixing 

Air blisters generally occur due to trapped air during the processing of the rubber in the calender or extruder or during hand building up of the products. The remedial measures to be undertaken are 1) Modify process temperature or pressure, 2) Feed the calender or extruder with adequate rubber stock, 3) Prick out visible blisters when the compound is in unvulcanized state, and 4) Use bleeder cords between metal and the rubber sheets applied at the welded places in the rubber lining process. 

This is due to over-vulcanization. The remedy is to reduce cure times and temperatures or modify the compound formulation to reduce the speed of cure. Tearing can also be due to method of removal of the product from the mould while it is in hot condition. Tearing can be eliminated by removing the products from the mould after sufficient cooling or by careful and slow removing. 

Volatile substances in the rubber or compounding ingredients or moisture can cause porosity. Insufficient stock of rubber in the mould and under cure also cause porosity. To prevent this defect from occurring, avoid use of raw materials containing volatile materials, test all raw materials for moisture content, allow the solvents or adhesives to dry up completely, check up the volume and shape of the finished product and increase curing pressure if feasible. 

It is often difficult to obtain satisfactory adhesion of rubbers of different compositions and types with metal. The amounts of sulphur and accelerators of various types influence adhesion. These must be chosen so that there is no under or over vulcanization. 

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