Shrinkage rate of rubber compound: During the process of pressing, heating and vulcanization, the inside of the compound is deformed and cross-linked, resulting in thermal expansion force. During the cooling process of the vulcanized compound, the stress tends to be eliminated. The linear dimensions of the compound are scaled down. Therefore, in the mold design, the size of the molding part needs to be increased accordingly. The shrinkage ratio is generally expressed as a percentage.
General Law of Compound Shrinkage
Factors affecting the shrinkage rate of the rubber compound: the higher the vulcanization temperature (over the positive vulcanization temperature), the greater the shrinkage rate. In general, the shrinkage rate increases by 0.1% to 0.2% for every 10°C increase in temperature.
The shrinkage rate of the rubber compound in the calendering direction and the flow direction in the mold is greater than that in the vertical direction; the longer the flow distance, the greater the shrinkage rate.
The more the amount of semi-finished compound, the higher the density of the finished product, and the smaller the shrinkage rate.
The greater the plasticity of the compound, the smaller the shrinkage rate; the higher the hardness of the compound, the smaller the shrinkage rate. (The exception is high hardness. According to the experimental measurement, the hardness of the rubber compound exceeds 90 degrees Shore, and its shrinkage rate has a rising trend)
The more filler dosage, the smaller the shrinkage rate; the higher the glue content, the greater the shrinkage rate.
In the multi-cavity mold, the shrinkage rate of the products extruded from the middle cavity is slightly smaller than that of the edge cavity products.
The shrinkage rate of injection-molded products is smaller than that of molded products.
The shrinkage rate of thin products (section thickness less than 3mm) is 0.2% to 0.6% larger than that of thick products (more than 10mm).
The shrinkage rate of general products decreases with the increase of the inner and outer diameters and cross-sections of the products. The shrinkage rate of different types of rubber is fluorine rubber, silicone rubber, EPDM, natural rubber, nitrile rubber, and neoprene. (The above rubber types are based on the type of rubber, not the formula brand of the type of rubber).
The shrinkage rate of commonly used rubber products
The shrinkage rate of cotton cloth products that are layered with rubber after being glued is generally 0 to 0.4%;
For clip polyester thread products, the shrinkage rate is generally 0.4 to 1.5%;
The shrinkage rate of nylon silk and nylon cloth products is generally 0.8 to 1.8%;
The shrinkage rate of rubber products lined with metal inserts is small, and shrinks toward the metal direction, and the shrinkage rate is generally 0 to 0.4%;
The shrinkage rate of unidirectional adhesive products is generally 0.4 to 1.0%; (for example, in the skeleton oil seal structure, the shrinkage rate of the insert bonding part is generally 0 to 0.4%; the shrinkage rate of the lip part (pure rubber part) is in the form of steps, The closer to one end of the insert, the smaller the shrinkage, and vice versa.)
Hard rubber (Shore hardness greater than 90 degrees), when the rubber content is about 20%, the shrinkage rate of the product is generally 1.5%;
The shrinkage rate of rubber and plastic products like plastic products is generally 1.1% to 1.6%; about 0.1% to 0.3% smaller than similar rubber products;
For square products with grooves, the shrinkage rate of the B direction is 0.2% to 0.4% larger than that of the A direction due to the extrusion direction of the rubber during pressing.
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