After vulcanizing the rubber suction cup products, the geometry is different from the size of the corresponding mold. All rubber suction cups will shrink after vulcanization. The shrinkage of various suction cups after vulcanization is generally between 1.2% and 3.5%. After vulcanization，The shrinkage of the mold are good for pulling products out from the mold, but the product size is unstable.
The amount of shrinkage mainly depends on the coefficient of thermal expansion difference between the suction cups material and the mold material at the vulcanization temperature. The main influencing factors are the vulcanization temperature，the type of raw rubber in the rubber mixture and the type and amount of filler.
- How to calculate the vulcanization shrinkage rate of the suction cup? And how to make sure the exact shrinkage rate of the suction cup?
There are many ways to determine the vulcanization shrinkage rate. Such as linear expansion coefficient method and rubber Shore A hardness method.
- Factors affecting the shrinkage of rubber compounds
There are many factors which can affect the shrinkage of the suction cup, such as the shape of the rubber suction cup (different parts of one suction cup shown the different shrinkage), the vulcanization temperature and the processing method. Here are some key factors below,
1). The hardness of the material
Hardness increased will caused the shrinkage of the rubber decreases. The hardness between 75 and 85 shoreA are the lowest shrinkage.
2). Rubber varieties
Rubber with different molecular structures has different effects on shrinkage. For example, when the same amount of reinforcing agent (20 parts of semi-reinforcing carbon black) is added, the shrinkage ratio of various vulcanized rubbers obtained by vulcanization at different temperatures is: fluororubber > silicone rubber > ethylene propylene diene rubber > styrene – Butadiene rubber>Natural rubber>Butyl rubber> Neoprene rubber. The specific shrinkage data of several rubbers at different temperatures are as follows,
Natural rubber 1.4-2.4
EPDM rubber 1.6-2.2
Styrene – Butadiene Rubber 1.5-2.0
Butadiene rubber 1.4-2.0
3). Filler effect on the shrinkage
The filler in the rubber mixture also has a large effect on the shrinkage of the suction cup. In particular, increased fillers will caused the shrinkage decreased. In addition, the difference in the type of filler will also effect on the shrinkage rate of the suction cup. Since the linear temperature coefficient of the filler material is small, it is much more close to the shrinkage of metal mold.
4). Vulcanization temperature
The vulcanization temperature effect on the shrinkage rate of the product. The vulcanization shrinkage rate increases while the temperature increases. The shrinkage rate can be lowered by appropriately lowering the vulcanization temperature. The vulcanization shrinkage of several rubber raw materials increases as the vulcanization temperature increases.
5). Skeleton material
When the suciton cups has a fabric skeleton, the shrinkage ratio of the cotton skeleton product is from 0.2% to 0.4%. The shrinkage of the polyester matrix product is from 0.4% to 1.5%, and the shrinkage of the nylon skeleton product is from 0.8% to 1.8%. The more layers you got, the smaller the shrinkage will be. The metal parts has a small shrinkage ratio, the metal-facing side shrinks, the shrinkage is from 0 to 0.4%, and the unidirectional adhesive product shrinkage is from 0.4% to 1.0%.