In the vulcanization process of rubber grommets, the geometry of the finished product will undergo different degrees of shrinkage after demolding. The rubber grommets industry is accustomed to calling this phenomenon “shrinkage.” The ratio of the difference between the size of the mold and the Size of the finished part of the finished product after vulcanization (in percentage) is called shrinkage (the shrinkage of the rubber is generally between 1% and 3%). For example, the rubber grommets have a mold diameter of 150 mm, and the Finished product has a diameter of 147 mm after vulcanization, and the shrinkage amount is 3 mm, and the shrinkage ratio is 2%.
In daily production, the exact meaning of shrinkage is:
- Make the dimensions of the rubber grommets meet the requirements, that is, the size of each part should be within the tolerance range, in order to formulate the acceptance criteria for the external dimensions;
- It is convenient to improve the data required for the mold finishing drawings.
There are many factors that affect the rubber shrinkage rate, but in general it can be summarized into two major aspects.
(1) Raw materials
The different types of rubber of different types of rubber have different degrees of difference due to their different molecular structures. The macromolecular chains of rubber are entangled with each other, and there is free space between the networks, so the room for shrinkage is also large When the other components in the compound enter, they are filled into the voids, compressing the free space and reducing the shrinkage. The shrinkage of different rubber types can be reflected by their coefficient of linear expansion.
The coefficient of linear expansion of each rubber type is not the same, but they are all in the same order of magnitude; while the shrinkage of filler and steel (mold material) is one order of magnitude smaller than that of rubber, because unvulcanized rubber During the vulcanization process, the grommets are expanded by heat, and conversely, the die shrinks with cooling. Metals also have thermal expansion and contraction, but the degree of shrinkage is small. In contrast, rubber shrinks much more than metal.
Another major factor affecting shrinkage of auxiliaries is the filler with a large amount of compound. Their variety and filling amount will affect the shrinkage rate.
The shrinkage of different fillers is not equal. When shrinking of different fillers is not equal. When filled in 100 equivalents, the order of shrinkage was as follows: sulfuric acid Dam (1.95%) > calcium carbonate (1.72%) > carbon black (1.4%) > magnesium carbonate (1.1%).
The amount of filling shrinkage is related to the amount of filling. taking calcium carbonate as an example of natural rubber, the relationship between filling amount and shrinkage rate is as follows:
(2) Process aspects
1) Crosslinking density As the degree of vulcanization deepens, the crosslink density also increases, and the space of the network structure is compressed, so that the shrinkage rate is reduced. This is related to the amount of sulfur used. Natural rubber is used in combination with 3 parts of sulfur, the shrinkage ratio is 1.5% to 2%, and when the amount of sulfur in the hard rubber is 35 or more, the shrinkage rate is drastically lowered.
2) Plasticity and rubber compounding period The plasticity of the rubber compound is larger, and the longer the parking time, the greater the shrinkage rate.