Most raw polymers are very tough and it requires a lot of energy to mix the powders and oils into them. There are two basic methods of mixing.
Open Mill Mixing
This consists of two large rotating steel rolls up to 2 metres long- the rubber is banded around one of them and this leaves a rolling bank of rubber between the nip of the rolls. The powders and oils can be added to this nip and sheared into the raw polymer. The band of rubber needs to be cut and blended by the mill man to ensure all the materials added have been adequately dispersed. This type of mixing can be messy as during the mixing the loose powders can fly about.
Industrial Rubber have rubber mixing mills on site for mixing and also for warming the rubber up prior to further processing such as extrusion.
Due to the very large capital investment required this process is generally only used by large rubber companies such as tyre manufactures for their own consumption. There are specialist custom compounders that mix and supply rubber compounds to the smaller processors. Industrial Rubber does not have an internal mixing facility on site so we use custom compounders to mix our rubber compounds often to our own formulations.
Rubber compounds are batch mixed in, for want of a better description, very powerful cake mixers. The internal mixer has two rotors inside a chamber. The rotors are driven by a very powerful electric motor. The raw rubber is loaded into the mixing chamber along with the fillers and oils and most of the small chemicals such as antioxidants. During the mixing when the powder and oils are sheared into the raw polymer a lot of heat is generated so the chemicals that make the compound vulcanise are generally added at the end of the cycle to prevent premature vulcanisation or scorching of the material. The mixer is usually situated above a two roll mill so when the compound has finished mixing it is dropped into the nip of the rolls where it is cooled and sheeted or stripped off and further cooled prior to stacking.
We will go into quality later but it is worth mentioning at this stage that a sample of the mixed compound is taken and tested for some basic QC checks before it is released for production.
A test button is moulded to test the hardness of the vulcanised material and also its specific gravity. The SG gives an indication that all the bulk materials in the mix have been added in the correct proportions. As all the ingredients have a different SG and they are added to the mix in specified proportions then there will be a theoretical SG for the combined mix. The finished mix should work out close to the theoretical SG. A general tolerance of +/- 0.2gms/cc is normally acceptable. The hardness of the test button is usually measured in Shore hardness or IRHD (International Rubber Hardness Degrees). The shore A scale is very similar to IRHD. A general commercial tolerance on the hardness of rubber is the nominal hardness +/- 5 points.
At this stage a sample is also taken to check the Rheological properties of the mix. This measures the viscosity of the material and also the speed of vulcanisation and the final state of cure of the compound. This information will give some indication of how the material will behave when extruded, moulded or further processed along with the final hardness of the finished article.
If the finished properties such as tensile strength, elongation at break, compression set, swelling in fluids etc, are for a critical application then we can ask for all the tests to be carried out on the vulcanised material prior to it being released for production.
Typically incoming raw material has a certificate note showing rheological information for basic release and tests on cured sheet