If the viscosity of a fluid is too high at low temperatures, it cannot be pumped. Fast-moving liquids like water have low viscosity and slow-moving liquids like honey have high viscosity. Viscosity is the resistance produced in a liquid to its flow. Viscosity is an essential feature of liquids that are used for lubrication, like lubricating oils and grease. The generalized expression in coordinate-free form is provided by. Where \, which is a particular case of fluid viscosity. The force F acting on the fluid particles is directly proportional to the fluid velocity ‘u’ and the area ‘A’ of the layer and is inversely proportional to the distance ‘y’ between them.Īccording to Newton, the viscosity formula is, In fluids, the flow velocity varies linearly at different points, being 0 at the bottom, and a velocity ‘u’ in the top. Sir Isaac Newton has postulated some assumptions regarding the fluid viscosity. Let us consider a liquid in which the fluid particles move in a specific direction in straight parallel lines. This makes it difficult for a user to decide the instrument type while experimenting with accurate measurement. Many different types are available for viscosity measurements simple ones involve counting of seconds as a liquid drop from a stick, while complex instruments involve sophisticated automatic recordings. Selecting the right viscometer that suits your experiment is quite a difficult task. There are various methods to measure viscosity, depending upon the type of materials used and the circumstances. This is the reason that fluids with high viscosity need more force to move as compared to those of low viscosity. Shearing occurs when a fluid moves or is distributed, like pouring, spraying, spreading, mixing, etc. More is the friction more is the amount of force required to move the layers this is called shear.
The internal forces or friction of fluid comes into play when one layer of fluid is subjected to move over another layer. It is a measure of the internal forces of a fluid. * Test method: 190☌/2.16kg.Viscosity is the tendency of a fluid to resist any change in its shape or motion. RELATIONSHIP BETWEEN MELT FLOW INDEX* AND PROCESSES OF TRANSFORMATION Blue cells indicate adequate fluidity for each process: The table below shows the relationship between the melt flow index and the different transformation processes for PE and PP. In the case of recycled, the MFI is very important to verify the uniformity of the batches, because if one batch presents fluidity 10 and the other 20 it means that there were big changes In the raw material used, which may cause some changes in process parameters of the transformer. What I mean by that is that there is not much difference between resins with 38, 39 and 40 MFI, for example but there is a lot of difference between resins with a MFI between 3 and 40. Besides those that I mentioned there are infinite variables. This happens because inside the plastometer channel there is not the same shear that occurs inside the barrel of the injector, just as the plastometer has no screw but a piston and its channel is vertical, unlike the horizontal injector. The laboratory test does not accurately reflect what occurs during processing in a plastic injector, for example. The result of this analysis is used for quality control and as a guideline for the process in which the plastic will be destined, with polymers with low melt flow being used in extrusion processes, while the medium and high are directed to injection molding. The melt flow index (MFI) is verified in a device called plastometer, common in petrochemical laboratories and plastics recyclers.