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Viscosity Plastics 001 Viscosity If volcano ‘A’ is erupting high viscosity lava and volcano ‘B’ is erupting low viscosity lava. Which volcano will be taller? Viscosity Plastics 001 KEY POINTS: After reviewing the Viscosity presentation, students should: • Understand why viscosity is an important property of plastic materials • Be able to explain several primary factors that affect a material’s viscosity • Be able to explain the difference between a Newtonian and a nonNewtonian fluid. Viscosity Plastics 001 Overview Viscosity is resistance to flow. Many people have a hard time understanding this concept. A high viscosity material is very thick and has trouble flowing – like honey A low viscosity material flows very easily –like water Viscosity is very important in the study of plastic materials because the viscosity of plastic materials is very dynamic –it changes very quickly based on the environmental conditions. Viscosity Plastics 001 Viscosity One of the primary advantages to using thermoplastic materials to produce items is the ability to produce a large number of components in a short period of time. The catch to this is that you need to produce consistent components that meet the customer’s specifications. It does not do us any good to make a large number of bad or scrap parts in a short period of time. Scrap parts are usually ground into smaller particles and sometimes used in the original process, but often used to make products like plastic lumber or flower pots. Viscosity Plastics 001 Viscosity The key to producing good parts is to maintain consistency in all aspects of the production process. There are four key aspects where it is important to maintain consistency: 1. Tooling 2. Equipment (Machinery) 3. Process 4. Material There is variation in every area, but one is the largest source of variation in plastic processing. Viscosity Plastics 001 Viscosity Of the four aspects, the one that is the most subject to variability is the material. By the nature of how it is produced, there is always variation in the molecular weight averages of different lots of material. Variation is the nemesis of consistency. Viscosity Plastics 001 Viscosity The characteristics of how the material flows is a very good indication of the molecular weight as well as other factors like: • Consistency of additives • Filler amounts • Degree of branching For this reason, Rheology or the study of flow is very important to maintaining consistency in plastic materials. Viscosity is the resistance to flow and directly affects how all materials flow. Viscosity Plastics 001 Viscosity There are two main things that will affect the viscosity of a plastic material. Temperature and Shear Rate As you increase the temperature, the polymer chains are further apart and can slide past one another more easily. It flows more easily As you increase the shear rate, the polymer chains orient in the direction of flow more and can more easily flow past each other. Viscosity Plastics 001 Viscosity Because of their high aspect ratio (high length compared to diameter), plastic materials do not behave the same way as most liquids when in a molten state. The entanglement of the polymer chains causes them to have very high viscosities – they don’t flow easily. Understanding how temperature affects viscosity should not be difficult. As you heat things, they expand. The further apart the molecules get, the easier it is for them to flow past one another. Shear Rate is a little more difficult and requires a longer explanation. Viscosity Plastics 001 Shear Rate In order to explain shear rate, first there are a few items that need to be discussed: – Reynolds Number – Turbulent and Laminar Flow – Newtonian and non-Newtonian fluids – Fountain Flow Viscosity Plastics 001 Reynolds Number When discussing fluid flow, there are two distinct types of flow: Turbulent and Laminar (there is also transitional flow which occurs between the two distinct types) Turbulent flow is very random and there is a lot of mixing that takes place within the molecules. Laminar flow is very ordered, the molecules move in layers. The Reynolds number is a dimensionless number that is used to describe the type of flow that is occurring. Viscosity Plastics 001 Reynolds Number The Reynolds Number (Re) of a fluid moving through a round channel is equal to; Re = density x velocity x diameter = ρ x v x d viscosity µ If a fluid has a Re of: 2300 > »Laminar Flow 2300-4000 »Transitional Flow > 4000 »Turbulent Flow Turbulent flow provides a higher degree of mixing and a more consistent temperature across the flow channel than laminar flow. In applications where we use water flow to cool tooling, we want turbulent flow in order to get better heat transfer. Viscosity Plastics 001 Laminar Flow While Turbulent flow is very chaotic an there is a lot of mixing that takes place, Laminar flow is much smoother and there is a difference in velocity across the flow channel. The molecules at the channel wall are flowing more slowly than the molecules in the middle due to frictional drag. Frictional Drag Faster Flow – less drag Frictional Drag Viscosity Plastics 001 Laminar Flow Laminar flow is sometimes envisioned as a series of plates or layers that flow across each other. If each laminate can be assumed to be moving at a constant velocity, the number of laminates will be determined by the thickness of the flow channel, the average velocity, and the viscosity of the liquid. The shear rate is determined by the velocity divided by the thickness of the laminate or γ = v/h Viscosity Plastics 001 Newtonian and Non-Newtonian Fluids A Newtonian fluid is a fluid that the viscosity stays the same when the shear rate changes. The viscosity will still change with a change in temperature or pressure, but as we push it faster, the viscosity stays constant. A non-Newtonian fluid is a fluid that the viscosity changes (drops) as we increase the shear rate. Polymer molecules are a lot like hair, long thin strands. As we push them faster, the drag on the outer walls causes the strands to line up and orient. As the strands orient more, it is easier for them to flow past each other and the viscosity drops. A good example of this behavior is with ketchup, when the ketchup is just sitting in the bottle, it is at a high viscosity and difficult to get to flow, once it starts flowing the tomato fibers align and it flows more easily. Viscosity Plastics 001 Fountain Flow When plastic molecules flow into a cold mold in order to make a component, they exhibit fountain flow due to their high viscosities and high aspect ratios. The molecules in the middle of the flow channel remain in a relatively amorphous arrangement with a low level of orientation. The molecules at the mold wall freeze in place almost instantly. The molecules just inside of the frozen layer see the highest shear rates. Viscosity Plastics 001 Fountain Flow The molecules just inside of the frozen layer also exhibit the highest degree of orientation due to the higher shear rate. Theoretically if we push the material fast enough, all of the molecules will align and the material will start to behave like a Newtonian fluid. Viscosity Plastics 001 Fountain Flow This phenomena of fountain flow allows for operations like in-mold decoration, where a decal is placed into a mold and the plastic is injected over it. The plastic does not race across the mold face, it lays down similar to if paint rollers were traveling across it. Viscosity Viscosity Plastics 001 If volcano ‘A’ is erupting high viscosity lava and volcano ‘B’ is erupting low viscosity lava. Which volcano will be taller? Questions? Volcano ‘A’ The higher viscosity lava will not flow away as quickly and will cool into a taller peak.