Melt Behavior – Plastic melt behavior inside the barrel and Mould
How does plastic flow؟
Study the behavior of the substance during injection.
1- Material behavior
Molten thermoplastic exhibits viscoelastic behaviors, which combines flow characteristics of both viscous liquids and elastic solids. When a viscous liquid flows, the energy that causes the deformation is dissipated and becomes viscous heat. On the other hand, when an elastic solid is deformed, the driving energy is stored. For example, the flow of water is a typical viscous flow, whereas the deformation of a rubber cube falls into the elastic category.
Deformation
Deformation In addition to the two types of material flow behavior, there are two types of deformation: simple shear and simple extension (elongation), as shown in (a) and (b) below. The flow of molten thermoplastics during injection Moulding filling is predominantly shear flow, as shown in (c), in which layers of material elements "slide" over each other. The extensional flow, however, becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (e.g., a gate region), as shown in (d).
2- Visco-elastic behavior
In response to an applied stress (force per unit area), molten thermoplastics exhibit viscoelastic behavior, which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid. In other words, under certain conditions, molten thermoplastics behave like a liquid, and will continuously deform while shear stress is applied, as shown below. Upon the removal of the stress, however, the materials behave somewhat like an elastic solid with partial recovery of the deformation, as shown in (b) and (c). This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the molten state, which allows the movement and slippage of molecular chains under the influence of an applied load. However, the entanglement of the polymer molecular chains also makes the system behave like an elastic solid upon the application and removal of the external load. Namely, on removal of the stress, chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery. The recovery is not instantaneous because of the entanglements still present in the system.
FIGURE 2. (a) Ideal viscous liquid deforms continuously under applied stress. (b) Ideal elastic solid deforms immediately upon the application of stress, but fully recovers when the stress is removed. (c) Molten thermoplastic deforms continuously under the applied stress (like a viscous liquid), but it also recovers partially from the deformation upon removal of the applied stress (like an elastic solid).
3- Melt shear viscosity
What is shear viscosity?
Melt shear viscosity is a material's resistance to shear flow. In general, polymer melts are highly viscous due to their long molecular chain structure. The viscosity of polymer melt ranges from 2 to 3,000 Pas (water 10-1, glass 1020). Viscosity can be thought of as the thickness of a fluid, or how much it resists flow. Viscosity is expressed as the ratio of shear stress (force per unit area) to the shear rate (rate change of shear strain), as shown in the equation and diagram below:
3.1 Newtonian fluid vs. non-Newtonian fluid
For Newtonian fluids, viscosity is a temperature-dependent constant, regardless of the shear rate. A typical example of Newtonian fluid is water. However, for non-Newtonian fluids, which include most polymer melts, the viscosity varies, not only with temperature, but with the shear rate.
3.2 Shear-thinning behavior
When the polymer is deformed, there will be some disentanglement, slippage of chains over each other, and molecular alignment in the direction of the applied stress. As a result, the resistance exhibited by polymer to flow decreases with the deformation, due to the evolution of its microstructure (which tends to align in the flow direction). This is often referred to as shear-thinning behavior, which translates to lower viscosity with a high shear rate. Shear-thinning behavior provides some benefits for processing the polymer melt. For example, if you double the applied pressure to move water in an open-ended pipe, the flow rate of the water also doubles, since the water does not have shear-thinning behavior. But in a similar situation using a polymer melt, if the pressure is doubled, the melt flow rate may increase from 2 to 15 times, depending on the material.
3.4 Shear rate distribution
Having introduced the concept of shear viscosity, let us look at the shear rate distribution in the cavity during injection Moulding. Generally speaking, the faster the adjacent material elements move over each other, the higher the shear rate is.
FIGURE 4. (a) A typical velocity profile with relative flow element movement and (b) the corresponding shear rate distribution in injection Moulding filling.
Therefore, for a typical melt flow velocity profile, shown in (a), it is clear that the shear rate is highest at the Mould-melt interface (or at the melt-solid interface if there is a frozen polymer layer). On the other hand, the shear rate approaches zero at the center line because there is no relative material element movement due to flow symmetry, as shown in Figure 4 (b). Shear rate is an important flow parameter since it influences the melt viscosity and the amount of shear (viscous) heating. The typical shear rate experienced by the polymer melt during the injection Moulding process ranges from 10^2 to 10^5 second^-1.
3.5 Effects of temperature and pressure
Since the mobility of polymer molecular chains decreases with decreasing temperature, the flow resistance of polymer melt also greatly depends on the temperature. As shown in Figure 5, the melt viscosity decreases with increasing shear rate and temperature due to the disentanglement and alignment of the molecules and enhanced mobility of polymer molecules, respectively. In addition, the melt viscosity also depends on the pressure. The higher the pressure, the more viscous the melt becomes.
Rheological material properties contain a mathematical description of the shear viscosity as a function of shear rate, temperature, and pressure. For a discussion on how high pressure increases the level of viscosity, see Pressure dependence of viscosity.
Pressure-Volume-Temperature (PVT) Behavior
PVT behavior refers to the change in specific volume with temperature and pressure changes.
The specific volume is defined as volume per unit mass. The specific volume, v of a polymer changes with variations in temperature and pressure.
Volumetric expansion data for polymeric materials are obtained under equilibrium. Such data represent fundamental thermodynamic properties of the material and reflect the transitions as the material moves from glassy to crystalline to melt state.
In the figure, as the pressure and the temperature change from P and T to P ' and T', the volume of the same mass m changes from V to V'.
P V T data can be measured using standard equipment. The specimen is heated in an enclosed cell and the change in its volume is measured when it is subjected to a range of pressures.
Specimens may be either in the form of polymer pellets or they may be cut from the molded plaques.
PVT behavior of materials plays a critical role in relating processing to the final part performance.
The shrinkage of a moulded plastic part can be as much as twenty percent by volume when measured between the processing temperature and the ambient temperature.
Semi-crystalline polymers have higher shrinkage than amorphous polymers because of the ordering and folding of chains in a semi crystalline polymer below its freezing point.
This leads to a greater difference in specific volume (dv) between the melt phase and the solid phase for semi-crystalline materials. The presence of fillers like talc or short glass fibers reduces the difference in specific volume (dv) between the melt phase and the solid phase.
Viscosity
Viscosity behavior of materials is important in determining the flow length and the amount of viscous heating generated during the melt flow.
Most polymer melts exhibit shear-thinning behavior, which translates to lower, viscosity with higher shear rate. Hence the viscosity of the melt varies across the thickness of the part due to the variation in shear rate.
Melt viscosity decreases with temperature but the sensitivity varies among thermoplastics. For example, the viscosity of polystyrene and polypropylene are considerably more sensitive to temperature than that of polyethylene. At pressures of several thousand p s i the viscosity increases with pressure. The presence of fillers increases melt viscosity.
The viscosity is critical for determining the injection pressure with a given rate or the flow length with a given maximum pressure.
The viscosity of a polymer melt is adequately described by the Cross-model. This model treats viscosity as a function of temperature (T), pressure (P), and shear rate (Y).
This model handles both the Newtonian and the shear thinning flow regimes. The transition between the two regimes is characterized by t*.
t* is the shear stress at which shear thinning behavior begins to manifest itself. The slope of the shear-thinning region can be characterized in terms of a shear thinning index, n.
This model is often adopted for simulating the filling stage of injection moulding.
Cooling time is a function of Mould wall temperature, melt temperature, material properties and part wall thickness.
4-Injection pressure
4-1 Pressure drives the melt
Pressure is the driving force that overcomes the resistance of polymer melt (see Pressure-driven flow), pushing the polymer to fill and pack the Mould cavity. If you place a number of pressure sensors along the flow path of the polymer melt, the pressure distribution in the polymer melt can be obtained, as schematically illustrated in Figure 6 below.
Equations Based on a simplification of classic fluid mechanics theory, the injection pressure required to fill the delivery system (the sprue, runner, and gate) and cavities can be correlated with several relevant material, design, and processing parameters. In the following equations, P is the injection pressure and n is a material constant (the power-law coefficient), which typically ranges from 0.15 to 0.36 (with 0.3 being a good approximation) for a variety of polymer melts. Figure 7 shows injection pressure as a function of several of these parameters
4-2 Circular channel flow
The melt flow in the sprue, runner, and cylindrical gates
4-3 Strip channel flow
Such as melt flow in a thin cavity.
4-4 Factors that influence injection pressure requirements.
The following diagrams illustrate the design and processing factors that influence injection pressure.
5- Pressure-driven flow
Flow of molten thermoplastics (in injection Moulding filling) is driven by pressure that overcomes the melt's resistance to flow. Molten thermoplastics flow from high pressure areas to the low-pressure areas, analogous to water flowing from higher elevations to lower elevations. During the injection stage, high pressure builds up at the injection nozzle to overcome the flow resistance of the polymer melt. The pressure gradually decreases along the flow length toward the polymer melt front, where the pressure reaches the atmospheric pressure, if the cavity is vented properly.
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