1. The effect of temperature
It has been known from the previous analysis that the viscosity of plastics is a function of shear rate, but the viscosity of plastics is also affected by temperature. Therefore, it is of practical significance to study the influence of temperature on viscosity only when the shear rate is constant. Generally speaking, the viscosity sensitivity of plastic melt is stronger than that of shear action. The results show that the viscosity of the melt decreases exponentially with the increase of temperature.
This is because the temperature increases, it will accelerate the movement between molecules and molecular chains, which will reduce the entanglement between plastic molecular chains and the distance between molecules, thus leading to the decrease of viscosity. It is easy to form, but the product has a large shrinkage rate, which will cause decomposition. The temperature is too low, the melt viscosity is large, the flow is difficult, the formability is poor, and the elasticity is large, which will also make the shape stability of the product poor.
But the viscosity of different plastics varies with the temperature. The most sensitive of the polyformaldehyde to the temperature is polyethylene, polypropylene and polystyrene, and the most sensitive is cellulose acetate
In practice, for the melt with good temperature sensitivity, it is possible to improve the flow performance of plastics by improving the molding temperature during the molding process, such as PMMA, PC, CA, PA. But for plastics with poor sensitivity, it is not obvious to improve the flow performance by increasing temperature, so it is not generally used to improve the flow characteristics.
For example, POM, PE, PP and other non-polar plastics, even if the temperature rise is large, the viscosity decreases very little. Moreover, the temperature must be limited by certain conditions, that is, the molding temperature must be within the allowable molding temperature range of plastic, otherwise, plastic will degrade. The loss of forming equipment is large, the working conditions are deteriorated, and the loss is not paid. It is of quantitative significance to express the relationship between viscosity and temperature of materials by using the size of activation energy.
2. The effect of pressure
There is a small space between molecules and chains inside the plastic melt, that is, the so-called free volume. So plastic can be compressed. During injection, the maximum external pressure on plastic can reach tens or even hundreds of MPa. Under the pressure, the distance between macromolecules decreases, the range of chain segment activity decreases, the distance between molecules decreases, and the force between molecules increases, which makes the dislocation of chains more difficult, which shows that the overall viscosity increases.
But the viscosity of different plastics is different under the same pressure. Polystyrene (PS) is the most sensitive to pressure, that is, when pressure increases, viscosity increases rapidly. Compared with LDPE, the pressure has little influence on viscosity, and polypropylene is equal to that of medium degree polyethylene.
The fact that the increase of viscosity caused by increasing pressure shows that it is not appropriate to increase the flow rate of plastic melt by increasing pressure alone. The high pressure can not only improve the filling of the fluid obviously, but also the filling performance may decrease due to the increase of viscosity. It not only causes excessive power loss and excessive equipment wear, but also causes the defects such as overflow and internal stress of the product.
In addition, the pressure is too high, and there will be injection defects such as product deformation, which leads to excessive power consumption. But the pressure is too low, which will cause the shortage of material.
The influence of temperature on viscosity can be found that the effect of increasing pressure on the viscosity of plastic melt is similar to that of reducing temperature on plastic viscosity within the range of normal processing parameters. For example, for many plastics, when the pressure increases to 100 MPa, the viscosity change is equivalent to reducing the temperature of 30-50 ℃.
3. The influence of shear rate
With the increase of shear rate, the viscosity of plastics generally decreases. But in the case of low shear rate and high shear rate, the viscosity almost does not change with the shear rate. Under the premise of certain temperature and pressure, the viscosity of different plastics is different.
Or, although the viscosity of most plastic melts decreases with the increase of shear rate, the sensitivity of different plastics to shear rate (shear stress) is different.
The Enlightenment of this point is that the viscosity of plastics will be significantly reduced and the flow performance will be improved by increasing the shear rate within a certain shear rate range. However, it is better to choose the process adjustment in the range of melt viscosity which is not sensitive to shear rate, otherwise, the fluctuation of shear rate will cause processing instability and defects in the quality of plastic products.
4. The influence of plastic structure
For plastics, the viscosity of plastics increases with the increase of relative average molecular weight at a given temperature. The larger the relative molecular weight, the stronger the intermolecular force, and the higher the viscosity.
The smaller the molecular weight of plastic, the less the viscosity depends on the shear rate; the larger the molecular weight, the greater the viscosity dependence on the shear rate. The melt viscosity and processability of resin with wide molecular weight distribution and double molecular weight distribution resin are low. Because the low molecular weight chain is helpful to improve the fluidity of resin melt.
5. The influence of low molecular weight additives
Low molecular can reduce the force between macromolecules, and play the role of "lubrication", thus reducing the viscosity of melt and reducing the viscosity fluidization temperature. If plasticizer and solvent are added, resin can be easily filled and molded.
In short, the viscosity of polymer melt directly affects the difficulty of injection molding. If the temperature of plastic forming is controlled below the decomposition temperature, the melt viscosity is 50-500 pa-s when the shear rate is 103 s-1, and the injection molding is easier. However, if the viscosity is too large, it is required to have a high injection pressure, the size of the product is limited, and the product is prone to defects; if the viscosity is too small, the overflow phenomenon is serious, the product quality is not easy to guarantee, in this case, the nozzle is required to have self-locking device.
