Theory of Viscosity of plastic always forgotten by plastic injection operators

Abstract:   The viscosity of the plastics is important for plastic injection process. But it is not constant. It will change due to plastics property,
                   external temperature, pressure,etc. This blog lists main factors in order to help plastic engineer or operators have better
                   understanding of plastic process.
Keywords: viscosity, melt flow index, temperature, injection pressure, additive, shear velocity

Melt flow index MFI

The nature of the friction between large molecules in the flow of molten plastics is called the viscosity of the plastic, and the coefficient of the viscosity is called the viscosity, so the viscosity is the reflection of the fluidity of the molten plastic, the greater the viscosity, the stronger the viscosity of the melt, the worse the fluidity and the harder the processing. The flow of a plastic is compared not to the value of its viscosity, but to the flow index of the melt (called MFI): the so-called MFI is the molten body weight of the standard die in a unit time (usually 10 minutes) under the rated pressure at a certain melting temperature, in unit of g/10min. The viscosity of the plastic is not constant, the changes in the properties of the plastic, the external temperature, the pressure and other conditions can contribute to the change of the viscosity.

The influence of molecular weight

The larger the molecular weight is, the narrower the molecular weight distribution is, the greater the viscosity is.

The influence of low molecular additives

The additives of low molecular weight can reduce the force between large molecules. Thus, the viscosity is reduced. Some plastic molding times are added to the solution or plasticizer to reduce the viscosity and make it easy to mold.

The effect of temperature on viscosity

The temperature has a great influence on the viscosity of most of the molten plastics. The higher the temperature, the lower the viscosity, but the decrease of the viscosity of various plastic melts is different. PE/PP plastics increase the temperature and improve the fluidity and viscosity of the melt. The viscosity of PMMA/PC/PA class and other plastics increased significantly. The increase of PS ABS temperature has great advantages for reducing viscosity and molding.

The effect of shear velocity

Effective increase of the shear speed of plastic can reduce the viscosity of plastics, but some plastics, such as PC, have exceptions. Its viscosity is almost not affected by screw speed.

The influence of pressure

The effect of pressure on viscosity is more complex, and the effect of PP&PE viscosity on the pressure is not very large, but the effect on PS is quite remarkable. In actual production, the high speed injection, that is, the high shear speed, should be paid attention to, and the pressure should not be increased blindly.

The effect of the control of injection temperature on the molding process

The control of the barrel temperature is the problem of how the plastic in the barrel has been heated evenly from the raw material to the plastic viscous fluid, that is, how the temperature of the barrel is configured. The adjustment of the barrel temperature should ensure that the plasticized plastic is good, and it can smoothly injecting and filling without causing decomposition. This requires that we can not consciously reduce the plasticizing temperature because of the sensitivity of the plastic to the temperature, and use the injection pressure or injection speed to force the mold. Plastic melting temperature mainly affects the processability, and also affects the surface quality and color. The control of the material temperature is related to the moulds of the parts. The large and simple parts are close to the weight of the parts and the injection quantity. It is necessary to use a high temperature, thin wall, and a high temperature for the complex shape. On the other hand, for thick wall parts, some additional operations, such as embedded parts, can be used to make use of low temperature to determine whether the temperature of the plastic solution can be used properly. Point movements should be observed at air pressure at low pressure. Suitable temperature should make the sprayed material strong and powerful, without bubbles, no curls and bright continuous. In order to prevent the overmature decomposition of the plastic and the change of the color of the parts, the misallocation of the temperature of the material can also cause the failure of the screw rod sometimes - the screw does not turn or the empty rotation. This may also be caused by the excessive injection pressure or the failure of the screw stop ring (the meson) to cause the barrel. The thin molten material at the end is reflux in the direction of the feed zone.

Control of injection pressure during injection period

The actual pressure should be higher than the full cavity pressure. During the injection, the mode control pressure rises sharply and finally reaches a peak. The peak is the usual injection pressure. The injection pressure is obviously higher than the full cavity pressure. The effect of pressure holding pressure: after the cavity is filled with plastic until the gate is completely cooled to the front, the plastic in the mold still needs a very high pressure support, that is, the pressure protection, its specific effect is: 1.Replenish the amount of material near the gate position, and reverse flow to the source direction of the gate under the effect of residual pressure on the plastic that has not been hardened in the mold cavity before closing the gate. 2.Prevent the contraction of the parts and reduce the vacuoles. 3.Reduce the excessive injection pressure of the parts to produce sticky burst or bending deformation. Therefore, the pressure pressure usually is the 50%~60%. pressure holding pressure of the injection pressure or the time is too long. It is possible to squeeze the cold material on the gate and the runner to make the cold spot near the gate position. It has a long cycle.

Selection of injection pressure:

• according to the shape of the piece.

• Selection for different plastic materials.

Under the conditions of production and quality standard licensing, it is recommended to adopt the technological conditions of temperature and low pressure.

Adjustment of pressure of back pressure:

· The back pressure is the pressure that the plastic plasticizing process takes. It is also called plasticizing pressure.

· The mixing effect of color is influenced by back pressure, the back pressure is increased, and the mixing effect is strengthened.

· Back pressure helps to eliminate all kinds of gases in plastic parts and reduce the phenomenon of crazing and bubbles.

· The proper back pressure can avoid partial delay in the barrel, so the back pressure is often increased when cleaning the barrel.

Control of injection speed

The influence of speed high and low: the advantages of low velocity filling are steady flow velocity, stable part size, low fluctuation, low internal stress and good consistency of internal and external stress. The disadvantage is that the melting point mark, water grain and so on are easy to appear in the parts, and the high speed filling can adopt lower injection pressure, and improve the gloss and smoothness of the products. The phenomenon of seam seam and stratification is found, and the shrinkage sag is smaller and the color is more uniform. The disadvantage is that "free jet" is easy to produce, that is, stagnation or eddy current. High temperature rise, yellow color, bad exhaust and sometimes difficult mold release. The plastic with high viscosity may produce melt rupture, the surface of the workpiece is foggy, and also increases the tendency of the wing caused by internal stress and the cracking of the thick parts along the seam.

The disadvantage is that "free jet" is easy to produce, that is, stagnation or eddy current. High temperature rise, yellow color, bad exhaust and sometimes difficult mold release. The plastic with high viscosity may produce melt rupture, the surface of the workpiece is foggy, and also increases the tendency of the wing caused by internal stress and the cracking of the thick parts along the seam.