Injection molding process parameters - viscosity conditions, pressure control, injection speed, etc.
The effect of viscosity and condition of plastic on viscosity
1. The influence of low molecular weight
The low molecular weight addition can reduce the force between the macromolecular linkages, thus reducing the viscosity. Some plastic molding time is added to the solvent or plasticizer to reduce the viscosity and make it easy to mold.
2, the impact of molecular weight
The larger the molecular weight, the narrower the molecular weight distribution, and the greater the viscosity reflected.
3, the effect of temperature viscosity
The effect of temperature on the viscosity of most molten plastics is very large. Generally, the temperature rises, the lower the viscosity is reflected, but the magnitude of the viscosity reduction of various plastic melts varies:
(1) PE/PP type plastics, the effect of increasing the temperature on improving the fluidity and reducing the melt viscosity is small, the temperature is too high, and the consumption is increased, but it is not worth the loss.
(2) For plastics such as PMMa/PC/Pa, the viscosity increases significantly when the temperature rises, and the elevated temperature of PS aBS is also beneficial for reducing the viscosity.
4, the impact of shear speed
Effectively increasing the shear rate of the plastic can reduce the viscosity of the plastic, but some plastics, such as PC, have an exception, and the viscosity is hardly affected by the screw speed.
5, the impact of pressure
The effect of pressure on viscosity is more complicated. Generally, the viscosity of PP&PE is not greatly affected by pressure, but the effect on PS is quite significant. In actual production, on machines with better equipment, attention should be paid to high-speed injection, ie high shear. The effect of cutting speed, rather than blindly increasing the pressure.
The influence of injection temperature control on molding processing
The so-called barrel temperature control refers to how the plastic is uniformly heated from the raw material particles into a plastic viscous fluid in the barrel, that is, how to arrange the barrel baking temperature.
1. The melting temperature of plastic mainly affects the processing performance, and also affects the surface quality and color.
2. The adjustment of the temperature of the barrel should ensure that the plastic plasticization is good, and the injection can be smoothly injected without causing decomposition.
This requires us not to consciously reduce the plasticizing temperature due to the sensitivity of the plastic to temperature, and to force the mold filling by injection pressure or injection speed.
3. The control of the material temperature is related to the mold of the workpiece. The large and simple parts, the weight of the parts and the injection volume are close, the higher the baking temperature is required, the thin wall. The shape is complicated and the high baking temperature is also used. Conversely, for thick-walled parts, some additional operations, such as inserts, can use a low baking temperature to identify whether the temperature of the plastic solution is appropriate. It can be observed by a jog action at low pressure. The temperature of the material should be such that the material ejected is strong and strong, without bubbles, not curled, and the light is continuous.
Pressure control during the injection cycle
1. The actual applied pressure should be higher than the full cavity pressure. During the injection process, the mold control pressure rises sharply and finally reaches a peak. This peak is the so-called injection pressure. The injection pressure is obviously higher than the full cavity pressure. High.
2, the role of pressure holding pressure: after the cavity is filled with plastic until the gate is completely cooled for a period of time before closing, the plastic in the cavity still needs a relatively high pressure support, that is, pressure retention, its specific role is:
(1) Replenish the amount of material near the gate position, and stop the plastic that has not been hardened in the cavity before the gate condensation to close, and return to the source of the gate under the residual pressure.
(2) Prevent the shrinkage of the parts and reduce the vacuum bubbles.
(3) Reducing the phenomenon of sticking or bursting due to excessive injection pressure of the part. Therefore, the holding pressure is usually 50%~60% of the injection pressure. If the holding pressure or time is too long, it is possible The cold material on the gate and the runner is squeezed into the workpiece to add a cold spot to the gate, and the cycle is extended without any benefit.
3, the choice of injection pressure
(1) According to the shape of the part. Thickness selection.
(2) Choose for different plastic materials.
In the case of production conditions and the quality standards of the parts, it is recommended to use the process conditions of temperature and low pressure.
4, the adjustment of back pressure
The back pressure represents the pressure that the plastic plasticizing process is subjected to. It is also called plasticizing pressure.
(1) The mixing effect of the color is affected by the back pressure, the back pressure is increased, and the mixing effect is enhanced.
(2) Back pressure helps to eliminate various gases in plastic parts and reduce the phenomenon of silver streaks and bubbles.
(3) Appropriate back pressure can avoid local stagnation in the barrel, so the back pressure is often increased when cleaning the barrel.
Injection speed control
The effect of speed: low speed filling is the smooth flow rate, the workpiece size is relatively stable, the fluctuation is small, the internal stress of the workpiece is low, and the internal and external stresses are consistent. The disadvantage is that the workpiece is prone to delamination and poor melting point. Traces, water marks, etc., high-speed filling can use lower injection pressure, improve the gloss and smoothness of the product, eliminate seam line phenomenon and delamination, shrink shrinkage and uniform color.
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