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Injection Molding Process Theory - Manufacturer Explains

Injection Molding Process Theory

The property of mutual friction between macromolecules when the molten plastic flows is called the viscosity of the plastic. And the viscosity is a reflection of the fluidity of the molten plastic. The higher the viscosity, the stronger the melt viscosity. Also, the fluidity will be worse with the process more difficult.

Comparing the fluidity of a plastic is not based on its viscosity value, but on its melt flow index (called MFI). 

What is MFI?

At a certain melting temperature, the melt is subjected to the rated pressure, and the weight of the melt passing through the standard die per unit time (usually 10 mins). 

Expressed in g/10min, the viscosity of plastic is not static. Changes in the characteristics of the plastic itself, and the influence of external temperature, pressure and other conditions can lead to changes in viscosity.

The effect of molecular weight

The larger the molecular weight, the narrower the molecular weight distribution, and the greater the reflected viscosity.

The effect of low molecular weight additives

The addition of low molecular weight can reduce the force between macromolecules, thus reducing the viscosity. Some plastics are added with solvents or plasticizers to reduce the viscosity and make them easier to mold.

The effect of temperature viscosity

Temperature has a great influence on the viscosity of most molten plastics. Generally, the higher the temperature, the lower the reflected viscosity. But the viscosity of various plastic melts varies in the magnitude of the reduction.

For PE/PP plastics, increasing the temperature has little effect on improving the fluidity and reducing the melt viscosity. If the temperature is too high, the consumption will increase instead.

For plastics such as PMMA/PC/PA, the viscosity decreases significantly when the temperature increases. The increase in temperature of PS ABS is also beneficial for reducing the viscosity and forming.

The effect of shear speed

Effectively increasing the shear rate of the plastic can reduce the viscosity of the plastic. But there are exceptions for some plastics, such as PC, whose viscosity is hardly affected by the screw speed.

The effects of pressure

The influence of pressure on viscosity is more complicated. Generally, the viscosity of PP&PE is not greatly affected by pressure, but the influence on PS is quite significant. In actual production, for machines with relatively complete equipment, we should use high-speed injection. That is, pay attention to the high shear rate.

Influence of injection temperature control on molding process

The barrel temperature control is to make the raw material of plastic in the barrel uniformly heated to a plastic viscous fluid. That is, how to configure the barrel baking temperature.

The barrel temperature should ensure that the plastic is well plasticized and can be injected and filled smoothly without causing decomposition.

Therefore, we can’t consciously reduce the plasticizing temperature due to the sensitivity of plastic to temperature. Also, we can’t force the mold to be filled by methods such as injection pressure or injection speed.

The plastic melting temperature mainly affects the processing performance, but also affects the surface quality and color.

The control of the material temperature is related to the part mold

  • For large and simple parts whose weight is close to the injection volume, a higher baking temperature and thin wall are required.
  • For complex shapes, a high baking temperature is needed.
  • For thick-walled parts, somemay require additional operations, such as embedding parts. Then, we can use a low baking temperature to identify whether the temperature of the plastic melt is appropriate. And we can use jog action to observe the air injection at a low speed. With the appropriate material temperature, the material sprayed out should be vigorously, without bubbles, without curling, and bright and continuous.

The configuration of material temperature is generally ascending sequentially from the feeding section to the discharging section. However, in order to prevent the overcooked decomposition of the plastic and the color change of the part, it can also be slightly lower than the middle section.

Improper configuration of material temperature can sometimes cause the screw to fail – the screw does not rotate or rotates idly. It may also be that the injection pressure is too large or the screw check ring (meson) fails. And then it further causing the thin melt at the front end of the barrel to flow back toward the feeding zone.

Pressure control during injection cycle

The actual applied pressure should be higher than the filling cavity pressure. During the injection process, the mold control pressure rises sharply and finally reaches a peak value, which is commonly referred to as the injection pressure. Besides, the injection pressure is obviously higher than the filling cavity pressure.

The role of holding pressure

During the time after the mold cavity is filled with plastic and before the gate is completely cooled and closed, the plastic in the mold cavity still needs a relatively high pressure support. That is, the pressure holding.

The function of holding pressure

  • Supplement the material close to the gate. And before the gate is closed by condensation, the unhardened plastic in the mold cavity is prevented from flowing backwards in the direction of the gate material source under the action of residual pressure.
  • Prevent shrinkage of parts and reduce vacuum bubbles.
  • Reduce the phenomenon of sticky mold bursting or bending deformation caused by excessive injection pressure of the part. Therefore, the holding pressure is usually 50%~60% of the injection pressure. If the holding pressureis too high or time is too long, It is likely that the cold material on the gate and the runner is extruded into the part. What’s worse, bright spots of the cold material will appear near the gate, with the cycle prolonged.

Choice of injection pressure

  • According to the shape and thickness of the parts.
  • Select for different plastic raw materials.
  • When the production conditions and the quality standards of the parts allow, it is recommended to use the process conditions of temperature and low pressure.

Back pressure adjustment

  • The back pressure represents the pressure that the plastic plasticizing process is subjected to. Sometimes it is also calledplasticizing pressure.
  • The color mixing effect is affected by the back pressure. If the back pressure increases, the mixing effect will be
  • Back pressure helps to remove all kinds of gas from plastic parts and reduce silver streaks and bubbles.
  • Appropriate back pressure can avoid local stagnation in the barrel. So the back pressure is often increased when cleaning the barrel.

Injection speed control

For low-speed mold filling

With the low-speed mold filling, the flow rate and the part size are stable. In addition, the internal stress of the part is low, which is well consistent with the external stress. The disadvantage is that the parts are prone to delamination, poorly combined melting point marks, water marks, etc.

For high-speed mold filling

In high-speed filling, we can adopt lower injection pressure. Then, it can improve the gloss and smoothness of the product. Besides, it can eliminate the seam lines and delamination, with small shrinkage dents and more uniform color.


The disadvantage is that it is easy to produce “free jet”. That is, stagnation or eddy current. That will make temperature rise overly high, the color yellow and the exhaust poor. And sometimes it is difficult to demold. The plastic with high viscosity may cause melt fracture and fog on the surface of the part. It also increases the tendency of fin buckling and cracking of thick pieces along the seam line caused by internal stress.

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