Theory of injection moulding process

Time:2020-07-02 08:41:28 / Popularity: / Source:

Influence of plastic viscosity and conditions on viscosity

Property of friction between macromolecules when molten plastic flows is called viscosity of plastic, and coefficient of this viscosity is called viscosity, so viscosity is a reflection of fluidity of molten plastic. The worse fluidity, the more difficult processing.
Comparing fluidity of a plastic is not based on its viscosity value, but on its melt flow index (called MFI): So-called MFI is that melt is weight of melt that passes through a standard die per unit time (usually 10 minutes) under a certain melting temperature and action of rated pressure of melt. Expressed in g/10min, viscosity of plastics is not static. Changes in characteristics of plastics, effect of external temperature, pressure and other conditions can all contribute to changes in viscosity.
injection moulding process 

Effect of molecular weight

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

Effects of low-molecular-weight additives

Addition of low molecular weight can reduce interaction between macromolecules. Therefore, viscosity is reduced. Some plastics are added with solvents or plasticizers during molding time to reduce viscosity and make it easy to mold.

Effect of temperature viscosity

Temperature has a great influence on viscosity of most molten plastics. Generally, the higher temperature, the lower reflected viscosity, but magnitude of decrease in melt viscosity of various plastics varies.
PE/PP plastics. Increasing temperature has little effect on improving fluidity and reducing melt viscosity. If temperature is too high, consumption will increase, but it will pay more.
PMMA/PC/PA and other plastics, viscosity will decrease significantly with increasing temperature. PS ABS increasing temperature will also have greater benefits for reducing viscosity and molding.

Effect of Shear Speed

Increasing shearing speed of plastics effectively can reduce viscosity of plastics, but some plastics, such as PC, have exceptions, and their viscosity is hardly affected by screw speed.

Effects of stress

Effect of pressure on viscosity is more complicated. Generally, viscosity of PP&PE is not greatly affected by pressure, but effect on PS is quite significant. In actual production, on high-quality machines, attention should be paid to role of high-speed injection, that is, high shear speed, and pressure should not be increased blindly.

Influence of injection temperature control on molding process

So-called barrel temperature control refers to how plastic in barrel is heated uniformly from raw material particles to a plastic viscous fluid, that is, how to configure barrel baking temperature.
Barrel temperature should be adjusted to ensure that plastic is well plasticized, can be smoothly injected and filled without causing decomposition.
This requires that we cannot consciously reduce plasticizing temperature due to plastic's sensitivity to temperature, and force mold to be filled by injection pressure or injection speed.
Melting temperature of plastic mainly affects processing performance, but also affects surface quality and color.
Control of material temperature is related to mold of part. For large and simple parts, weight of part is close to injection volume, a higher baking temperature and thin walls are required. If shape is complex, a high baking temperature is required. Conversely, For thick-walled parts, some that require additional operations, such as inserts, can use a low baking temperature to identify whether temperature of plastic solution is appropriate, can use jog action to observe air injection at low pressure. Appropriate material temperature should make sprayed material stiff and strong, no bubbles, no curl, bright and continuous.
Configuration of material temperature generally increases from feeding section to discharging section in order, but in order to prevent over-ripe decomposition of plastic and change of color of product, it can also be slightly lower than middle section. Improper configuration of material temperature can sometimes cause screw failure-screw does not rotate or is idling. This may also be caused by excessive injection pressure or failure of screw check ring (meson), and thin melt at the front end of barrel will flow back to feeding area.

Control of pressure during injection cycle

Actual applied pressure should be higher than pressure of filled cavity. During injection process, mold control pressure rises sharply and eventually reaches a peak, which is so-called injection pressure. Injection pressure is obviously higher than pressure of filled cavity. .

Role of holding pressure:

After cavity is filled with plastic until gate is completely cooled, plastic in cavity still needs a fairly high pressure support, that is, holding pressure, its specific role is:
Replenish amount of material near gate, stop unhardened plastic in cavity before gate condensing and closing, under action of residual pressure, flow back to gate source.
Prevents shrinkage of parts and reduces vacuum bubbles.
Reduce phenomenon of sticky mold burst or bending due to excessive injection pressure of part. Therefore, holding pressure is usually 50% ~ 60% of injection pressure. If holding pressure or time is too long, it is possible to squeeze cold material on gate and runner to add bright spots of cold material near gate, cycle is extended for no good at the same time.

Choice of injection pressure:

According to shape of workpiece.
Choose for different plastic raw materials.
Where production conditions and product quality standards permit, it is recommended to use low-temperature process conditions.

Adjustment of back pressure:

Back pressure represents pressure experienced by plastics during plasticizing, which is also called plasticizing pressure.
Effect of color mixing is affected by back pressure, which increases as back pressure increases.
Back pressure helps to exclude all kinds of gas from plastic parts, reducing silver streaks and bubbles.
Proper back pressure can avoid local material stagnation in barrel, so back pressure is often increased when cleaning barrel.

Control of injection speed

Impact of speed: low-speed filling has advantages of stable flow rate, relatively stable product size, small fluctuations, low internal stress of part, good consistency of internal and external isotropic stress. Disadvantage is that part is prone to delamination, poor melting marks, water marks, etc. High-speed filling can use lower injection pressure, improve gloss and smoothness of product, eliminate phenomenon of seam line and delamination, reduce shrinkage and depression, and make color more uniform.
Disadvantage is that it is easy to produce "free spray", that is, stagnation or vortex, high temperature rise, yellow color, poor exhaust and sometimes difficult to demould. Plastics with high viscosity may cause melt fracture and fog on the surface of product. Spotting also increases tendency of warpage and thick parts to crack along seam line caused by internal stress.

Go To Top