The viscosity of plastics and the influence of conditions on viscosity theory of injection molding process
When the molten plastic flow between the large molecules of the nature of mutual friction is called plastic viscosity, and the coefficient of the size of this viscosity is called viscosity, so viscosity is a reflection of the liquidity of molten plastic, the greater the viscosity, the stronger the viscosity of the melt, the poorer the liquidity, the more difficult processing.
Compare the fluidity of plastic is not to see its viscosity value, but to see the size of its melt flow index (called MFI): the so-called MFI, that is, at a certain melting temperature, the melt is subject to rated pressure, unit time (generally 10 minutes) through the standard mouth mold of melt weight.
The viscosity of plastics is not constant, but changes in the properties of the plastic itself, external temperature, pressure, and other conditions can contribute to changes in viscosity.
Influence of molecular weight theory of injection molding process
The larger the molecular weight, the narrower the molecular weight distribution, and the greater the viscosity reflected in it.
Influence of low molecular weight addition theory of injection molding process
Low molecular weight addition can reduce the force between large molecules. Some plastics can be molded by adding solvents or plasticizers in order to reduce the viscosity and make it easier to mold.
Effect of temperature on viscosity theory of injection molding process
The effect of temperature on the viscosity of most molten plastics is great, generally, the higher the temperature, the lower the viscosity reflected, but there is a difference in the magnitude of the reduction in the viscosity of various plastic melts.
For PE/PP plastics, increasing the temperature has little effect on improving fluidity and reducing melt viscosity, and too high a temperature will increase consumption, which is not worth the cost.
PMMA/PC/PA and other plastics, temperature rise viscosity will drop significantly, PS ABS to raise the temperature to reduce viscosity in the molding also has greater benefits.
Impact of shear speed theory of injection molding process
An effective increase in the sheer speed of plastics can reduce the viscosity of plastics, but some plastics, such as PC, have exceptions, and their viscosity is hardly affected by the speed of the screw.
Influence of pressure theory of injection molding process
The effect of pressure on viscosity is complex, generally PP & PE viscosity is not greatly affected by pressure, but the impact on PS is quite significant, in practice, in a better-equipped machine, attention should be paid to the role of high-speed injection, i.e. high shear speed, rather than blindly increase the pressure.
The influence of injection temperature control theory of injection molding process
The so-called barrel temperature control refers to how the plastic in the barrel from the raw material particles has been uniformly heated for plastic viscous fluid, that is, how the barrel baking temperature configuration of the problem.
The barrel temperature should be adjusted to ensure that the plastic is well plasticized and can be injected smoothly into the mold without causing decomposition.
This requires us not to be constrained by the sensitivity of the plastic to temperature and consciously lower the plasticization temperature, using injection pressure or injection speed and other methods to force the mold to fill.
Plastic melting temperature mainly affects the processing performance, but also affects the surface quality and color.
The control of the material temperature and the mold of the parts, large and simple parts, the weight of the parts, and the injection volume is close to the need for higher baking temperature, thin-walled. Thin-walled and complex shapes also require a high baking temperature.
On the contrary, for thick-walled parts, some need additional operations, such as the installation of inserts, you can use a low baking temperature, to identify whether the temperature of the plastic solution is appropriate can be used to point action in low-pressure speed to the empty injection observation, the appropriate material temperature should make the spray out of the material rigid and powerful, without bubbles, not curled, bright and continuous.
The configuration of the material temperature is generally from the incoming section to the outgoing section in ascending order, but in order to prevent the decomposition of overripe plastic and changes in the color of the parts can also be slightly lower than the middle section, the material temperature is not properly configured sometimes resulting in screw failure – screw does not rotate or idle, which may also be the injection pressure is too large or screw non-reverse ring (meson) failure caused by the front of the barrel of thin melt backflow in the direction of the incoming material area.
Control of pressure during the injection cycle theory of injection molding process
The actual pressure applied should be higher than the cavity-filling pressure. During the injection process, the mold control pressure rises sharply and eventually reaches a peak, which is commonly referred to as the injection pressure. The injection pressure is obviously higher than the cavity filling pressure.
The role of holding pressure: after the mold cavity is filled with plastic until the gate is completely cooled to close, the plastic in the mold cavity still needs fairly high-pressure support, i.e. holding pressure, the specific role of which is:
Replenish the amount of material close to the sprue and stop the plastic in the cavity, which has not yet hardened, from flowing back towards the sprue source under residual pressure before the sprue is condensed and closed.
Prevents shrinkage of the part and reduces vacuum bubbles.
Reduces the risk of sticky mold bursts or bending deformations caused by excessive injection pressure. Therefore, the holding pressure is usually 50% to 60% of the injection pressure. If the holding pressure or time is too long or too large there is a risk that the cold material on the gate and runners will be squeezed into the part, adding a bright spot of cold material near the gate and extending the cycle time without benefit.
Selection of injection molding pressure
According to the shape of the article, the thickness is selected
Selection of different plastic raw materials
Where production conditions and part quality standards permit, it is advisable to use low pressure and temperature process.
Adjustment of the backpressure
The back pressure represents the pressure under which the plastic is plasticized. It is also known as plasticizing pressure.
The mixing effect of the color is influenced by the backpressure, which is increased to enhance the mixing effect.
Backpressure helps to remove all kinds of gases from the plastic part, reducing silvering and air bubbles.
Adequate back pressure can avoid the phenomenon of local stagnation in the barrel, so the backpressure is often increased when cleaning the barrel.
Control of injection speed theory of injection molding process
Influence of speed:
The advantage of low-speed mold filling is that the flow rate is smooth, the size of the part is more stable and fluctuates less, the internal stress of the part is low, the consistency of the stress inside and outside is better, the disadvantage is that the part is easy to appear layered with bad melting point marks, water lines, etc.
High-speed mold filling can use lower injection molding pressure, improve the gloss and smoothness of the product, eliminate the phenomenon of seam lines and layering, shrinkage depression is small, the color is more uniform Consistency.
The Control of injection speed theory of injection molding process disadvantage is the tendency to produce “free jetting”, i.e. stagnation, swirling, high-temperature rise, yellow color, poor venting, and sometimes difficult release.
High viscosity plastics have the potential for melt breakage and fogging of the surface of the part, as well as an increased tendency for internal stresses to cause wing bending and cracking of thick parts along the seam line.
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