The viscosity of plastics and the influence of standards on viscosity
The characteristics of molten plastic that rub against each other in the middle of biomolecules during the flow of molten plastic are called the viscosity of plastic. The index of this viscous size is called viscosity, so viscosity is the reflection of the high and short circulation of the molten plastic. The greater the viscosity, the stronger the viscosity of the melt, the worse the flow, and the more difficult the production process.
Industrial production, more than a plastic circulation is not to look at its viscosity value, only to see its melt fluidity index value size (called MFI): to be clear MFI, that is, at a certain melt temperature, the melt suffers from the rated value of the working pressure under the effectiveness of the unit time (generally ten minutes) according to the standard mold mouth of the net weight of the melt. To g/10min show, such as injection molding grade PP material, the model is not the same, the value of the MFI can change from 2.5 to 30, the viscosity of plastic is not invariable, the transformation of the characteristics of the plastic itself, the external temperature, working pressure, and other standards, can lead to changes in viscosity.
1.1 Influence of content
The greater the content, the narrower the content spread, the greater the viscosity reflected out.
1.2 Influence of low molecular structure and economy
The low molecular structure and the economy can reduce the interaction force between the biomolecules and the middle. Therefore, the viscosity is reduced, and some plastics are molded with solvents or plasticizers to better reduce the viscosity and make it easier to mold.
1.3 Effect of temperature on viscosity
The effect of temperature on the viscosity of most melted plastics is very large, generally the temperature rises, the lower the reflected viscosity, but there is a difference in the size of the reduction in the viscosity of various plastic melts:
PE/PP plastic, rising temperature to enhance the circulation, reduce the melt viscosity is not very effective, the temperature is too high, the consumption increases, but because of the small loss of large
PMMA/PC/PA plastics, temperature rise viscosity will be significantly reduced, PS ABS rising temperature for reducing viscosity in the molding also has a great benefit
1.4 Effect of cutting rate
A reasonable increase in the cutting rate of plastics can reduce the viscosity of plastics, but there are some plastics, such as PC also have the exception that their viscosity is almost not affected by the extruder screw speed ratio.
1.5 Influence of working pressure
The influence of working pressure on viscosity is very complex, generally PP & PE viscosity by working pressure is not very big, but the influence of PS is very obvious, specific production and manufacturing, in the machinery and equipment
more sound equipment, should pay attention to give full play to high-speed injection, that is, the effectiveness of high shear rate, rather than blindly follow the trend of working pressure to enhance.
Influence of injection temperature on forming production process
To put it bluntly, the manipulation of the cylinder temperature refers to how the plastic in the cylinder from the raw material particles has been uniformly heated into plastic deformation of the mucus body, that is, how the cylinder baking temperature configuration of the problem.
2.1 The adjustment of the barrel temperature should ensure that the plastic melts well and can be successfully injected into the filling mold without causing dissolution.
This means that we cannot purposefully reduce the melting temperature due to the temperature sensitivity of the plastic, or force the mold to be filled by the working pressure or injection rate of the injection molding process
2.2 The melting temperature of plastics critically affects the production and processing characteristics, as well as the process properties and color.
2.3 The manipulation of mold temperature is related to the product mold shell, large and simple products, the net weight of the product, and the injection volume is close to the higher baking temperature, thick walls. The look of the complicated also uses high baking temperature. On the contrary, for thick-walled tube products, some must be additional actual operation, such as the installation of inserts, can be applied to low baking temperature, identify the plastic melt temperature is not appropriate can be used to point to move the posture of the bottom pressure speed under the empty injection view check, the suitable material temperature should make the material sprayed powerfully, no bubble, no roll, bright and continuous.
2.4 die temperature is generally equipped with a succession of rising to the feed section into the section, but in order to better avoid overripe dissolution of plastic and the transformation of the product tone can also be slightly lower in the middle zone, die temperature is not well equipped sometimes lead to the card extruder screw common failures - extruder screw does not rotate or high speed, which may also be injected into the work pressure is too large or the extruder screw non-return ring (meson) ineffective lead to the front of the barrel developed more The thin abrasion-resistant material returns to the feed zone orientation.
When this backflowing material is filled into the small gap between the inner hole of the male thread and the inner cavity of the barrel and suffers from lower temperature cooling, it will be chilled into a layer of the plastic film firmly stuck in the middle of the two boundary layers, so that the extruder screw can not rotate or runoff. And then affect the feeding. At this time, do not force the loosening back or injection, the proposed feeding mouth cooling circulating water temporarily turned off, strengthen the temperature of the feeding section until 30 ~ 50 ℃ higher than the plastic melting point, and another feeding section temperature reduced to the melt temperature around, to be 10 ~ 20 minutes later, carefully rotate the extruder screw, can rotate before restarting, followed by slow feeding.
Manipulation of the working pressure in the injection cycle time
3.1 The specific working pressure to be used should be higher than the working pressure of the filled concave die, during the whole process of injection, the die control working pressure rises significantly and finally achieves a maximum value, which is generally known as the injection working pressure. The injection working pressure is obviously higher than the filling working pressure of the die.
3.2 The efficacy of pressure-holding work pressure: after the mold core is filled with plastic until the plastic inlet is completely cooled for a period of time before closing, the plastic in the mold core must still be a very high working pressure applied, that is, pressure-holding, its actual efficacy is:
A: to fill the amount of material near the mouth of the plastic, and in the mouth of the plastic into the cold suspicion of the closed before discouraging the core of the plastic did not harden in the residual work pressure effect, to the mouth of the plastic material source orientation backflow.
B: avoid the products of the collection, reduce the vacuum pump bubble.
C: Reduce the situation of sticky mold cracking or bending deformation caused by the excessive injection pressure of products. Therefore, the pressure-holding working pressure is generally 50%~60% of the injection working pressure. If the holding working pressure or time is too long and too big, it may push the cold material into the product on the inlet and overflow channel, which will add a cold material bright spot on the part near the inlet, and increase the cycle time with little benefit.
3.3 Selection of working pressure for injection
A. According to the appearance of the product. Thin and thick selection.
B. For different plastic raw materials selection.
In the case of manufacturing standards and product quality standards approved, it is proposed to use the processing standards of the bottom pressure on the temperature.
3.4 Adjustment of the working pressure of condensate type
Condensate means the working pressure assumed during the melting process of plastic. It is also called melt working pressure.
A. The actual effect of the mixing of tones is influenced by the condensation type, and the mixing efficiency will be improved when the condensation type increases.
B. The condensation type is good for removing all kinds of vapor from plastic parts and reducing the condition of silver and vapor bubbles.
C. Moderate condensation can prevent some stagnant material in the barrel, so the condensation is usually increased when cleaning the barrel.
The manipulation of injection rate
Influence of high or low rate: The advantage of low-speed filling is that the water flow is stable, the product specification is relatively stable, the fluctuation is small, the thermal stress of the product is low, and the consistency of the internal and external stress is good, but the defect is that the product is prone to the melting point marks and wavy lines, etc., which are not well integrated with the level. The color is more uniform and consistent.
The disadvantage is that it is easy to cause "random gushing", i.e., stagnation or vortex. If the temperature is too high, the color becomes yellow, the exhaust pipe is not good, and sometimes it is difficult to get out of the mold. The high viscosity of the plastic may cause the melt to crack, the surface of the product caused by fog, in addition to enhancing the tendency of the fin bending and thick parts caused by thermal stress along the seam line cracking.