Second, the extrusion process is unreasonable
1. Excessive or insufficient plasticization of materials
This is related to the process temperature setting and feed ratio. If the temperature is set too high, the material will be over-plasticized. Some lower molecular weight components will decompose and volatilize. If the temperature is too low, there will be no molecules between the components.
Completely fused, the molecular structure is not strong.
However, if the feed ratio is too large, the heated area and shearing force of the material increase, the pressure increases, and it is easy to cause excessive plasticization; if the feed ratio is too small, the heating area and shearing force of the material will decrease, which will Causes less plasticization.
Whether it is excessive plasticization or superplasticization, it will lead to the cutting and folding of profiles.
2, insufficient head pressure
On the one hand, it is related to the mold design (this will be described separately below). On the other hand, it is related to the feed ratio and temperature setting. When the pressure is insufficient, the compactness of the material is poor and the looseness of the material appears to be loose.
The metering feed rate and the extrusion screw speed should be adjusted to control the pressure between 25 MPa and 35 MPa.
3. The low molecular components in the product are not discharged.
There are generally two ways to produce low molecular weight components in the product, one that is produced during the hot mixing process and can be discharged through the dehumidification and exhaust system during hot mixing.
The second is partially residual and heated water and hydrogen chloride gas generated during heating.
This is usually forced out through a forced exhaust system in the exhaust section of the main engine. The vacuum is usually between -0.05 MPa and 0.08 MPa. If it is not opened or too low, low molecular components will remain in the product, resulting in a decrease in the mechanical properties of the profile.
4, the screw torque is too low
The torque of the screw is the value of the reaction machine under force. Set the value of the process temperature. The feed ratio is directly reflected in the screw torque value. The screw torque is too low to be reflected to some extent.
The temperature is low or the feed ratio is small, so the material is not completely plasticized to the extent of extrusion and the mechanical properties of the profile are also reduced.
Depending on the extrusion equipment and mold, the screw torque is typically between 60% and 85% to meet the requirements.
5. The traction speed does not match the extrusion speed.
If the pulling speed is too fast, the mechanical properties of the profile wall will be reduced and the pulling speed will be too slow. The resistance of the profile is large and the product is in a high tensile state, which also affects the mechanical properties of the profile.
Third, the mold design is not reasonable
1. The design of the mold part is unreasonable, especially the distribution of the inner ribs and the treatment of the interface angle.
This can lead to stress concentrations, and it is necessary to improve the design and eliminate the right and acute angles at the interface.
2. The mold pressure is insufficient.
The pressure of the mold is directly dependent on the compression ratio of the mold, especially the length of the straight section of the mold.
If the compression ratio of the mold is too small or the straight portion is too short, the product will not be dense and affect physical properties.
On the one hand, the change in mold pressure can be adjusted by changing the length of the flat portion of the mold; on the other hand, different compression ratios can be selected to change the extrusion pressure at the mold design stage, but attention must be paid to the compression ratio of the indenter.
The compression ratio of the extruder screw is adapted; the extrusion process parameters can also be adjusted and the porosity can be increased by changing the formulation.
The plate changes the amount of melt pressure.
3. For performance degradation caused by poor convergence of the split ribs, the length of the ribs and the outer surface, the ribs and ribs at the confluence should be appropriately increased, or the compression ratio should be increased to solve.
4. The mold discharge is uneven, resulting in inconsistent wall thickness or inconsistent profiles.
This also results in a difference in mechanical properties between the two faces of the profile. We sometimes failed to pass the test when we were cold, which proves this.
As for non-standard profiles such as thin walls, we will not repeat them here.
5. The cooling rate of the sizing die.
The cooling water temperature usually does not cause enough attention. The function of the cooling water is to cool and shape the macromolecular chain of the profile in time for the purpose of use.
Slow cooling allows the molecular chain to extend for a sufficient time to promote shaping.
Rapid cooling, the temperature difference between the water temperature and the extruded parison is too large, and the product is quenched, which is not conducive to the improvement of the low temperature performance of the product.
From the explanation of polymer physics, PVC macromolecular chains undergo crimping and stretching under the action of temperature and external force. When the temperature and external force are taken out, the macromolecular chain does not recover in a free state and is out of the glass state.
The disordered arrangement of chains leads to low temperature impact properties of macroscopic products.
From
Plastic processing techniques explain the existence of a stress relaxation process after removal of the PVC profile after removal of the product from temperature and external forces.
A suitable cooling water temperature is beneficial to the process.
When the temperature of the cooling water is too low, the stress in the product does not disappear, resulting in a decrease in product performance. Therefore, the profile cooling is slowly cooled, and warpage, bending, and shrinkage of the molded product can be prevented, and the impact strength of the product can be prevented from being lowered due to internal stress.
Usually, the water temperature is controlled at 20 °C.
In order to gently cool the parison without quenching, the water pipe connected to the cooling sizing sleeve is connected to the rear of the molding so that the flow direction of the water in the sizing sleeve is opposite to the moving direction. The front part is discharged.
This does not cause the parison to be quenched due to low water temperature, excessive internal stress, and the profile is fragile, and the impact resistance of the profile is lowered. (to be continued)