The factors of underwater cutter damage and improvement solutions are here

Influencing factors of cutter damage

1.Pelletizing process

The contact pressure of the cutter is regulated by the feeding air pressure and the returning air pressure. The feeding air pressure increases and the returning air pressure decreases. The closer the contact between the cutter and the die plate, the greater the friction between the cutter and the die plate, resulting in an increase in the current of the cutter and an increase in the wear of the cutter.

In addition, when the extruder is started, in order to make the cutter and the die plate closely contact without generating padding material, the initial feeding air pressure is set to 0.6 MPa in manual mode, and then the feeding air pressure is reduced by 0.01 MPa each time until when the feeding air pressure is 0.48 MPa, the automatic mode is enabled; during the process, the maximum current of the cutter is as high as 206A, which is about 30A higher than the normal value (see Figure

When the rotation speed of the cutter increases, the force of the pelletizing water on the cutter to press the die plate increases (its effect is equivalent to increasing the feeding air pressure), and the number of frictions between the cutter and the die plate per unit time increases, so the cutter wears faster. It should be noted that when the cutter speed is too high, the cutter speed becomes the main factor affecting the cutter current. The cutter current increases obviously (see Table 1).

2.PP Grade

For different grades of PP, the physical properties are quite different. In general, the lower the MFR of PP, the greater the hardness of the molten material, the greater the friction on the cutting edge, and it is easier to passivate the cutting knife. In actual production, grade switching is often carried out according to market demand, and transition materials will be generated during the process of grade switching.

The MFR of the transition material continues to change, so the pelletizing process (water temperature, die plate temperature and barrel temperature, etc.) needs to be adjusted accordingly, so that the hardness change of the transition material passing through the barrel and the die plate is uncertain. Due to the change of the hardness of the transition material, the cutter is subjected to repeated action of alternating stress, resulting in passivation of the cutter.

3.Polymerization/Modification Process

For the same grade of PP, the MFR fluctuates within a certain range due to the characteristics of the polymerization/modification process. When the MFR fluctuates greatly, it is also unfavorable to prolong the life of the cutter. In actual production, due to the influence of changes in process and raw materials, such as fluctuations in the amount of additives added, inconsistent feeding, uneven catalyst preparation, and low purity of raw materials, the uniformity and stability of product properties may be affected, which is not conducive to Knife protection during granulation.

The fluctuation of the post-processing section of PP polymerization can also have an adverse effect on the properties of the powder. For example, due to the fluctuation of the steam in the drying unit of the polymer raw material, the drying effect is not good, so that the raw material contains a certain amount of water, and finally the knife wrapping phenomenon occurs in the pelletizer.

4.Others

Equipment factors include the flatness of the cutter plate, the perpendicularity of the cutter shaft and the die plate, the flatness of the cutter surface and the material of the cutter.