Improve The "feeding" And "retracting" of Underwater Cutting To Achieve Stable Production of The Pelletizing System

In the process of extrusion granulation, the feed system plays a key role in the force and coordination between the cutter and the template for the smooth operation of the unit. 

Let's take a look at the relationship between the feed hydraulic oil pressure and the retract balance system hydraulic oil pressure in the actual production process, and the practical effect after improvement.

1 Problems currently faced by underwater pelletizers

The underwater pelletizing unit is mainly composed of a pelletizing chamber, a cutting knife, a knife disc, a template and other components. This unit is the realization unit for converting molten resin into finished pellets. It is the core unit for the success of pelletizing and directly affects product quality, waste volume, production cost, and whether the unit can operate smoothly for a long period of time. At the same time, this unit has many forms of failure, and the reasons are sometimes extremely complicated. It is also the unit with the highest failure frequency of the unit. It can be said that the smooth operation of the underwater pelletizing unit is the determining factor for the long-term smooth operation of the extrusion pelletizing unit.

The principle of pelletizing by the cutter and the template is also called the "scissors principle", that is, the high-speed rotating cutter and the fixed template discharge hole shear each other to cut off the resin material extruded from the die hole.

The cut material rolls in cold water to cool and solidify into round particles, which are then taken out of the pelletizing chamber by the pellet cooling water to complete the pelletizing work. Therefore, when the cutter and the template complete the "cutting" work, if the force is uneven, it will cause improper fitting, which will easily lead to abnormal gaps between the cutter and the template, and then lead to the production of unqualified pellets, such as wire drawing materials, tailing materials, string materials, snake skin materials, etc. In severe cases, abnormal pelletizing will cause the unit material to "wrap around the knife" failure, and even "enema" (that is, the molten material fills the upstream and downstream conveying pipelines without pelletizing, and blocks the pelletizing conveying pipelines after cooling and solidification) accidents.

Once the unit has abnormal faults such as knife entanglement, the unit becomes more difficult to handle. Sometimes it takes several days to completely clean and dredge the pellet conveying pipeline. Sometimes the unit downtime is extended due to replacement of templates and cutters, which further causes production fluctuations and affects the output of the device. On the other hand, under long-term abnormal stress, the wear of the cutter and template will also increase, seriously affecting the service life of the cutter and template.

2 Principle of the hydraulic oil system for tool retraction balance

When the extrusion granulation unit is working normally, the forces between the cutter and the template are complex and mostly variable. For example, when the molten resin material is extruded from the die hole of the template, the positive pressure on the cutter will increase with the increase of production load, and at the same time, it will increase with the increase of the viscosity coefficient of the resin material. The viscosity coefficient is related to the screw speed, screw shape, resin material temperature, and resin material melt index. In the actual production process, only the resin material temperature, resin material melt index, and production load often change. Therefore, when these three parameters change, the template discharge pressure of the barrel extrusion section will change accordingly. The feed oil pressure set by the feed system is a fixed value, which can only compensate for the normal wear of the cutter to a limited extent. When the discharge pressure increases, the knife will often retreat, and a large number of defective products will be produced. In severe cases, it will cause the cutter to be entangled.

In order to solve the above problems, a new balancing force must be introduced into the pelletizer feed shaft, which can instantly and effectively calculate and compensate the force on the cutter's feed or retract, and on the other hand, can balance the propulsion force of the feed system according to the force conditions of the cutter under different working conditions, so that the cutter always maintains a slightly positive pressure working state on the template, which can not only improve product quality, but also reduce the loss of wearing parts, and at the same time reduce the number of unit failure shutdowns.

Through the force analysis of the feeder, in order to ensure the balanced force on the cutter, a reverse force must be added to the cutter feed system, which is achieved by adding an additional retractor.

3 On-site improvement of the hydraulic oil system for tool retraction balance

The specific improvement method is to install two knife retractors on both sides of the pelletizer body. Install a knife retractor balance system under the unit, connect the system to the instrument air as the power source, and add a pressure difference indicator with remote transmission between the hydraulic oil circuit of the feeder and the hydraulic oil circuit of the knife retractor. Introduce the hydraulic oil pressure of the feeder, the hydraulic oil pressure of the knife retractor, the pressure difference and other signals into the instrument control cabinet, and configure and install them in the unit control system. The feeder and the knife retractor are controlled by the pneumatic control valve, and the control signal is introduced into the instrument control cabinet. At this point, all equipment is installed.

4 Automatic adjustment of oil pressure in tool retraction balance system

After filtering and reducing pressure, the instrument air of about 0.2MPa is delivered to the hydraulic oil cylinder. The air pressure in the hydraulic oil cylinder is initially adjusted through the damping regulator. At this time, the instrument air pushes the piston of the hydraulic oil cylinder to do work and delivers the hydraulic oil to the retractor. The hydraulic oil pressure delivered to the retractor can be read from the output value of the pressure sensor.

The knife retractor is fixed on the support of the pelletizer body by fixing bolts. When the hydraulic oil enters the knife retractor, the spring of the knife retractor is compressed to exert a force on the feed shaft in the opposite direction of the feed shaft. The differential pressure gauge with remote signal draws pressure from a point on the hydraulic oil pipeline of the knife retractor at one end and draws pressure from a point on the hydraulic oil pipeline of the knife retractor at the other end. The differential pressure gauge can accurately measure the differential pressure value of the two pressure points and transmit it to the control system. The control system makes judgments based on the collected differential pressure signal. When the differential pressure value is too high or too low, it can trigger the unit alarm and interlock switch.

The operator can obtain the pressure difference value from the operation station, and adjust the knife oil pressure setting value of the feeder and the retractor in time according to the melt index of the production resin, the temperature of the extrusion section, and the production load, so as to ensure that the pressure difference indication of the unit feed shaft is the specified value.

The specific operation method is: when the pressure difference value increases, it proves that the pressure of the resin material on the feed shaft decreases. At this time, the pressure of the feed shaft on the template increases. The hydraulic oil pressure of the ejector needs to be increased synchronously to balance the pressure of the feed shaft, so that the pressure difference is reduced to the original pressure difference value, and the reverse adjustment is completed; when the pressure difference value decreases, it means that the reverse force of the resin material on the feed shaft increases. At this time, the hydraulic oil pressure of the ejector needs to be reduced in time to increase the pressure difference to the original value and complete the forward adjustment. When the pressure difference value suddenly increases or decreases to the interlocking value, it can also trigger the interlocking shutdown of the extrusion section of the unit to ensure that the feed shaft will not be entangled with the material due to the ejection of the knife, or the cutter and the template will not be damaged due to excessive pressure.

The improved feed system has significantly reduced the number of unit shutdowns and improved the equipment's ability to operate smoothly over a long period of time. Online sampling and analysis of the finished particles before and after the improvement showed that the appearance of the particles was significantly improved compared to before.

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