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 hydraulic oil pressure of the retract balance system in the actual production process, and the practical effect after improvement.
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 "scissor principle", that is, the high-speed rotating cutter and the fixed template discharge hole shear each other, cutting 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 taken out of the pelletizing chamber by the particle 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, and it is easy to form an abnormal gap between the cutter and the template, which will 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 pipeline 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.
Principle of hydraulic oil system for tool retraction balance
When the extrusion granulation unit is working normally, the force between the cutter and the template is complex and mostly variable. For example, when the molten resin material is extruded from the template die hole, 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 cutter will often retreat, and a large number of unqualified 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 to the pelletizer feed shaft, which can instantly and effectively calculate and compensate the force of the cutter feed or retract. On the other hand, it can balance the propulsion force of the feed system according to the force 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 also play a role in reducing the number of unit failure shutdowns.
Through the force analysis of the feeder, in order to ensure the balanced force of the cutter, it is necessary to add a reverse force to the cutter feed system, which is realized by adding an additional retractor.
Figure 1 Improved underwater pelletizing unit control system diagram
Products Description
As can be seen from Figure 1, the main idea of the balance system is: when the instrument air is reduced by the pressure reducing valve, it is supplied to the hydraulic cylinder of the feeder and the hydraulic cylinder of the retractor at the same time. When the feeder pushes the feed shaft to the template to a certain distance, the feed shaft micrometer reaches a certain constant value. At this time, the cutter is close to the template. The hydraulic oil pressure of the retractor is adjusted to the preset value to ensure that the excess feed thrust of the feed shaft axis is offset. The high-speed rotating cutter on the feed shaft is in a slightly positive pressure working state close to the template for pelletizing. The pressure difference between the feeder and the retractor, and the hydraulic oil pressure signals of the feeder and the retractor are transmitted to the relevant cards of the instrument control cabinet of the unit through the communication cable, and after processing, they are transmitted to the operation station together with the alarm and interlocking information.
The operator can directly observe various parameters on the operation station and adjust the feed shaft in real time according to the actual situation. The adjustment signal controls the output of the on-site equipment through the instrument control cabinet and feedbacks in real time, as shown in Figure 2.
Figure 2 Specific improvement plan for the newly added tool retraction balance system
On-site improvement of hydraulic oil system for tool retraction balance
Figure 3 On-site installation diagram of the hydraulic oil system for retracting the cutter
The specific improvement method is to install two knife retractors on both sides of the pelletizer body, as shown in Figure 4. Install the knife retractor balance system shown in Figure 3 below the unit, connect the system to the instrument wind 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, as shown in Figure 5. 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 H51Q-4RS control system of the unit. 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.
Figure 4: On-site installation diagram of the knife retractor
Figure 5 SCHALTER differential pressure meter installation diagram
Automatic adjustment of oil pressure in the tool retraction balance system
After filtering and reducing pressure, the instrument air gas of about 0.2MPa is delivered to the hydraulic oil cylinder. The air pressure in the hydraulic oil cylinder is initially adjusted by the damping regulator. At this time, the instrument air pushes the piston of the hydraulic oil cylinder to 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. As shown in Figure 3.
The retractor is fixed to the pelletizer body bracket by fixing bolts. When the hydraulic oil enters the retractor, the compressed retractor spring exerts a force on the feed shaft in the opposite direction of the feed shaft, as shown in Figure 4. The differential pressure measuring meter with remote transmission signal takes a point pressure from the hydraulic oil pipeline of the feeder at one end, and takes a point pressure from the hydraulic oil pipeline of the retractor at the other end. The differential pressure meter can accurately measure the differential pressure value of the two pressure points and transmit it remotely to the control system. The control system makes a judgment 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 back-up knife 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 feed shaft of the unit 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 back-up knife 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 back-up knife 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 cause material entanglement or damage to the cutter and template due to excessive pressure due to back-up. The adjustment operation is shown in Figure 6.
Figure 6 Improved automatic adjustment diagram of oil pressure in the tool retraction system
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.
