Common Problems of Twin-screw Plastic Granulators

2021-10-15 09:20:58

This article introduces the development status, operating principle, and structural characteristics of twin-screw plastic granulators. This article is from the perspectives of the processing, assembly, and trial operation of the primary vital components of the granulator. It analyzes some common problems that may occur during the manufacturing process of twin-screw plastic granulators

Twin-screw plastic granulators are essential equipment in the industrial production process of polyolefins. The kit makes polymer raw materials into pellets through methods such as mixing, extruding, and pelletizing. And thus, effectively improves the performance of polyolefin products and brings much more convenience for subsequent measurement, transportation, and other operations. In the 1930s, the first twin-screw plastic granulator was successfully developed in Italy. In the late 1960s and early 1970s, the granulators industry was developed rapidly. At present, only a few companies can design and manufacture complete sets of large-scale plastic granulators.

Twin-screw plastic granulators

At present, twin-screw plastic granulators are widely used in the industrial production of polyolefins. The maximum screw diameter of domestic co-rotating twin-screw plastic granulators reaches 350mm. The top screw diameter of counter-rotating twin-screw plastic granulators is 480mm. With the expansion of production scale and the enlargement of the equipment, higher requirements have been put forward for the polymer resin mixing capacity and mixing effect and the stability of the unit operation. This makes large plastic granulators use twin screws structure more frequently, plus configuration equipment such as melt extruder and underwater pelletizing. However, with the increasing requirements in production, higher standards have been pointed out on the quality of twin-screw plastic granulators. This makes the production process of plastic granulators focus more on the quality problems that occur in the various production processes of the equipment. We can only manufacture efficient and stable twin-screw plastic granulators by avoiding the common issues in production.


Common Problems Existing in the Production of Twin-screw Plastic Granulators

①. Common issues in the production of crucial components of twin-screw plastic granulators
During the operation, twin-screw plastic granulators have to withstand high-temperature environments. The abrasion and flow erosion of the molten polyolefin mixture operate under high torque load conditions for mixing and extrusion. Therefore, the components in contact with the raw materials must have the characteristics of high-temperature resistance, high strength, wear resistance, and stability of deformation resistance. The main features are the screw rotor for mixing, the barrel, the spool for the screen changing device, the rotor of the melt gear pump, the device for underwater pelletizing, etc. Different components may involve various problems.

②. The mixing rotor problem of plastic granulators
The mixing rotors are long-running in a high temperature and high-strength environment. The main problems include the chromium layer not being firmly bonded to the base metal, pores, and the outer layer peels off. These possible issues will seriously impact the production efficiency of plastic granulators. Therefore, the rotor material is generally high-strength alloy steel, integrally forged, and quenched and tempered. The rotor is processed by CNC machine tools and then chrome-plated, and the bonding force and hardness of the finished product are tested. Generally, the hardness of the rotor is no less than 880HV. In addition, the surface roughness of the side and root diameter of the rotor screw edge also affects the work efficiency to a large extent. If the roughness is too large or too small, the raw material will be unevenly formed. Therefore, the surface roughness Ra value of the rotor shall be no less than 0.8μm, and the surface roughness Ra value of the outer diameter no less than 3.2μm.

③. The barrel problem of plastic granulators
The main concern for pelletizing machine barrels is their hardness and roughness. Only when the hardness and roughness meet the requirements can the corresponding production requirements be fulfilled. The barrel of the feeding section can be made of alloy steel castings, and the barrels of the exhaust section and the closed area shall be forged with alloy steel as a whole. Components of this material can guarantee the following strength requirements. After finishing using the machine tool, the surface of the inner hole should be chrome-plated. The hardness value shall meet no less than 880HV, and the surface roughness Ra value no less than 1.6μm.

④. Melt gear pump rotor problem of pelletizing machine
The possible main problem of the melt gear pump rotor is strength problems. Once the gear pump rotor encounters strength damage, it will cause the gear pump to fail as a whole. Therefore, the material should be forged with alloy steel. After finishing, the tooth's surface is nitride, the nitride layer's depth is Not less than 0.4mm, and the hardness should not be less than 800HV.

⑤. The problem of the screen changer sliding column of pelletizing machine
The sliding column material used in the screen changer is forged with alloy steel as a whole. The primary purpose is to ensure the strength requirements. Strength failure is a common issue. Do surface nitriding treatment after finishing. The depth of the nitrided layer shall be no less than 0.4mm. The hardness should not be less than 800HV. The runner surface is chrome-plated, the hardness of the chrome layer is above 650HV. The chrome layer and the base metal must be firmly bonded to prevent the chrome layer from peeling off.

⑥. The underwater pelletizing template problem of pelletizing machine
The underwater pelletizing template is made of austenitic stainless steel, which may cause rust and corrosion failure after long-term use. The pelletizing belt should be made of composite material. The thickness of the hardened layer is greater than 3mm, and the hardness exceeds 60HRC. The surface flatness of the pelletizing belt should be less than 0.015mm, and the surface roughness Ra should not be greater than 0.8μm. The inner surface roughness Ra of the granulation hole shall not exceed 0.8μm. Thus to ensure the stable operation of the granulation belt without failure.


Common Problems Existing in the Assembly Process of the Critical Components of the Twin-screw Pelletizing Machine

The assembly process of the twin-screw pelletizing machine is a vital link to ensure the safe and stable operation of the equipment. During the operation of the equipment, the heating, mixing, plasticizing, forming, and cooling of the production materials will be affected by the equipment assembly. In addition, for the pelletizing machine, The stability of working conditions such as automatic operation, automatic monitoring, lubrication, and cooling also depends on the assembly process of the main components. The assembly process is mainly divided into barrel assembly, main reducer assembly, melt gear pump assembly, and underwater pelletizing device assembly.

①. The cylinder problem of pelletizing machine
During the assembly process, the barrel often has the problem of being out of concentricity with the screw rotor. The inner holes of the adjacent barrels must be overlapped after the barrel is assembled to ensure concentricity with the screw rotor. There is no significant deviation, and the variation should not be More than 0.10mm.

②. Main reducer box problem
The common problems of the main reducer in the operation process are mainly the decrease of stability and the production of significant noise. To ensure the stable operation of the main reducer and reduce noise, it is required that the three gears' meshing clearance and meshing area must meet the design standards. On the basis, ensure that the axiality coaxiality meets the design standards; in addition, the leakage problem of the lubrication pipeline system is also relatively common. During the assembly process, it is necessary to ensure no oil leakage at the joint surfaces and sealing positions of the box body. The lubricating oil pipeline is firmly connected.

③. Melt gear pump problem of pelletizing machine
The main problem of the melt gear pump is often caused by the misalignment of the input and output shafts of the reduction box. In addition, the equipment will also fail to operate normally when the input and output shafts are not concentric with the main motor shaft and the gear pump rotor. The concentricity of the transmission system formed by the gear pump and the reduction box is not greater than 0.05mm. The gear side clearance of the gear pump rotor must be adjusted to ensure that the gear pump rotor does not contact each other during operation.

④. Underwater pelletizing device problems
The common problem of the underwater pelletizing device is the immense verticality during installation. During the underwater pelletizing device installation, the water chamber and the pelletizing device shall keep concentric. The deviation requirements are less than 0.05mm. To ensure the verticality of the water chamber and the knife shaft. The verticality of the knife shaft and the pelletizing device shall also meet the requirements.

running test of twin-screw plastic granulators


Common Problems Existing in the Running Test of Twin-screw Plastic Granulators

Plastic granulators cannot carry outstrip tests in the production workshop. Therefore, only the mechanical operation test can check whether the processing meets the requirements, the assembly accuracy, and the intuitive control design are reasonable.

①. The whole process of plastic granulators mechanical operation test
The assembled plastic granulators can carry out the mechanical operation. When the heating system and cooling system are operating stably, the automatic operation test is started. First of all, plastic granulators need to be heated to the required operating temperature. The throttle valve, start-up valve, and screen changer are all working at the operating temperature. Then the deformation of the barrel and other parts is detected. The temperature range of the cooling system should keep the oil inlet temperature of the primary reducer lubrication system within the specified oil inlet temperature range. When the gear pump is full of materials, start to operate the crank and motor drive. Therefore, the gear pump can only be tested under the support of a particular test bearing.

②. Vibration problem during mechanical operation test of extrusion granulator
Vibration testing is the best standard to measure whether the overall rotating parts' design, processing, and assembly accuracy meet the requirements. Sometimes, the vibration will cause unstable operation of the granulator, significantly shortening the unit's service life. Excessive vibration will cause the quality of the produced product to drop. Therefore, during the test, the main motor and primary reducer vibration should be a focus. The vibration speed of each monitoring point of the direct motor and main reducer should be less than 2.2mm/s.
With the continuous increase of production requirements, higher requirements are put forward for the production process of the twin-screw extrusion granulator.
Paying attention to the main components of the equipment and the problems that occur in each production process. By doing so, we can effectively avoid failures during operation, and significantly improve the production efficiency of the twin-screw extrusion granulator, and maintain efficient and stable operation.

For more information about the equipment and operation of plastic granulators, please contact Packer engineers.

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