PVC powder extruder is a type of twin-screw extruder.

1. Introduction

Plastic extruders are divided into twin-screw extruders and single-screw extruders, as well as multi-screw extruders and screwless extruders that are not often seen. They can be divided according to the number of screws. At present, single-screw extruders are the most widely used and suitable for extrusion processing of general materials. Twin-screw extruders have less heat generated by friction, more uniform shearing of materials, larger conveying capacity of screws, more stable extrusion volume, longer material stay in the barrel, and uniform mixing. The PVC powder extruder is one of its twin-screw extruders, a conical twin-screw extruder. It has the characteristics of forced extrusion, high quality, wide adaptability, long life, low shear rate, difficult material decomposition, good mixing and plasticizing performance, direct powder molding, temperature automatic control, vacuum exhaust and other devices. It is suitable for the production of products such as pipes, plates, and special-shaped materials.

2. Function and Use

The conical twin-screw extruder has the characteristics of uniform plasticizing and mixing, high output, stable quality, wide adaptability, long service life, and direct molding of PVC powder. Equipped with corresponding molding head molds and auxiliary machines, various thermoplastic plastics, especially rigid polyvinyl chloride powder, can be directly extruded into plastic products such as tubes, plates, sheets, rods, films and profiles, and can also complete the modification of various plastics and powder granulation process.

The conical twin-screw extruder has stable performance and can make the melt plasticized and extruded well at a lower temperature. The barrel is equipped with a cast aluminum heater with high thermal efficiency, fast and uniform heating, and a cooling fan.

The specially designed transmission part adopts a new variable frequency motor drive or DC motor drive, which has stable operation, large transmission torque and high efficiency. Through imported frequency converters or DC speed regulators, stepless and stable speed regulation can be achieved, with high precision and energy saving. The intelligent dual-display digital temperature controller is adopted, with high control accuracy and small temperature fluctuation. It is equipped with overload protection and fault alarm, screw core oil circulation constant temperature, barrel oil cooling and other functions, and is equipped with a vacuum exhaust pipe device and a quantitative feeding device.

III. Operation process

When making different plastic products, the operating points of the extruder are different, but there are also similarities. The following briefly introduces the same operating steps and the operating points of the extruder that should be paid attention to when extruding various products.

1. Preparation before starting

(1) For different extrusion molding plastics. The raw materials should meet the required drying requirements and need to be further dried if necessary. The raw materials should be sieved to remove agglomerates and mechanical impurities.

(2) Check whether the water, electricity and gas systems in the equipment are normal, ensure that the water and gas lines are unobstructed and leak-free, whether the electrical system is normal, and whether the heating system, temperature control and various instruments are working reliably; run the auxiliary machine at a low speed and observe whether the equipment is operating normally; start the vacuum pump of the calibrating table and observe whether it is working normally; add oil and lubricate the smooth parts of various equipment. If any fault is found, it should be eliminated in time.

(3) Install the die head and the mold sleeve. Select the die head specifications according to the product type and size. Install the die head in the following order.

2. Start the machine

(1) After the temperature is constant, the die head and the extruder flange bolts should be tightened again before starting the machine to eliminate the difference in thermal expansion between the bolts and the die head. The order of tightening the die head bolts is to tighten them diagonally and apply force evenly. When tightening the die head flange nuts, the tightness should be consistent around the edges, otherwise the material will escape.

(2) Start the machine, select the "Ready to Start" button, then the "Start" button, and then slowly rotate the screw speed adjustment knob to start the screw speed slowly. Then gradually speed up and add a small amount of material at the same time. When adding material, pay close attention to the changes in the indications of the host ammeter and various indicator heads. The screw torque cannot exceed the red mark (generally 65%-75% of the torque gauge). Before the plastic molding is extruded, no one is allowed to stand in front of the die to prevent injuries caused by bolts being pulled off or raw materials being wet and bubbling. After the plastic is extruded from the die of the die head, the extrudate needs to be slowly cooled and led to the traction device and the shaping die, and these devices are started. Then, according to the indication value of the control instrument and the requirements for the extruded product, each part is adjusted accordingly to make the entire extrusion operation reach a normal state. And add enough material as needed. The twin-screw extruder uses a metering feeder to feed the material evenly and at a constant speed.

(3) When the die discharge is uniform and the plasticization is good, it can be pulled into the shaping sleeve. The judgment of the degree of plasticization needs to be based on experience. Generally, it can be judged based on the appearance of the extruded material, that is, the surface is glossy, free of impurities, foaming, charring and discoloration. The extruded material is squeezed by hand to a certain degree without burrs or cracks, and has a certain elasticity. At this time, it means that the material is well plasticized. If the plasticization is poor, the screw speed, barrel and die head temperature can be appropriately adjusted until the requirements are met.

(4) During the extrusion production process, various process parameters should be checked regularly according to process requirements to see if they are normal, and the process record sheet should be filled in. Check the quality of profile products according to quality inspection standards, and take timely measures to solve problems.

3. Stop

(1) Stop feeding, squeeze out the plastic in the extruder, and when the screw is exposed, turn off the power of the barrel and the head, and stop heating.

(2) Turn off the power of the extruder and auxiliary machines to stop the screw and auxiliary machines.

(3) Open the head connection flange and disassemble the head. Clean the porous plate and various parts of the head. To prevent damage to the inner surface of the head during cleaning, the residual material in the head should be cleaned with steel bars and steel sheets, and then the plastic adhering to the head should be ground off with sandpaper, polished, and coated with engine oil or silicone oil to prevent rust.

(4) Cleaning of the screw and barrel. After removing the die, restart the main machine, add the stopping material (or crushed material), clean the screw and barrel. At this time, the screw should be operated at a low speed (about sr/min) to reduce wear. After the stopping material is crushed into powder and completely extruded, the residual granular material and powder can be repeatedly blown out from the feed port and exhaust port with compressed air until there is no residual material in the barrel. Then reduce the screw speed to zero, stop the extruder, and turn off the main power supply and cold water valve.

(5) Safety items that should be paid attention to during extrusion of the extruder include: electrical, thermal, mechanical rotation and loading and unloading of heavy parts. The extruder workshop must be equipped with lifting equipment to install and disassemble heavy parts such as the die head and screw to ensure safe production.