The plastics processing industry is a highly comprehensive technology-based industry. It involves polymer chemistry, polymer physics, interface theory, plastic machinery, plastic processing molds, formula design principles and process control. The extrusion theory mainly studies the movement and changing laws of plastics in the extruder. The relationship between the three physical states of the polymer in different temperature ranges of the plastic material in the extruder under the action of a certain external force, and the screw structure, plastic properties, and processing conditions. So as to carry out reasonable process control. In order to achieve the purpose of improving the output and quality of plastic material products. Plastic polymer materials, when heated under constant pressure, appear in three physical states: glassy state, high elastic state, and viscous flow state in different temperature ranges. Generally, the molding temperature of plastics is above the viscous flow temperature. The control parameters of the extrusion molding process include molding temperature, working pressure of extruder, screw speed, extrusion speed and traction speed, feeding speed, cooling and shaping, etc.

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1. The pretreated polyolefin of the raw material is a non-water-absorbent material, usually with a very low moisture content, which can meet the needs of extrusion, but when the polyolefin contains water-absorbent pigments, such as carbon black, it is not suitable for Humidity sensitive. In addition, when using regrind and filler, the water content will increase. Moisture not only causes rough inner and outer surfaces of the tube, but can also cause bubbles to appear in the melt. The raw material should usually be pretreated. Generally, drying treatment is adopted, and corresponding auxiliary agents with dehumidification function can also be added. Such as defoamer and so on. The dry temperature of PE is generally 60-90 degrees. At this temperature, the yield can be increased by 10%-25%.

2. Temperature control Extrusion temperature is a necessary condition to promote the plasticization of the molding material and the flow of the plastic melt. It has a very important impact on the plasticization of materials and the quality and output of products. The theoretical temperature window for plastic extrusion is between the viscous flow temperature and the degradation temperature. The temperature range is wider for polyolefins. Usually above the melting point, it can be processed below 280 degrees. To correctly control the extrusion temperature, it is necessary to understand the relationship between the temperature limit of the material being processed and its physical properties. Only by finding out its characteristics and laws can a better temperature range be selected for extrusion molding. Therefore, the following aspects should be considered in the temperature setting of each section: First, the properties of the polymer itself, such as melting point, molecular weight size and distribution, melt index, etc. Next, consider the performance of the device. In some equipment, the temperature of the feeding section has a great influence on the current of the host. Again, observe whether the surface of the extruded tube blank is smooth by observing the tube die. Check for the presence or absence of bubbles. Plastic RecyclingPelletizer

The extrusion temperature includes the set temperature of the heater and the melt temperature. The heating temperature refers to the temperature provided by the external heater.

The melt temperature refers to the temperature of the material between the front end of the screw and the connection of the die.

Barrel temperature distribution, from the feed zone to the die head may be flat, increasing, decreasing and mixed

Mainly depends on the material point and the configuration of the extruder.

Geometry parameters of the screw

The geometrical parameters of the screw include diameter, aspect ratio, compression ratio, groove depth, helix angle, clearance between screw and barrel, etc., which have a significant impact on the working characteristics of the screw. Generally, the length-diameter ratio of the extruder is 15 to 25, but the extruder developed in recent years has reached 40 or even larger.

The large aspect ratio can improve the temperature distribution of the plastic, make the mixing more uniform, reduce the backflow and leakage during extrusion, and improve the production capacity of the extruder. If the aspect ratio is too small, it is unfavorable for the mixing and plasticization of plastics. Therefore, for hard plastics and powdery plastics, which require a long plasticizing time, a larger one should be selected. The screw with large aspect ratio has strong adaptability and can be used for extrusion of various plastics. However, if the aspect ratio is too large, the heat-sensitive plastic will be decomposed due to too long heating time, and at the same time, the self-weight of the screw will be increased, making it difficult to manufacture and install, and will also increase the power consumption of the extruder. At present, the aspect ratio is mostly 25.

The compression ratio of the screw; refers to the ratio of the volume of the first screw groove of the screw feeding section to the volume of the next screw groove of the homogenization section Z, which indicates the degree of compression of the plastic through the whole process of the screw. The greater the compression ratio, the greater the effect of extrusion on the plastic, and the greater the ability to remove air from the material. However, if the compression ratio is too large, the mechanical strength of the screw itself decreases. The general compression ratio is between 2 and 5. The size of the compression ratio depends on the type and shape of the extruded plastic. For example, the relative density of the powdery plastic is small and the air is more entrained, and the compression ratio should be greater than that of the granular plastic. In addition, when extruding thin-walled products, the compression ratio ε should be larger than that of extruding thick-walled products.

The depth of the screw groove: the depth of the screw groove affects the plasticization and extrusion efficiency of the plastic. When the groove depth is small, the plastic can be affected.

High shear rate, good for heat transfer and plasticization, but reduced extrusion productivity. Therefore, heat-sensitive plastics are suitable. The deep groove screw with large groove depth should use plastic with low melt viscosity and high thermal stability.

In actual production, the depth of the screw groove is often changed according to the needs of the process. The depth of the screw groove is different according to the function of each section of the screw. The most common is the gradual change screw. For example, the screw groove depth H1 of the feeding section is a A fixed value, generally H1>0.1Ds; the screw groove depth H2 of the compression section is gradual and is a changing value; the screw groove depth H3 of the homogenization section is a fixed value, according to experience H3 = 0.02 ~ 0.06 Ds . The helix angle θ is the angle between the thread and the cross section of the screw. With the increase of θ, the production capacity of the extruder increases, but the extrusion shearing effect of the screw on the plastic decreases. For the convenience of machining, take Ds = Ls, then θ is 17.26. For Z commonly used screw.

Gap between screw and barrel: The gap between screw and barrel affects the production capacity of the extruder and the plasticization of the material. The gap value of the barrel is large, the heat conduction is poor, and the shear rate is low, which is not conducive to the melting and mixing of materials, and the production efficiency will not be high. However, when the gap of the barrel is small, the heat conduction and the shear rate are increased accordingly. However, if the gap of the barrel is too small, it is easy to cause material degradation.