In-depth analysis and manufacturing craftsmanship of screw barrels

Screw barrels play an important role in the plastic processing industry and are one of the core components of plastic molding equipment. They affect the quality and production efficiency of plastic products, and are also related to the stability and cost control of the entire production line.

In the injection molding process, the screw barrel is responsible for melting the plastic raw materials and conveying them to the mold cavity. Its precise melting and metering capabilities ensure the consistency and high precision of the injection molded parts. The design and structural optimization of the screw can effectively improve the mixing uniformity and melting efficiency of the plastic, reduce energy consumption and scrap rate.

In the extrusion process, the screw barrel plays a more complex role. It not only melts the plastic raw materials, but also extrude the molten plastic at a constant pressure and flow rate through precise screw speed and temperature control to form continuous profiles, films or pipes and other products. The performance of the screw barrel directly affects the dimensional accuracy, surface quality and production efficiency of the extruded products.

In the blow molding process, the screw barrel also plays a key role. It is responsible for melting the plastic raw materials and injecting them into the blow mold, and then blowing the molten plastic into the desired shape of the hollow product through high-pressure gas. The melting effect and temperature control of the screw barrel are crucial to the wall thickness uniformity, surface gloss and strength of the blow molded products.

With more than 30 years of manufacturing experience for the world’s top brand machinery companies, Barrelize is recognized as the largest and professional Plasticizing Equipment manufacturer in China. We provide standard and customized single and twin (cylindrical, parallel) screw and barrel for injection and extrusion machineries.

The screw barrel is a key component in plastic processing equipment, mainly composed of two parts: the screw and the barrel. The following is a brief introduction to these two parts, as well as the effects of parameters such as the length ratio of each section of the screw, the depth of the screw groove, and the thread design on the plasticizing effect.

1. The main components of the screw barrel

Screw

The screw is the core component in the screw barrel, which is responsible for the melting, conveying, shearing and homogenizing of the plastic.

The screw is usually divided into three sections: the feeding section, the compression section and the homogenizing section.

Feeding section: mainly responsible for the conveying and preliminary preheating of plastic raw materials. The screw groove of this section is deeper and the pitch is larger, so as to accommodate more plastic raw materials and gradually heat them up through the friction generated by rotation.

Compression section: The screw groove of this section gradually becomes shallower and the pitch gradually becomes smaller, exerting greater shear force and compression force on the plastic raw materials, making them gradually melt and become more uniform.

Homogenization section: Mainly responsible for further homogenization and transportation of molten plastic. The screw groove depth, pitch and thread design of this section of the screw are carefully optimized to ensure that the temperature, pressure and composition of the molten plastic are highly uniform.

Barrel

The barrel is the channel for the screw to rotate and the channel for the plastic melt.

The inner wall of the barrel is usually precision machined and polished to reduce friction and resistance and ensure that the plastic melt can flow smoothly.

Heating elements and temperature sensors are usually installed on the barrel to accurately control the temperature of the melt.

2. The influence of the parameters of each section of the screw on the plasticizing effect

Length ratio

The length ratio of each section of the screw has an important influence on the plasticizing effect. Generally speaking, a longer feeding section is conducive to the full preheating and conveying of plastic raw materials; a moderate compression section can ensure the full melting and homogenization of plastic raw materials; a longer homogenization section is conducive to the further homogenization and stabilization of molten plastic.

The selection of length ratio should be comprehensively considered according to factors such as the type of plastic, processing temperature and screw speed.

Screw groove depth

The screw groove depth directly affects the screw's conveying capacity and shearing effect on plastic raw materials. The deeper the screw groove of the feeding section is, the more plastic raw materials can be accommodated, and the friction generated by the rotation gradually heats it up. While the screw grooves of the compression section and the homogenization section gradually become shallower, greater shearing and compression forces can be applied to the plastic raw materials, causing them to gradually melt and become more uniform.

The selection of screw groove depth should be comprehensively considered according to factors such as the type of plastic, processing temperature and screw speed. Generally speaking, the deeper the screw groove depth, the stronger the screw's conveying capacity for plastic raw materials, but the shearing effect is relatively weak; conversely, the shearing effect is strong, but the conveying capacity is relatively weak.

Thread design

The thread design also has an important influence on the plasticizing effect of the screw. Different thread designs can produce different shear and compression forces, thus affecting the melting and homogenization of plastics.

Common thread designs include equal distance and unequal depth, equal depth and unequal distance, and unequal depth and unequal distance. Equal distance and unequal depth are the most commonly used designs, which can make the contact area between plastic and barrel larger and the heat transfer effect better. The designs of equal depth and unequal distance and unequal depth and unequal distance can be flexibly adjusted according to different processing requirements.

The role of screw in plastic melting, conveying, shearing, homogenization and other processes

Melting: The screw exerts shear force and friction force on the plastic raw material through its unique spiral design and rotational motion, causing it to gradually heat up and melt. In this process, the geometry of the screw, the speed and the coordination of the heating system are crucial to ensure that the plastic can be melted evenly and efficiently.

Conveying: The molten plastic is pushed by the screw and moves along the axial direction of the barrel and is conveyed to the mold or other molding equipment. The conveying efficiency of the screw directly affects the production speed of plastic processing.

Shearing: The spiral groove and thread design of the screw enable it to produce shearing action in the plastic melt, which helps further mix and homogenize the plastic. The shearing action also helps remove bubbles and impurities in the plastic and improve the quality of the product.

Homogenization: During the melting and conveying process, the screw ensures that the plastic melt is highly uniform in temperature and composition through its cleverly designed compression section and homogenization section. This helps to eliminate temperature gradients and composition differences in the melt, thereby producing products of consistent quality.

The cooperation between the barrel and the screw ensures temperature control and pressure stability of the plastic during processing. As a channel for the plastic melt, the inner wall of the barrel is usually precision machined and polished to reduce friction and resistance. The barrel is usually also equipped with heating elements and temperature sensors to accurately control the temperature of the melt.

When the screw rotates in the barrel, it not only pushes the plastic melt forward, but also forms a relatively closed space by closely fitting with the inner wall of the barrel. The temperature and pressure of the plastic melt in this space are effectively controlled and regulated under the shearing and compression of the screw.

In addition, the cooling system of the barrel also plays a key role. It helps control the temperature of the outer wall of the barrel and prevents heat from being transferred to the external environment too quickly, thereby ensuring the temperature stability of the melt during processing.

Material selection and surface treatment technology

Commonly used screw barrel materials include high-quality alloy steel and chrome-molybdenum alloy steel.

High-quality alloy steel: This material has high strength and hardness and can withstand large mechanical and thermal stresses. At the same time, it also has good wear resistance and corrosion resistance, and can maintain stable performance in harsh processing environments.

Chrome-molybdenum alloy steel: This material is based on high-quality alloy steel, and alloy elements such as chromium and molybdenum are added to further improve its wear resistance, corrosion resistance and high-temperature strength. Therefore, chrome-molybdenum alloy steel is particularly suitable for high-temperature, high-pressure and highly corrosive plastic processing environments.

In addition to the selection of materials, surface treatment technology is also an important means to improve the wear resistance and service life of the screw barrel. Commonly used surface treatment technologies include nitriding and spray welding alloys.

Nitriding: Nitriding is a technology that forms a layer of hard nitride on the surface of the screw barrel through a chemical reaction. This layer of nitride has high hardness and wear resistance, which can significantly improve the wear resistance and service life of the screw barrel.

Spray welding alloy: Spray welding alloy is a technology that sprays alloy powder onto the surface of the screw barrel through a high-temperature flame and fuses it with the base material. This technology can form an alloy layer with excellent wear resistance and corrosion resistance on the surface of the screw barrel, further improving its service life.

The manufacturing process of the screw barrel is a complex and delicate process, including multiple processes such as quenching and tempering, shaping, molding, fine grinding, and polishing.

Quenching and tempering: Quenching and tempering is the first step in the manufacture of the screw barrel. It improves its internal organization and performance by heating and cooling the raw materials, laying a good foundation for subsequent processes.

Shaping: Shaping is the process of cutting and preliminarily shaping the quenched and tempered raw materials according to the design requirements. This step requires precise control of size and shape to ensure the smooth progress of subsequent processes.

Molding: Molding is the process of accurately molding the shaped raw materials through a mold. This step requires strict control of parameters such as temperature and pressure to ensure the accuracy and performance of the screw barrel.

Fine grinding: Fine grinding is the process of fine grinding the screw barrel after molding. This step can remove burrs and unevenness on the surface and improve the surface quality and precision of the screw barrel.

Polishing: Polishing is the process of further processing the screw barrel after fine grinding. Through polishing, the surface of the screw barrel can be made smoother and brighter, improving its aesthetics and service life.

Screw barrel selection guide

Select according to plastic type

Plastics with poor thermal stability: such as PVC, it is recommended to choose a gradual screw, which has a longer compression section and a gentle energy conversion during plasticization, which helps to reduce the thermal decomposition of the plastic.

Crystalline plastics: such as polyolefins, PA, etc., the mutant screw is more suitable, with a shorter compression section, more intense energy conversion during plasticization, and can quickly melt the plastic.

General plastics: General screws have strong adaptability and can adapt to the processing of a variety of plastics. They are suitable for production scenarios where the type of plastic needs to be frequently changed.

Consider the compression ratio and aspect ratio

Compression ratio: It is the ratio of the volume of the first screw groove in the feeding section of the screw to the volume of the last screw groove in the homogenization section. It determines the degree of compression of the plastic in the screw, which in turn affects the plasticization effect. Generally, when processing thermoplastics, a screw with a moderate compression ratio should be selected; when processing thermosetting plastics, a higher compression ratio may be required.

Aspect ratio: It is the ratio of the length of the working part of the screw to the diameter of the screw. The larger the aspect ratio, the longer the plastic is heated in the screw, and the more complete and uniform the plasticization. However, an excessively large aspect ratio may lead to problems such as increased power consumption and screw bending. Therefore, when selecting the aspect ratio, it is necessary to weigh it according to the processing characteristics and production requirements of the specific plastic.

2. The influence of parameters on the plasticization effect

Compression ratio

The appropriate compression ratio can fully plasticize and compact the granular plastic and improve the product quality. However, a compression ratio that is too high may cause the screw to bear too much pressure, increase wear and energy consumption; a compression ratio that is too low may cause poor plasticization of the plastic, affecting product quality.

Aspect ratio

Increasing the aspect ratio is beneficial to the mixing and plasticization of plastics and improves the quality of plasticization. However, too large an aspect ratio may cause the plastic to stay in the screw for too long, causing thermal decomposition problems; at the same time, it may also increase the difficulty of processing and assembling the screw and barrel.