What is the difference between an extruder screw and an injection molding machine screw?

There are significant differences between extruder screws and injection molding machine screws in many aspects. These differences are mainly reflected in the design structure, functional characteristics, application scenarios and working principles.

Design structure

Length-to-diameter ratio (L/D): The length-to-diameter ratio of the injection screw is small, usually between 10 and 15, while the length-to-diameter ratio of the extrusion screw is relatively large, which is related to their respective working requirements.

Screw groove depth: The groove depth of the injection screw is designed to be deeper, especially in the homogenization section, to improve productivity and plasticizing effect. The groove depth of the extrusion screw is gradually reduced according to the extrusion needs to achieve compression and melting of the plastic.

Head shape: The head of the injection screw is mostly pointed in order to fit well with the nozzle and achieve high-pressure injection of plastics. The head of the extrusion screw is mostly round or flat to meet the needs of continuous extrusion.

Features

Injection screw: The main functions include feeding, conveying, plasticizing and injection. It is an intermittent operation process. Although the plasticizing capacity, stability and operation continuity requirements of the material are not as strict as those of the extrusion screw, it needs to withstand higher pressure and temperature during the injection process.

Extrusion screw: It is responsible for heating and melting the plastic raw material from the solid state, pushing it forward through the rotation and shear force of the screw, and extruding it through the die to form a continuous profile. The function of the extrusion screw is more focused on continuous extrusion molding, which has higher requirements on the melting, mixing and extrusion processes of plastics.

Application scenarios

Injection screw: Mainly used in injection molding machines to inject molten plastic into the mold cavity through high-pressure injection to form various complex plastic products. These products usually feature high precision, quality and variety.

Extrusion screw: It is widely used in plastic extruders to produce continuous profiles such as pipes, plates, and films. The design of the extruder screw pays more attention to the continuous melting of plastic and the stability of the extrusion process.

Working principle

Injection screw: During the injection process, the screw rotates and moves forward to transport molten plastic from the barrel to the nozzle. Through the high-pressure injection action of the nozzle, the molten plastic is injected into the mold cavity and cools and solidifies.

Extrusion screw: It generates shear force and extrusion force through rotation, heating and melting the plastic raw material from solid state and pushing it forward. During the extrusion process, the groove depth and pitch of the screw are gradually reduced to achieve compression and melting of the plastic. The molten plastic is extruded through the die driven by the screw to form a continuous profile.

Compression ratio and plasticizing effect

Compression ratio: The compression ratio of the injection screw is small, usually between 2 and 2.5, which means that the melting section compresses the plastic to a relatively low degree. The compression ratio of the extrusion screw is usually larger to better achieve melting and mixing of the plastic.

Plasticizing effect: The plasticizing effect of the extrusion screw is usually better than that of the injection screw, because the extrusion screw can melt and mix the plastic more fully through higher shear force and temperature in the longer melting section.

Length of feeding section and homogenizing section

Feeding section: The feeding section of the injection molding screw is longer, about half the length of the screw, so that it can accommodate more plastic raw materials and perform preliminary compaction before plasticization. The feeding section of the extrusion screw is relatively short because its main task is to quickly introduce plastic raw materials into the melting section.

Homogenization section: The homogenization section of the injection molding screw is shorter in length but has a deeper groove depth to improve productivity and plasticizing effect. The homogenization section of the extrusion screw is relatively long to better achieve uniform melting and mixing of the plastic and control the stability of the extrusion pressure.

Movement methods and operating modes

Movement mode: The movement mode of the injection screw includes rotation and axial movement (ie, injection stroke), while the extrusion screw mainly performs rotational motion. This difference leads to different performances between the two during the work process.

Operation mode: The injection molding machine is an intermittent operation process, and each cycle includes stages such as feeding, plasticizing, injection and pressure holding. The extruder is a continuous operation process. Once started, the plastic can be continuously melted and extruded.

Head design and cooling method

Head design: The head of the injection screw is mostly pointed to fit well with the nozzle and achieve high-pressure injection. The head of the extrusion screw is mostly round or flat to meet the needs of continuous extrusion. In addition, the head of the injection screw may also have special structures such as a check valve to prevent plastic from flowing back.

Cooling method: Although both involve the melting and molding process of plastic, the extruder usually requires cooling and shaping (such as water cooling) after extruding the profile, while the injection molding machine uses the cooling system of the mold to achieve cooling and solidification of the product.

Scope of application and material requirements

Scope of application: Injection molding machines are widely used to produce various complex plastic products, such as automobile parts, electronic casings, daily necessities, etc. Extruders are mainly used to produce continuous profiles such as pipes, plates, and films.

Material requirements: Injection molding machines have relatively high requirements for raw materials. They usually require the raw materials to have moderate viscosity so that they can be injected smoothly into the mold under high pressure. The extruder is more adaptable to raw materials and can handle many different types of plastic raw materials.