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The screw of the plastic extruder machine comprises a shaft with a flight wrapped around it. Double and triple flights screws are used commonly in plastic extrusion. The barrel, on the other hand, is a long cylinder that contains the screw. It is aimed at mixing and melting the plastic as the material moves forward. The barrel is often made of cast iron or steel and is lined with brass or chrome.
Extruder screws can be categorized based on the specific characteristics of the screw design:
Conical screws
Older extruder models rely mainly on conical screws. They have a two-flight setup where the back sections melt the plastic, while the front ones build pressure before the material enters the die. The screw is arranged in a cone shape (wide in the rear and narrow in the front), which aids in the mixing of the material. Some benefits of conical screws include their relatively simple design and ability to handle a wide range of materials. However, they suffer critical drawbacks such inconsistency and poor mixing.
Cylindrical screws
Cylindrical screws feature a single flight setup. The pressure buildup happens through the friction of the barrel. It consists of a straight shaft with a groove around it. Due to the simplistic nature of cylindrical screws, manufacturing isn't costly. Plus, they do well in extruding homogenous products. That said, cylindrical screws are not flexibly suited for different material viscosities. Also, they lack the complex thermal profiles required to deal with certain materials.
Homogeneous screws
This type of screw is designed to extrude homogenous materials. It is manufactured with a general-purpose deep flight groove and a 3:1 compression ration. Besides, homogeneous screws are ideal for processing conventional polymer such as PS, PP, and PVC. While they have some advantages such as lower costs and simple design, homogeneous screws may not be suitable for high-efficiency processing and complex material requirements.
These specifications are critical for achieving the correct flow and mixing of plastic materials through the barrel.
Screw Diameter
The screw diameter varies depending on the capacity of the extruder. Usually, industrial extruder screws range from 15mm to 300mm. A more prominent screw diameter increases the extruder's capacity.
Screw Length
The screw length varies depending on the type of extruder and the materials processed. Generally, the length-to-diameter ratio for screws is between 20:1 and 40:1. Longer screws provide better mixing and melting of the plastic material.
Screw Design
The screw design includes elements like flights, barriers, and grooves. The specific flight and barrier configurations decide how well the material gets conveyed, mixed, and melted. Some common screw designs are general purpose, mixing, and devolatilizing types.
Barrel Material
Barrel materials are alloys with high corrosion and wear resistance. They are capable of withstanding high temperature and pressure during plastic extrusion. The barrel materials are carbon steels, alloy steels, and stainless steel, among others.
Temperature Zones
Extrusion barrels have several temperature control zones. The temperature zones enable precise regulation of material heating and melting. Typically, there are 3 to 7 temperature zones along the barrel and heater bands surrounding it.
Coating
Extruder barrels may have coatings to enhance their wear resistance and reduce material degradation during processing. Some typical coatings are nitride coatings, ceramic coatings, and chrome plating.
Routine maintenance of the screw and barrel helps to keep the plastic extruder operating optimally. It prevents untimely breakdowns and extends the life span of the equipment significantly.
Daily Checks
Please, on a daily basis, check for any signs of damage to the barrel heating bands and thermocouples. Look at the temperature readings to see if they are stable within the specified ranges. Ensure that the fasteners connecting the feeding inlet and outlet are tightly secured. Conduct electrical safety checks on the feeding motor and its cables.
Weekly Maintenance
Perform a detailed cleaning of all the feeding hopper parts and the barrel repress sections. Lubricate the moving components of the feeding mechanism according to the stipulations of the lubrication manufacturer. Examine the extrusion screw to look for areas of wearing out, damage, or material buildup, and clean the drive belt from any debris.
Monthly Servicing
Replace barrel heating bands and thermocouples if they show signs of damage or degradation. Inspect the feeding motor to check for unusual vibrations or noises. Ensure that all electrical connections are securely fastened. Have a certified technician calibrate the temperature controllers and adjust the screw speed.
Quarterly Repairs
Take out the extrusion screw and thoroughly inspect it for wear or damage. Clean any material buildup along the flights. Based on the extent of the wearing out, recoat or repair the screw. Also, inspect the barrel for similar damages, and recalibrate the feeding gearbox's lubrication system.
The plastic extruder screw and barrel are used in many industries. Here are some sectors that have benefited from the extruder and its parts.
Packaging industry
The packaging sector makes flexible and rigid packs for many consumables. It uses PVC, PS, PP, and other materials to create injection-molded containers, shrink, stretch, and other types of wrappers.
Automotive
Automobile makers use the extruder to create interior and exterior parts. Some of these parts are non-metallic engine covers, dashboards, trim plates, light cases, door and window seals, bumpers, and internal piping.
Extruded plastic parts are lightweight and reduce the car's energy usage. Also, extrusion replaces many assembly parts, reducing the need to use adhesives and mechanical fasteners.
Construction
The construction industry extrudes PVC and other materials to create drain pipes, door and window frames, roofing sheets, siding, flooring, and insulation material for electrical conduits.
Electronics
Extruders help to produce electronic items like television sets, computer monitors, and refrigerators. They provide the plastic covers and insulating material for wires and electrical parts.
Medical
Medical equipment also uses extruded plastic for smooth tubing, drip IV bags, medical device casings, and other flow components inside devices.
Consumer Goods
Many everyday items like toys, kitchen tools, and furniture are made from plastic parts created by injection molding and 3D printing. High-quality extruders are used to make raw material filaments for 3D printers.
The following tips will help business buyers select the right screw barrel set for their plastic extrusion machine.
Understand material application
Know the types of plastics the extruder will work with. Some materials need special treatment, like higher temperatures or specific friction levels. To avoid wear and damage, choose screws and barrels that match the materials' needs.
Consider production volume
For high production amounts, select durable, tough, and wear-resistant screw and barrel sets. However, for low production, budget-friendly standard sets will be just fine.
Know the extrusion type
Match the screw design to the extrusion's type. For example, use a feed screw for screw feeders or an archimedean screw for extracting liquids.
Assess cost and budget
High-performance screws and barrels cost more, so balance the budget and equipment needs. Think about long-term expenses like maintenance and replacement parts.
Q1: What are the latest trends in screw and barrel technology for plastic extruder machines?
A1: Some of the latest trends in the screw barrel for extruder machines include wear-resistant materials, ECML technology for efficient energy conversion, smart barrels and screws with sensors, co-rotating screws for better mixing, and self-cleaning extruder screws to reduce machine downtime.
Q2: What is the relationship between extruder screws and barrels?
A2: An extruder screw connects to the motor and pumps the plastic through the barrel, while the extruder barrel receives the screw and contains the material as it moves and heats. Both components work closely together to melt and mix the plastic while resisting wear from abrasive materials.
Q3: What are the specific applications of screws and barrels in different types of plastic extruder machines?
A3: In a single-screw extruder, the screw and barrel mix the melted plastic before shaping it into sheets, films, and other flat products. For multi-screw or intermeshing extruders, the screws and barrels perform more complex tasks of shearing, mixing, and conveying, usually for making high-performance plastics and engineering materials.
Q4: How do the designs of screws and barrels vary for different types of plastics?
A4: The buyer may need to customize the screw and barrel design according to the specific characteristics of the material, such as its viscosity and thermal sensitivity. If the material needs to be blended with additives, the mixing and filtering parts should be included. Also, the conveying speed and temperature along the way.
