What is an extruder? How to set up an extruder?

What is an extruder?

An extruder is a piece of equipment widely used in the manufacturing process. It is mainly used to shape raw materials into products of specific shapes by heating, melting and extruding them. This process is called extrusion and is commonly used to produce a variety of materials including plastics, rubber products, metals, food, and more.

Working principle

The working principle of the extruder is based on a simple and efficient process. First, raw materials (usually granular materials) are fed into the extruder through a feeding system. Inside the machine, a screw rotates and pushes the raw material into a heated barrel. Here, the raw material is heated and melted into an extrudable flow state. Finally, the molten material is extruded through a die to form the desired cross-sectional shape, and is finally cooled and solidified into the final product.

Main components

The extruder is mainly composed of several key components such as the feeding system, screw and barrel, and die.

• Feeding system: Responsible for transporting raw materials to the interior of the extruder.
• Screw and barrel: The screw rotates in the barrel to compress, heat and melt the raw materials.
• Die head: It determines the final shape of the extruded material. Die heads of different shapes can be designed according to product requirements.

Application areas

• Plastic industry: Produces various plastic products, such as pipes, sheets, films, etc.
• Rubber industry: manufacturing rubber seals, hoses, etc.
• Metal processing: Extruding metal materials for use in construction, automobiles and other fields.
• Food industry: Produces various foods, such as noodles, snacks, etc.

The wide range of applications of extruders has made them indispensable equipment in modern manufacturing, and their efficiency and flexibility make them play a key role in various industries. With the continuous advancement of technology, the design and control system of extruders are also constantly innovating to adapt to changing market needs.

Different types of extruders

Extruders can be divided into many types according to their structures and working principles, among which single-screw extruders and twin-screw extruders are two common types.

Single screw extruder

Structure and working principle:

A single-screw extruder consists of a screw rotating in a barrel. The rotation of the screw pushes the raw material to the outlet of the extruder, where it is heated and melted to finally form the desired product shape.

Scope of application:

Single-screw extruders are suitable for processing some relatively simple materials, such as ordinary plastic pellets. They are commonly used in the production of pipes, films, wires, etc.

Twin-screw extruder

Advantages and application scenarios:

A twin-screw extruder has two screws that work together or rotate in opposition to each other. Compared with single-screw extruders, twin-screw extruders have higher extrusion efficiency and wider applicability. They enable more uniform mixing and processing of a variety of materials, including complex blends. Therefore, twin-screw extruders are widely used in rubber, plastic alloys, food, medical equipment and other industries.

Compare the advantages and disadvantages of single-screw extruders:

Compared with single-screw extruders, twin-screw extruders have better mixing effects and higher production capabilities. However, the structure of a twin-screw extruder is complex, and maintenance and operation may be relatively cumbersome. When selecting an extruder type, there are trade-offs based on specific production needs and material characteristics.

How to set up the extruder?

The performance and production efficiency of the extruder are affected by multiple key parameters. Correct parameter settings are the key to ensuring a stable extrusion process and excellent product quality.

Temperature control:

Effect of extrusion temperature: Temperature directly affects the melting and fluidity of raw materials, which has an important impact on the performance and appearance of the final product.

How to set it up: Different types of raw materials and products may require different extrusion temperatures. Typically, extruders have multiple heating zones where temperatures can be set individually to ensure proper melting and formation.

Pressure and flow control:

The importance of pressure: Pressure directly affects the extrusion speed and shape of raw materials, and has a direct impact on product size and appearance.

Optimization of flow rate: Optimizing flow rate can improve production efficiency and ensure uniform extrusion.

Screw speed

Affects the extrusion speed and mixing effect: The rotation speed of the screw directly affects the extrusion speed and mixing effect, and needs to be adjusted according to the characteristics of the raw materials and product requirements.

Setting method: Control the rotation speed of the screw by adjusting the speed of the motor.

Cooling system:

Control of cooling speed: The cooling speed directly affects the size and appearance of the product. Too fast or too slow cooling may cause problems.

Setup method: Adjust the water flow and temperature of the cooling system to ensure proper cooling rate.

Die design and adjustment:

Affects product shape: The design and adjustment of the die head directly determines the shape of the extruded material and needs to be accurately adjusted according to product design requirements.

Setting method: Control the product shape by adjusting the opening and shape of the die.

Machine running speed:

Production efficiency: The operating speed is directly related to the production efficiency of the extruder and needs to be reasonably set according to product specifications and quality requirements.

Barrel pressure:

Affects the melting of raw materials: The setting of the cylinder pressure is directly related to the melting process of the raw materials in the cylinder, and needs to be adjusted according to the melting point and fluidity of the raw materials.

Properly setting these key parameters can improve the production efficiency of the extruder and ensure stable product quality. When adjusting parameters, it is necessary to closely monitor the operating status of the extruder and make timely adjustments according to the actual situation to meet the processing requirements of different materials and products.

How to select screw parameters for different raw materials?

PC

Features:

Amorphous plastic, no obvious melting point, glass transition temperature 140°~150°C, melting temperature 215°C~225°C, molding temperature 250°C~320°C.

It has high viscosity and is sensitive to temperature. It has good thermal stability within the normal processing temperature range. It basically does not decompose when staying at 300°C for a long time. It starts to decompose when it exceeds 340°C. The viscosity is less affected by the shear rate.

Highly absorbent.

Screw parameter selection:

• L/D has the characteristics of good thermal stability and high viscosity. To improve the plasticizing effect, a large aspect ratio should be selected as much as possible. Because of its wide melting temperature range, a progressive screw is used. L1=30% of the total length, L2=46% of the total length.
• The compression ratio ε needs to be adapted to the melting rate from gradient A, but the melting rate cannot be calculated at present. According to the processing characteristics of PC from melting from 225°C to 320°C, the gradient A value can be relatively medium. The upper value, when L2 is larger, the ordinary gradual screw ε=2~3.
• Other parameters such as e, s, φ and the gap with the barrel can be the same as other ordinary screws.

PMMA

Features:

• The glass transition temperature is 105°C, the melting temperature is greater than 160°C, the decomposition temperature is 270°C, and the molding temperature range is very wide.
• High viscosity, poor fluidity, and good thermal stability.
• Strong water absorption.

Screw parameter selection:

• L/D selects a gradual screw with an aspect ratio of 20 to 22. Depending on the accuracy requirements of the product molding, generally L1=40% and L2=40%.
• Compression ratio ε is generally selected from 2.3 to 2.6.
• In view of its certain hydrophilicity, a mixing ring structure is adopted at the front end of the screw.
• Other parameters can generally be designed according to the universal screw, and the gap with the barrel should not be too small.

PA

Features:

• There are many types of crystalline plastics, different types have different melting points, and the melting point range is narrow. The melting point of commonly used PA66 is 260°C ~ 265°C.
• Low viscosity, good fluidity, relatively obvious melting point, and poor thermal stability.

Screw parameter selection:

• L/D selects a mutant screw with an aspect ratio of 18 to 20.
• The compression ratio is generally selected from 3 to 3.5, among which h3=0.07 to 0.08D to prevent overheating decomposition.
• Because of its low viscosity, the gap between the non-return ring and the barrel should be as small as possible, about 0.05, and the gap between the screw and the barrel is about 0.08. If necessary, depending on the material, the front end can be equipped with a non-return ring, and the nozzle should be Self-locking.
• Other parameters can be designed according to the general screw.

PET

Features:

• The melting point is 250℃~260℃, and the molding temperature of blow molding grade PET is wider, about 255℃~290℃.
• Blow molding grade PET has a high viscosity, temperature has a great influence on the viscosity, and has poor thermal stability.

Screw parameter selection

• L/D is generally taken as 20, three-section distribution L1=50%-55%, L2=20%.
• Use a screw with low shear and low compression ratio. The compression ratio ε is generally 1.8 to 2. At the same time, shearing and overheating will cause discoloration or opacity h3=0.09D.
• There is no mixing ring at the front end of the screw to prevent overheating and material accumulation.

PVC

Features:

• It has no obvious melting point, becomes soft at 60℃, becomes viscoelastic at 100℃~150℃, melts at 140℃ and decomposes at the same time, decomposes rapidly at 170℃, the softening point is close to the decomposition point, and it decomposes and releases HCl gas.
• Thermal stability is poor, temperature and time will cause decomposition, and fluidity is poor.

Screw parameter selection:

• The temperature control is strict, and the screw design should be as low as possible to prevent overheating.
• The screw and barrel must be corrosion-proof.
• The injection molding process needs to be strictly controlled.
• Generally speaking, the screw parameters are L/D=16~20, h3=0.07D, ε=1.6~2, L1=40%, L2=40%.
• In order to prevent material accumulation, there is no non-return ring, and the head taper is 20°~30°, which is more suitable for soft rubber. If the product requirements are higher, a separated screw without metering section can be used. This kind of screw is suitable for hard PVC. It is more suitable, and in order to cooperate with temperature control, cooling water or oil holes are added inside the screw in the feeding section, and cold water or oil tank is added outside the barrel. The temperature control accuracy is about ±2℃.