The screw of the twin-screw puffing extruder, as the core component of the equipment, is subjected to harsh working environments such as high temperature, high pressure, high shear force, and material friction for a long time. Its material directly determines the service life, puffing efficiency, and product quality of the equipment. At present, the commonly used screw materials in the industry are mainly divided into five categories, each with different characteristics, advantages and disadvantages, and adaptation scenarios. The specific introduction is as follows:
1, Nitrided steel (commonly used 38CrMoAlA, 1.8550)
Nitrided steel is the most widely used basic material in twin-screw puffing extruders. The core uses 38CrMoAlA or German standard 1.8550 forgings as the substrate, and after quenching and tempering treatment, the surface nitriding process is carried out to form a high hardness nitride layer (hardness HV860-980). The substrate maintains good toughness and is a conventional choice that balances cost-effectiveness and practicality, suitable for most general puffing scenarios.
Advantages:
1 Extremely cost-effective, easy to obtain raw materials, mature processing technology, low manufacturing cost, suitable for small and medium-sized enterprises and general production lines;
2. The matrix has excellent toughness, strong fatigue and impact resistance, and is not prone to fracture or deformation problems;
3. The surface nitride layer has good wear resistance and can meet the puffing needs of ordinary grains, feed and other materials without high hardness fillers;
4. The processing difficulty is low, and the screw tooth shape and size can be flexibly customized according to the needs.
Disadvantages:
1 Poor corrosion resistance. When exposed to chlorine containing, acidic additives, or high humidity materials for a long time, the nitride layer is prone to corrosion and detachment, leading to accelerated screw wear;
2. Limited high-temperature stability. When working for a long time above 350 ℃, the surface hardness will decrease and the wear rate will accelerate; 3. Not suitable for strong wear conditions such as high glass fiber and mineral filling. In such scenarios, the service life is significantly shortened, only 1/3-1/5 of that of bimetallic alloy screws.
2, Bimetallic alloys (nickel based, iron-based, tungsten carbide based)
Bimetallic alloy screws are currently the mainstream choice for mid to high end equipment, using a composite structure of "substrate+surface alloy layer". The substrate is mostly steel with good toughness such as 38CrMoAlA and SKD61. The surface is compounded with a layer of wear-resistant alloy (commonly nickel based, iron-based, tungsten carbide based) through processes such as centrifugal casting and plasma welding. The thickness of the alloy layer can reach 1.5-6mm, and the highest hardness can reach HRC62-65, achieving the dual advantages of "toughness+high wear resistance". The service life is 3-6 times that of nitride steel screws.
1. Nickel based alloy (Ni-20)
Nickel based alloys are based on nickel, with added elements such as carbon, silicon, chromium, boron, and some containing 20% -25% tungsten carbide. They are formed by centrifugal casting process and have balanced corrosion resistance and wear resistance. They are one of the most widely used bimetallic materials.
Advantages:
1 Outstanding corrosion resistance, able to effectively resist the corrosion of PVC, halogen-free flame-retardant plastics, acidic materials, etc., avoiding rust spots and pitting on the surface of the screw;
2. Excellent high temperature resistance, able to maintain high hardness and stability even when working at around 400 ℃ for a long time;
3. The wear resistance is better than that of nitride steel, and it is suitable for expanding materials containing medium proportions of glass fiber (10% -30%) and minerals;
4. The alloy layer is tightly bonded to the substrate, without pores or slag inclusions, and has good impact resistance and is not easily detached.
Disadvantages:
1 The cost is relatively high, 2-3 times that of nitride steel screws, and the initial investment is relatively large;
2. The processing technology is complex, centrifugal casting and precision machining are difficult, and the customization cycle is long;
3. The hardness is slightly lower than that of tungsten carbide based alloys, and it is not suitable for strong wear scenarios with ultra-high glass fiber filling (over 50%).
2. Iron based alloy (Fe-30)
Iron based alloys are based on iron and add elements such as chromium, boron, molybdenum, and nickel. Their cost-effectiveness is between that of nitride steel and nickel based alloys, making them suitable for cost sensitive and wear-resistant scenarios. They are the preferred choice for general-purpose bimetallic materials.
Advantages:
1 High cost-effectiveness, with manufacturing costs lower than nickel based and tungsten carbide based alloys, and wear resistance 3-5 times that of nitride steel;
2. The processing technology is relatively simple, the customization cycle is short, and it is suitable for mass production;
3. It has good wear resistance and corrosion resistance, and can meet the expansion needs of nylon with 10% -30% glass fiber, PVC with calcium powder, recycled materials, etc;
4. High mechanical strength and superior deformation resistance compared to pure nitride steel.
Disadvantages:
1 Corrosion resistance and high temperature resistance are not as good as nickel based alloys, and they are prone to wear and tear when exposed to highly corrosive materials for a long time;
2. Low hardness (HRC59-61), not suitable for harsh working conditions with high wear and high temperature; 3. The toughness of the alloy layer is average, and it may crack after long-term exposure to severe impacts.
3. Tungsten carbide based alloy (Wc-10)
Tungsten carbide based alloy is a high-end wear-resistant material, containing 50% -60% tungsten carbide, with added elements such as carbon, silicon, chromium, cobalt, etc. The hardness can reach HRC62-65, making it one of the most wear-resistant screw materials currently available, suitable for extreme wear conditions.
Advantages:
1 The wear resistance is extremely strong, 5-6 times that of nitride steel, and it can resist the erosion of strong wear materials such as high glass fiber (30% -50%), ceramic powder, aluminum magnesium powder, etc;
2. Excellent high temperature resistance and corrosion resistance, able to maintain stable performance even in high temperature and corrosive environments for a long time;
3. Long service life, can significantly reduce equipment maintenance costs and downtime, suitable for large-scale and continuous production;
4. The surface is smooth, the material is not easy to adhere, reducing material retention and degradation issues, and ensuring product quality uniformity.
Disadvantages:
1 The cost is extremely high, 4-5 times that of nitride steel, with a large initial investment, suitable for large enterprises with extremely high requirements for production efficiency and service life;
2. The material is brittle and has poor impact resistance. If metal impurities are mixed into the material, it is easy to cause screw teeth to break or fracture;
3. The processing difficulty is extremely high, with high requirements for manufacturing processes, long customization cycles, and high maintenance costs.
3, Powder metallurgy high-speed steel (Km-22, imported superhard powder steel)
Powder metallurgy high-speed steel is made of materials imported from Sweden, such as Km-22, through powder metallurgy integral sintering process. The carbide distribution is uniform, and the hardness can reach HRC59-64. It has high hardness, toughness, and thermal fatigue resistance, and is suitable for high-speed and high shear expansion scenarios. It is one of the specialized materials for high-performance equipment.
Advantages:
1 Excellent wear resistance and high temperature resistance, able to meet the expansion needs of special materials such as nylon with 50% -60% glass fiber, electric wood, magnetic powder, etc;
2. It has better toughness than tungsten carbide based alloys, strong impact resistance and fatigue resistance, and is less prone to fracture and tooth breakage;
3. High dimensional accuracy, smooth surface, can reduce material adhesion and shear loss, and improve puffing efficiency;
4. Long service life, 5-8 times longer than nitride steel, with lower overall cost for long-term use.
Disadvantages:
1 High cost, dependence on imported raw materials, and manufacturing costs far higher than ordinary bimetallic alloys;
2. The processing technology is complex, sintering and precision machining are difficult, and the customization cycle is long;
3. The corrosion resistance is average, and additional surface anti-corrosion treatment is required when in long-term contact with highly corrosive materials.
4, Stainless steel (special stainless steel, 42CrMo chrome plated)
Stainless steel material is mainly used in food grade and pharmaceutical grade twin-screw puffing extruders. The core is made of special stainless steel or 42CrMo as the substrate, which is quenched and chrome plated to highlight corrosion resistance and hygiene. It is suitable for scenarios with extremely high material cleanliness requirements.
Advantages:
1 Highly corrosion-resistant, able to resist corrosion from acidic, alkaline materials and high humidity environments, avoiding material contamination;
2. Good hygiene, smooth surface, easy to clean, no impurities falling off, in line with the hygiene standards of the food and pharmaceutical industry;
3. Good toughness, strong impact and deformation resistance, suitable for scenarios such as food puffing and pharmaceutical excipient processing;
4. After surface hardening treatment, the wear resistance of some stainless steels can be close to that of ordinary nitride steels.
Disadvantages:
1 Poor wear resistance, stainless steel that has not undergone surface hardening treatment will also wear out quickly after long-term contact with ordinary materials;
2. The cost is relatively high, especially for special stainless steel, which has a higher manufacturing cost than nitride steel;
3. High temperature stability is average. When working at temperatures above 300 ℃ for a long time, hardness and toughness will decrease, making it unsuitable for high-temperature expansion scenarios.
5, Non metallic materials (polytetrafluoroethylene, nylon, ultra-high molecular weight polyethylene)
Non metallic material screws are mainly used in special lightweight and anti adhesion scenarios, with a narrow application range, only suitable for slight expansion needs with low pressure, low temperature, and no hard impurities, such as small food and soft material expansion. Core materials include polytetrafluoroethylene, nylon, ultra-high molecular weight polyethylene, etc.
1. Polytetrafluoroethylene material
Advantages: Excellent corrosion resistance, high and low temperature resistance, able to resist corrosion from various acidic and alkaline materials, with a wide working temperature range; Smooth surface, materials are not easily adhered, suitable for puffing easily sticky materials; Lightweight, easy to install and maintain.
Disadvantages: Low mechanical strength, poor rigidity, easy creep, poor impact resistance, unable to withstand high pressure and high shear force; Extremely poor wear resistance, easily worn by materials, and short service life; Not suitable for conventional puffing scenarios.
2. Nylon material
Advantages: Medium strength, better wear resistance than polytetrafluoroethylene, good mechanical strength (tensile, impact); Moderate cost, low processing difficulty, and flexible customization of sizes; Lightweight and low operating noise.
Disadvantages: weak acid and alkali resistance, poor dimensional stability after water absorption; The upper limit of working temperature is low (only 120 ℃), which cannot adapt to high-temperature puffing scenarios; Limited wear resistance, not suitable for materials containing hard impurities.
3. Ultra high molecular weight polyethylene material
Advantages: Super strong wear resistance, good self-lubricating performance, extremely high impact resistance, and not easy to break; Lightweight, low operating resistance, and can reduce energy consumption; Good corrosion resistance, suitable for slightly corrosive materials.
Disadvantages: Poor rigidity, easy deformation, unable to withstand high pressure and high shear force; The upper limit of working temperature is low (only 80 ℃), and the adaptation scenario is extremely narrow; Poor dimensional stability, long-term use is prone to deformation and wear.
summary
The selection of screw material for twin-screw puffing extruder depends on the characteristics of the puffing material (whether it contains hard fillers, corrosiveness), working temperature and pressure, and budget: for general scenarios and limited budget, nitrogen doped steel is selected; Choose nickel based/iron-based bimetallic alloys for medium to high-end wear-resistant and moderately corrosive scenarios; Choose tungsten carbide based alloys or powder metallurgy high-speed steel for extreme wear and high demand scenarios; Stainless steel is selected for hygiene scenarios such as food and medicine; Non metallic materials can be selected for special lightweight and anti adhesion scenarios. Reasonable selection of materials can significantly improve the service life of equipment, reduce maintenance costs, and ensure stable quality of puffed products.