Why we equip corrosion-resistant screw and barrel for WPC extruder?
The screw of WPC extruder serves as the core transmission and plasticizing component of the equipment. It directly contacts the molten material of the wood-plastic composite and withstands high temperatures and high pressures. Its corrosion resistance is crucial to ensuring stable operation, product quality, and longevity. Therefore, Yongte's wood-plastic composite extruders typically use screws and barrels made of wear-resistant and corrosion-resistant materials.
1. The Inherent Corrosiveness of Wood-Plastic Composite Materials
The raw materials of wood-plastic composite (WPC) contain a variety of components that can corrode the screw, which is the core reason why the screw needs to be corrosion-resistant:
Wood fiber (such as wood flour, bamboo flour, and straw flour) inherently contains a certain amount of organic acids (such as acetic acid, formic acid, and phenolic acids produced by lignin degradation). Under the high temperatures of the extrusion process (typically 160-220°C), these acids are activated and released, directly contacting the screw surface. Long-term exposure can cause slow corrosion of the screw's metal substrate, resulting in surface pitting and flaking. To improve the compatibility of wood fiber with the plastic matrix, some processes pre-treat the wood fiber (such as alkali treatment or coupling agent modification). If the pre-treatment is not thoroughly cleaned, residual alkaline substances (such as sodium hydroxide) or polar groups in the coupling agent can chemically react with the screw metal, accelerating corrosion.
To meet performance requirements for weather resistance, aging resistance, and flame retardancy, wood-plastic materials often incorporate various additives, some of which are known to be corrosive:
Antioxidants/Light Stabilizers: Some phenolic antioxidants and benzotriazole light stabilizers may decompose at high temperatures to produce acidic substances, which can corrode the screw surface.
Flame Retardants: Commonly used halogenated flame retardants (such as decabromodiphenyl ether) may release trace amounts of hydrogen halide gas during high-temperature processing. Hydrogen halide is highly corrosive and can react with the alloy components of the screw (such as chromium and nickel), damaging the passivation layer on the screw surface.
Fillers: Some inorganic fillers (such as calcium carbonate and talc) contain trace impurities (such as chloride ions and sulfate ions). Under high temperature and pressure, these impurities can create a "corrosive environment," exacerbating pitting or intergranular corrosion of the screw.
The WPC industry currently widely uses recycled plastics (such as PE and PP). These materials may contain residual printing ink, adhesives, and metal impurities (such as copper and iron debris). Residual solvents in inks (such as esters and ketones) can react chemically with the metal in the screw at high temperatures. These metal impurities can form a "micro-battery effect" in the gap between the screw and the barrel, triggering electrochemical corrosion and accelerating wear on the screw surface.
2. Extrusion Conditions Exacerbate the Synergistic Effect of Corrosion and Wear
The high temperature, high pressure, and high shear conditions of wood-plastic extrusion not only activate the corrosive properties of the material but also amplify the damaging effects of corrosion on the screw, creating a vicious cycle of "corrosion + wear":
Wood-plastic extrusion temperatures typically range from 160-220°C. At these temperatures, the chemical reaction rate between the acidic and alkaline components in the material and the screw metal (commonly 38CrMoAlA alloy) increases significantly. According to chemical kinetics, the reaction rate increases approximately 2-3 times for every 10°C increase in temperature. This means that under sustained high temperature conditions, the passive layer (such as the nitride layer) on the screw surface is more rapidly destroyed, exposing the base material and rapidly corroding.
To achieve uniform mixing and continuous extrusion of wood powder and plastic, the screw must provide sufficient pressure (typically 10-30 MPa) and shear force. Under high pressure, molten material containing corrosive components adheres tightly to the screw surface, accelerating the penetration of the corrosive medium into the metal. Simultaneously, high shear forces continuously scrape away weak areas of the screw surface caused by corrosion, rapidly stripping away the oxide layer and exfoliated material, exposing fresh substrate and further exacerbating the corrosion process.
If the extrusion process experiences unstable feed or fluctuating screw speed, some material may remain within the screw channel. This retained material degrades and carbonizes at high temperatures, producing even more corrosive substances (such as small molecule organic acids and carbides). These can cause concentrated corrosion in localized areas of the screw, resulting in defects such as deep grooves and pits, seriously impacting the screw's conveying efficiency and plasticizing performance.
3. Corrosion resistance is fundamental to ensuring equipment life and product quality.
Screw corrosion not only shortens equipment life and increases maintenance costs, but also directly impacts the quality of WPC products. This is reflected in:
If a screw's corrosion resistance is insufficient, surface corrosion and wear will typically reduce conveying capacity after 3-6 months of use, necessitating downtime and screw replacement. The manufacturing cost of a single WPC extruder screw (65-120mm diameter) can reach tens of thousands of yuan, and frequent replacements significantly increase equipment maintenance costs. Corrosion-resistant screws (such as those with nitriding, chrome plating, or Hastelloy alloy) can extend their service life to 1-2 years, significantly reducing downtime and replacement costs.
Screw surface corrosion alters the geometry of the screw channel (such as flight height and pitch), reducing material conveying efficiency and uneven residence time within the channel. This, in turn, can lead to fluctuating extrusion output and uneven material plasticization. For example, pitting on the screw surface increases friction between the material and the screw, potentially leading to localized overheating and degradation, affecting the mechanical properties of the final product (such as tensile strength and impact strength).
Corrosion on the screw surface can produce metal debris or oxide flakes. These impurities can mix into the molten WPC material, ultimately forming "impurities" in the product. For WPC products with high aesthetic requirements, such as outdoor flooring and decorative panels, these impurities can directly lead to product failure. For interior decorative materials with demanding requirements, metal impurities can also affect the product's environmental performance (e.g., heavy metal migration), potentially violating industry standards (such as GB/T 24137-2021 "Wood-Plastic Composite Panels").
In summary, the corrosion resistance of WPC extruder screws is a core requirement for addressing the corrosive components of WPC materials, resisting damage from high-temperature and high-pressure conditions, and ensuring stable equipment operation and product quality. Therefore, the industry usually enhances the corrosion resistance of the screw through material selection (such as corrosion-resistant alloy steel) and surface treatment (such as gas nitriding and PVD coating) to ensure long-term and stable operation of the equipment.
