Substandard raw materials can directly lead to abnormal equipment wear, increased failure rates, and frequent product defects, and can affect the stability of the entire production process and production costs from the raw material stage onward.
The Impact of Key PVC Resin Properties
Degree of Polymerization (K-value) and Molecular Weight: The higher the degree of polymerization, the better the mechanical properties of the pipe, such as tensile strength and impact strength. However, at the same time, the resin's processability and flowability deteriorate, resulting in higher torque requirements and increased energy consumption during extrusion, which places greater demands on the extruder's drive system and screw strength. Therefore, in production, it is necessary to select an appropriate resin grade based on the performance objectives of the pipe and the capabilities of the equipment.
Thermal Stability: PVC is a heat-sensitive material, and its decomposition temperature is very close to its processing temperature. If the resin itself has poor thermal stability, it is prone to decomposition even at normal processing temperatures, producing hydrogen chloride (HCl) gas as well as discoloration and charred residues.
Consequences: These decomposition products not only corrode the screw, barrel, and die but also cause surface defects on the pipe's inner wall, such as yellowish-brown streaks and black spots. At the same time, the HCl gas produced by decomposition further catalyzes decomposition, creating a vicious cycle.
Volatile (Moisture) Content: If the resin is damp or has excessively high volatile content, it will vaporize during the extrusion process.
Consequences: If moisture cannot be expelled in a timely manner, it will cause bubbles in the pipe wall and an uneven inner surface; in severe cases, it will result in the pipe failing to meet mechanical performance standards. Excessive moisture content can also trigger HCl generation, corroding equipment.
Impurities and Black/Yellow Spots: Physical impurities or black/yellow spots in the raw material cannot be plasticized and will become stress concentration points.
Consequences: These impurities form pitting, pits, or raised spots on the pipe surface, severely affecting appearance and mechanical strength. As these hard impurities pass through the extruder, they act like abrasives, accelerating wear on the screw and barrel.
The Impact of Additives and Fillers
Stabilizers: Their function is to inhibit the thermal degradation of PVC during processing.
Consequences: If the stabilizer is of poor quality or used in insufficient quantities, the material will degrade prematurely. In production, this manifests as discoloration of the pipe surface and a decrease in the product's impact and tensile strengths.
Lubricants: The balance between internal and external lubrication must be precisely controlled to ensure the material flows smoothly through the equipment without adhering to metal surfaces.
Consequences: Insufficient lubrication leads to a sharp increase in torque and higher die head pressure, increasing equipment load and energy consumption. Excessive lubrication may cause the material to slip on the screw, reducing output and potentially triggering decomposition.
Fillers (e.g., calcium carbonate CaCO₃): Adding them in appropriate amounts can reduce costs, but excessive or improper use does more harm than good.
Consequences: Excessive filler reduces material flowability, leading to a significant decrease in the pipe's impact strength and tensile strength. More importantly, highly filled dry blends have poor flowability, which can cause feeding difficulties and even block the feed inlet. Their high hardness and abrasiveness severely accelerate wear on the extruder screw and barrel.
Impact on Equipment Lifespan
The most direct harm that low-quality raw materials pose to pipe extruders is the accelerated physical wear and tear of their core components.
Screw and Barrel Wear: Hard fillers, unplasticized resin particles, or impurities are the primary sources of wear.
Consequences: Accelerated wear of the screw and barrel leads to poor plasticization, reduced output, and may ultimately result in screw damage, directly increasing equipment maintenance and replacement costs.
Equipment Corrosion: HCl gas released during the thermal decomposition of PVC is a highly corrosive substance.
Consequences: It corrodes the screw, barrel, die, and venting system, shortening the overall service life of the equipment.
Impact on Production Efficiency and Product Yield
Production Instability: Variations between raw material batches can alter material rheological behavior, forcing operators to frequently adjust process parameters such as temperature and screw speed, making it difficult to maintain a stable, high-output state.
Increased Product Defects: Various raw material issues directly manifest as product defects. For example:
Rough, Dull Surface: Often related to improper lubricant dosage, poor plasticization, or excessively low die temperature.
Pipe Warping: Caused by uneven wall thickness or uneven cooling, and related to the plasticization effectiveness and formulation of the raw material.
Cross-sectional Bubbles: Directly caused by excessive moisture in the raw material or material decomposition.
In summary, the quality of raw materials is directly linked to the operating condition of the extruder, the extent of wear and tear, energy consumption, as well as the quality and output of the final product. Treating raw material quality control as the primary step in the production process is of fundamental importance for ensuring the long-term, stable, and efficient operation of the production line.