Surface gloss of PVC wire is a crucial indicator of its appearance quality, and its formation is closely related to multiple stages of the processing. From raw material preparation to final molding, process control at each stage can significantly impact gloss.
The uniformity of raw material mixing is a fundamental factor affecting gloss. The mixing effect of PVC resin with various additives (such as plasticizers, stabilizers, and lubricants) directly determines the material's flowability in subsequent processing. Insufficient mixing can lead to unstable melt flow due to localized differences in additive concentration, resulting in surface unevenness during extrusion and reduced gloss. For example, uneven distribution of plasticizers can create areas of varying hardness on the wire surface, causing diffuse reflection and weakening the overall gloss.
Extrusion temperature control is particularly critical to gloss. PVC wire is typically formed using an extruder, and the melt temperature must be strictly controlled within a reasonable range. Too low a temperature results in excessively high PVC melt viscosity, poor flowability, and a rough surface; too high a temperature can trigger material decomposition, producing bubbles or carbides. These defects damage surface smoothness and reduce gloss. Furthermore, temperature fluctuations can cause uneven wire diameter, further affecting surface reflectivity.
Matching the screw speed and extrusion speed is a core aspect of process control. Screw speed directly affects the degree of plasticization and shear force of the melt. If the speed is too high, excessive shear force may cause PVC molecular chains to break, degrading material properties and resulting in surface cracks or flow marks. If the speed is too low, insufficient plasticization will leave unmelted particles in the melt, forming surface defects. Extrusion speed and screw speed must be adjusted in tandem to ensure uniform melt flow at the die, avoiding surface ripples caused by uneven flow rates.
Die design and maintenance play a decisive role in gloss. The flow channel structure of the die needs to be optimized to ensure uniform force on the melt during flow, avoiding surface stress marks caused by differences in shear stress. The surface finish of the die directly affects the surface quality of the wire. If there are scratches or material buildup on the inner wall of the die, the same defects will be replicated on the wire surface. Regularly cleaning the die and polishing the die cavity surface are necessary measures to maintain gloss. Furthermore, precise control of the die temperature can reduce melt expansion upon leaving the die, preventing surface shrinkage marks.
A proper cooling process is the final hurdle to ensuring gloss. After extrusion, PVC wire needs to be cooled and shaped; cooling too quickly or too slowly will affect surface quality. Insufficient cooling may lead to uneven wire shrinkage and surface wrinkling; excessive cooling may cause cracking due to residual internal stress. Using a segmented cooling method to gradually reduce the wire temperature helps release internal stress and maintain a smooth surface. At the same time, the cleanliness of the cooling medium must also be considered, as impurities can cause surface spots.
The stability of the traction and winding processes is equally important. Fluctuations in traction speed will cause periodic changes in the wire diameter, creating bright and dark stripes when the surface reflects light; uneven winding tension may cause the wire to bend or indent, damaging the surface gloss. Using a constant tension control system to ensure the wire remains straight during winding is an important means of maintaining gloss.
Post-processing techniques such as surface coating or polishing can further enhance gloss. For high-end applications, a transparent brightener can be coated onto the PVC wire surface to form a continuous film, enhancing light reflection; or mechanical polishing can be used to eliminate minor imperfections and improve surface smoothness. While these processes increase costs, they significantly enhance the added value of the product.
