During PVC wire production, the stability of the pulling speed is a key factor in ensuring wire diameter uniformity. The pulling device ensures a stable extrusion process by uniformly pulling the PVC wire and controlling its cross-sectional area. Fluctuations in the pulling speed directly disrupt the dynamic balance between extrusion and pulling, resulting in uneven wire diameter in the longitudinal or transverse directions. This unevenness not only affects the product's appearance but also reduces the PVC wire's electrical performance, mechanical strength, and reliability.
Excessively fast pulling speeds increase the tensile forces applied to the PVC wire during the shaping process, leading to excessive wire thinning. This increases the directional alignment of the PVC molecular chains within the wire, while weakening the intermolecular forces. This can lead to localized shrinkage or breakage. Especially when producing fine-diameter PVC wire, even slight fluctuations in the pulling speed can cause the wire diameter to exceed tolerances, resulting in "bamboo-knot" defects. Conversely, if the drawing speed is too slow, the PVC wire's residence time between the die and the shaping die is prolonged, allowing material to accumulate due to gravity or die expansion, resulting in localized thickening of the wire diameter and even die blockage, disrupting production continuity.
The impact of drawing speed fluctuations on wire diameter uniformity is also reflected in the thermal history of PVC. As a heat-sensitive material, PVC's melt viscosity is extremely sensitive to temperature. Unstable drawing speeds can lead to inconsistent cooling times for the wire in the shaping zone. If the speed is too fast, the wire is pulled out before sufficient cooling occurs, leading to residual stress accumulation and subsequent shrinkage. If the speed is too slow, the wire cools excessively, increasing its brittleness and prone to cracking during the drawing process. This thermal history difference further amplifies wire diameter non-uniformity, resulting in periodic or random dimensional fluctuations.
From the perspective of process parameter coordination, the drawing speed must be strictly matched with parameters such as extrusion speed, screw speed, and cooling efficiency. For example, if the extrusion speed decreases due to material supply fluctuations and the pull-out speed isn't adjusted promptly, the wire can accumulate in the sizing area, thickening the wire diameter. Conversely, if the extrusion speed remains stable while the pull-out speed suddenly increases, the wire can become thinner. This parameter mismatch can also lead to internal structural defects in the PVC wire, such as air holes, delamination, or uneven filler distribution, further reducing the mechanical stability of the wire diameter.
In actual production, pull-out speed fluctuations are often caused by equipment failure or improper operation. For example, insufficient clamping force on the pull-out track, belt slippage, and inverter control failure can all lead to unstable speed output. Furthermore, an improper ratio of heat stabilizer and lubricant in the PVC raw material formula can affect the material's fluidity during extrusion, indirectly exacerbating the impact of pull-out speed fluctuations on wire diameter. For example, excessive lubricant can cause material slippage at the die, resulting in a momentary mismatch between the pull-out speed and the extrusion speed.
To address the problem of pull-out speed fluctuations, comprehensive optimization of equipment, processes, and raw materials is necessary. At the equipment level, a high-precision servo motor-driven pulling device should be used, coupled with real-time data feedback from an online diameter gauge to form a closed-loop control system. At the process level, experiments should be conducted to determine the optimal matching range between pulling speed and extrusion speed, and strict operating procedures should be established. At the raw material level, the PVC formulation should be optimized to ensure the synergistic effect of additives such as heat stabilizers, lubricants, and fillers, thereby enhancing material stability during the extrusion process.
Strengthening pulling speed stability is a critical step in ensuring uniform wire diameter in PVC wire production. Through equipment upgrades, process optimization, and raw material control, the impact of speed fluctuations on wire diameter can be effectively reduced, improving the overall quality of PVC wire. This not only helps meet the stringent wire diameter accuracy requirements of applications such as wire and cable, and piping systems, but also reduces scrap rates during production, creating significant economic benefits for enterprises.
