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A brief analysis of the auxiliary equipment for pipe extruders and extrusion machines: what are they?

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2019-03-04

The auxiliary equipment for pipe haul‑off machines and plastic extrusion lines typically includes pay‑off units, straightening devices, preheating units, cooling systems, traction mechanisms, metering counters, spark testers, and take‑up units. Depending on the specific application of the extrusion line, the accompanying auxiliary equipment may vary; additional components can include cutters, air‑drying units, and marking devices. Straightening devices: Among common defects in plastic extrusion, eccentricity is frequently observed, and various forms of core bending are among the primary causes of such eccentricity. In sheath extrusion, surface scratches on the sheath are often attributable to core deformation. Consequently, straightening devices are indispensable in many extrusion lines.

The auxiliary equipment for pipe haul-off machines and plastic extrusion lines typically includes pay-off stands, straightening units, preheating devices, cooling systems, traction units, meter counters, spark testers, and take-up reels. Depending on the specific application of the extrusion line, the accompanying auxiliary equipment may vary; additional options can include cutters, air‑drying units, and printing devices.

Straightening Devices: Among the common defects in plastic extrusion, eccentricity is frequently encountered, and various forms of core bending are one of the primary causes of such eccentricity. In sheath extrusion, surface scratches on the sheath are often attributable to core bending as well. Consequently, straightening devices are indispensable in many extrusion lines. The main types of straightening devices include: roller-type (available in horizontal and vertical configurations); pulley-type (offering single-pulley and pulley‑block options); take‑up‑and‑straighten‑with‑tension‑stabilizing units; and pressure‑roller‑type (also available in horizontal and vertical arrangements), among others.

Preheating Device: Preheating the cable core is essential for both bare‑core extrusion and jacket extrusion. For bare‑core applications, where the insulation layer is thin and air pockets are unacceptable, high‑temperature preheating of the conductor prior to extrusion removes surface moisture and oil contaminants. In jacket extrusion, its primary function is to dry the cable core, thereby preventing the formation of voids in the jacket caused by moisture—whether from ambient humidity or residual moisture in the cable’s inner bedding layers. Preheating also mitigates the residual internal stresses that can arise when the extruded plastic undergoes rapid cooling. During the extrusion process, preheating eliminates the temperature gradient created when a cold conductor enters the high‑temperature die head; this gradient, which occurs at the die exit upon contact with the molten polymer, helps stabilize extrusion pressure by minimizing fluctuations in polymer temperature, thus ensuring consistent extrusion output and quality. All extrusion lines employ electrically heated core‑preheating units, which must have sufficient capacity and enable rapid temperature rise to achieve efficient core preheating and core drying. The preheating temperature is constrained by the take‑up speed and is typically set close to the die‑head temperature.

Cooling system. Once the molded plastic extruded coating leaves the die, it must be immediately cooled and set; otherwise, it will deform under gravity. Cooling is typically achieved by water, which can be categorized into rapid cooling and slow cooling depending on the water temperature. Rapid cooling involves direct immersion in cold water; it promotes effective shaping of the extruded layer but, for crystalline polymers, the sudden temperature drop can leave residual internal stresses within the structure, leading to cracking during service. Rapid cooling is generally employed for PVC coatings. Slow cooling, on the other hand, aims to minimize internal stresses in the finished product. This is accomplished by arranging water at progressively lower temperatures in staged sections within the cooling tank, allowing the product to cool and solidify gradually. For the extrusion of PE and PP, slow cooling is used, involving three stages: hot water, warm water, and cold water.