The inherent properties of the material itself form the foundation.
Certain intrinsic characteristics of PP fundamentally determine the difficulty and nature of the cooling process. As a semi-crystalline polymer, PP possesses a low thermal conductivity, meaning heat transfers slowly within the pipe walls. Additionally, PP possesses a high specific heat capacity and contains crystallization latent heat during melting. This necessitates the removal of substantial heat during cooling from the molten state. Consequently, PP pipes require the slow and uniform dissipation of large amounts of heat during cooling. Improper cooling can easily lead to the generation of internal stresses.
The design of cooling equipment and molds is critical.
The design of the sizing sleeve is paramount: it serves as the core component for cooling and shaping the pipe. Its length is a key parameter: too short may result in insufficient cooling and shaping, potentially causing pipe deformation; too long may induce excessive frictional resistance, increasing traction power and potentially generating internal stresses in the pipe. Typically, its length requires careful selection based on the pipe diameter. Additionally, the sizing sleeve should be manufactured from metals with excellent thermal conductivity (such as wear-resistant copper alloys). The structural design of its internal vacuum chamber directly impacts cooling efficiency and tube surface quality.
Spacing between die and sizing sleeve: During production, a specific distance must be maintained between the two. This allows the extruded billet to undergo air pre-cooling before entering the sizing sleeve, facilitating vacuum cooling and shaping of the molten billet. It also prevents direct contact-induced heat exchange, avoiding die temperature drops or sizing sleeve temperature rises that could compromise process stability.
The selection of cooling methods is crucial.
For PP pipes, cooling primarily involves two methods: immersion cooling and spray cooling. The choice of method significantly impacts cooling efficiency.
Immersion Cooling: This method is typically used for small-diameter pipes. Its main issues include potential vertical temperature differences within the cooling tank and buoyancy forces that may cause deformation, particularly in large-diameter pipes.
Spray cooling: For PP pipes, spray cooling is preferred, especially for large-diameter pipes. Spray cooling employs nozzles uniformly distributed around the pipe circumference, delivering not only higher cooling intensity but also more uniform heat transfer, effectively avoiding the drawbacks of immersion cooling. To achieve higher cooling efficiency, mist cooling can be employed, utilizing water vaporization to dissipate substantial heat.
Precise control of process parameters is essential to ensure quality.
Even with superior equipment, improper process parameters can lead to cooling issues.
Cooling water temperature and flow rate: As a crystalline polymer, PP typically requires gradual cooling to minimize internal stress in the finished product. This necessitates segmented cooling tanks with varying temperatures to create a temperature gradient, allowing the product to cool and set progressively-for example, through hot water, warm water, and cold water stages. Cooling water flow rate also requires precise adjustment: excessive flow may cause surface roughness, spots, or pitting; insufficient or uneven flow may result in bright spots, susceptibility to breakage, wall thickness inconsistencies, or excessive ovality.
Vacuum Level: Controlling the vacuum level in the vacuum sizing tank is also critical. Generally, the vacuum level should be kept as low as possible while maintaining acceptable pipe appearance quality. Excessive vacuum increases internal stress in the pipe, making the product more prone to deformation during storage.
Drawing Speed: Drawing speed directly affects the pipe's dwell time in the cooling water tank. Faster speeds result in shorter dwell times, potentially leaving more residual heat inside the pipe and increasing subsequent shrinkage rates.