Polyethylene Pipe Extrusion: Advanced Manufacturing Process

Polyethylene Pipe Formation

Polyethylene pipe processing is relatively straightforward, as various specifications of pipes can be easily manufactured using the polymer extrusion method. This versatile material offers excellent flexibility, non-toxicity, corrosion resistance, electrical insulation, cold resistance, and impact performance. These properties make low-density polyethylene pipes suitable for applications such as coiled water pipes, agricultural irrigation pipes, and electrical insulation tubes.

High-density polyethylene exhibits superior mechanical properties, relative hardness, and tensile strength compared to low-density polyethylene. It can withstand certain pressure and possesses excellent electrical performance and chemical resistance, making it widely used in pipelines for transporting water, oil, gas, and chemical liquids, as well as in cable protection sleeves. The polymer extrusion process allows for precise control over these material properties during manufacturing.

Linear low-density polyethylene can be used for direct polymer extrusion of various liquid transport pipes and cable sheaths. It can also be mixed in specific proportions with low-density or high-density polyethylene to take advantage of its excellent environmental stress crack resistance, higher rigidity, and heat distortion temperature.

Medium-density polyethylene offers good mechanical properties and a long service life, making it suitable for producing gas pipes with a pressure rating of ≤0.1MPa. The polymer extrusion technique ensures that these material characteristics are maintained consistently throughout the production process.

1.1 Raw Material Selection

Polyethylene pipes are generally produced using polyethylene resin as the primary raw material without the need for additional additives. There are two main types of polyethylene resins: high-pressure polyethylene and low-pressure polyethylene. The selection of raw materials depends primarily on the usage requirements of the processed product, followed by the characteristics of the processing equipment, as well as the availability and cost of the raw materials. In polymer extrusion, the choice of raw material directly impacts the quality and performance of the final product.

The polymer extrusion process relies heavily on the consistent quality of raw materials. Even minor variations in resin properties can affect the entire production process, leading to defects in the final pipe product. Therefore, strict quality control measures for raw materials are essential before they enter the polymer extrusion line.

2. Process Flow

The process flow for high-density polyethylene pipes and low-density polyethylene pipes is fundamentally similar. The primary difference is that high-density polyethylene pipes, being rigid, undergo fixed-length sawing, while low-density polyethylene, being semi-rigid, can be coiled into rolls of 200~300 meters. The following describes the process flow using low-density polyethylene as an example, highlighting the key stages of polymer extrusion.

Raw Material Preparation

First, the specific grade of raw material is determined. LDPE granules are then prepared for introduction into the polymer extrusion system. Proper storage and handling of raw materials are crucial to prevent contamination and moisture absorption, which could affect the polymer extrusion process.

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Extrusion Melting

The LDPE granules are fed into a single-screw extruder from the hopper, where they are heated to a molten state. The rotational force of the screw propels the molten material through the die head's annular channel, forming a tubular shape. This stage is critical in the polymer extrusion process as it determines the initial material properties.

Sizing Process

Due to the high temperature of the molten material, sizing measures must be implemented to fix the plastic pipe's shape. Vacuum sizing or internal pressure sizing methods are commonly used in polymer extrusion. This step ensures the pipe maintains its dimensional accuracy after exiting the extruder.

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Cooling Process

Although the plastic pipe has been shaped after passing through the sizing sleeve, it may still deform due to insufficient cooling. It must continue cooling through a cooling device, typically consisting of one or more cooling water tanks, each 2~4 meters long. Proper cooling is essential in polymer extrusion to maintain dimensional stability.

Pulling and Coiling

After cooling, the pipe is pulled forward by a traction device and coiled on a winding device. The traction speed must be precisely matched to the polymer extrusion rate to ensure uniform wall thickness and prevent stretching or compression of the pipe.

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Finishing Operations

When a certain length is reached, the pipe is cut. The finished pipes then undergo inspection, weighing, packaging, and other subsequent processes. Each stage of the polymer extrusion line must maintain strict synchronization, and process conditions must be carefully controlled to produce qualified pipes that meet quality requirements.

3. Production Operations

The following are key operational points for pipe extrusion molding, focusing on the critical aspects of polymer extrusion that operators must monitor and control:

4. Determination of Production Process Conditions

With raw materials and equipment determined, the selection and control of process conditions during production become particularly important. It is essential to establish production process conditions that have both theoretical basis and practical production applicability, especially in polymer extrusion where small variations can affect final product quality.

Temperature Control

Polyethylene raw materials with different melt flow rates require different temperature controls during production. The control temperature should be determined based on the material's melt flow rate. In general, high-density polyethylene has a higher crystallinity and larger crystallization melting latent heat, so its processing temperature is somewhat higher than that of low-density polyethylene. The polymer extrusion temperature profile must be carefully adjusted for each specific material formulation.

In polyethylene pipe temperature control, the die temperature is generally set lower than the maximum barrel temperature. This practice in polymer extrusion serves three main purposes:

1. Polyethylene melt has low viscosity and a wide processing temperature range. Lowering the temperature helps improve formability and makes the product more dense, which is crucial in polymer extrusion for achieving proper mechanical properties.
2. Lower die temperatures facilitate better sizing, which can increase production efficiency in the polymer extrusion process by allowing faster cooling and higher line speeds.
3. Energy savings can be achieved, reducing waste and operational costs associated with polymer extrusion.

Maintaining precise temperature control throughout the polymer extrusion process ensures consistent melt viscosity, which is essential for uniform wall thickness, dimensional stability, and overall product quality in polyethylene pipe production.