I. Process Flow

PVC → Various additives → Mixing and blending → Extrusion → Cooling and vacuum forming → Pulling → Cut-to-length → Finished product → Inspection → Packaging and warehousing.

II. Main Equipment

1. Main machine: The majority of the workshop uses 90 single-screw extruders.

2. Auxiliary Equipment:

(1) Cooling and shaping unit: Composed of an air–water separator, a movable table plate, and an adjustment mechanism. The vacuum cooling section comprises four stages; the water-ring vacuum pump is driven by a 10 kW motor.

(2) Traction Device: Large-flat rubber belt traction;

(3) Sawing Unit: A pneumatic sawing machine equipped with a length‑measuring device, capable of tracking the cutting process.

(4) Stacking device: Pneumatically operated tilting bracket.

III. Key Points of the Production Process: Extrusion molding of profiles involves using a rotating screw to apply pressure, continuously forcing the plasticized material from the extruder barrel into the die head. The molten material is then shaped into a parison that closely matches the die’s cross-sectional profile. A pulling device continuously draws the formed product out of the die while it is simultaneously cooled and set, yielding a finished part of the desired shape. To produce profile products with excellent surface appearance and intrinsic quality, it is essential to carefully control the extrusion process parameters. The primary control points include material blending, molding temperature, screw cooling and rotational speed, extrusion pressure, and cooling‑and‑setting conditions.

3.1 Mixing: In the mixing process, PVC resin and the other ingredients specified in the formulation are first charged into a hot mixer. The mixture is stirred and heated to 120°C until uniform. Once thoroughly blended, it is transferred to a cooling mixer, where it is cooled while being mixed down to 45°C, yielding a loose, free-flowing powdered blend that is then discharged for further processing. Temperature control during mixing is critical: excessively high temperatures can cause the material to become sticky, form lumps, or result in uneven plasticization; conversely, too low a temperature may lead to insufficient blending, failing to achieve the desired pre‑plasticization. Therefore, under normal conditions, the final temperature of the hot mix is maintained at 115–120°C, with a hot‑mixing duration of 10–15 minutes, and the discharge temperature from the cooling mixer is set at 45°C.

3.2 Extrusion Molding Temperature: The production process employs a co-rotating, counter‑rotating extruder equipped with a venting system. After preheating and feeding, the material is conveyed, vented, and melted, resulting in uniform plasticization and the formation of a molten polymer. Upon reaching the die head, the melt undergoes further homogenization; under die‑head pressure, it is compacted into a dense parison, which then flows continuously through the die to form the final product.

Extrusion molding temperature is a critical factor that promotes the plasticization of the feedstock and the flow of the melt, significantly influencing both the degree of plasticization during extrusion and the quality and throughput of the extruded profiles. The temperatures of the barrel and the die are the primary parameters to be controlled. Given that PVC’s processing temperature is very close to its decomposition temperature, precise temperature control is essential.

Typically, temperature control in an extruder focuses on the barrel—from the feeding zone to the extrusion zone—ensuring that the material gradually transitions from a solid powder or pellet form to a molten state, achieving optimal plasticization. The temperature settings for each zone are generally determined based on the extruder’s design, the formulation and processing characteristics of the resin, and the desired quality of the final product. In extrusion, the operating temperature usually refers to the melt temperature, which is largely influenced by the barrel and screw temperatures. For practical measurement and control, the barrel temperature is often used as an approximation of the melt temperature, with thermocouples employed for monitoring and regulation. During extrusion, the temperature in the feeding zone should be maintained above the resin’s melting point, and the temperature profile across the feeding, compression, venting, and metering zones typically follows a saddle-shaped curve.

The die temperature has a significant impact on extrusion quality. To achieve a desirable profile appearance and mechanical properties while minimizing melt die swell, the die temperature should be maintained within an appropriate range; typically, the die setpoint is higher than the barrel temperature.

According to the formulation requirements, the extrusion temperature is set between 170 and 180°C; if it exceeds this range, the PVC will decompose and discolor. The die temperature at the die head can reach as high as 190–210°C.

3.3 Screw Cooling and Screw Rotational Speed

Due to the high melt viscosity of PVC, excessive heat generated by friction can lead to material degradation on the screw, resulting in a rough inner surface of the profile. Therefore, screw cooling is employed to reduce the frictional heat between the PVC melt and the screw surface; however, the cooling temperature should be maintained within the range of 70–90°C. If the cooling temperature is too low, it will decrease the extrusion output and compromise plasticization quality, thereby adversely affecting product performance.

Screw speed is a critical process parameter that governs extrusion rate, production output, and product quality. Increasing screw speed and shear rate reduces the apparent viscosity of the melt, promoting better material homogenization and enhancing mechanical properties such as impact strength, flexural strength, and tensile strength. However, excessively high screw speeds can lead to increased die swell and insufficient residence time of the material in the barrel, which may compromise product quality.

3.4 Extrusion Pressure. During the molding process, a certain level of pressure is required to overcome flow resistance and the material’s own viscous friction. Extrusion pressure is essential for ensuring proper plasticization, producing uniform, dense, and high‑quality parts; moreover, die pressure is a key factor influencing both product quality and production output. The operating pressure of an extruder is determined by the screw design, die geometry, and the flow characteristics of the molten polymer. When extruding a melt, sufficient pressure is necessary to obtain products with appropriate density and favorable physical properties. However, excessive die pressure can lead to flash and thermal degradation due to excessive shear heating; it is typically maintained within the range of 18–22 MPa. Conversely, insufficient die pressure may also result in quality issues, such as inadequate densification and increased warpage.

3.5 Traction Speed

Traction and plasticization directly affect the wall thickness, dimensional tolerances, mechanical properties, and surface appearance of the finished product. Therefore, a stable traction speed is essential, and the traction‑plasticization rate must be matched to the extrusion speed of the profile. During normal production, the traction speed should be slightly higher than the extrusion speed to counteract die swell.

3.6 Cooling and Vacuum Setting

The cooling and shaping of profile products employs a combined dry–wet cooling approach. While wet vacuum cooling can increase extrusion speed, the impact of cold water can induce internal stresses in the profiles, leading to warping and deformation. In contrast, dry vacuum cooling prevents direct contact between water and the profile surface; heat is removed by the cooling jacket through the water, resulting in lower cooling efficiency. To enhance cooling performance, the length of the cooling and shaping unit must be extended. Therefore, a serial cooling method that combines dry vacuum shaping with wet vacuum shaping both ensures product dimensional accuracy and reduces internal stresses while significantly boosting forming speed. Specifically, after gentle cooling in the dry vacuum stage, the entire profile is subjected to reduced‑pressure cooling in a wet vacuum tank, thereby preventing deformation during cooling, minimizing internal stresses, and enabling high‑speed production of dimensionally precise parts.

During the extrusion of profiles, the molten material exiting the die is directly fed into a vacuum‑forming mold. Under the action of vacuum suction, the material is tightly drawn onto the inner wall of the mold. It is then cooled and solidified by circulating cooling water within the mold. Proper control of the vacuum level directly affects product quality: if the vacuum is too low, the suction force on the parison is insufficient, making it difficult to achieve the desired shape and resulting in poor surface appearance and dimensional accuracy; if the vacuum is too high, increased resistance can lead to material buildup at the mold inlet, and in severe cases, even cause the profile to break.

During profile extrusion, an ideal cooling water temperature is 18–22°C, and the effective water pressure should exceed 0.2 MPa.

Finished‑product control requirements: The manufacturing process is a critical factor influencing the quality of extruded plastic profiles. To ensure product quality, it is essential to rigorously control material blending and the extrusion process. Process parameters during extrusion must be closely monitored and regularly assessed to promptly evaluate surface finish, dimensional accuracy, and all relevant physical and mechanical properties, enabling timely adjustments and continuous analysis of quality trends to enhance process control.