Extrusion molding of profiles involves using the rotational pressure of a screw to continuously force plasticized material from the extruder barrel into the die. The molten material is shaped into a parison that conforms to the die’s cross-sectional geometry, and a pulling device continuously draws the formed product out of the die while it is cooled and set, yielding a finished part of the desired shape. To produce profile products with both excellent surface appearance and intrinsic quality, it is essential to carefully control the extrusion process parameters, with key factors including compounding, molding temperature, screw cooling and speed, extrusion pressure, and cooling‑setting conditions.

(1) Mixing materials

During the compounding process, PVC resin and the formulated additives are first charged into a hot mixer. The mixture is stirred and heated to 120°C until uniform, after which it is transferred to a cooling mixer for simultaneous mixing and cooling down to 45°C, yielding a loose, free-flowing powdered blend that is then discharged for further processing. Temperature control during compounding is critical: excessively high temperatures can cause the material to become sticky, form lumps, and result in uneven plasticization, while too low a temperature leads to insufficient mixing and failure to achieve the desired pre‑plasticization. Therefore, under normal operating 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 cooled‑mix discharge temperature is set at 45°C.

(2) Extrusion molding temperature

The production process employs a counter‑rotating twin‑screw extruder equipped with a venting system. After preheating and feeding, the material undergoes conveying, venting, and melting stages, achieving uniform plasticization to form a molten polymer. Upon reaching the die head, the melt is further homogenized; under die‑head pressure, it is compacted into a dense parison and continuously extruded through the die to take its final shape. Extrusion temperature is a critical parameter that facilitates plasticization and melt flow, significantly influencing both the degree of plasticization during processing and the quality and throughput of the finished profile. Temperature control is particularly focused on the barrel and die, as PVC’s processing window is very close to its decomposition temperature, necessitating strict regulation. Typically, extruder temperature control spans from the feed zone to the die‑exit zone, ensuring gradual melting of solid powder or pellets and attaining optimal plasticization. The specific temperature settings for each zone are determined based on the extruder design, the formulation and processing characteristics of the resin, and the desired product quality. In extrusion, the relevant temperature generally refers to the polymer melt temperature, which is largely governed by the barrel and screw temperatures. For practical measurement and control, the barrel temperature is often used as an approximation of the melt temperature, monitored and regulated via thermocouples. When using a twin‑screw extruder, the feed zone should be maintained above the resin’s melting point, while the temperature profile across the feed, compression, venting, and homogenization zones typically follows a saddle‑shaped curve. Die‑head temperature exerts a substantial influence on extrusion outcomes. Maintaining the die‑head within an appropriate temperature range is essential to achieve desirable surface appearance and mechanical properties, minimize melt‑out expansion, and generally set the die‑head temperature higher than the barrel temperature.

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

(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 frictional heating between the PVC melt and the screw surface; however, the cooling temperature must be maintained within the range of 70–90°C. If the cooling temperature is too low, extrusion output will decrease and plasticization quality will be compromised, adversely affecting product quality.

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

(4) Extrusion pressure

During the molding process, a certain level of pressure is required to overcome the flow resistance of the material and its internal viscous friction. Extrusion pressure is essential for achieving proper plasticization, producing uniformly dense, high‑quality parts, and ensuring consistent product quality; 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, excessively high die pressure can lead to flash and cause excessive shear heating, resulting in thermal degradation; therefore, it is typically maintained within the range of 18–22 MPa. Conversely, too low a die pressure may also compromise product quality, leading to insufficient densification and increased warping.

(5) Traction Speed

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

(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. By 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. Consequently, a serial cooling method that integrates dry vacuum shaping with wet vacuum shaping both improves 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 residual stresses, and enabling high‑speed production of dimensionally precise parts.

Hebei Shuangda Rubber and Plastics Machinery Co., Ltd. is a modern enterprise integrating research and development, manufacturing, and sales of complete sets of extrusion equipment for rubber, silicone, and plastics. The company’s main products include rubber extruders, rubber hose extrusion lines, three‑layer composite sealing strip extrusion lines, silicone sealing strip production lines, plastic sealing strip production lines, plastic profile production lines, plastic pipe production lines, elastomer extruders, and other extrusion equipment, as well as production machinery for engineering seals, automotive seals, wire and cable applications, air‑conditioning hoses, braided hoses, automotive hoses, waterstop strips, and related products.