1. Definition: Plastic profiles refer to extruded products—such as pipes, sheets, rods, and films—that are continuously formed by extrusion and have a uniform cross-sectional shape. Plastic profiles exhibit excellent properties, including light weight, corrosion resistance, superior load-bearing capacity, strong decorative appeal, and ease of installation. Soft plastic profiles are primarily used for cushioning and sealing, while rigid plastic profiles are mainly employed as structural materials.

2. Classification. Based on the hardness of the profile, it can be divided into soft profiles and rigid profiles. According to the type of plastic, it can be classified into PVC profiles, ABS profiles, PE profiles, and others, with PVC profiles being the most common.

3. Performance characteristics. Plastic profiles exhibit excellent service properties, including light weight, corrosion resistance, superior load-bearing capacity, strong decorative appeal, and ease of installation.

4. Applications. These products are used in construction, electrical appliances, furniture, transportation, civil engineering, water conservancy, and everyday consumer goods. Soft extruded profiles are primarily employed as gaskets and seals, while rigid extruded profiles are mainly utilized as structural materials.

5. Molding methods (using PVC profiles as an example). These are divided into single-screw extrusion molding and twin-screw extrusion molding.

(1) Single-screw extrusion molding. PVC resin and plastic additives → mixing → pelletizing → single-screw extrusion molding → vacuum sizing → cooling → traction → cutting → stacking. The single-screw extrusion process is suitable for the production of small batches or small‑size profiled products.

(2) Twin-screw extrusion molding. PVC resin and plastic additives → mixing → twin-screw extrusion molding → vacuum sizing → cooling → traction → cutting → stacking. The twin-screw extruder process allows direct molding from powdered materials, offers high production capacity, and is well suited for the mass production of standard profiles and large‑size custom‑shaped profiles.

6. Composition of the Molding Unit

(1) Extruder.

Single-screw extruder: For the molding of polyolefin profiles, a single-screw extruder with a gradually varying screw design should be used. The screw diameter typically ranges from 45 to 120 mm, with an L/D ratio of at least 20. PVC profiles with relatively small cross-sectional dimensions are suitable for production on a single-screw extruder. In such cases, the screw diameter is generally between 45 and 65 mm, the L/D ratio is around 20, and the screw channels should be as deep as possible.

Twin-screw extruders used for molding are generally well suited for processing PVC, particularly rigid PVC (RPVC). Conical twin-screw extruders can directly convert powdered materials into finished profiles. They are ideal for the mass production of standard profiles and large‑size custom‑shaped sections, offering excellent plasticization and high throughput.

7. Die Head. The die head is the primary component for shaping and forming the plastic material. Its function is to continuously and uniformly convert the cylindrical molten polymer supplied by the extruder into a well‑plasticized parison that matches the cross‑sectional shape and geometric dimensions of the die channel. Subsequently, through cooling, setting, and other processing steps, it yields high‑performance profiled products.

8. Types of profile extrusion dies.

(1) Streamlined die head. The flow path, which transitions gradually from the circular cross-section at the screw exit to a shape approximating the final product’s geometry, can be either integral or segmented. This type of die head features a cavity meticulously designed to meet material‑flow requirements, with smoothly contoured surfaces that eliminate material stagnation. It enables long‑term continuous production, accommodates heat‑sensitive materials, delivers high‑quality finished parts, and keeps operating costs low, making it an ideal choice for extruding profiled products. It is particularly well suited for the mass production of standardized, fixed‑shape components.

(2) Plate-type die head. In a plate-type die head, the flow channel undergoes a sharp transition from a circular cross-section at the screw exit to a shape that closely resembles the final product’s geometry.

Features: simple structure, low cost, rapid manufacturing, and easy adjustment and installation. Drawbacks: due to abrupt changes in the flow path, the material flow within the die is poor, leading to localized material stagnation and incomplete flow at the corners. Therefore, it is not suitable for RPVC extrusion.

Characteristics: This design reduces the overall machining difficulty of the runner system; however, during assembly, some degree of uneven or non‑smooth transitions in cross-sectional dimensions is inevitable at the joints. Consequently, the melt flow conditions are less favorable than in an integral runner, and molding quality is more difficult to control.

9. Shaping Device

(1) Vacuum forming apparatus.

The vacuum sizing unit employs indirect water cooling to cool and shape the high‑temperature molten profile extruded from the die. The vacuum sizing sleeve consists of two sections—the vacuum sizing zone and the cooling zone—each with suction slots on its inner wall, arranged in an alternating pattern. A negative pressure is generated around the vacuum zone, ensuring tight contact between the profile’s outer surface and the inner wall of the sizing sleeve, thereby enabling effective cooling and shaping.

(2) Corrugated‑plate slip‑type shaping device. The material is first extruded into a tubular parison, then slit and flattened before being fed into the shaping die for final forming.

Features: It can be molded into complex, large-scale, irregular profiles using molds with simple geometries.

10. Traction and film‑applying unit. The traction device used in pipe production—specifically, a tracked traction system—can be employed. If the rubber blocks on the tracks are machined to match the contour of the profile being pulled, the traction performance will be improved. Subsequently, a protective film is applied, and the assembly is fed into the cutting unit.

11. Cutting devices. Cutting methods include sawing, shearing, and grinding. Examples include circular saw cutters, pneumatic shear machines, rotary shear machines, and abrasive‑wheel grinders.