Plastic Extrusion Process Flow
Plastic extrusion is a continuous, high-efficiency molding method primarily used to produce pipes, profiles, films, sheets, and wire/cable coatings. The basic process flow is as follows:
**Raw Material Preparation and Drying**
Mix plastic pellets (e.g., PVC, PE, PP, ABS) with additives such as masterbatches, stabilizers, and lubricants according to the formulation.
Dry hygroscopic materials (e.g., PA, PET) to prevent moisture from causing bubbles or material degradation in the finished product.
**Extruder Heating and Screw Plasticization**
Feeding: The mixed material enters the extruder barrel via the hopper.
Conveying and Compression: The rotating screw pushes the material forward; the material gradually melts under the combined influence of external barrel heaters and internal frictional shear heat.
Homogenization: The material is further mixed and plasticized in the screw's homogenization zone to form a uniform melt and build up pressure.
**Filtration and Flow Distribution (Process-Dependent)**
The melt passes through a screen changer/filter to remove impurities or unmelted particles.
It enters an adapter or flow distributor to adjust the flow pattern in preparation for entering the die.
**Die Forming**
The melt flows through a shaped die to form a continuous extrudate with a cross-section approximating that of the die opening (e.g., tubular, sheet-like, or complex profiles).
The internal flow channel design of the die ensures uniform material outflow, preventing stagnation or weld lines.
**Cooling and Sizing**
Sizing: For products like pipes and profiles, a sizing sleeve or vacuum sizing box is used to maintain the shape and provide initial cooling.
Cooling: Methods such as water baths (immersion or spray), air cooling, or cooling rolls are used to cool the product to below room temperature, allowing it to solidify and set.
**Haul-off (Pulling)**
A haul-off unit pulls the formed and cooled product from the die exit at a constant speed.
The haul-off speed must be matched to the extrusion rate to control wall thickness and dimensional accuracy.
**Cutting or Winding**
Depending on the product type:
Rigid products (pipes, profiles): Cut to specified lengths using a saw or planetary cutter. Flexible products (films, sheets): Trimmed and then wound into rolls by a winder.
Post-processing (optional)
Examples include annealing (to relieve internal stress), heat treatment (to increase crystallinity), and surface corona treatment (to improve print adhesion).
Inspection and packaging
Inspection of appearance, dimensions, mechanical properties, etc.
Packaging and warehousing of qualified products.
Examples of typical applications:
Pipes: Raw material → Extrusion → Vacuum sizing → Spray cooling → Haul-off → Cutting.
Films: Raw material → Extrusion → T-die → Cooling roll sizing → Haul-off → Winding.