Understanding the Extrusion Process
Extrusion is a fundamental manufacturing process used to create continuous profiles of plastic materials with consistent cross-sectional shapes. It is widely employed across industries such as construction, automotive, appliances, and consumer goods due to its efficiency, repeatability, and ability to produce complex geometries. The extrusion process transforms raw plastic resin into usable components through a sequence of precisely controlled steps. Understanding these extrusion steps is critical for optimizing production, ensuring dimensional accuracy, and maintaining material integrity.
Step 1: Feeding and Conveying (Hopper to Feed Section)
The first stage of the extrusion process begins with material feeding. Raw plastic pellets or granules—typically PVC, PE, ABS, or UPVC—are loaded into the hopper above the extruder barrel. Gravity moves the material into the feed throat, where it enters the rotating screw housed within the heated barrel.
Screw Design and Material Flow
The screw, a core component of the extruder, features three distinct zones: the feed zone, transition (compression) zone, and metering (pumping) zone. In the feed zone, the screw channels are deep to maximize material intake. As the material advances, the channel depth decreases in the transition zone, compressing the resin and initiating melting through mechanical shear and external barrel heaters.
Proper hopper design and screw geometry are essential to prevent bridging and ensure consistent volumetric feeding, which directly affects output stability.
Step 2: Melting and Homogenization (Barrel Heating and Shearing)
During this phase, the plastic transitions from solid pellets to a homogeneous molten state. Barrel temperatures are precisely controlled in segmented heating zones, typically ranging from 150°C to 230°C depending on the polymer type.
Thermal and Mechanical Energy Input
Heat is applied via external electric heaters, while internal shear forces generated by the rotating screw contribute significantly to melting. The combination of conduction, convection, and viscous dissipation ensures complete melting and uniform melt temperature. The metering section further refines the melt, removing air pockets and eliminating unmelted particles.
Maintaining consistent melt viscosity is crucial for downstream shaping and surface quality. Melt temperature sensors and pressure transducers monitor process stability in real time.
Step 3: Pressurization and Pumping (Die Entry)
Once homogenized, the molten plastic is pressurized as it approaches the die. The screw acts as a positive displacement pump, generating high pressure (up to 1000 psi or more) to push the melt through the die orifice.
Role of the Melt Pump and Filtering
In precision applications, a gear pump (melt pump) may be installed between the extruder and die to decouple pressure generation from screw speed, enabling tighter control over flow rate and reducing pulsation. A breaker plate with a screen pack filters contaminants such as gels, carbon specks, or foreign particles before the melt enters the die.
This step ensures a clean, uniform flow profile critical for achieving dimensional accuracy and surface finish in the final product.
Step 4: Shaping (Die Design and Profile Formation)
The die is a custom-machined tool that imparts the desired cross-sectional geometry to the extruded profile. Die design accounts for polymer rheology, shrinkage, and drawdown effects to ensure the final product meets specifications.
Die Land and Calibration
The die land provides a straight channel for melt stabilization before exiting. For hollow or complex profiles, internal mandrels or spiders support the structure. Post-die, the extrudate enters a calibration stage—either in air or a vacuum sizer—for dimensional control. In some cases, such as refrigerator plastic profiles, vacuum calibration tanks are used to cool and shape the profile while maintaining tight tolerances.
Step 5: Cooling, Pulling, and Cutting
After shaping, the extrudate must be cooled rapidly to set its shape. Cooling methods include water baths, spray tanks, or air cooling, depending on the material and profile size.
Take-Off and Cutting Systems
A puller (haul-off) unit maintains constant line speed, applying controlled tension to prevent sagging or stretching. The puller speed is synchronized with the extruder output to maintain dimensional consistency. Finally, the continuous profile is cut to length using a flying cutoff saw or guillotine cutter. For coiled products, a coiler may be used instead.
For applications like door and window frames or custom plastic profiles for appliances, inline printing, embossing, or notching may also be integrated into this stage.
Conclusion
The five extrusion steps—feeding, melting, pressurization, shaping, and cooling—form an integrated system where each stage impacts the quality and efficiency of the final product. Mastery of these extrusion steps enables manufacturers to produce high-performance plastic components with repeatable precision. Dalang specializes in custom extrusion services for PVC, UPVC, ABS, PE, and other plastic materials, widely used in windows, doors, refrigerators, and various applications requiring precision plastic components.