Plastic Extrusion: Professional Terminology
This vocabulary provides a solid foundation for communicating effectively about the plastic extrusion process, from machine operators and process engineers to product designers
Category 1: The Extrusion Line & Major Components
Term | Definition & Context |
Extrusion Line | The complete set of equipment used in the extrusion process, from raw material handling to the final product. |
Extruder | The machine that melts and pumps the plastic. It consists of a barrel, screw, and drive motor. |
Hopper | The container that holds and feeds the plastic resin (usually in pellet form) into the extruder barrel. |
Barrel | The heated, hardened steel cylinder that houses the extruder screw. |
Die | The precision-machined tool mounted at the end of the extruder that shapes the molten plastic into the desired profile. |
Screen Pack / Filter Pack | A stack of metal wire meshes placed between the screw and the die to filter out contaminants and create backpressure. |
Breaker Plate | A thick, perforated metal plate that supports the screen pack. |
Downstream Equipment | All equipment that handles the product after it exits the die. This includes cooling, sizing, pulling, and cutting units. |
Upstream Equipment | Equipment that handles material before the extruder, such as dryers, blenders, and feeders. |
Category 2: The Screw & The Plasticating Process
Term | Definition & Context |
Screw | The rotating shaft inside the barrel that conveys, melts, and pumps the plastic. Its design is critical to the process. |
Flight | The helical ridge on the screw. |
Root | The central shaft of the screw. The root diameter often changes along the screw's length. |
Channel | The space between the flights. |
Channel Depth | The distance from the root to the top of the flight. Changes in depth define the screw zones. |
Feed Zone (Solid Conveying Zone) | The first section of the screw. Its deep flights convey solid pellets forward from the hopper. |
Compression Zone (Transition Zone) | The middle section where the channel depth decreases. This compresses the material, which, combined with heat, melts the plastic. |
Metering Zone (Melt Pumping Zone) | The final section with a shallow, constant channel depth. It generates pressure to pump the homogenized melt through the die at a consistent rate. |
Length-to-Diameter Ratio (L/D Ratio) | The ratio of the screw's flighted length to its diameter (e.g., 24:1). A higher L/D ratio allows for better melting and mixing. |
Compression Ratio | The ratio of the channel volume in the feed zone to the channel volume in the metering zone. It indicates the screw's melting capability. |
Mixing Section | A specialized section on the screw (e.g., Maddock mixer, pineapple mixer) designed to homogenize the melt temperature and composition. |
Vented Barrel (Two-Stage Screw) | A barrel with a vent port to remove moisture or volatiles. Requires a special two-stage screw with a decompression zone under the vent. |
Category 3: Dies & Tooling
Term | Definition & Context |
Die | The tool that shapes the extrudate. |
Die Body / Die Holder | The main block of the die assembly. |
Mandrel (Spider Mandrel, Torpedo) | A central pin used in dies for hollow profiles (like pipes) to form the inner surface. |
Die Land | The final, parallel section of the die channel where the melt is stabilized before exiting. The length of the die land is critical for controlling surface finish and dimensions. |
Adapter (Crosshead) | A section that connects the extruder barrel to the die, often used to change the flow direction (e.g., in wire coating). |
Die Swell (Extrudate Swell) | The phenomenon where the extrudate expands as it exits the die due to the relaxation of polymer molecules. Die design must compensate for this. |
Spider Legs | The arms that hold a mandrel in place inside a die. They leave witness lines (spider lines) on the extrudate where the plastic flows around them. |
Coathanger Die / Manifold | The internal channel design in a flat sheet or film die that distributes the melt evenly across the width of the die. |
Category 4: Process Parameters & Control
Term | Definition & Context |
Throughput (Output Rate) | The mass of material extruded per unit of time (e.g., kg/hour, lbs/hour). |
Screw Speed (RPM) | The rotational speed of the extruder screw. A primary control for throughput. |
Melt Temperature (T melt) | The actual temperature of the molten plastic as it exits the die. |
Melt Pressure | The pressure of the molten plastic, typically measured just before the screen pack or die. High pressure can indicate a blockage. |
Barrel Temperature Zones | The barrel is divided into multiple heating/cooling zones (e.g., rear, middle, front, adapter) to precisely control the melting profile. |
Backpressure | The resistance to flow. Created by the screen pack, breaker plate, and die. Essential for proper melting and mixing. |
Residence Time | The amount of time the plastic spends inside the extruder. |
Parison | The tubular extrudate used in extrusion blow molding to make bottles. |
Category 5: Downstream Equipment & Processes
Term | Definition & Context |
Sizing (Calibration) | The process of setting the final dimensions of the still-molten extrudate. Often done with a vacuum sizer or sizing sleeve that uses vacuum and cooling water. |
Cooling Tank / Water Bath | A long tank filled with water that cools the extruded product. |
Haul-Off (Puller) | A device (often with caterpillar tracks) that grips the cooled product and pulls it at a constant speed, stabilizing the line. |
Cutter (Saw, Guillotine Cutter) | Cuts the continuous extrudate to the desired length. |
Winder / Coiler | A machine that winds flexible extrudates (like film, sheet, or tubing) onto rolls. |
On-The-Fly Perforation / Punching | Downstream units that add holes or cutouts to the profile while it is moving. |
Category 6: Products & Specific Processes
Term | Definition & Context |
Profile Extrusion | The extrusion of any continuous shape with a constant cross-section (e.g., window frames, deck railings). |
Tube/Pipe Extrusion | Extrusion of hollow, cylindrical profiles. |
Sheet Extrusion | Production of wide, flat sheets using a flat die and a set of cooling rolls (calendar stack). |
Blown Film Extrusion | A process where a tube of plastic is extruded upward and inflated like a balloon, then collapsed to form a double layer of film. |
Cast Film Extrusion | A process where the melt is extruded through a flat die directly onto a chilled roll to produce film. |
Coextrusion | The process of extruding two or more different materials simultaneously through a single die to create a multi-layer product. |
Extrusion Coating | The process of extruding a thin film of molten plastic onto a substrate like paper, foil, or fabric. |
Category 7: Defects & Quality
Term | Definition & Context |
Surging | An unstable, cyclical variation in output, leading to inconsistent dimensions. Often caused by poor screw design or feeding problems. |
Die Lines | Visible lines running in the machine direction on the extrudate, caused by a scratch in the die or contamination. |
Melt Fracture | A surface defect where the extrudate appears rough or wavy, caused by excessive shear stress at the die entrance. |
Degradation | The breakdown of the polymer due to excessive heat or shear, leading to discoloration, black specs, and loss of properties. |
Shark Skin | A surface defect characterized by a matte, rough finish, often occurring at high output rates. |
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