Extrusion molding is the core production process for HDPE plastic pipes and directly determines dimensional accuracy, wall-thickness uniformity, and appearance quality. Longkang uses advanced extrusion production lines with strict control of process parameters. This article explains the extrusion molding process in detail.
- Principle of Extrusion Molding
Extrusion molding is a process in which plastic pellets are heated and melted in an extruder, continuously pushed forward by a screw, extruded through an annular die head into a pipe parison, and then cooled and sized into finished pipe.
- Extruder System
- Main machine
- Single-screw extruder, models SJ-65/SJ-75/SJ-90
- Screw material: 38CrMoAlA, nitrided
- Screw diameter: 65–90 mm
- L/D ratio: 25:1–33:1
- Drive: variable-frequency speed-regulated motor
- Heating system
- Barrel 4-zone heating
- Temperature: 180 °C–220 °C
- Cast aluminum heating bands
- PID temperature control, accuracy ±2 °C
- Cooling system
- Barrel water cooling
- Prevents degradation from overheating
III. Die Head and Tooling
- Die head structure
- Screen pack (screen changer)
- Flow divider
- Die body
- Mandrel
- Die
- Key dimensions
- Die inner diameter: 15%–25% larger than the product outer diameter (allowing for drawdown)
- Mandrel outer diameter: determines inner diameter
- Die gap: determines wall thickness
- 500 mm product: die about 600 mm
- Die material
- Alloy tool steel
- Chrome-plated and polished
- Surface roughness Ra<0.8
- Vacuum Sizing
- Sizing principle
- The pipe parison enters the vacuum sizing sleeve
- Vacuum is drawn inside the sleeve, drawing the pipe wall against the inner wall of the sizing sleeve
- Simultaneously cooled by cooling water
- Precisely controls the outer diameter
- Sizing sleeve
- Length: 200–300 mm
- Material: brass or stainless steel
- Inner diameter: product outer diameter +0.2–0.5 mm
- Vacuum level: -0.06–-0.08 MPa
- Sizing accuracy
- Outer diameter tolerance: ±0.5 mm
- Roundness: ≤1 mm
- Cooling System
- First-stage cooling in the vacuum sizing sleeve
- Cooling water tanks (3–4 stages)
- Water temperature gradient: first stage 20–30 °C, subsequent stages at ambient temperature
- Spray or immersion cooling
- Sufficient cooling to prevent pipe deformation
- Haul-off and Cutting
- Haul-off machine
- Crawler-type haul-off
- Haul-off speed: 0.5–3 m/min
- Synchronized with the extrusion speed
- Ensures uniform wall thickness
- Cutting machine
- Planetary saw cutting
- The pipe continues moving during cutting
- Cut face perpendicular and flat
- Fixed length: 1.5 m
VII. Process Parameter Control
| Parameter |
Control range |
| Barrel temperature |
180–220 °C |
| Die head temperature |
190–210 °C |
| Screw speed |
20–50 rpm |
| Haul-off speed |
0.5–3 m/min |
| Vacuum level |
-0.06–-0.08 MPa |
| Cooling water temperature |
20–30 °C |
VIII. Impact on Process Quality
- Temperature effect
- Too low: poor plasticating, rough surface
- Too high: material degradation, reduced performance
- Longkang strictly controls temperature to ±2 °C
- Haul-off speed effect
- Too fast: wall thickness thins, longitudinal drawdown
- Too slow: wall thickness too thick, high internal stress
- Precisely matched to the extrusion output
- Vacuum level effect
- Too high: pipe deformation
- Too low: out-of-round outer diameter
- Precise control of the vacuum level
- Longkang Extrusion Advantages
- Variable-frequency extrusion, stable process
- Vacuum sizing, precise dimensions
- Uniform wall thickness, tolerance ≤0.5 mm
- Smooth appearance, no bubbles
- Consistent performance across batches
