Polyolefin Pipe Production

Common Problems and Solutions in Polyolefin Pipe Production

With the wide application of polyolefin pipes such as HDPE and PP in municipal engineering, water supply and drainage, gas pipelines, and agricultural irrigation systems, the stability of pipe quality during production has become a major concern in the industry. In order to help manufacturers improve production efficiency and reduce rejection rates, the following is a systematic summary of common problems, causes, and solutions in the extrusion process of polyolefin pipes for industry reference.

Polyolefin melts have viscoelastic properties. During processing, two common phenomena often occur: die swell and melt fracture. These are not discussed in detail here. Below are the common abnormalities in pipe production, their causes, and recommended solutions:

Common Problem: Dull and mat surface
Causes:

1. Moisture in raw materials

2. Improper melt temperature

3. Uneven melting and plasticizing from the extruder

4. Sizing sleeve too short

5. Die forming section too short

Recommended Solutions:

1. Pre-dry and pre-treat raw materials

2. Adjust processing temperature

3. Increase back pressure, use finer screen packs, and optimize screw structure design

4. Extend the length of the sizing sleeve

5. Extend the die forming section

Common Problem: Surface spots
Causes:

1. Moisture in raw materials

2. Air bubbles attached to the pipe surface in the water tank

Recommended Solutions:

1. Dry raw materials thoroughly

2. Remove air bubbles and adjust processing temperature

Common Problem: Bright transparent lumps on outer surface (commonly called “fish eyes”)
Causes:

1. Excessively high die head temperature

2. Insufficient or uneven cooling water

Recommended Solutions:

1. Lower the die head temperature

2. Increase cooling water flow or clean the sizing sleeve water channels

Common Problem: Regular streaks on smooth outer surface
Cause:
Pipe tends to stick to the sizing sleeve

Recommended Solutions:
Increase cooling water flow, clean water lines, or reduce line speed

Common Problem: Deep waves on outer surface
Cause:
Misalignment between sizing sleeve and die

Recommended Solutions:
Align the center and keep the vacuum sizing box and die on the same axis

Common Problem: Rough inner surface
Causes:

1. Moist raw material

2. Low mandrel temperature

3. Excessive gap between die and mandrel

4. Too short die calibration section

Recommended Solutions:

1. Dry or pre-treat raw materials

2. Increase temperature or extend holding time

3. Replace the mandrel

4. Use a die with a longer calibration section

Common Problem: Corrugated inner wall
Causes:

1. Unstable extruder output or unstable feeding

2. Haul-off slippage

3. Uneven cooling

Recommended Solutions:

1. Lower the temperature of the screw feeding zone

2. Adjust haul-off pressure

3. Adjust cooling water flow

Common Problem: Pits on inner wall
Causes:

1. High moisture content in raw materials

2. Poor dispersion of fillers or impurities

Recommended Solutions:

1. Preheat and dry the raw materials

2. Change materials, adjust temperature, and clean raw materials

Common Problem: Burnt particles on inner wall
Causes:

1. Dirty inner surface of die head and die

2. Local overheating

3. Serious material accumulation inside the die

Recommended Solutions:

1. Clean the die

2. Check whether thermocouples work normally

3. Clean the die and reduce die temperature appropriately

Common Problem: Fluctuation of outer diameter or wall thickness
Causes:

1. Changes in extrusion speed

2. Changes or slippage of haul-off speed

3. Unstable feeding (uneven regrind particle size)

4. Melt instability

5. Uneven cooling

Recommended Solutions:

1. Check haul-off unit

2. Increase pressure appropriately

3. Sieve or re-pelletize raw materials

4. Increase material temperature, reduce line speed, and increase die gap

5. Clean and optimize water channels

Common Problem: Uneven wall thickness
Causes:

1. Die not centered

2. Uneven die temperature

3. Misalignment among haul-off, sizing sleeve, and die

4. Excessive distance between sizing sleeve and die

Recommended Solutions:

1. Adjust die concentricity

2. Balance die temperature

3. Keep all units on the same axis

4. Reduce the distance

Common Problem: Poor weld seam
Causes:

1. Die forming section too short

2. Low melt temperature

3. Poor plastic dispersion in the die head

4. Unreasonable die head structure

Recommended Solutions:

1. Use a longer die forming section

2. Increase melt temperature

3. Clean the die head

4. Replace or modify the die head

Common Problem: Premature pipe failure – pinholes
Causes:

1. Water bubbles

2. Air bubbles

3. Impurities

4. Poor dispersion of pigments or fillers

Recommended Solutions:

1. Dry raw materials

2. Dehumidify or reduce processing temperature

3. Clean raw materials or use screen filters

4. Adjust temperature or change raw material

Common Problem: Premature pipe failure – brittle fracture
Causes:

1. Low material temperature

2. Excessively high temperature causing material degradation

Recommended Solutions:

1. Increase material temperature

2. Clean the die and reduce processing temperature

Common Problem: Pipe cracking
Causes:

1. Low die head temperature and high extrusion speed

2. Excessive cooling water

Recommended Solutions:

1. Increase temperature and reduce speed

2. Reduce cooling water flow

Common Problem: Poor roundness and pipe bending
Causes:

1. Die and mandrel not concentric

2. Uneven circumferential temperature of die head

3. Cooling water too close to the die

4. Excessive spray force

5. Insufficient spray volume

6. Water level too high

7. Excessive haul-off pressure

Recommended Solutions:

1. Adjust concentricity

2. Balance die head temperature

3. Adjust the position of cooling water

4. Adjust spray nozzle angle

5. Clean water channels

6. Control water level

7. Adjust haul-off pressure