5 Common DWC Pipe Production Defects: Causes and Quick Solutions

Stable double wall corrugated pipe production depends on the extrusion system, die head, corrugator, cooling circuit and haul-off working as one process. When a visible defect appears, changing several settings at once usually makes diagnosis harder. A better method is to identify the symptom, check the most likely cause and make one controlled correction at a time.

This guide covers five defects frequently encountered on HDPE double wall corrugated pipe lines. The values below are practical starting points, not universal recipes. Always confirm the allowable processing window for the resin grade, pipe diameter, wall distribution and required SN class before changing production parameters.

Quick Diagnostic Order

Before stopping the line for a major adjustment, check the process in this order:

1. Raw material condition, formulation and contamination.
2. Actual melt temperatures, extrusion pressure and output stability.
3. Die gap, die-to-mould alignment and forming-air stability.
4. Mould-block cleanliness, wear and corrugator movement.
5. Cooling uniformity, haul-off alignment and downstream support.

1. Rough or Uneven Outer Wall

Typical symptoms: longitudinal marks, pitting, an orange-peel texture or incomplete reproduction of the corrugation profile.

Likely Causes and Corrective Actions

1. The melt freezes before fully contacting the mould cavity. Check the actual die-head and melt temperatures rather than relying only on the controller setpoints. Increase the relevant temperature in small steps and remove excessive early-stage cooling that is chilling the mould blocks.
2. Deposits or carbonized resin remain in the mould grooves. Stop the machine safely and clean each groove with a non-damaging tool such as a copper brush. Inspect the mould surface for scratches, dents and accumulated residue.
3. Melt fluidity at the die exit is too low. If permitted by the resin supplier, raise the die-head temperature by approximately 5-10°C and observe the surface before making another change. Many PE formulations run in a broad range around 190-210°C, but the correct value depends on grade, output and residence time.
4. The formulation is too stiff or crystallizes too quickly. Review the resin blend with the material supplier. A controlled amount of LLDPE may improve melt extensibility and cavity replication, but it must be validated against ring stiffness, impact performance and product standards.

2. Inner-Wall Blisters, Bubbles or Burn Marks

Typical symptoms: raised bubbles, voids, pinholes, local delamination or dark degraded spots on the smooth inner layer.

Likely Causes and Corrective Actions

1. Moisture or volatile contamination is entering with the material. Virgin HDPE is not strongly hygroscopic, but condensation, wet regrind, additives and surface contamination can still create gas. Check storage conditions and dry affected material according to the supplier’s recommendation before reuse.
2. The inner-layer melt is overheating or remaining in the barrel too long. Reduce the affected barrel or die zone by 5-10°C, check for dead zones and confirm that output is high enough to avoid excessive residence time. Do not allow the material to remain at degradation temperature during prolonged stops.
3. Interlayer forming air is too high or unstable. Reduce the pressure gradually and inspect the regulator, valve and air passage. A stable low pressure is more useful than a high pressure that locally separates the inner and outer layers.
4. Regrind contains incompatible material or foreign particles. Improve sorting, clean the regrind system and replace the screen pack with the mesh specified for the current output. Record the pressure before and after the screen to detect blockage.

3. Uneven Wall Thickness

Typical symptoms: circumferential thickness variation, an eccentric inner wall or periodic thick-and-thin bands along the pipe.

Likely Causes and Corrective Actions

1. The co-extrusion die gap is not uniform. Measure the gap at 0°, 90°, 180° and 270° with the appropriate gauge, then make small symmetrical adjustments. A 0.2 mm setup tolerance can be a useful initial reference, but the acceptable value depends on die size and pipe specification.
2. The corrugator or mould-block train is vibrating. Inspect guides, bearings, drive components and the foundation. Remove mechanical play before compensating with extrusion settings.
3. The die head and mould-block centreline are not aligned. Re-establish the mechanical centreline from the die exit through the forming tunnel. Confirm alignment under operating temperature because thermal expansion can shift the hot die.
4. Extruder output or forming pressure is fluctuating. Check screw speed, melt pressure, feeding stability and pressure-control components. Wall thickness cannot remain stable if melt delivery is pulsing.

4. Ring Stiffness Below the Required Class

Typical symptoms: SN4, SN8 or SN16 test results fall below the specified value even though the pipe looks acceptable.

Likely Causes and Corrective Actions

1. The corrugation profile is too shallow because the mould blocks are worn or not fully filled. Compare the actual profile with the approved drawing. Measure mould wear and replace or repair blocks that can no longer form the required rib geometry.
2. Wall mass is too low or distributed incorrectly. Verify pipe weight per metre, inner-wall thickness and material allocation to the corrugated outer layer. Increasing total output is not enough if the additional material does not reach the structural ribs.
3. The formulation has insufficient modulus. Check resin grade, filler dispersion and recycled-material consistency. Set the regrind percentage according to the product standard and verified test data rather than using one fixed limit for every diameter.
4. Cooling does not allow a stable geometry and material structure. Check water temperature, flow and residence time together. Over-aggressive cooling can freeze the surface before complete forming, while insufficient cooling can deform the profile downstream. Optimize the circuit against measured pipe temperature and ring-stiffness results.

5. Pipe Bending or Poor Straightness

Typical symptoms: the pipe curves after leaving the forming section or deviates from the line centre during haul-off.

Likely Causes and Corrective Actions

1. Cooling spray is uneven around the pipe. Clean blocked nozzles and verify comparable flow on all sides. Uneven shrinkage is one of the most common causes of bending.
2. Haul-off pressure differs from side to side. Equalize the pneumatic or hydraulic settings and inspect belts or tracks for asymmetric wear and contamination.
3. The corrugator outlet, cooling section and haul-off are not on one centreline. Use a laser or mechanical reference to realign the downstream equipment. For many lines, a deviation within approximately 2 mm is a practical starting target, subject to machine size.
4. A one-sided heat source or cold draft affects the pipe. Shield the line from direct sunlight, open doors and local heaters, and stabilize airflow around the cooling and haul-off sections.
5. The pipe already has eccentric wall thickness. Correct the die gap, output stability and centre alignment described in Section 3 before trying to compensate at the haul-off.

Five-Minute Shift-Start Checklist

• Confirm that actual temperatures are stable and close to the approved setpoints.
• Check screen-pack service records and melt-pressure trend.
• Confirm that cooling nozzles are clear and water flow is balanced.
• Inspect mould blocks and haul-off surfaces for resin deposits.
• Listen for abnormal vacuum-pump, bearing or drive noise.
• Check raw material for contamination, condensation and incorrect blending.
• Record one good pipe sample and key process values for comparison during the shift.

Use Data, Not Repeated Guesswork

For each correction, record the original parameter, the change, the time and the measured result. Wait long enough for the new material and temperature condition to reach the inspection point before judging the effect. This simple discipline prevents operators from chasing a defect with multiple conflicting adjustments.

For equipment configurations, see our high-speed water-cooled DWC pipe line, air-cooled aluminium mould corrugator and steel mould corrugated pipe machine. If a defect remains after systematic checks, contact the Wings Plastic technical team with pipe photos, material grade, diameter, line speed and current process settings.