01 What is Water Ripple?
Typically, the surface of freshly extruded film should be smooth. However, sometimes longitudinal wavelike patterns (usually parallel to the direction of traction) similar to rippling water may appear on the surface or inside the film. This phenomenon is known as “water ripple” in the industry. Water ripple affects the appearance, reducing the product grade; in terms of performance, it indicates uneven film thickness and localized degradation of physical properties (such as tensile strength and barrier properties). In short, water ripple has a significant impact on product quality. This article will briefly analyze the causes of water ripple and the solutions when it occurs.
02 Causes of Water Ripple
(1) Unstable Melt Pressure
Uneven Plasticization: Screw wear (especially in the compression and metering sections), improper screw design (inappropriate compression ratio), or unstable screw speed lead to uneven mixing of molten plastic with large viscosity differences.
Feeding Interference: Poor hopper discharge (bridging, agglomeration), fluctuations in the feeding screw, excessive proportion of recycled materials, or large differences in particle size/density result in unstable raw material input.
Temperature “Roller Coaster”: Failure of heating coils, poor accuracy of temperature control systems, improper thermocouple positions cause severe fluctuations in barrel or die temperature, and thus the melt viscosity.
Back Pressure “Asthma”: Improper adjustment or instability of the back pressure valve, or severe filter blockage with delayed replacement, cause the melt pressure to fluctuate.
(2) Uneven Flow in the Die
“Traffic Jam” or “Vacancy”: Improper adjustment of die lip gap (locally too wide or too narrow), damaged die lips, degraded carbon deposits or impurity blockages inside the die cavity, which disrupt the uniform distribution of melt at the die exit.
“Temperature Difference” Troubles: Uneven temperature control of the die leads to significant differences in melt viscosity in different regions and inconsistent outflow speeds.
(3) Uneven/Unstable Cooling
Air Ring “Eccentricity”: The air outlet of the air ring is not concentric with the bubble, the internal deflector of the air ring is damaged or poorly designed, resulting in uneven cooling air impact on the bubble.
Airflow “Madness”: Fluctuations in fan speed, leakage or unreasonable design of air supply pipelines, interference from workshop airflow (doors, windows, fans) cause unstable cooling air pressure/volume.
Water Temperature “Drift”: Unstable water temperature control or poor circulation of the internal cooling system (IBC) affects the uniformity of internal cooling of the bubble.
(4) Mechanical Vibration and Traction Slippage
“Equipment Vibration”: Unstable equipment foundation, severe wear of transmission components (such as gearboxes, traction roller bearings), poor dynamic balance of rotating components (such as winding), causing abnormal vibration of the entire production line.
Traction “Slippage”: Uneven pressure of traction rollers, aging and hardening of rubber rollers or surface contamination, improper speed ratio setting, causing the film to slide or have uneven stress during traction.
(5) Raw Material Issues
“Hybrid” Conflict: Mixing of raw materials from different batches or grades with large differences in rheological properties such as melt flow index (MFI) and viscosity.
“Physical” Decline: Unstable melt flow index of the raw material itself, excessive moisture content (insufficient drying), uneven dispersion or precipitation of additives (such as slip agents), use of excessively degraded materials.
03 Solutions
(1) Eliminate Explicit Faults
Observation: Carefully observe the starting position of water ripple (near the die? Traction section?), form (dense? Sparse? Regular?), and accompanying phenomena (such as bubble shaking?).
Inspection: Immediately check hopper discharge, filter pressure difference (is it time to replace?), temperature display of each section (whether there are abnormal fluctuations?), cooling air ring position and airflow (can be observed with a smoke pen), traction roller pressure and surface condition, and whether the equipment has obvious abnormal vibration or noise.
Adjustment:
Stabilize Cooling: Prioritize checking and adjusting the air ring position (ensure concentricity), stabilize air pressure/volume (check fans, valves, pipelines). Stabilize IBC water temperature and flow.
Optimize Traction: Check and adjust the uniformity of traction roller pressure, clean the surface of traction rollers, and confirm that the speed ratio is reasonable (avoid slipping or excessive stretching).
Basic Parameters: Ensure that the temperature settings of each section of the extruder are reasonable and the temperature control is stable. Check and stabilize the screw speed. Ensure that the raw materials are fully dried (especially hygroscopic materials such as PA and PET).
(2) In-depth Investigation and Process Optimization (Solve Potential Problems)
Plasticization and Homogenization:
Evaluate the Screw: Check the screw wear (especially the metering section), evaluate whether the screw design (compression ratio, length-diameter ratio) matches the current raw materials and processes. Repair or replace if necessary.
Optimize Parameters: Adjust the back pressure to a stable and appropriate level (increase homogenization effect, but avoid excessive degradation). Optimize the matching of screw speed and feeding speed to avoid frequent and large adjustments of the main machine speed.
Manage Raw Materials: Strictly control the consistency and stability of raw material batches. Optimize the proportion, particle size and cleanliness of recycled materials, and ensure uniform dispersion and no precipitation of additives.
Die Fine Adjustment:
Cleaning and Maintenance: Regularly (according to plan or when fluctuations occur) disassemble and clean the die to remove carbon deposits and impurities. Check and repair die lip damage.
Precise Adjustment: Use a special thickness gauge for guidance, and extremely finely adjust the die lip gap (usually micron-level adjustment) to pursue uniform circumferential discharge. Ensure that the temperature in each area of the die is uniform and stable (check heating coils and thermocouples).
Equipment Status Improvement:
Vibration Reduction and Reinforcement: Check and reinforce the equipment foundation, replace worn bearings, drive belts and other components, and perform dynamic balance correction on rotating components.
Maintain Air Ring/IBC: Clean the inside of the air ring and check whether the deflector is intact. Maintain the IBC system to ensure smooth water circulation and precise temperature control.
(3) Parameter Linkage and Fine-tuning (Synergistic Effect)
Understand the interaction between parameters (for example, increasing the extrusion speed may require simultaneous increase in cooling intensity and adjustment of the traction ratio).
When making fine-tuning, follow the principle of “single variable, gradual fine-tuning, and observation of effects”.
Pay attention to the stability of melt pressure, as it is a key indicator reflecting the comprehensive status of plasticization, feeding, back pressure, etc.
04 Summary
Water ripple is an external manifestation of uneven melt flow or cooling during the stretch film blowing process, which seriously affects the appearance quality of products. The solution lies in systematically investigating raw materials, equipment and process parameters, then accurately intervening (prioritizing stabilizing cooling and traction, then in-depth optimizing plasticization and die), finely adjusting (die lip gap is the key), and deeply understanding the synergistic effect between parameters. In short, only by maintaining good equipment maintenance status and stable control of process parameters can water ripple be prevented.
Post time: Jul-18-2025