Publication date: 2025-03-25
Introduction
The stability of the HDPE/LDPE film extrusion process plays a crucial role in maintaining product quality, reducing material waste, and improving operational efficiency. The introduction of LDPE regranulate into the production mix provides economic and environmental benefits, but also presents technological challenges, including melt flow irregularities, pressure fluctuations, film bubble instability, and surface defects.
This article analyzes the key factors affecting process stability, discusses common issues, and presents proven technological solutions to minimize disruptions in HDPE/LDPE film production with regranulate.
Key Challenges in Processing LDPE Regranulate
Adding LDPE regranulate to HDPE can cause processing irregularities due to differences in the flow properties of both materials. The most common issues include:
Film Bubble Instability
In blown film extrusion, bubble stability is critical for achieving uniform thickness and mechanical consistency. Instability appears as vibrations, deformations, or asymmetrical bubble shapes, leading to thickness variations and surface quality issues.
Causes of the problem:
- Variability in melt viscosity – LDPE regranulate may have a lower or inconsistent viscosity compared to virgin HDPE.
- Excessive LDPE content – higher LDPE flexibility reduces the bubble's resistance to deformation.
- Improper cooling parameters – too rapid or uneven cooling can lead to distortions.
Solutions:
- Optimize processing temperature – too low a temperature can cause melt instability, while too high can increase flowability and reduce bubble control.
- Adjust cooling intensity – fine-tuning air ring settings for optimal airflow direction and intensity.
- Ensure homogeneous material blending – proper mixing of HDPE and LDPE for consistent melt viscosity.
Pressure Fluctuations and Extrusion Instability
Melt pressure fluctuations during extrusion can lead to irregular material flow through the die, causing thickness inconsistencies and surface defects.
Causes of the problem:
- Non-uniform regranulate quality – variations in MFI and contamination levels can cause inconsistent melt flow.
- Improper filtration – mechanical impurities (poorly processed regranulate) can clog filters and cause pressure fluctuations.
- Incorrect process settings – too high a screw speed can create sudden pressure changes.
Solutions:
- Use proper melt filtration – fine filters (<100 μm) help remove impurities and stabilize pressure.
- Monitor regranulate MFI – selecting material with consistent flow index reduces viscosity fluctuations.
- Optimize screw speed and die pressure – avoiding excessive acceleration or pressure drops for a stable melt flow.
Surface Defects (Roughness, "Fish Eyes," Microbubbles)
Impurities in regranulate, significant differences in MFI between HDPE and LDPE, and cooling inconsistencies can lead to surface imperfections in the film.
Causes of the problem:
- Moisture in regranulate – while LDPE is less hygroscopic than other polymers, improperly stored regranulate can contain residual moisture.
- MFI mismatch between HDPE and LDPE – extreme MFI differences can lead to inclusions and fish-eye defects.
- Excessive extrusion speed – can cause microbubbles and uneven melt distribution.
Solutions:
- Proper regranulate drying – using polymer dryers prevents moisture-related defects.
- Blend HDPE and LDPE with similar MFI – avoiding extreme viscosity differences for a uniform melt.
- Adjust extrusion speed – slowing down may improve surface smoothness and eliminate defects.
Conclusion
The stability of HDPE/LDPE film extrusion with regranulate depends on precise control of process parameters. The most common challenges can be minimized by:
- Optimizing processing temperature – ensuring uniform melt viscosity.
- Effective melt filtration and homogenization – removing impurities and stabilizing pressure.
- Proper cooling adjustments – achieving even cooling of the film bubble.
- Monitoring regranulate quality – controlling MFI and moisture content to prevent flow inconsistencies.
By implementing these solutions, manufacturers can increase the proportion of regranulate in film production while improving process stability and product quality.
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