Corrugating Medium vs Linerboard:

What’s the Difference?

Introduction

In the competitive world of packaging, understanding the quality and material properties of different components is essential for producing high-quality, cost-effective products. This guide aims to provide machine operators and managers with a clear and comprehensive understanding of the distinct characteristics and applications of corrugating medium and linerboard. By grasping these concepts, you can ensure that your packaging solutions meet the highest standards of performance and durability, ultimately giving your products a competitive edge in the market.

Corrugating medium and linerboard are the two primary materials used in the production of corrugated cardboard, each playing a unique role in the final product’s performance. The corrugating medium, typically made from semi-chemical hardwood pulp, is designed to form the fluted layer that provides structural strength and cushioning. On the other hand, linerboard, which can be made from either virgin Kraft fibres or recycled fibres, serves as the flat outer layers that protect and enhance the appearance of the packaging.

Understanding the differences in material composition, manufacturing processes, and key physical properties of these components is crucial for optimizing the production process and achieving the desired quality in your packaging products. This guide will delve into the specifics of each material, offering insights into their respective advantages and applications, and providing practical tips for ensuring consistent quality and performance.

By the end of this guide, you will have a thorough understanding of the essential properties of corrugating medium and linerboard, enabling you to make informed decisions and effectively communicate the importance of these materials to your team. Whether you are a machine operator looking to optimize production efficiency or a manager aiming to improve product quality, this guide will equip you with the knowledge needed to excel in the packaging industry.

Material Composition

Note: The following section provides the equivalent metric and imperial units if possible.

                MSF = thousand square feet,    and g/m² = grams per square metre,

Corrugating Medium

The corrugating medium is primarily composed of semi-chemical hardwood pulp, which provides the necessary compressibility and fluting formation. The typical basis weights for standard applications range from 26-36 lb/msf (127-176 g/m²), while lightweight solutions use 20-23 lb/msf (98-112 g/m²). The high proportion of short fibres in the corrugating medium enhances its compressibility, making it ideal for forming stable flutes.

Linerboard

Linerboard, on the other hand, can be made from either virgin Kraft fibres or recycled fibres. Premium grades use Kraft fibres, while standard grades use recycled fibres. The typical basis weights for linerboard range from 35-90 lb/msf (171-439 g/m²), with the most common weights being 42 lb/msf (205 g/m²) and 69 lb/msf (337 g/m²). The long fibres in linerboard provide superior strength, dense formation for improved printability, and high burst and tear resistance.

Key Physical Properties

Corrugating Medium Properties

• Concora Medium Test (CMT) value: Typically ranges from 50-65 lb for 26 lb/msf, which is critical for flute formation and structural integrity.
• Ring Crush Test (RCT): Standard requirement is 35-45 lb for 26 lb/msf.
• Moisture content: Optimal range is 7.0-7.5%.
• Porosity: Higher porosity allows better adhesive penetration.

Linerboard Properties

• Mullen Burst Test: Minimum requirement is 100 psi for 42 lb/msf.
• Edge Crush Test (ECT) contribution: Typically accounts for 60-70% of the total ECT value.
• Moisture content: Optimal range is 6.5-7.0%.
• Smoothness: Crucial for print quality, measured using Bendtsen or Sheffield tests.

Manufacturing Process Differences

Corrugating Medium Production

The corrugating medium is produced using the neutral sulphite semi-chemical (NSSC) pulping process, which has a higher yield (75-85%) compared to the Kraft process. The refining specifications are more intensive to improve compressibility, and the formation requirements ensure even distribution of short fibres and controlled porosity for optimal performance.

Linerboard Production

Linerboard production involves the Kraft process for virgin grades and recycled fibre processing for standard grades. The refining approach is less intensive to maintain fibre length, focusing on strength development. Surface treatments such as calendar finishing for smoothness and potential sizing for water resistance are also applied.

Performance in Converting pert

Corrugating Medium Behaviour

• Fluting characteristics: Forms stable flutes under heat and pressure, maintaining flute caliper after cooling.
• Adhesion properties: Rapid adhesive penetration forms strong fibre-to-fibre bonds.
• Running speed impact: Optimal performance at 600-1000 ft/min, with potential speed adjustments based on basis weight.

Linerboard Performance

• Tensile strength: Machine direction (MD) is 30-40 lb/in for 42 lb/msf, and cross direction (CD) is 15-20 lb/in for 42 lb/msf.
• Bonding characteristics: Forms strong surface bonds with the medium, critical for box performance.
• Converting considerations: Higher tensile strength allows faster running speeds and better dimensional stability.

Quality Control Considerations

Testing Protocols for Medium

• Regular CMT testing: Conducted every 2 hours during production to ensure consistent performance.
• Moisture monitoring: Continuous online measurement with adjustments to prevent runnability issues.

Linerboard Quality Assurance

• Burst testing: Standard requirement for customer specifications.
• Print quality assessment: Includes smoothness testing and ink absorption evaluation.

Common Issues and Troubleshooting

Medium-Related Problems

• Poor flute formation: Caused by inadequate CMT values, which can be resolved by adjusting refining or basis weight.
• Fracturing during converting: Caused by low moisture content, which can be resolved by steam conditioning before corrugating.

Linerboard Challenges

• Delamination: Caused by poor internal bonding, which can be resolved by reviewing wet-end chemistry.
• Print quality issues: Caused by surface roughness, which can be resolved by evaluating calendar stack pressure.

Industry Standards and Specifications

Medium Grades

• TAPPI standards: For CMT and RCT.
• ISO specifications: For testing methods.
• Common industry-specific requirements: For medium grades.

Linerboard Classifications

• Kraft linerboard specifications: For premium grades.
• Recycled linerboard standards: For standard grades.
• High-performance grade requirements: For specific applications.

Cost Considerations and Economic Factors

• Raw material pricing trends: Impact on final board cost.
• Performance vs. cost trade-offs: Considerations for selecting materials.

Future Trends

• Lightweight grades development: For cost and material efficiency.
• Sustainable sourcing initiatives: For environmental responsibility.
• Enhanced performance characteristics: For improved product quality.

Conclusion

Understanding the fundamental differences between corrugating medium and linerboard is essential for optimizing corrugated board production. Each component plays a unique and critical role in the final product’s performance and selecting the right specifications for both medium and linerboard ensures the production of high-quality corrugated packaging solutions.