Analyzing the Causes of Insufficient Hardness in Corrugated Board from Semi-Automatic Production Lines

Analyzing the Causes of Insufficient Hardness in Corrugated Board from Semi-Automatic Production Lines

In the corrugated packaging industry, the hardness (or rigidity) of the board is a critical property directly impacting the performance and stacking strength of the final boxes. While semi-automatic production lines offer a cost-effective solution, they are particularly prone to producing board that feels soft, flexible, and lacks the necessary stiffness. This issue is rarely due to a single fault but is often a cumulative result of several interrelated factors across four key areas:Raw Material, Production Process, Equipment Condition, and Environmental Management.

1. Raw Material Issues

The quality of the input materials sets the fundamental ceiling for the board's potential hardness.

Insufficient Basis Weight:Using facing paper or corrugating medium (fluting) that is too light simply means there is less fibrous material to provide structural integrity. The board lacks a solid foundation.

Poor Paper Grade:Low-quality paper with short fibers, high recycled content without proper reinforcement, or low Ring Crush Test (RCT) and Short Span Compression Test (SCT) values will result in a weak substrate. The fluting medium, which acts as the "I-beam" of the board, is especially critical; if it is weak, the entire structure will be compromised.

Improper Moisture Content:This is a primary culprit.

Excessively High Moisture:Overly damp paper becomes soft and pliable. The hydrogen bonds between fibers weaken, drastically reducing rigidity. Paper that enters the production line with high moisture content is a recipe for a soft board.

Excessively Low Moisture:Brittle paper can fracture its fibers during the corrugating process, creating micro-failures that undermine strength.

Poor Storage Conditions:Raw paper rolls stored in damp environments will absorb ambient moisture, leading to the "high moisture" problem before production even begins.

2. Production Process Parameters

Semi-automatic lines heavily rely on operator skill. Incorrect machine settings are a direct path to poor board hardness.

The Adhesive Bonding Process:

Excessive Starch Application:Too much adhesive penetrates and saturates the fluting tips, causing them to collapse and lose their arch shape. This destroys the core "I-beam" structure and introduces excess water into the board, making it soft.

Insufficient Starch Application:Too little adhesive creates a weak bond between the liners and the fluting. The layers can delaminate under pressure, and the board fails to act as a cohesive, rigid unit.

Faulty Starch Recipe:An improper balance of starch, caustic soda (alkali), and water can lead to incorrect viscosity. Low viscosity causes over-penetration, while high viscosity causes poor bonding and "skip glue."

The Corrugating Forming Process:

Incorrect Nip Pressure:The pressure between the corrugating roll and the pressure roll is critical.

Excessive Pressure:This crushes the fluting tips, flattening the corrugations and destroying their load-bearing capability.

Insufficient Pressure:Results in poorly formed, shallow flutes that do not bond properly to the liners, creating a weak structure.

Insufficient Heat on Corrugating Rolls:Heat is essential for proper flute formation and initial starch gelatinization. Low roll temperature leads to poor flute shape and weak initial bond, contributing to a soft board.

Drying and Curing:

Speed & Heat Mismatch: If the line speed is too high for the temperature settings of the hot plates, the water in the starch does not fully evaporate. The board exits the line with high residual moisture, feeling warm and soft.

Insufficient Cooling:The board exiting the dryer is hot and plastic. The fibers are flexible, and the starch is not fully set. Without adequate cooling (via air fans and natural cooling on the conveyor), the board does not achieve its final cured rigidity. Semi-automatic lines often have limited cooling sections.

3. Equipment Condition

The mechanical state of a semi-automatic line is a major factor.

Worn Corrugating Rolls:This is a common and severe issue. Worn rolls cannot form a precise, consistent flute profile. The resulting poorly shaped flutes have significantly reduced compression strength.

Uneven or Warped Hot Plates:If the heating plates are not level or have temperature inconsistencies, the board will be unevenly dried and cured, leading to soft spots and warping.

Misaligned or Worn Pressure Rolls:These cause inconsistent pressure across the web width, leading to uneven bonding and variable board quality.

4. Environmental & Post-Production Handling

High Ambient Humidity: A humid production or storage environment allows the finished corrugated board to re-absorb moisture from the air, a phenomenon known as "regain," which rapidly decreases its hardness.

Improper Stacking and Handling:

Stacking Too High: The weight of the stack can crush the bottom sheets, permanently deforming the flutes and reducing their hardness.

Uneven Stacking: Causes the board to warp and bend, putting stress on the flutes and compromising their structural integrity.

Conclusion and Recommended Action Plan

Troubleshooting soft board on a semi-automatic line requires a systematic approach. Begin with the simplest and most common causes:

Conduct a "Touch and Listen" Test: Feel the board for dampness and coolness. Try to flex it; a rigid board will make a sharp cracking sound, while a soft board will produce a dull, muffled sound.

Check the Glue Line: Peel the layers apart. A good bond will tear the paper fibers. A poor bond will show clean, easy separation.

Audit Raw Materials: Verify the specification and storage conditions of the current paper rolls.

Inspect the Machine: Focus on the corrugating rolls for wear and check the temperature/pressure settings at each stage.

By methodically investigating these areas—Material, Process, Machine, and Environment—operators of semi-automatic lines can identify the root causes of insufficient hardness and implement precise corrective actions to produce a stiffer, stronger, and more reliable corrugated board.