Duplex boards, primarily used in carton manufacturing, significantly affect customer perception through their quality, printing, and overall appearance. Therefore, strict quality control is crucial at both the board mill and the printer-converter stages. This ensures the desired quality and aesthetics of the final carton.

While some argue that it's impossible to achieve ideal duplex board qualities using unconventional, short-fibered pulpable materials, it's possible to produce high-quality boards by implementing proper care, precautions, and control measures. This article marks the beginning of a three-part series that explores common defects in duplex boards and outlines potential control methods to minimize their occurrence and ensure optimal board quality

Stay tuned for the following parts, where we'll explore the additional defects and their preventive measures.

List of Common Defects in Duplex Board

Curl and waviness
Variation in Caliper and G.S.M
Poor stiffness
Poor creasing quality
Poor smoothness and gloss
Burn mark and streaks
High or low bulk density
Fluff
High or low ink absorbency
Low brightness
Delamination
Shade variation
Poor Physical strength properties
Poor Glueability
Poor vamishability
Colour reversion
Poor waxing anG high wax absorbency
Poor dimension and stability

Implementing proactive measures and maintaining rigorous control throughout the entire production chain, including pulping, bleaching, beating, and paper machine operations, can significantly reduce the occurrence of these defects.

This article, the first in a three-part series, explores the listed initial six commonly encountered defects in duplex board production, delving into their causes and potential solutions.

1) CURL

Curling and waviness arise from **uneven drying** during the production process. Here are key strategies to tackle them, incorporating insights from the provided info:

Moisture Management:
- Aim for a uniform distribution of moisture throughout the board, ideally at 40% or more for the top liner. This ensures proper flexibility during drying and minimizes warping.
- Maintain a uniform moisture content of 4.5-7.5% after drying. This minimizes moisture gain or loss upon exposure to the atmosphere, preventing imbalances that contribute to curling and waviness.
- Since the top and back liner have different compositions (bleached virgin pulp vs. recycled and ground wood pulp), their contraction and expansion properties differ. This can lead to uneven drying and warping. For optimal drying efficiency and uniform temperature in the dryer, efficient condensate removal is crucial. In M.G. dryers specifically, precise control of both the pre-dryer temperature and the moisture content on both the top and back sides of the board is essential. This ensures the board exits the dryer with minimal moisture variation between its surfaces, preventing uneven drying and warping.
Machine Control and Optimization:
- Ensure proper temperature control throughout the drying process, especially in the pre-dryer section of M.G. dryers. This minimizes uneven drying and warping.
- Maintain efficient condensate removal from dryers to ensure maximum drying efficiency and uniform temperature distribution.
- When using M.G. cylinder, Apply adequate pressure from the M.G. press roll to ensure good contact between the board and the cylinder, promoting even and smooth drying.
Additional Measures:
- Uniform thickness and grammage (G.S.M.): Maintain consistent thickness and grammage across the board to prevent imbalances that can cause waviness.
- Well-polished M.G. dryer surface: Minimize friction and allow for smoother board passage by keeping the M.G. dryer surface well-polished.

2) Variation in Caliper and G.S.M

Inconsistent thickness (caliper) and weight (grammage per unit area, or G.S.M.) are common problems in duplex board production. Here are some approaches to address these variations:

Material and Consistency Management:
- Maintain consistent pulp consistency (concentration) in the headbox, ensuring even fiber distribution across the board web.
- Regularly clean the wire mesh of the cylinder mould and paper machine using high-pressure oscillating water showers. This minimizes fiber build-up and promotes uniform board formation.
- Maintain consistent moisture content throughout the board during production, as uneven moisture can lead to warping and thickness variations during drying.
Machine Control and Optimization:
- Precisely control the basis weight (weight per unit area) of each individual board layer during formation. This ensures consistent overall board thickness and grammage.
- Maintain proper crowning (shape) of press rolls and cylinders, as well as M.G. dryer press rolls. This ensures uniform pressure across the board web during pressing and calendering, minimizing thickness variations. Additionally, adjust calendering settings (pressure and temperature) to achieve the desired thickness and smoothness without over-compressing the board, which can affect grammage.
Additional Considerations:
- Fiber properties: Consider the impact of fiber refining and ash content. Well-refined fibers generally have reduced thickness, while higher ash content (e.g., from fillers) can increase thickness. Aim for an optimal balance of these factors.
- Pulp selection: Utilize a blend of pulp types that contribute to desired caliper. For instance, short fiber pulps like hardwood or straw pulp can reduce caliper, while mechanical pulps like BCTMP can increase it.
- Drying control: Maintain consistent and relatively low temperatures in the first group of dryers to minimize thickness variations.
- Moisture control before calendering: Ensure proper moisture content before calendering to achieve the desired final board bulk.
Monitoring and Adjustments:
- Regularly monitor caliper and grammage throughout the production process using online measurement systems.
- Quickly identify and address any deviations from the target values by adjusting process parameters (basis weight, headbox consistency, calendering settings, etc.) as needed.

3) Poor Stiffness

Stiffness in duplex board is a critical property that influences its performance in various applications, including packaging. Poor stiffness can result in reduced structural integrity, handling issues, and overall dissatisfaction with the product. Here are some suggestions to avoid poor stiffness defects in duplex board:

Control Furnish Properties and Beating:

Use stronger pulps in the furnish mix. Explore options like higher content of long-fibered wood pulp or recycled paper with good fiber strength.
Optimize beating process to develop good fiber bonding and network, improving inherent strength and stiffness. This might involve adjusting beating time, refining intensity, and other parameters.

Moisture Content Management:

Ensure proper drying of individual plies before duplexing. High moisture content can contribute to poor stiffness when the board dries further.
Control moisture content during web formation and pressing to achieve the desired final board properties.

Minimize Calendering:

Excessive calendering can reduce stiffness by flattening and weakening the fiber network. Optimize calendering pressure and temperature to achieve desired smoothness without compromising stiffness.

Consider Beater Additives:

Explore stiffness-enhancing additives like sodium silicate during beating. These can improve internal bonding and fiber network strength, leading to increased stiffness.

4) Poor Creasing Quality

Creasing defects in duplex board can affect the folding and creasing performance of the material, particularly in applications such as packaging. To avoid poor creasing defects, consider the following measures:

Board Selection:

Choose duplex board with appropriate basis weight and caliper for the desired crease quality. Thicker and heavier boards generally offer better creasing performance.
Consider grammage (weight per unit area) and ply composition. Boards with higher grammage and balanced ply composition will offer better crease integrity.

Scoring:

Experiment with different crease depths, wheel diameters, and scoring pressures to find the optimal balance for your specific board and folding needs.
Choose scoring wheels and equipment suitable for the thickness and density of your duplex board.
Maintain sharp scoring wheels. Dull wheels can tear the board instead of creating a clean crease, leading to poor folding and cracks.

Varnishing:

Control varnish application. Excessive varnish can reduce the board's flexibility, hindering proper creasing and potentially causing cracking.
Consider flexible varnishes specifically formulated for applications requiring good creasing performance.

Ensuring the robust strength characteristics of the top liner is crucial in preventing the front liner from cracking during the creasing process. The formation of ribs on the inside generates tension in the front liner, which becomes compressed during folding, leading to potential cracking. To avoid cracking, it is advisable to follow these precautions :-

Using stronger and extensible pulp
Reducing ash content in pulp
Increasing G.S.M. of front liner
Avoiding over drying of the board
Avoiding low moisture content

Similarly, the cracking of the back liner may also be avoided by following these precautions :-

Using low precentage of poor quality waste paper
Keeping mixed usage of ground wood
Making board with better ply adhesion
Avoiding over drying of the board

5) POOR SMOOTHNESS AND GLOSS

Smoothness and gloss are crucial properties in duplex board, especially when it comes to printing and packaging applications. Poor smoothness and gloss can result in a lower-quality appearance and reduced printability. Here are some steps to avoid poor smoothness and gloss defects in duplex board:

Furnish Selection and Refining:

Use fibers with good inherent smoothness in the furnish mix. Explore options like refined wood pulp or recycled paper with smooth fibers.
Optimize beating process to achieve uniform fiber distribution and minimize fiber entanglement, leading to a smoother surface.

Calendering Control:

Optimize calendering pressure and temperature to achieve the desired smoothness without compromising other properties. Excessive calendering can negatively impact both smoothness and gloss.
Proper temperature control of the M.G. cylinder is crucial, as an overheated surface hinders proper board drying.

Coating and Sizing:

Apply a smooth and uniform coating to the surface of the board. Ensure proper coating formulation and application techniques to avoid unevenness or surface imperfections.
Consider using surface sizing with appropriate chemicals to improve surface smoothness and gloss receptivity.

Drying Control:

Uniform and controlled drying is crucial. Uneven drying can lead to surface irregularities and impact smoothness.
For optimal finish, the M.G. dryer's surface must be well-polished, and the moisture content of the front side (top liner) should be maintained at or above 40%.

6) Burn marks and streaks

Burn marks and streaks on duplex board can result from various factors in the manufacturing process. To avoid burn marks and streaks defects, consider the following measures :-

Temperature Control:
- Ensure that the drying and curing temperatures are well-controlled during the manufacturing process. Monitor and adjust the temperature parameters to prevent overheating, which can lead to burn marks.

Drying Control:

Excessive drying temperatures can contribute to burning, especially for boards with high recycled content or specific coating formulations.
Keep the M.G. dryer clean and avoid air pockets between the board and dryer to ensure uniform drying and prevent scorching.

Coating Considerations:

Choose coatings with higher scorch resistance and adjust drying parameters accordingly.
Ensure consistent coating application. Uneven coating thickness can lead to localized burning during drying. Regularly monitor and maintain coating equipment for proper application.

Roll Cleaning:
- Regularly clean and maintain calendering and coating rolls to prevent the accumulation of debris or coating materials that can contribute to streaks.

Lubrication:
- Ensure proper lubrication of equipment, especially in areas prone to friction and heat generation. Inadequate lubrication can contribute to overheating and burn marks.

Summary

Always work with reliable suppliers: Choose duplex board manufacturers known for consistent quality. If the issue persists, consider seeking professional guidance from paperboard manufacturers, converting companies, or printing experts to diagnose the root cause and recommend appropriate solutions.

At Paxert LLP, we offer not just a diverse range of pulp and paper products, but also free consultations and assistance in identifying and solving any defects encountered in duplex boards and other paper products. and the guarantee of quick, efficient service tailored to your specific needs.

For a free consultation and personalized solutions to your duplex board and paper product defects, contact us today.

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Common defects in Duplex board and their Solutions - Part 1