Solve the problem of curling gift box wrapping paper by keeping these key factors in check!

Outer wrapping paper is one of the main components in gift box production. It plays an important role in printing, surface finishing, die-cutting, and lamination. However, during production, many factors often cause paper curling, seriously disrupting the orderly progress of manufacturing. The main causes of curling are as follows: First, the moisture on the front and back sides of the paper is unbalanced; Second, the surface process changes the tension on one side; Third, environmental changes interfere with the paper's stability; fourth, water molecules in the adhesive weaken the paper's surface resistance. In addition, factors such as material quality adaptability, rationality of surface process design, and standardized process operation are all causes of outer packaging paper curling. This paper provides a comprehensive problem analysis and formulates effective preventive measures, ultimately achieving the goal of controlling outer packaging paper curling.

Material selection

First, analyzing the raw materials required for gift boxes, besides using gray board cardboard of different thicknesses, the packaging materials, edge covering, inner labeling, sand top, edge insertion, and pull paper are all mainly packaged with coated paper of 128g/m2, 157g/m2, and 200g/m2. When these components curl during production, it has a significant impact on quality and production efficiency. Therefore, before warehousing or putting paper into use, each batch of coated paper should be tested for key technical indicators, such as moisture, smoothness, gloss, coating strength, absorbency, dryness, and other items related to printing and post-process suitability. Paper exceeding technical standards should be used with caution; only reasonable selection is beneficial for normal production.

01/ The effect of moisture content on paper flatness

After production, the moisture content of coated paper remains stable, and the paper surface is smooth and flat. When the external environment changes, humidity inevitably affects the flatness of the paper. When the ambient humidity is higher than the relative humidity of the paper, such as in rain, snow, or hot and humid weather, the paper absorbs varying amounts of moisture. If one side absorbs more moisture, the paper will curl toward the opposite side; When the ambient humidity is lower than the relative humidity of the paper, the paper releases moisture outward. The side that loses more moisture loses its ability to maintain balance and curls toward the surface where water is not lost. Therefore, the deviation between the relative humidity of the environment and the relative humidity of the paper should be controlled within 5% to avoid obvious curling.

02/ The relationship between paper quantification and flatness

The flatness of paper is closely related to the quantity of the paper. In terms of resisting environmental changes, high-yield paper outperforms low-quantification paper due to the thickness of its substrate. The stress on both sides of low-quantity paper is weaker, so when the surface is affected by environmental humidity, curling is inevitable. Also, during lamination, the moisture in the adhesive penetrates low-quantification paper faster than high-quantification paper. Under the action of water molecules, the curling rate of low-quantification paper is much higher than that of high-quantification paper. Given the characteristics of the paper, when selecting outer packaging paper, as long as process requirements are met, small-sized products should use coated paper with 128g/m², and large-sized products should use coated paper or art paper with at least 128g/m². This choice ensures product quality and improves production efficiency.

03/ The relationship between single- and double-sided coating and paper flatness

Coated paper can be coated on both sides and single-sided. Due to differences in processing technology, double-coated coated paper has basically equal stress on both sides and maintains good flatness. For single-sided coated paper, one side is protected by paint, while the other side cannot effectively isolate moisture ingress. When the moisture content increases, the balance between front and back is disrupted, causing the water-absorbing side to curl toward the coated side. This is why single-sided coated paper tends to curl easily. Therefore, when designing gift box processes, single-sided coated paper should be avoided as much as possible.

04/ The impact of laminating processes on paper flatness

Laminated coated paper is made by attaching a thin film to one side of the paper. This type of paper mainly features gold and silver finishes and is printed using UV printing. The most critical technical indicator for this type of paper, besides the flatness of the paper, is the adhesion between the surface film and the paper. Flatness is determined by the tension of the surface film. For high-tension laminated paper, as long as it comes into contact with the natural environment, especially during seasons with large humidity changes, the front side of the paper is completely moisture-resistant due to the different absorbency of the front and back, while the back side is very sensitive to the environment. If the process is not strictly controlled, water molecules can take advantage of the gap, causing curling.

To reduce paper curling, suppliers are required to add backcoating processes to prevent further moisture absorption on the back side. When both sides of the paper are isolated from the environment, curling is minimized. Therefore, when designing gift box craftsmanship, the back-coating process should be taken into account, especially when using low-volume paper. Additionally, considering the stiffness and adhesion requirements of the outer packaging paper, the backing coating amount should be controlled at around 3g/m2. This composite process design basically solves the curling of the printed surface, improves stiffness, and lays a stable foundation for subsequent processes.

05/ The impact of the storage cycle of roll paper on paper flatness

Sheet-fed paper is made by cutting roll paper, with the core part having a small diameter. If the roll paper is stored for a long time before cutting, the grain direction will be remembered, and after cutting, the paper will be left for a short time. When environmental changes occur, the paper may curl at both ends. Therefore, when purchasing paper, be sure to check the production date. For paper urgently needed during production, carefully consider its potential impact on production to prevent uncertainties caused by curling during normal production.

Printing operation standards and control

Gift box outer packaging printing differs from cardboard box printing, which uses high-volume materials and paper flatness that meets printing requirements well. In contrast, the surface material of gift boxes is mostly low-quantity material. Environmental control, machine operation, ink selection, and employee skill performance are all unstable in each stage, all of which can lead to issues in print quality. Therefore, regardless of the type of printing equipment used, it must be operated strictly according to process specifications. Only with a strong sense of responsibility can high-quality products be printed. Conversely, to provide a smooth print surface for the next process, the following aspects should be addressed.

01/ Workshop temperature and humidity control

The optimal working environment for a printing workshop is 18~23°C, with relative humidity of 55%~65%. Creating a production environment with relatively stable temperature and humidity is essential to ensure the quality of printed products. As everyone knows, paper is a material that is sensitive to changes in temperature and humidity. When the ambient humidity changes, the moisture content of the paper also changes. The moisture content of paper is also closely related to printability. Under the interference of these factors, paper deformation is inevitable, and it can also cause changes in other properties of the paper, such as increased or decreased moisture content, decreased tensile strength and surface strength, and static electricity during printing, causing double-sheet printing. Therefore, controlling temperature and humidity in the printing workshop is the primary task in preventing paper curling.

02/ Control of paper moisture content

During gift box production, the main factor affecting product quality is moisture. From environmental humidity to paper moisture content, from printing machine water to controlling the water content of laminating glue, all these can interfere with stable production. Therefore, before printing production, the paper moisture content must first be tested. Paper below 8% is basically stable, but whether the paper loses water or absorbs water, the flatness of the paper will be affected. Therefore, controlling the paper moisture content mainly focuses on controlling the base paper. Second, sealing the non-printed side of the paper reduces the paper's sensitivity to environmental factors. Generally, if the moisture content of the base paper is within the controlled range, the print surface will have the least degree of curl. Additionally, at every stage of production and transportation, fully wrapped wrapping protection is used to maintain stable moisture content in the paper, thereby reducing product curl.

03/ Other moisture control and use of additives

During printing, moisture transfer on the plate surface is indispensable, but surface moisture must be strictly controlled. Under the principle of preventing plate contamination, the less moisture you control, the better. If the surface of the printing plate contains too much moisture, not only will the coating on the paper peel off affecting ink quality, but the paper may also shrink, which in turn affects the complete reproduction of graphic and text registration and dot reproduction. Additionally, some moisture can penetrate the ink, causing ink emulsification. After emulsification, the ink's penetration on the paper is enhanced, weakening the effect of ink trace oxidation and filming, and causing slow drying or sticky surfaces of printed products. To more effectively prevent curling of the printed surface, anti-crimp additives can be added to the ink. These additives can be twice as effective for curling with half the effort, but attention should be paid to the amount added, which should be determined based on the ink's area on the print surface and the type of paper. Proper and standardized use is highly beneficial for anti-curl.

Control of temperature and humidity differences in different workshops

The entire production process of gift boxes is longer than that of cardboard boxes. From front-end printing to back-end production, from individual processing of components to assembly and assembly, all are processed in different workshops in sequence. Because multiple processes are involved, consistency in temperature and humidity is required. Only when temperature and humidity are close can product quality stability be ensured. If the discrepancies exceed the controlled range, the product's suitability will decrease, affecting smooth production. Therefore, we require that the temperature and humidity in the downstream workshop be close to that in the printing workshop Under the same environment, only the physical changes in materials can remain stable.

However, reality tells us that downstream workshops generally find it difficult to meet this standard. Because many pieces of equipment are involved, heat dissipation between machines absorbs some moisture from the environment, and this operating state keeps temperature and humidity constantly fluctuating. Coupled with frequent entry and exit of loaders and paper trailers, the effectiveness of the workshop's isolation from the outside world is limited and weakened. Fully meeting control targets remains challenging.

Given these issues, only the entrance and exit can be modified, and like in a printing workshop, a moisture spray system should be installed to ensure temperature stability while maintaining stable ambient temperature and humidity. Additionally, an automatic lifting system should be installed on the main door, meaning the car rises and closes, so the door is effectively isolated. Only with a stable indoor environment can the semi-finished products brought in during the previous process be kept at constant temperature and humidity. Under these conditions, the processing quality of the semi-finished products will not be affected.

The laminating process, like the subsequent hot stamping and die-cutting processes, has many heat sources. Most of these heat sources come from laminating machines and hot melt gluing machines. The heat and moisture emitted during operation can cause subtle changes in the local environment. Since these devices are all at the start of the production line, multiple production lines are lined up side by side, limiting heat and moisture to local circulation, resulting in temperature and humidity differences between the front and rear of the production line. This difference affects the external packaging after glue coating. To address this, it is necessary to improve air circulation in the workshop. Use an electric fan to blow in the opposite direction to ensure full air circulation, while adjusting the air conditioner's direction to avoid blowing directly on the product. This adjustment stabilizes the workshop temperature between 18~23°C and relative humidity between 55%~65%, ensuring consistent balance and smooth production of products.

Production management and control for outsourced factories

Gift box production is a multi-process design process, where surface finishing techniques are layered to make the product more attractive, thereby attracting more people's favor. To achieve perfect process results, collaboration between production processes is necessary. However, most factories do not have multiple process manufacturing and equipment configurations, which requires support and coordination from multiple outsourcing manufacturers to complete product manufacturing. However, most of the outsourcing manufacturers we face have varying levels of control. Some are open-type, some without temperature control systems, and some with single equipment. Their doors and windows lack integrity, with the same temperature inside and outside, and dry-wet conditions are out of the question. When semi-finished products enter such environments, the paper is affected by environmental uncertainties, which can alter its function. Without basic guarantees, no matter what product it is, you can only rely on yourself.

In light of this, choosing qualified and compliant outsourcing factories is especially important. First, it must be reviewed and evaluated, i.e., whether the factory's management meets requirements, whether the skill level meets process standards, whether process operations are stable for multi-batch production, and whether production plans are orderly and standardized. Only manufacturers that pass inspection can be included in the list of qualified suppliers. At the same time, these outsourcing factories should be categorized and managed, with targeted outsourcing for different products. This is the best way to eliminate substandard products.

Standards for Semi-finished Product Transfer Process Management

Every product is considered a semi-finished product before production is completed. These semi-finished products undergo transfer processes during surface processing, such as workshop to workshop, downstairs to upstairs, indoor to outdoor work. Each scenario has differences, and unregulated operations or arbitrary adjustments to process programs will affect the physical properties of the semi-finished products themselves. If each process is not strictly controlled, it is impossible to know which link is faulty, making problem-solving more difficult.

Therefore, it is necessary to establish operational standards for the process, and to promptly use stretch film to protect finished components. Different paper wraps have clear requirements for the number of layers, and arbitrary wrapping or gaps between films cannot provide isolation. The most critical issue is semi-finished products left outdoors. Affected by spring, summer, autumn, winter, rain, snow, temperature, and humidity, these semi-finished products absorb or release some moisture. Over time, in addition to stretching and shrinking, they develop ruffles, wavy surfaces, and curling is unavoidable. These issues prevent normal production in the next production process, seriously affecting production progress and causing delivery delays.

Control of the laminating process

The gluing process is an indispensable part of gift box production. Scientific and standardized control of the mounting process is an important step to ensure the paper is flat. Key control points such as temperature and humidity, processing conditions, and operational standards are closely related to product quality. In addition, the amount of coating is also an important part of the operation. Excessive coating causes the polymers in the glue to crowd together, preventing good tensile force, and the strongest attraction between them is affected, serving only as a filling rather than bonding. Large amounts of glue accumulate and prevent moisture between polymers from evaporating easily, leaving most of it on the paper surface, disrupting the balance on both sides and creating curling conditions under unilateral stress.

Therefore, the laminating process must be digitally controlled. First, check whether the temperature reaches 18~23°C and the relative humidity reaches 55%~65%. Next, the adhesive viscosity is tested: the white latex viscosity must reach above 3000 cPs, solid content above 45%, and the coating amount should be 31±2 g/m2. After fully dissolving animal protein gel and stirring evenly, the concentration reaches 55±2%, and the coating amount is generally controlled at 33±2g/m2.

Of course, seasonal changes cause temperature and humidity changes, so the glue's technical specifications and opening times should also be adjusted. For example, animal protein glue has formula standards for winter, spring, and summer. In summer, when temperatures are higher, the glue opening time must be extended; in winter, when temperatures are lower and volatility is low, the glue opening time is correspondingly shortened. Regardless of the type of glue, quantity control is crucial. The amount applied depends on the gap between the rollers. The most accurate method is to use a tape measure to correct the process. After the technical parameters on both sides are consistent, visually observe the uniformity of the coating on the print surface. Then, dip your index finger into the adhesive layer on the paper. Wherever your finger touches, the outer packaging paper is lifted and instantly falls off, indicating that the coating amount basically meets the requirements for mounting and adhesion.

Although this testing method is simple, it serves the purpose of initial detection. After completing the initial inspection, the weighing method is used for precise calibration. The specific method is to calculate the packaging area, weigh the packaging paper, apply glue, and re-weigh the total packaging material. Production can only begin once the coating amount reaches the standard value. After determining the coating amount, the relationship between the glue opening time and the operating interval must also be considered. If the transmission distance is too long, the adhesive film formation will fail prematurely, severely affecting the quality of the adhesive. Therefore, it is recommended that after glue coating is completed, the laminating should be done with the gray board and cardholder as quickly as possible, aiming to finish laminating before the paper deforms. This can minimize the impact of curling on production to the greatest extent possible.

Control of the lamination process

Most gift box outer wrapping paper uses a lamination process, which provides excellent beautification and protection for the product surface. However, due to certain irregular operations, the flatness of the outer packaging paper is affected. Generally, the tension of the film surface determines the direction of curling. If the outer packaging paper curls upward, it means the film is too tense; If the outer wrapper curls downward, it means the film tension is too low.

During the lamination process, the tension of the film during operation varies slightly. The tension at the beginning and near the end of the film differs; the tension at the start of the entire roll is relatively low, but the tension increases as the roll progresses. Therefore, during the lamination process, attention should be paid to tension changes, timely adjustments, and the tension difference between the previous and subsequent stages should be minimized to ensure controllability and stable tension. Whether tension control meets process technical requirements can be verified using simple methods: first, take the laminated print surface and allow it to fully release heat. Then, use a utility knife to cut 5cm ×20cm long strips of paper on the print surface. Select strips on each side of the print surface along the grain direction, then cut strips perpendicular to the top and bottom of the print surface. Place the film side down on a table and carefully observe the changes for about 20 minutes. If the surface is flat and shows no change, the film tension is appropriate; if it curls upward, This indicates the membrane tension is too tight; downward curling indicates insufficient tension. In this case, the membrane tension should be adjusted according to the specific situation. Different types of equipment should be determined according to actual conditions.

Additionally, while controlling tension well, the operating speed should be regulated in a standardized manner, with the lamination speed at 20~30m/min being reasonably maintained. In addition, controlling the temperature of the roller should not be taken lightly. Excessive temperatures not only cause the film to shrink and deform, but also cause wrinkles and bubbling, and store excess heat inside the paper, prolonging the release time. If the temperature is not dissipated in time, it will fluctuate due to environmental factors, causing difficulties in subsequent processes. Finally, it should be mentioned that regardless of the type of paper used, as long as the surface finishing process uses lamination, it is recommended that the paper grain intersect perpendicularly with the film's grain. This not only improves the flatness of the print surface but also facilitates subsequent processing.

Back-end process control

Many gift box packaging papers often use post-printing processes such as reverse printing, snowflake, and frosted coating. These processes rely heavily on UV curing. Although UV ink curing improves suitability and adhesion, the paper loses a large amount of moisture under high-temperature, intense light. The reduced moisture content maximizes heat storage. Due to surface processing additives, the full dissipation of heat is hindered. The only area capable of rapid release is the back of the paper, which can be quickly released when exposed to environmental factors or water molecules. This is where the moisture absorption process takes place.

Once enough moisture has soaked in, the paper will inevitably curl upward. Therefore, during post-press operations, manual paper collection should be set at the end of the equipment. The main purpose is to fan the finished print surface by shaking the paper, allowing an appropriate amount of air to enter the gaps between the papers, helping to release internal heat and improve environmental adaptability. After the temperature of the print surface drops, pour the foot plate again, then wrap it with stretch film to seal it. Before use, it should be left in the machining workshop for no less than 12 hours to allow for a fully adapted process. Only after testing and fully meeting process technical requirements can production be scheduled. These simple and convenient operations reduce the impact of surface curling on production, ensuring product quality to some extent.

By optimizing the paper processing technology, the surface tension of the paper is stabilized while the moisture content is kept within a reasonable range. With materials that meet the requirements of the printing process, production between processes can be orderly. Orderliness means stability, and stability improves efficiency and reduces costs. In addition, it also positively promotes on-site management levels and lean production management, thereby creating conditions for the healthy development of enterprises.