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Paper serves as the primary substrate in printing, and its quality directly influences the final printed output. Therefore, for printing and packaging enterprises, ensuring the quality of incoming paper stock is a fundamental prerequisite for producing high-quality printed materials.
Inspection of Outer Packaging
Currently, flat paper is primarily packaged using two standard methods: plywood encasement or wooden crate packaging. Qualified sheet paper typically features at least two layers of protective wrapping-such as wax paper, asphalt paper, kraft paper, or snake-skin paper-followed by rigid wooden board reinforcement and secure strapping with metal bands. Packaging should clearly display markings such as "Moisture-Proof," "Do Not Drop," and, where applicable, fiber direction indicators. A product conformity certificate must also be affixed to the exterior, specifying key details including product name, specifications, unit weight, sheets per bundle, paper grade, applicable standards, production date, and manufacturer's name.
The outer packaging must remain intact, free from deformation or damage. If severe damage has exposed the paper, immediate rejection or corrective action is required to prevent potential losses. This initial inspection constitutes a critical control point and must be rigorously enforced.
Appearance Inspection After Unpacking
Following removal of the outer packaging, the first step is to verify the presence of a quality inspection certificate, followed by a visual assessment of the paper's appearance and color consistency.
A well-manufactured paper stack should exhibit good flatness, with both surface and edge faces appearing smooth and even. Assessing individual sheets for visible defects can be challenging unless significant deformation is present. A common method involves unpacking and stacking the sheets to a height of approximately one meter to evaluate overall stack uniformity. Abnormal stack profiles may manifest as wavy, disc-shaped, mountain-shaped, valley-shaped, angular, bag-shaped, skewed, curled, saddle-shaped, or other irregular forms. These defects generally arise from several factors: (1) uneven moisture distribution across the sheet leading to differential shrinkage; (2) non-uniform thickness during the papermaking process; and (3) inconsistent expansion or contraction between the front and back sides of the paper, particularly evident in single-sided coated grades.
Additionally, when viewed from the side of the stack, the paper color should appear consistent and uniform. All sheets within the same batch must exhibit minimal variation in whiteness, tone, and hue to ensure print consistency.
Single Sheet Inspection
Select a random sheet from the stack and examine it for surface defects and uniformity. Tactile and visual assessments-by hand or under flat viewing conditions-are used to evaluate surface smoothness, uniformity, and cleanliness. Ideally, the paper surface should be smooth, free from blemishes, protrusions, or foreign particles. There should be no spots exhibiting differing shades, reflective inconsistencies, or dust that contrasts with the base color. Hold the sheet up to a light source to assess internal uniformity; under transmitted light, the sheet should not display translucent patches or variations in brightness. The presence of large, cloud-like mottling indicates poor formation and will significantly compromise print quality.
Examination of Multiple Consecutive Sheets
In some cases, flat paper is produced by simultaneously cutting and stacking multiple paper rolls. Due to slight variations in manufacturing processes between rolls, adjacent sheets within a single stack may differ in physical properties. If a substandard roll is introduced, periodic anomalies may occur during printing, resulting in recurring print defects at regular intervals. Therefore, it is essential to extract and compare four to five consecutive sheets from the stack to detect any such inconsistencies. While this scenario is uncommon, such inspections can be crucial in preventing downstream printing issues.
Paper Dimension Verification
Paper dimensions should be verified in accordance with national standards using a 2-meter steel tape measure accurate to 1 mm. Three samples should be randomly selected from the same package. Measurements must record all observed values, reported to the nearest millimeter. The permissible tolerance for length and width is ±3 mm. Skewed cutting may result in dimensional inaccuracies and non-right angles between edges. Therefore, skew must also be measured, with an allowable deviation of 3 to 5 mm.
Basis Weight Verification
To verify basis weight, cut five samples measuring 200 × 250 mm using a precision paper cutter. Weigh each sample on a high-precision balance, then divide the mass by the area to calculate the basis weight (g/m²). The average value should closely match the figure stated on the product certificate. Minor fluctuations are acceptable within nationally permitted tolerances, but deviations beyond these limits are unacceptable.
Dimensional Stability Assessment
Dimensional stability is evaluated through the paper's expansion rate. One practical method involves measuring elongation after water immersion. Cut a 2 cm × 20 cm strip, immerse it in water for 1–2 hours until fully expanded, then place it on a glass plate and measure its final length using a transparent ruler. Calculate the elongation percentage and compare it with reference data. Excessive swelling upon exposure to moisture indicates poor dimensional stability and may lead to registration issues during printing. Such paper should be rejected to avoid future complications.
Surface Strength Evaluation
Surface strength is a critical property for printing papers, as defined by national standards. Insufficient surface strength may result in linting, powdering, or delamination during printing. Despite its importance, this parameter is often overlooked, leading to avoidable pressroom problems.
Simple field tests can provide preliminary assessments. One widely used method is the Denison wax rod test: melt the tip of a specified-grade wax rod and apply it firmly to the paper surface. After 15 minutes, rapidly peel it off and inspect the residue for detached fibers or powder. Alternatively, lightly moisten the thumb, press firmly onto the surface, gently twist, and quickly lift. Observe whether short fibers or dust adhere to the skin, indicating weak surface integrity.
Conclusion
In the absence of specialized testing equipment, the aforementioned methods enable a reasonable evaluation of flat paper quality. However, precise measurement of paper properties requires dedicated instruments and controlled environmental conditions.
