How to Test Qualified Packaging Paper
Packaging Inspection for Paper
Currently, flat paper packaging primarily comes in two basic forms: wood clipboard packaging and wooden box packaging. Qualified flat paper usually has more than two layers of wrapping paper on the outside, further reinforced with wood paneling and tightly bound with metal straps. The wrapping materials used include wax paper, tar paper, kraft paper, and snake-skin paper. On the wood clipboard or wooden box, labels such as "Moisture-proof" and "Handle with care" should be displayed, and in some cases, the grain direction of the paper may also be indicated.
A product qualification certificate should be affixed to the wood clipboard or wooden box, indicating the product name, specifications, ream weight, number of reams per package, paper grade, applicable standards, production date, and the name of the manufacturer. The outer packaging should be intact without deformation or damage. If the packaging is seriously damaged and the paper is exposed, it should be rejected without hesitation, or other measures should be taken to avoid unnecessary losses. Outer packaging inspection is a basic quality control step, and it must be handled with strict attention.
Appearance Inspection After Removing the Outer Packaging
Once the outer packaging is removed, check for a quality inspection sheet, and then inspect the paper's appearance and color.
The normal appearance of paper is characterized by good flatness, meaning the surface should be smooth and the edges neat. For a single sheet of paper, it can be relatively difficult to judge if its appearance is normal unless there is significant deformation.
A common method is to unbundle the paper and stack it about 1 meter high to check for irregularities in the paper’s appearance. If the paper has an abnormal appearance, it may take on various shapes such as wavy, dish-shaped, or others like mountain-shaped, valley-shaped, corner-lifted, bag-shaped, slanted, or curled.
The causes of these undesirable shapes are varied but generally fall into three categories:
1. Uneven moisture distribution across the entire sheet, causing localized shrinkage.
2. Uneven thickness formed during the papermaking process.
3. Unequal expansion and contraction between the front and back of the sheet, especially in single-sided coated paper.
Additionally, from the side of the stack, it is possible to determine whether the paper color is uniform. The whiteness and color tone of the paper from the same batch should be consistent. Otherwise, when the printed materials are bound into a book, obvious tonal differences may appear on the top, bottom, or cut edges of the pages.
How to Measure Paper Size
According to national standards, paper size should be measured with a steel tape measure that is 2 meters long and accurate to 1 mm. Three samples should be randomly selected from the same packaging for measurement, and the results should be accurate to 1 mm. The allowable tolerance for paper size is ±3 mm. Any skewing of the paper will cause the dimensions to be incorrect, resulting in the long and short edges not forming a right angle. Therefore, besides measuring the dimensions, the skewness of the paper should also be checked. The allowable skewness tolerance is between 3 mm to 5 mm.
The dimensional stability of paper can also be expressed by its expansion rate. During acceptance, the paper’s expansion rate in water can be tested. Cut a 2 cm x 20 cm strip of paper, submerge it in water for 1-2 hours until it is fully expanded and its dimensions no longer change, then remove it and lay it flat on a glass plate. Use a transparent ruler to measure the dimensions and calculate the expansion rate, comparing it to the standard.
If the paper shows significant deformation after unpacking and a high expansion rate in water, it is best to reject the batch to avoid future problems.
How to Test Paper Surface Strength
Surface strength is a very important characteristic of paper, and each type of printing paper has specific requirements for it. National standards also specify the surface strength required for different types of printing paper.
If the paper's surface strength does not meet printing requirements, it may shed fibers or particles during the printing process, or even delaminate. Therefore, checking the surface strength of paper is an important part of acceptance testing, although many printing companies overlook this step, leading to printing issues later.
A simple method to test surface strength is the Dennison wax stick method. Though it has some limitations, it is still a convenient way to measure the lint-shedding resistance of a particular paper.
To test, melt the end of the wax stick and press it against the surface of the paper. After 15 minutes, quickly pull it off and examine the end for any paper fibers or particles. Another simple method is to press a slightly damp finger (usually the thumb) firmly on the surface of the paper, rub slightly, then quickly lift it off. Check the finger for any small fibers or powder residue.
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