How to test and test the carton strength
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First, the appearance quality:
1. Printing quality: The pattern and handwriting should be clear, with uniform color and bright finish. The printing position error should not exceed 7mm for large boxes and 4mm for small boxes.
2. Sealing quality: The box should be tightly sealed without any leakage. The cover and seams should align perfectly without any noticeable difference.
3. Dimensional tolerance: The inner diameter of the box must adhere to a tolerance of ±5mm for large boxes and ±3mm for small boxes. The outer dimensions should remain consistent.
4. Cover folding strength: After folding the corrugated box back and forth 180 degrees more than five times, the total length of cracks on the surface layer of first- and second-class boxes, and on the inner layer of third-class boxes, should not exceed 70mm. Additionally, the joints should be neat, with no overlapping angles. The surface must not show any visible damage or smudges.
Second, the pressure resistance of the carton and influencing factors:
The compressive strength of the carton is one of the most critical quality indicators for many product packaging requirements. During testing, the corrugated carton is placed between two pressure plates and gradually pressurized until it collapses. The pressure at which this happens is referred to as the compressive strength, measured in kilonewtons (kN).
1. Predicted compressive strength: A carton must have sufficient compressive strength to prevent collapse under the weight of stacked boxes during storage and transportation. The formula used to calculate compressive strength is P = KW(n-1), where P represents the compressive strength, W is the weight of the loaded carton, n is the number of stacking layers, and K is the safety factor based on the storage duration. According to national standards: if the storage period is less than 30 days, K = 1.6; for 30–100 days, K = 1.65; and for over 100 days, K = 2.0.
2. Calculating compressive strength based on raw materials: Once the required compressive strength is determined, appropriate corrugated board and paper should be selected to avoid unnecessary waste. The Kellicutt formula is commonly used to estimate the compressive strength of 0201-type boxes, offering a practical and concise method for estimation.
3. Determining compressive strength through testing: In production, the actual compressive strength may differ from estimated values due to various factors. Therefore, the most accurate way to determine the compressive strength is by using a carton compression tester after subjecting the box to controlled temperature and humidity conditions. For smaller factories without testing equipment, a wooden board can be placed on top of the carton, followed by an equal amount of weight to roughly assess whether the compressive strength meets the requirements.
4. Factors affecting compressive strength:
- **Raw material quality**: The base paper is the primary determinant of compressive strength, as reflected in the Kellicutt formula. However, other factors such as adhesive amount, impregnation, coating, and composite processing also play a role.
- **Moisture content**: High moisture levels in corrugated cardboard can significantly reduce its compressive strength, especially when stored in humid environments. Paper is highly absorbent, so maintaining proper moisture balance is essential.
- **Box type and size**: The design and proportions of the box greatly influence its strength. Double-layer corrugated boxes typically offer better strength compared to single-layer ones. Taller boxes tend to be less stable and have lower compressive strength.
- **Printing and opening**: Any printing or cutouts on the box, such as ventilation holes or handle openings, can weaken the structure. Larger or asymmetrical openings can have a more pronounced effect on strength.
- **Manufacturing defects**: Poorly formed flutes, excessive grooving, or weak seams during the manufacturing process can all reduce the carton’s resistance to pressure.
Third, dynamic performance testing:
For specific products like ceramics, glassware, electronics, and instruments, additional tests are conducted to evaluate the carton's cushioning performance. These tests simulate real-world transport conditions, including drops, impacts, vibrations, and rotations.
1. **Drop test**: The packaged carton is dropped from a specified height in a set posture. The condition of the product inside is checked after a certain number of drops, or the number of drops before the carton is damaged is recorded.
2. **Bevel impact test**: The carton is rolled on a trolley, then slides down a slope and hits a baffle, simulating sudden braking during transport.
3. **Vibration test**: The carton is placed on a vibration table and subjected to horizontal or vertical movement, or both. The condition of the contents is assessed after a set time, or the carton is checked for damage.
4. **Hex drum rotation test**: The carton is placed in a hexagonal drum equipped with impact plates and rotated for a specified number of revolutions. The condition of the product and the carton is then inspected.
These dynamic tests are destructive, but they help identify weaknesses in the packaging. To improve the carton's resistance to damage, cushioning materials, baffles, or other protective measures are often used during packaging. In addition, some cartons may undergo weather resistance, water spray, or other tests depending on the contractual requirements between the parties.