How to avoid cracking or warping of paper cups

If the mouth of a paper cup cracks or curls during the molding or use process, it will seriously affect the sealing, appearance integrity and user experience of the cup body. Cracking usually manifests as cracks along the edge of the cup mouth along the direction of the paper fiber, which may leak liquid or cut the user's lips; curling is manifested as the cup mouth curling edge obviously curling outward or inward, resulting in the cup cover not being able to close or structural deformation.
Such defects are more common in the mass production and transportation of paper cups, and are one of the key factors affecting the stability of product quality. Its causes are complex, involving multiple links such as paper performance, process parameters, and equipment status.

The influence of raw material selection on cracking and curling
The selection of paper substrate is the first hurdle to control cracking and curling. High-quality paper cups should use food-grade virgin wood pulp paper with high tensile strength and transverse toughness. If the paper fiber distribution is uneven or the proportion of recycled fiber is too high, the probability of paper breaking will increase during stress or curling.
The moisture content of paper is another key parameter. Too high a moisture content will lead to uneven evaporation of water vapor during the heat sealing process, shrinkage internal stress at the curling edge, and warping or fine cracks. Too low a moisture content will make the paper too brittle and hard, and it is easy to crack during the curling process. The standard moisture content should be controlled between 6% and 8%.
The uniformity of the coating layer also affects the molding quality. If the inner wall PE or PLA coating is unevenly distributed, especially if the coating is thin at the curling edge, the paper will soften locally and stress will accumulate under the action of high temperature and pressure, thereby increasing the risk of cracking.

Control of mold and curling process parameters
Curling is a key process in paper cup molding, which directly determines the stability of the cup mouth structure. Cracking and warping often occur when the curling edge is poor, so the size, angle, speed and temperature of the curling mold must be accurately controlled.
The curling mold should have high finish and accurate curvature to ensure that the paper cup is formed along a fixed trajectory during high-speed curling to avoid local wrinkles or stretching ruptures. If the curling radius is too small, the paper will be stressed and the edge will tear; if the radius is too large, the curling will be loose or warped.
Temperature control of hot curling is crucial. If the temperature is too high, the PE or PLA layer will melt prematurely, lose the protection of the fiber layer, and expose the base paper at the curling edge; if the temperature is too low, the paper cannot be fully softened, resulting in incomplete curling or edge cracking. The ideal temperature should be precisely set according to the material properties, generally controlled within the range of 140℃～160℃.
The curling speed should be coordinated with the heat sealing and molding steps. If the speed is too fast, the curling time is insufficient, the pressing is not tight, and the edge is easy to warp; if the speed is too slow, the paper may overheat and tear. Modern curling machines are usually equipped with servo systems to accurately adjust the speed and pressing rhythm to improve molding consistency.

Equipment status and maintenance management
The maintenance of curling equipment directly affects production stability. Problems such as mold wear, uneven heating, and loose transmission system may lead to frequent curling defects. Regularly calibrating the mold position, checking the heating element temperature control system, and removing carbon deposits and oil stains are the basis for maintaining stable operation of the equipment.
For enterprises producing in large quantities, an automatic visual inspection system should be equipped to monitor the curling defects in real time. If problems such as warping, missing edges, and cracks are found, they should be removed in time to prevent defective products from entering the market.

Storage and transportation and environmental condition control
Finished paper cups may also cause curling deformation or cracking due to environmental factors during storage and transportation. Under high temperature and high humidity conditions, the cup mouth coating layer may stick and soften, resulting in edge warping. The brittleness of paper increases in a low temperature and dry environment, and handling vibration can easily cause fine cracks.
Therefore, paper cups should be stored in an environment with good temperature and humidity control, with a recommended temperature of 15℃~25℃ and a relative humidity of 45%~65%. The packaging method should be stacked in rows and covered with dustproof bags to avoid external force squeezing the curling.