A cupcake liner's pleated ridges act as flexible joints that allow the paper to expand without tearing during baking.

The modern pleated cupcake liner is a marvel of structural engineering designed to handle the thermal expansion of cake batter. Most standard cupcake liners are manufactured with exactly 27 to 30 pleats. These folds function similarly to bridge expansion joints or bellows, providing the paper with a high degree of 'hoop stress' resistance. Without these pleats, the rigid circular shape of the paper would simply rupture as the cake increases in volume.During the baking process, chemical leaveners like baking powder or baking soda react to heat and moisture. This reaction releases carbon dioxide gas, which expands the batter by up to 100 percent of its original volume. The pleated design allows the circumference of the liner to increase uniformly. This ensures that the pressure is distributed evenly across the paper surface rather than concentrating on a single point.The paper used is typically greaseproof cellulose, often treated with a slip agent like silicone or quilon. This treatment, combined with the mechanical advantage of the pleats, prevents the cake from sticking to the liner. The pleats also increase the surface area of the paper, which helps in heat distribution during the initial stages of baking. This ensures the edges of the cupcake set at the same rate as the center.The geometry of the pleats also provides vertical rigidity. While the liner can expand horizontally, the vertical folds act as pillars that prevent the sides from collapsing under the weight of the rising batter. This specific industrial design has remained largely unchanged since the early 20th century because it perfectly balances flexibility with structural integrity. It remains the most efficient way to mass-produce disposable baking molds that fit standardized muffin tins.