Material Response
Polymers and natural fibers like cellulose exhibit a complex reaction to stress that combines the properties of both liquids and solids. When a load is applied, viscoelastic strain describes how the material deforms instantly like a spring and then continues to flow over time like a thick fluid. This means the final shape of the paperboard depends not just on how much force was used, but also on how long that force was held.
In a printing press, the speed of the nip directly affects how the fibers respond.
Load Duration
Short impacts result in a mostly elastic response where the board springs back, while a longer dwell time leads to more permanent change. This behavior is why slow, heavy presses are often better for deep embossing than fast, light ones. If the stress is removed too quickly, the viscoelastic strain has no time to become permanent and the image will be shallow.
The temperature of the board also plays a role, as heat increases the liquid like flow of the fibers.
Recovery Profile
After the pressure is gone, the material continues to move as it slowly tries to return to its original state. This delayed recovery can cause cartons to change their dimensions slightly in the hours after they are produced. Converters must account for this movement when designing high precision packaging.
The rate of recovery is determined by the specific moisture content and fiber type of the board.
