Managing Thermal Expansion in Brass Embossing Dies
Managing thermal expansion in brass embossing dies requires pre-scaling vector artwork by calculating precise isotropic expansion across operating press temperatures.
Substrate Hygroexpansivity
Dimensional Property
Physical changes in the length and width of a paper or board sheet occur as it absorbs or releases moisture in response to ambient humidity. The coefficient of substrate hygroexpansivity describes how much a material will swell or shrink for every percentage point change in its water content. Managing this movement is a primary challenge in multi-pass printing and precision converting.
Fibre Response
Alignment between different print stations becomes impossible if the paper dimensions change between the first and last colour. Because substrate hygroexpansivity is higher in the cross-machine direction, printers often orient the grain to minimize the impact on critical registration points. The internal stress caused by uneven moisture absorption can lead to curling or waviness at the edges of the stack.
Conditioning the stock in the pressroom environment for forty-eight hours helps to stabilize these dimensions.
Printing Tolerance
Plastic films and foils do not exhibit this property to the same degree as paper, leading to curling in laminated structures. If the humidity in a warehouse is not controlled, the substrate may grow enough to cause jams in high-speed feeders. The limit of acceptable movement is defined by the registration tolerance of the specific printing process.