Relative Humidity

Atmospheric State

Ratio of the amount of water vapour present in the air to the maximum amount the air could hold at that temperature dictates the moisture exchange with porous materials. Controlling relative humidity on the press floor is the primary factor in maintaining the dimensional stability of paper and board. Fluctuations in air moisture cause the paper to absorb or release water, leading to changes in size, flatness, and runnability.

Moisture Equilibrium

Paper is a hygroscopic material that naturally seeks to balance its internal moisture with the surrounding air. When the humidity rises, the paper absorbs moisture and expands, while dry air causes it to release moisture and shrink. These dimensional changes can lead to tight edges, wavy edges, or overall sheet curl, which can cause feed jams and registration errors on the press.

In extreme cases, rapid humidity shifts can cause the paperboard to split or crack when it is folded on the high-speed gluer lines because the fibres have lost their natural elasticity.

Environmental Control

Pressrooms use automated humidification and air conditioning systems to maintain a constant humidity level, typically between forty-five and fifty-five percent. Paper is stored in its protective wrapping until it is ready to be printed, allowing it to acclimatize to the pressroom temperature first. This careful management ensures that the paper remains flat and stable throughout the printing and converting process.

An automated mechanical chuck engages a preformed light gray molded pulp section inside a specialized industrial mounting station for material testing.

Creasing Matrix Hemicellulose Plasticization Capillary Condensation

Interfiber Bond Disruption and Mechanical Modulus Decay in Humid Converting Environments

Elevated humidity causes gas-phase moisture sorption that disrupts interfiber hydrogen bonds, reducing sheet elastic modulus and causing converting failure.

A glass jar containing a stainless steel extraction cell sits on a substrate sheet in a print production facility.

Fickian Diffusion ISO 187 Dew Point

Thermal Gradients and Vapor Migration in Stretch Wrapped Unitized Pallet Loads

Thermal gradients across stretch-wrapped pallets drive internal vapor to cold perimeters, causing condensation that cuts box strength by over forty percent.

A gloved hand inspects aligned substrate samples spanning brown kraft paper and colored polymer coatings mounted on a mechanical testing track.

LW Stiffness CTMP Core GC2

Standard Laboratory Conditioning Protocols for Coated Folding Boxboard Evaluation

Valid boxboard physical testing requires ISO 187 conditioning at 23°C and 50% RH to prevent hysteresis errors in stiffness, yield, and creasing metrics.

A digital analytical scale with a glass chamber rests on a dark surface alongside a stack of paper samples in this synthetic laboratory rendering.

Bulk Conditioning Standard Solid Bleached Sulfate

Standard Laboratory Methods for Paperboard Flexural Rigidity

Paperboard flexural rigidity depends on the cube of caliper, requiring exact ISO 2493 or TAPPI T 489 instrument alignment and strict 23°C/50% RH conditioning.

Stacks of dark specialty cardstock feature deep tactile surface patterns including linear hatching and concentric grooves arranged on a geometric colored surface.

Viscoelastic Creep Equilibrium Moisture Content ISO 187

Quantifying Time Dependent Viscoelastic Creep and Moisture Recovery in Heat Embossed Recycled Paperboard Blanks

Heat embossing above lignin glass transition temperature locks paperboard relief depth, while controlled dwell time minimizes post-press viscoelastic creep.

Industrial shelving holds stacked paper substrates and a dark equipment stand within a controlled warehouse environment for printing production.

Thermal Mass Dwell Time Temperature Differential Cross Direction Expansion

Cold Warehouse Thermal Reconditioning Procedures to Prevent Edge Waviness and Fluting Defects

Cold paper pallets demand staged thermal acclimation inside unopened vapor wrap to eliminate edge waviness and fluting caused by moisture sorption gradients.

Stacked white paper cups move uniformly along a stainless steel conveyor system inside a clean industrial production environment.

Cobb 60 Dew Point Condensation Delamination Stress

Managing Paperboard Hygroexpansion and Moisture Balance in Converting Operations

Targeting 5.5 to 6.5 percent board moisture and maintaining 50 percent relative humidity prevents hygroexpansion fan-out, registration drift, and score cracking.

A technician wearing protective gear inspects a textured fiber substrate sheet near industrial machinery and raw material conveyor belts in a production facility.

Folding Boxboard Multi-Ply Recycled Board Crease Stiffness

Predicting Interlaminar Shear Shear Breakdown Thresholds in Recycled Multi-Ply Boxboards under Variable Ambient Humidity Conditions

Recycled boxboard interlaminar shear thresholds decay non-linearly above 65% RH due to fiber hornification and starch matrix plasticization.

A digital illustration depicts industrial binding machinery alongside a stack of printed sheets and an abstract paper form.

Hydrogen Bond Thermal Stretching Nip Frictional Heat Polyethylene Terephthalate

Substrate Core Thermal Expansion Limits in High Speed Sheetfed Converting Operations

Substrate core thermal expansion limits dictate sheet dimensional stability, requiring precise thermal control to prevent misregister and bimetallic warp.

Relative Humidity

Atmospheric State

Moisture Equilibrium

Environmental Control

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