The thickness or weight of paper rolls, often referred to as caliper (thickness) or basis weight (weight per unit area), is determined during production through careful control of the papermaking process. The following steps and factors ensure consistency and precision:

1. Raw Material Preparation

  • Fiber Selection:
    • Softwood fibers (longer) are used for strength, while hardwood fibers (shorter) enhance smoothness and printability.
    • The type and blend of fibers influence the paper’s density and thickness.
  • Pulp Consistency:
    • Adjusting the water-to-pulp ratio in the slurry (fiber suspension) determines fiber concentration, directly impacting the final sheet’s weight.

2. Formation of Paper

a. Wet-End Control

  • Headbox:
    • The headbox sprays the slurry onto the moving wire mesh in the papermaking machine.
    • Flow rate and speed control at the headbox determine the paper’s uniformity and thickness.
  • Fiber Orientation:
    • The alignment and settling of fibers on the wire affect the paper’s density and caliper.

b. Pressing

  • After water is drained from the slurry on the wire mesh, the wet paper passes through press rollers.
  • The pressure applied removes excess water and compacts the fibers, influencing the paper’s density and caliper.

c. Drying

  • Heated drying cylinders evaporate remaining moisture, solidifying the paper structure.
  • The drying tension and temperature can affect thickness and basis weight.

3. Thickness (Caliper) Control

  • Definition: Thickness refers to the distance between the two surfaces of the paper, typically measured in microns (µm) or mils.
  • Factors:
    • Fiber density and compression during pressing.
    • Coatings or additives applied to the surface.
  • Measurement:
    • Specialized calipers measure the thickness at multiple points for consistency.

4. Weight (Basis Weight) Control

  • Definition: Basis weight is the weight of the paper per unit area, typically measured in grams per square meter (GSM) or pounds (lb) per ream (a ream is usually 500 sheets).
  • Factors:
    • The pulp’s fiber concentration.
    • The speed of the papermaking machine (faster speeds can produce thinner sheets).
    • Additives like fillers (e.g., calcium carbonate) can increase weight without significantly changing thickness.
  • Calculation:
    • Basis weight is calculated during production by weighing a known area of the paper (e.g., 1 square meter).
    • Machines are equipped with sensors and gauges to ensure consistent weight across the roll.

5. Quality Assurance During Production

a. Online Sensors and Scanners

  • Modern papermaking machines use sensors and scanners to monitor thickness and weight in real time.
  • Devices like beta gauges or infrared sensors measure the paper’s mass and density as it moves through the production line.

b. Adjustments:

  • Operators adjust slurry flow, roller pressure, or drying temperature based on sensor feedback to maintain desired specifications.

6. Post-Production Testing

a. Thickness Testing

  • Measured using calipers or micrometers at controlled points on the roll.
  • Ensures the roll meets specifications for uniformity and caliper tolerance.

b. Weight Testing

  • Small paper samples are cut and weighed to verify GSM or basis weight.
  • Spot checks are conducted to ensure consistency throughout the roll.

7. Influence of Coatings and Finishes

  • Impact:
    • Coatings (e.g., clay, gloss, or wax) and surface treatments can add weight and slightly alter thickness.
    • Additional layers or laminates increase caliper while enhancing functionality (e.g., moisture resistance, printability).

8. Environmental Factors

  • Moisture Content:
    • The moisture content in the paper affects its weight and thickness. Higher moisture levels can make the paper heavier without changing its caliper significantly.
  • Temperature and Humidity:
    • Controlled environments in production facilities prevent fluctuations in paper properties caused by changes in humidity.