Classification and Characteristics of Headboxes

Headbox is a key component of paper machines, which undertakes the functions of stock distribution, flow homogenization and jetting. Its performance directly determines paper formation, basis weight profile and overall product quality. According to structural design and working principles, headboxes are mainly divided into five categories: open type, air-cushion type, full-flow hydraulic type, combined full-flow air-cushion type and multi-layer forming type.

1. Open Type Headbox

Working Principle

The upper part of the box is open to the atmosphere. Stock jet velocity is controlled by static pressure generated by the stock level inside the headbox.

Advantages

• Simple structure, low investment cost and easy maintenance.
• Easy operation, suitable for low-speed production and low-basis-weight paper grades.

Disadvantages

• Limited operating speed, generally below 200 m/min.
• Unstable stock level leads to poor cross-machine basis weight uniformity and easy generation of streaks.
• Air is easily entrained, which adversely affects paper formation.

Application

Low-speed production lines for cultural paper, tissue paper, paperboard and nonwoven fabrics.

2. Air-Cushion Type Headbox

Working Principle

The entire box is fully enclosed. Compressed air is filled above the stock surface to form an air cushion for pressure stabilization. Stock velocity is regulated by adjusting air pressure, and flow homogenizing rolls are equipped for flow rectification.

Advantages

• Low stock level with stable air cushion pressure; wide and responsive adjustment range for stock speed.
• Better cross-machine uniformity and less air entrainment compared with open type.
• Applicable machine speed: 200 ~ 500 m/min.

Disadvantages

• Relatively complex structure, requiring a stable compressed air supply system.
• Homogenizing rolls are prone to stock adhesion, demanding regular cleaning and maintenance.
• Insufficient micro-turbulence at speed over 600 m/min, causing easy fiber flocculation.

Application

Medium-speed production lines for cultural paper, packaging paper and white cardboard.

3. Full-Flow Hydraulic Headbox

Working Principle

Designed as full-flow structure without large stock pond. Stock is delivered by pump pressure through tapered header, step diffusers and foils. High-intensity micro-turbulence is generated to disperse fibers effectively.

Advantages

• High-intensity and fine-scale turbulence realizes excellent fiber dispersion and minimizes flocculation.
• Outstanding cross-machine basis weight uniformity and superior paper formation.
• Suitable for high-speed paper machines with operating speed up to 500 ~ 1200 m/min.
• Equipped with dilution water regulation system for high-precision cross-machine basis weight control.

Disadvantages

• Sophisticated structure with high manufacturing and installation precision requirements and high cost.
• Complicated flow passages are easy to clog, requiring meticulous daily maintenance.
• Sensitive to fluctuations of stock consistency and pressure, hence high requirements for automatic control system.

Application

High-speed production lines for high-grade cultural paper, coated paper and thin paper.

4. Combined Full-Flow Air-Cushion Headbox

Working Principle

Adopts full-flow structure as the main body, combined with auxiliary air cushion. The air cushion chamber is connected to the stock pond via small holes to suppress liquid level fluctuation.

Advantages

• Integrates the merits of high turbulence of hydraulic headbox and pressure stabilization of air-cushion headbox.
• Strong anti-fluctuation capacity, stable operation at 600 ~ 1000 m/min.
• Excellent cross-machine uniformity and flexible dilution water regulation.

Disadvantages

• The most complex structure with high procurement and maintenance costs.
• Difficulties in coordinated control between air cushion and full-flow system.

Application

High-speed wide-width production lines for high-grade paper and paperboard.

5. Multi-Layer Forming Headbox

Working Principle

Realizes Z-direction layered forming. Each layer is equipped with independent stock distribution, flow rectification and feeding units. Different stock formulations are jetted simultaneously for composite forming.

Advantages

• Supports layered batching: using high-quality pulp for surface layer and recycled pulp for core layer to reduce production cost while improving quality.
• Independent control over basis weight and fiber orientation of each layer.
• Finished paper features high strength, good surface performance and strong interlayer bonding force.

Disadvantages

• Large overall structure, high investment cost and complex control system.
• Strict requirements for layer alignment and interlayer bonding control.

Application

High-grade paperboard, linerboard, corrugated board and liquid packaging paper.

Comparison of Main Types

Selection Guidelines

1. Machine Speed: Choose open type for speed below 200 m/min; air-cushion type for 200 ~ 500 m/min; hydraulic type or combined type for speed over 500 m/min.
2. Quality Requirement: Select full-flow hydraulic headbox for high-grade paper and thin paper; air-cushion type for standard paper; multi-layer type for paperboard.
3. Budget: Prioritize open type or air-cushion type for cost control; choose hydraulic type or multi-layer type for premium quality pursuit.
4. Stock Characteristics: Apply high-turbulence hydraulic headbox for short fibers and flocculation-prone stock; air-cushion type for long fibers.