Pressboard is a technical material made from compressed cellulose fibres. Through mechanical pressing and targeted use of binders, a solid, homogeneous material with good electrical insulation properties is created. Simply put, pressboard is „compressed paper“ that, thanks to its homogeneous structure, is used versatilely in the electrical industry and mechanical engineering.
In the electrical insulation industry, pressboard plays a central role: from transformers through electric motors to electrical devices. It is one of the oldest and at the same time most established cellulose-based insulating materials.
Manufacturing and structure
Pressboard is obtained from high-purity cellulose (often from sulphate or sulphite pulp). Manufacturing involves:
- Fibre preparation: removal of lignin, resins and contaminants.
- Fibre bonding: felting through hydrogen bonds and press pressure.
- Pressing: compressed under pressure and heat into homogeneous sheets or rolls.
- Drying and smoothing: for mechanical stability and a uniform surface.
Depending on the application, different qualities are produced: from simple board sheets to highly compacted insulation pressboard for high-voltage transformers.
Properties of pressboard
Electrical properties
- High dielectric strength (up to >10 kV/mm depending on thickness).
- Good insulation resistance.
- Low dielectric losses.
Mechanical properties
- Compression and bending resistant.
- Good processability (stamping, cutting, bending).
- Dimensional stability when stored per standard.
Thermal properties
- Heat resistant up to approx. 105 degrees C (thermal class A).
- Can be combined with other materials for higher requirements (for example Nomex).
Ecological
- Cellulose-based, largely recyclable.
Standards and qualities
Pressboard is standardised and available in various types:
- DIN 7733: defines pressboard for electrical engineering purposes.
- IEC 60641: international standard for electrical pressboard papers.
- Thermal class A (105 degrees C) per IEC 60085.
- Typical qualities: pressboard 3010, 3020 etc., depending on strength and density.
Applications in electrical industry and mechanical engineering
Transformers
- Winding insulation, cylinders, spacers, cooling channels.
- Use in oil-filled transformers thanks to good oil impregnation.
Electric motors
- Slot insulation, layer insulation, spacer discs.
- Combination with other insulating materials (Nomex, polyester films).
Cable and electrical engineering
- Separator layers in cables and coils.
- Stamped parts for electrical devices.
Mechanical engineering
- Pressboard sheets as insulating parts in switchgear.
- Spacers, spacer rings, simple covers.
Advantages and limits
Advantages
- Cost-effective, established material.
- Good electrical insulation.
- Easy to process (stamping, cutting, bending).
- Compatible with oil impregnation processes.
Limits
- Limited thermal resistance (class A, max. 105 degrees C).
- Hygroscopic, therefore sensitive to moisture.
- Not suitable for high-temperature applications (here: Nomex, Kapton or Mica required).
Examples from practice
- Transformer construction: pressboard cylinders are manufactured as stable formed bodies in which windings are embedded.
- Electric motors: slot insulation made of pressboard serves as an economical alternative to PET or aramid films in standard motors.
- Electrical devices: small stamped parts such as washers, discs and covers.
GOBA Takeaway
Pressboard is one of the classic insulating materials in the electrical industry. Its strengths lie in cost-efficiency, good electrical properties and easy processing. In applications with moderate temperatures and voltages it is the first choice, particularly in transformers and standard motors. For higher loads, however, modern high-temperature materials such as Nomex, Kapton or mica products are required. For design, purchasing and quality assurance: pressboard remains an indispensable material, as long as it is specified for the right operating environment.