Press Felt Weight, Permeability and Void Volume: How to Read a Specification

Press-felt quotations often list weight, air permeability and caliper. Mills may also discuss void volume or water-handling capacity. These values are useful, but none is a stand-alone prediction of dryness, marking, startup or service life.

The correct comparison asks three questions: what was measured, how was it measured, and why does the value fit this specific press position?

Basis Weight

Basis weight is the mass of felt per unit area, commonly reported in g/m². It reflects the combined base fabric and batt mass, but does not reveal how that material is distributed.

Two felts with similar weight can differ in base construction, batt fineness, needling, surface density and internal openness. More weight may provide material for wear or water management, but can also affect startup, saturation and compaction. Do not treat GSM as a durability score.

Air Permeability

Air permeability measures airflow through a sample under a defined pressure difference. It helps characterize openness and compare controlled samples. Always record the unit, test pressure, sample condition and test standard.

New-felt permeability does not equal dynamic water flow in a loaded nip. Water viscosity, saturation, compressive pressure, rebound, contamination and Uhle-box conditions all change actual behavior. Values from different methods should not be placed in one table without conversion and method review.

Caliper and Compressibility

Caliper is felt thickness under a specified measuring load. The load matters because a soft sample compresses during measurement. Initial caliper, compressed caliper and recovery describe different aspects of the structure.

On the machine, the felt repeatedly compresses and rebounds. A design must maintain useful void behavior without creating excessive marking or instability. Therefore, a single unloaded thickness cannot define nip performance.

Void Volume

Void volume describes the internal space available within a structure, but suppliers may calculate or measure it differently. Ask whether the value is geometric, laboratory-derived or measured under compression, and request the units and test condition.

Effective water-handling space changes with saturation, load and felt age. Contamination can occupy pores; compaction can reduce recoverable space. A high initial value is not automatically better if the structure cannot release water or remain stable.

Read the Numbers as a System

For one press felt, evaluate the specification with:

• base-fabric type and yarn system;
• batt fiber size, blend and surface design;
• basis weight and tolerance;
• permeability, unit and test method;
• caliper and measurement load;
• seam or endless construction;
• finished dimensions and tolerances;
• heat or chemical resistance requirements;
• intended press position and startup procedure.

The same nominal permeability can be produced by different pore distributions. The same weight can be placed in different layers. That is why the construction and application rationale belong beside the numbers.

Compare Against Machine Evidence

Use the current felt as a baseline. Provide original and used-felt data together with press type, speed, nip load, paper grade, furnish, sheet moisture profile, felt moisture, vacuum, shower settings, service life and failure mode.

If the problem is high sheet moisture, follow the systematic troubleshooting workflow before changing one specification value. For introductory structure information, see What Is Press Felt?.

Questions to Put on the Purchase Order

Require units and test methods, manufacturing tolerances, finished-size tolerances, seam details, installation direction, storage requirements and the agreed application position. Record any approved substitution. A bare line such as “press felt, 1,500 GSM” is not sufficient to reproduce performance.

Ask the supplier to explain the tradeoff behind a proposed change. For example: which layer changes, what operating symptom it targets, what machine adjustment may be needed, and what trial data should be recorded?

Conclusion

Weight, permeability, caliper and void volume are coordinates, not a complete design. They become useful when test conditions are explicit and they are connected to the felt construction and actual press position. Contact HengXing with your current specification and operating baseline for a controlled comparison.