Essential Fabric Terminology
Understanding PMC specifications is the foundation for selecting the right fabric. This guide explains the key terms you will encounter when evaluating forming fabrics, press felts, and dryer fabrics.
Air Permeability (CFM)
What it is: The volume of air (in cubic feet) passing through one square foot of fabric per minute under a pressure differential of 125 Pa.
Why it matters:
- Higher CFM = Faster dewatering, but lower fiber retention
- Lower CFM = Better retention, but slower dewatering
- Must be matched to paper grade and machine speed
Typical Ranges by Section:
| Section | CFM Range |
|---|---|
| Forming (Tissue) | 250-400 |
| Forming (Packaging) | 400-650 |
| Forming (Kraft) | 450-700 |
| Dryer (Tissue) | 300-500 |
| Dryer (Packaging) | 500-800 |
Fiber Support Index (FSI)
What it is: A measure of the number of support points the fabric provides per unit area for fiber retention.
Why it matters:
- Higher FSI = Better sheet formation, better fiber retention
- Each warp-weft intersection = One anchor point for fibers
Typical FSI by Structure:
| Structure | FSI Range |
|---|---|
| Single Layer | 50-80 |
| Double Layer | 80-120 |
| Triple Layer | 130-170 |
| SSB | 150-200+ |
Mesh Count
What it is: The number of warp (machine direction) yarns per centimeter.
Why it matters:
- Higher mesh count = Finer surface, suitable for lightweight sheets
- Lower mesh count = More open, suitable for heavy basis weight dewatering
Selection Guide:
| Paper Grade | Recommended Mesh |
|---|---|
| Tissue (<40 gsm) | 70-95 threads/cm |
| Culture Paper (40-120 gsm) | 50-70 threads/cm |
| Packaging (80-200 gsm) | 35-55 threads/cm |
| Heavy Weight (200-400+ gsm) | 25-40 threads/cm |
Caliper (Thickness)
What it is: The thickness of the fabric, measured in millimeters.
Why it matters: Affects water-carrying capacity in press felts and sheet handling characteristics.
Tensile Strength
What it is: The maximum load a fabric can withstand before breaking, measured in kN/m.
Why it matters: Must exceed running tension with a safety margin (typically 5-8× running tension).
Void Volume (Press Felts)
What it is: The amount of open space within the felt structure available for water storage, measured in cc/m².
Why it matters: Higher void volume = Greater water-handling capacity, longer effective life.
Basis Weight (Press Felts)
What it is: The weight of the fabric per square meter (gsm).
Typical Ranges:
| Application | Basis Weight (gsm) |
|---|---|
| Tissue | 1100-1400 |
| Culture Paper | 1300-1600 |
| Packaging | 1400-1700 |
| Shoe Press | 1500-1800 |
Continue Learning
Once you understand the terminology, continue to PMC Selection Basics to learn how to apply these specifications when choosing fabrics.
