In recycled containerboard production, variability in OCC fiber quality—driven by origin, grade, and recycling history—has become a long-term operating condition rather than a temporary fluctuation. These differences directly influence refining response, final paper strength, and paper machine runnability.
Based on PMTEC’s engineering involvement in recycled fiber stock preparation and approach flow systems, refining performance is increasingly determined by fiber condition and system coordination, rather than refining energy input alone.
Ⅰ、 Refining Effects on Strength Properties: A Differentiated Response
Refining improves fiber bonding mainly through increased fibrillation and flexibility. However, different strength properties exhibit distinct response patterns.
Typical Strength Trends with Increasing Refining Degree
| Strength Property | General Trend | Technical Explanation |
| Burst strength | Gradual increase | Dominated by inter-fiber bonding |
| Ring Crush / Compression | Gradual increase | Sensitive to bonding and sheet density |
| Folding endurance | Increase → optimum → decrease | Fiber fatigue and cutting at high intensity |
| Tear strength | Increase → optimum → decrease | Strongly dependent on effective fiber length |
While burst and compression often improve continuously, tear and folding strength usually peak at moderate refining levels and decline with over-refining.
Ⅱ、Refining Influence on Paper Machine Runnability
Refining alters not only fiber morphology but also drainage characteristics and stock rheology, directly affecting paper machine operation.
Common Operational Impacts of Higher Refining Levels
- Reduced drainage rate in the forming section
- Increased vacuum demand in wet-end dewatering elements
- Higher sensitivity to retention aid dosage and shear conditions
- Lower press section dryness at equal loading
- Increased specific steam consumption in the dryer section
In practice, these effects often appear before strength gains reach saturation.
Ⅲ、 OCC Quality as the Primary Refining Constraint
The refining response of OCC is strongly governed by intrinsic fiber condition.
| OCC Category | Typical Fiber Characteristics | Refining Tolerance |
| High-grade OCC (imported A-grade) | Longer fibers, low contaminants | High |
| Medium-grade OCC | Mixed fiber population | Moderate |
| Low-grade / mixed OCC | Short, hornified fibers, higher fines | Low |
Low-grade OCC generally shows limited bonding improvement under aggressive refining.
Ⅳ、 Typical Failure Mechanism with Low-Quality OCC
When refining intensity exceeds fiber tolerance, mills commonly observe:
- Decline in tear and folding strength
- Increased fines generation and water retention
- Deterioration of drainage
- Reduced stable machine speed
- Increased specific energy consumption
At this stage, refining becomes counterproductive.
Ⅴ、PMTEC-Recommended Refining Optimization Logic
Effective OCC refining requires fiber differentiation and system-level alignment, rather than uniform energy input.
Fiber Allocation Strategy
- Higher-quality OCC → surface or performance-critical layers
- Lower-quality OCC → core layers with conservative refining
Typical Refining Degree Ranges (Reference)
| OCC Quality | Refining Range (°SR) | Objective |
| High-grade OCC | 34–38 | Strength development |
| Medium-grade OCC | 31–34 | Strength–runnability balance |
| Low-grade OCC | 28–32 | Drainage protection |
Ⅵ、Supporting Measures Beyond Mechanical Refining
Refining is most effective when combined with complementary measures:
- Chemical strength agents (starch, CPAM, CMC)
- Screening and cleaning to control fines
- Coordination with retention and wet-end shear conditions
These measures often improve strength-to-energy efficiency.
Ⅶ、Key Parameters That Should Be Evaluated Together
| Stock Preparation | Wet End | Dry End |
| Refining degree (°SR / CSF) | Vacuum level | Dryer steam |
| WRV | Formation stability | Final moisture |
| Specific refining energy | Press dryness | Strength variability |
Ⅷ、Engineering Perspective from PMTEC
Refining is a controlled adjustment tool, not a universal correction method.
Sustainable OCC-based papermaking depends on:
- Accurate fiber quality evaluation
- Refining intensity matched to fiber condition
- System-wide coordination
The objective is maximum effective fiber utilization per unit of energy and water consumed.
