Corrugated medium — also called fluting paper — forms the arched inner layer of corrugated board. It requires a unique combination of properties: high compression strength (CMT — Concora Medium Test), adequate stiffness to maintain flute geometry during converting, and sufficient surface quality for starch adhesion during corrugator operation. The stock preparation approach for fluting differs significantly from that of linerboard.

What Makes Corrugated Medium Different from Linerboard

While kraft linerboard prioritizes burst and tensile strength, corrugated medium is judged by:

• CMT (Concora Medium Test): Typically 150–250 N for standard grades (120–150 g/m²); this measures the flat crush resistance of the formed flute
• SCT (Short-span Compression Test): CD-SCT of 2.0–3.5 kN/m is a better predictor of box compression strength than CMT alone
• Ring crush: Related to edgewise compression; target 1.2–1.8 kN/m for 127 g/m² medium
• Porosity: Critical for even starch absorption during corrugating — Gurley porosity target of 3–10 seconds
• Water absorption (Cobb): 80–150 g/m² to ensure adequate starch pickup without over-wetting

Furnish Sources for Corrugated Medium

Corrugated medium is predominantly made from recycled fiber:

• Mixed waste paper: The most common furnish, typically 60–85% of total fiber input
• OCC (Old Corrugated Containers): Provides longer fiber for strength; 15–40% depending on quality requirements and availability
• Semi-chemical pulp (NSSC): Used for high-performance medium grades (CMT >200 N). The NSSC process uses sodium sulfite cooking at 160–175°C with yield of 70–80%, preserving hemicelluloses that contribute to fiber bonding

Pulping for Corrugated Medium: Low-Intensity Is Key

Unlike kraft liner production where fiber strength development is paramount, corrugated medium pulping prioritizes fiber preservation and contaminant dispersion at minimum energy input:

• Pulper type: D-type or drum pulper at 12–15% consistency for mixed waste. Lower consistency (12–14%) compared to OCC pulping for liner (14–18%) because mixed waste contains more short fiber that is sensitive to cutting
• Extraction plate: 10–14 mm holes — larger than for liner OCC pulping (8–12 mm) since downstream screening handles contaminant removal
• Pulping time: 12–20 minutes — shorter than OCC-to-liner because over-pulping generates fines that reduce CMT without any compensating benefit
• Specific energy: 18–28 kWh/t — the lowest among paper grades; every extra kWh in pulping is essentially wasted energy for medium production

Screening: Balancing Contaminant Removal with Yield

Corrugated medium tolerates higher contaminant levels than linerboard since it’s an interior layer. This changes the screening calculus:

• Primary screens: 0.30–0.40 mm slots (vs. 0.25–0.35 mm for kraft liner). The wider slots increase throughput and reduce energy while still removing problematic large contaminants
• Reject rate: 15–20% — lower than for liner (20–25%) since fiber yield is a larger economic driver
• Cascade configuration: 2-stage screening is usually sufficient for medium; 3-stage adds cost with marginal quality improvement for this grade

Refining for Fluting: Less Is More

This is where corrugated medium diverges most sharply from linerboard. For medium production:

• Refining intensity: Light refining at most. SEL of 0.5–1.0 J/m — about 50% of what kraft liner requires
• Target freeness change: 30–60 mL CSF reduction. The goal is to improve formation and sheet consolidation, NOT to develop strength
• Specific energy: 30–50 kWh/t — compared to 60–120 kWh/t for kraft liner OCC. Heavy refining actually damages CMT by shortening fibers
• Many lines skip refining entirely for 100% mixed waste medium grades, relying on the pulper alone for fiber treatment

The physics here is straightforward: CMT depends on fiber stiffness and flute geometry, not on inter-fiber bonding. Refining that increases bonding area through fibrillation doesn’t translate to higher CMT — but it does increase refining energy cost and drainage resistance.

Approach Flow and Wet-End Considerations

The wet end for corrugated medium is simpler than for liner or coating board, but a few considerations matter:

• Headbox consistency: 0.4–0.8% — slightly higher than for liner because formation requirements are less stringent
• Wire section: Single Fourdrinier with 35–45 mesh forming fabric. Vacuum foil boxes with progressively increasing vacuum (5–25 kPa) provide adequate drainage
• Press section: Double-felted roll press with nip pressure 60–90 kN/m achieves 42–46% solids
• Drying: 20–30 drying cylinders at 100–140°C surface temperature. Specific steam consumption: 1.6–1.9 tonnes steam per tonne of paper

Energy Benchmarks for Corrugated Medium

Compared to other paper grades, corrugated medium has lower specific energy requirements:

• Stock preparation (total): 120–180 kWh/t from 100% recycled fiber
• Paper machine: 280–380 kWh/t (including drives, vacuum, auxiliaries)
• Total production electrical: 400–560 kWh/t
• Steam consumption: 1.6–1.9 t steam/t paper

For mills producing both corrugated medium and test liner on adjacent machines, sharing a common OCC pulping and screening system with separate refining branches for each grade can yield capital savings of 25–35% compared to two independent lines.

📧 Contact us at leizhanzhang@gmail.com for corrugated medium production line equipment recommendations.