company news about Why Corrugated Paper Slitting Requires Upper and Lower Blade ？

In the slitting process of corrugated paper processing, many professionals wonder: Why can’t a single knife be used like cutting ordinary paper, but instead a combination of "upper knife + lower knife" is necessary? The core reason lies in the special multi-layered wave structure of corrugated paper—it consists of face paper, corrugated medium, and inner paper, with the middle corrugated medium forming a wave shape (with peaks and valleys). A single knife will collapse the peaks, cause edge delamination or burrs, while the cooperation of upper and lower knives achieves smooth, efficient slitting while protecting the corrugated structure through the "active cutting + passive support" method. This article breaks down the necessity of this design from the perspective of corrugated paper structural characteristics, the functional division of upper and lower knives, cooperation advantages, and key parameters, helping you understand the underlying logic of "why upper and lower knives are indispensable" and how to optimize their cooperation to improve slitting results.

To understand the necessity of upper and lower knives, we first need to clarify the essential difference between corrugated paper and ordinary paper: it is not a single-layer flat structure but a "sandwich-like" composite structure. This structure means slitting must balance two core needs—"cutting through" and "protecting the structure"—which a single knife cannot satisfy simultaneously.

Concrete Problem: If a single knife (e.g., a top-mounted circular knife) is used, the blade first presses the face paper and then acts on the corrugated medium. Without bottom support, the medium’s peaks are crushed (like pressing a wave-shaped paper with a finger). This not only reduces the corrugated paper’s compressive strength by 30%–50% but also leaves the inner paper uncut, leading to "delamination and cracking" during subsequent box folding.

The "upper knife + lower knife" combination is not simply "two knives stacked" but a professional system designed for corrugated paper structure. The two knives differ completely in type, material, installation position, and function—yet neither can be missing.

Compared with single-knife slitting, the upper-lower knife combination has significant advantages in slitting quality, efficiency, and cost—all directly affecting the qualification rate and productivity of downstream corrugated paper processing (e.g., carton forming).

The lower knife’s groove fits precisely with the corrugated medium’s flute type (e.g., 4.5mm width for A-flute, 2.5mm for B-flute). During slitting, only the upper knife cuts the medium’s peaks, while the valleys remain supported by the lower knife. This "cut peaks only, not valleys" method preserves 100% of the medium’s wave structure.

• Practical Test Data: Corrugated paper slit by upper-lower knives has a compressive strength 25%–35% higher than that by single knives. Cartons made from it can be stacked 1–2 layers more (e.g., 5 layers for single-knife slitting vs. 6–7 layers for upper-lower knife slitting).

The "pinch cutting" of upper and lower knives enables "synchronous cutting": the upper knife first cuts the face paper and peaks, the lower knife supports the inner paper from below, and as the upper knife presses further, the two work together to cut the inner paper. No fiber pulling occurs during the process, and burr length can be controlled to ≤0.1mm.

• Comparison Effect: Single-knife slitting often produces burrs of 0.3–0.5mm, leading to cracking during subsequent box folding due to "fiber breakage"; upper-lower knife slitting results in flat edges, increasing box folding qualification rate from 85% (single knife) to over 99%.

Modern slitters with upper-lower knives can achieve slitting speeds of 200–300m/min (equivalent to 120,000–180,000 meters per hour) and run continuously for 4–6 hours without stopping. This is because the servo motor precisely synchronizes the rotation speeds of upper and lower knives (speed difference ≤0.5%), avoiding "knife jamming" or "misalignment" common in single-knife slitting.

• Efficiency Comparison: Single-knife slitting speed is only 80–150m/min, requiring stops every 1–2 hours to clean burrs or adjust the knife. The daily output of upper-lower knife slitting is 1.5–2 times that of single-knife slitting.

A single knife bears all cutting pressure and the medium’s reaction force, leading to fast wear (e.g., ordinary alloy single knives have a service life of only 300–500 hours). In contrast, upper-lower knife cooperation distributes force: the upper knife mainly endures cutting force, and the lower knife mainly bears support force, resulting in more uniform wear.

• Cost Calculation: If tungsten carbide is used for the upper knife (800–1200 hours service life) and alloy for the lower knife (1500–2000 hours service life), the knife cost per ton of corrugated paper is approximately 1.2–1.5 yuan. For single-knife slitting, the cost is 2.0–2.5 yuan—upper-lower knife solutions reduce costs by 30%–40%.

Even with the right upper and lower knife materials, improper cooperation parameters can cause burrs or peak collapse. Below are 3 parameters that must be strictly controlled in industry:

The gap refers to the distance between the upper knife’s edge and the lower knife’s groove. It must be flexibly adjusted based on the corrugated paper’s total thickness (face paper + medium + inner paper):

• Thin Corrugated Paper (e.g., B-flute, total thickness 2–3mm): Gap 0.1–0.15mm to avoid uncut inner paper due to excessive gaps.
• Thick Corrugated Paper (e.g., A-flute, total thickness 4–5mm): Gap 0.2–0.3mm to prevent accelerated edge wear from friction between upper and lower knives due to too small gaps.
• Common Myth: "Smaller gaps are better"—excessively small gaps cause direct contact between upper and lower knives, leading to chipping of tungsten carbide edges; excessively large gaps allow the medium to squeeze through, causing "burrs and delamination."

The upper knife’s edge angle directly affects cutting resistance and edge quality. It must be adjusted based on the medium’s basis weight (hardness):

• Soft Medium (basis weight ≤110g/m²): Angle 30°–35° to reduce cutting resistance and avoid medium tearing.
• Hard Medium (basis weight ≥140g/m²): Angle 40°–45° to enhance cutting force and ensure complete peak cutting.
• Consequence of Improper Angle: Angles <25° cause the upper knife to "slip" and fail to cut the medium; angles >50° squeeze the medium, deforming peaks.

During slitting, the linear speed of the upper knife (circular knife) (π*diameter*rotational speed) must be completely synchronized with that of the lower knife to avoid "relative sliding":

• If Upper Knife Rotates Too Fast: It pulls the paper, causing "tailing burrs" on edges.
• If Lower Knife Rotates Too Fast: It pushes the paper, leading to slitting width deviation exceeding ±0.3mm (e.g., target width 500mm vs. actual 500.5mm).
• Solution: Modern slitters are equipped with dual servo motors, achieving speed synchronization accuracy of ±0.5% without frequent manual adjustments.

Fact: The lower knife’s support effect directly determines slitting quality. Worn or deformed lower knife grooves (e.g., flattened groove bottoms) fail to fit valleys, causing medium collapse. Even with a sharp upper knife, the slit corrugated paper will suffer structural damage. For high-wear scenarios (e.g., daily slitting >100,000 meters), tungsten carbide is recommended for lower knives; alloys work for ordinary scenarios, but groove flatness must be checked every 800 hours.

Fact: Lower knives must be replaced based on the corrugated paper’s flute type. For example, slitting A-flute (peak height 4.5mm) and B-flute (peak height 2.5mm) requires adjusting the lower knife’s groove depth and width—using A-flute lower knives for B-flute paper leaves valleys unsupported (excessive groove width), while using B-flute lower knives for A-flute paper squeezes peaks (insufficient groove width). Both cause poor slitting.

Fact: Knife wear changes gaps, requiring regular checks and adjustments. For example, after 500 hours of use, the upper knife’s edge wears by approximately 0.05–0.1mm. Failure to reduce the gap in time causes burrs. It is recommended to check gaps with feeler gauges every 200 hours and make fine adjustments (0.02–0.05mm each time) based on wear.

The design of upper and lower knives for corrugated paper slitting essentially solves the dual pain points of "difficulty cutting through" and "difficulty protecting the structure" caused by its "multi-layered wave structure." A single knife cannot balance these needs, but the "active cutting + passive support" cooperation of upper and lower knives ensures edge quality, protects the corrugated structure’s compressive performance, and adapts to the efficiency and cost requirements of industrial mass production.

For professionals in the tungsten carbide industry, focus on upper knife material selection (tungsten carbide is optimal) and lower knife adaptability (choose tungsten carbide or alloy based on scenarios). Additionally, communicate the importance of "parameter adjustment" to customers—high-quality knives require precise parameters to maximize their value.

If your enterprise faces issues like fast knife wear or poor edge quality in corrugated paper slitting, feel free to reach out. We can recommend suitable tungsten carbide upper knife specifications and lower knife materials based on the flute type and thickness of your slit corrugated paper, and provide optimized gap and angle parameter solutions.