Cutting and Sealing: How a Shutter Turns a Filled Rope into Individual Pieces

The Final Step from Rope to Ball

What co-extrusion produces is a continuous rope with filling in the middle and dough on the outside. Turning it into individual buns, daifuku, or croquettes depends on the cutting mechanism directly below the nozzle, which the industry often calls the "shutter," because the way it closes looks a lot like a camera's iris.

Shutter Construction

RHEON's U.S. patents US6709256B2 (Japanese priority filed in 2000, with inventors including Torahiko Hayashi) and US8359973B2 (priority in 2007) both describe the same basic structure:

• A ring-shaped housing with multiple pivots spaced evenly around its circumference.
• Each pivot is connected to a shutter blade. The patents say "a plurality" of blades; the number varies by model.
• When the pivots turn one way, the blades open outward, leaving a round opening in the center for the rope to pass through; when they turn the other way, all the blades close toward the center at once until the opening is fully shut.
• The blades are linked by gears or linkages, so they all open and close in sync.

The patents describe blades with an inward-projecting shape at the middle of their thickest part. When closed, the lower edges of the blades meet at the center point, giving the product a flat bottom.

What the Conveyor Below Does During Cutting

If the shutter simply closed, the rope would be "choked off," but the bottom of the product would have no support and could easily deform. So part of the conveyor directly below the shutter can move up and down:

1. As the rope is extruded downward, the conveyor rises to catch its bottom.
2. As the shutter closes, the conveyor supports the bottom, and the shutter and conveyor descend together.
3. The shutter closes completely, the rope is separated, and the product stays on the conveyor to be carried away.
4. The shutter opens and returns to its upper position, and the conveyor rises back into place for the next piece.

US8359973B2 notes that it is this combination of "support from below + shutter cutting" that gives the product its near-spherical appearance. This patent also drives both the shutter's open-close motion and its up-down motion with the same motor and shaft, completing one cycle per revolution, which simplifies the structure and control.

Why Doesn't Filling Leak When the Shutter Cuts?

The key is "gathering" rather than "snipping." Scissor-type cutting leaves a cross-section with exposed filling at the cut. A shutter closes from all sides toward the center at once, pushing the dough inward as it closes, so the dough covers the cut first and then joins at the center point. In the patent's words, the cut is made "while keeping the filling enclosed."

Of course, whether filling leaks also depends on other conditions:

• Dough extensibility: If the dough is too dry or brittle, it cracks when gathered.
• Dough-to-filling ratio: If the dough is too thin, there is not enough of it at the seal to close up; see Encrusting Ratio.
• Sticking to the blades: Dough that sticks to the blades gets torn, which is why there are dusting systems and non-stick shutter designs; see Dusting Flour and Anti-Stick Measures.

How Cutting Speed Affects Shape

US6709256B2 divides one shutter cycle into four phases: closing, holding closed, opening, and holding open, and allows the speed of the closing phase to be adjusted on its own while the total cycle time stays the same. This shows that manufacturers treat "how fast it closes" as an adjustable parameter.

Mechanically, the relationship between closing speed and shape is roughly as follows:

• If the shutter closes slowly, the rope keeps extruding while it is being sealed, so the product tends to be elongated and the dough at the seal is thicker.
• If it closes too fast, the dough does not have time to be drawn toward the center, so the seal may be thin or show a gap.
• The stroke and timing of the receiving conveyor's descent affect whether the product is a round ball, slightly flatter, or slightly taller.

In addition, the shutter's "interval" determines the length of each section of rope. For length adjustment, RHEON's KASEIJIN (火星人) CN700 lists a shutter interval of 1–99 seconds. The longer the interval at the same extrusion speed, the longer each section, so the product can go from a round ball to a tawara (bale) shape or a bar; see Three Product Shapes.

Cutting Methods Other Than a Shutter

Not every product uses a shutter. A continuously extruded bar can be cut with a rotating wire (suited to cookie dough), an ultrasonic knife (smooth cut faces, suited to dough with nuts or dried fruit), or a guillotine. Large multi-row machines also use "rotary shutters"; RHEON's Mega Former uses this design.

FAQ

Q: How does a filled rope become a round ball?
A: The shutter closes from all sides to pinch off the rope and gather the dough, while the conveyor below supports the bottom and descends in sync, so the product becomes a near-spherical piece.

Q: Why doesn't the filling leak when the shutter cuts?
A: The shutter closes from all sides toward the center, gathering the dough to cover the cut as it closes. As long as the dough is extensible and thick enough, the cut is sealed by the dough.

Q: Does cutting speed affect the shape?
A: Yes. How fast the shutter closes and the timing of the receiving conveyor's rise and fall both affect how round the product is and how thick the seal is, while the shutter interval determines the length of each piece.

Q: Do the shutter blades wear out?
A: All moving parts wear. If the shutter does not open and close smoothly or the seals are uneven, stop the machine and cut the power before inspecting it. For parts for a specific model, please call 06-257-4644 to ask.