The Co-Extrusion Principle: How Dough Wraps Filling in a Continuous Flow

First Extrude a "Rope," Then Cut It into "Pieces"

When wrapping by hand, you start with a wrapper and then place the filling on it. An encrusting machine works the other way around: it first extrudes the dough and filling together into a continuous rope with filling in the middle and dough on the outside, then cuts and seals it section by section. The first half of this process is called co-extrusion, and the second half is cutting; see Shutter Cutting and Sealing.

The term co-extrusion is also common in manufacturing plastic pipe and wire insulation, and the principle is similar: two materials pass through a multi-layer die at the same time and come out as one layer wrapped around another. The challenge with food is that dough, bean paste, and meat filling are all sticky, elastic, soft materials. The extrusion must be steady, yet it cannot damage the materials.

What a Concentric Double-Layer Nozzle Looks Like

RHEON's U.S. patent US6408899B2, filed in 2000 (with inventors including founder Torahiko Hayashi), describes this structure clearly: the nozzle has two parts, an inner nozzle that delivers filling and an outer dough passage that surrounds it, the two being concentric. The filling travels downward through the center while the dough is extruded in a ring around it. They meet at the outlet and form a two-layer rope with the dough enclosing the filling.

When the American science magazine Popular Science interviewed RHEON, it explained it in very plain terms: the outer ring determines the thickness of the dough, and the center nozzle determines the amount of filling. Change the nozzle diameter, and the product size and dough thickness change along with it.

Benefits of Feeding Dough and Filling Separately

In the same patent, the dough and the filling each have their own independent feed: a pair of horizontal, counter-rotating screws pushes the material toward a metering pump (a vane pump), and the vane pump delivers it into the nozzle at a fixed volume. Each feed system is controlled by its own motor, which brings several practical benefits:

• Adjustable dough-to-filling ratio: Feed the dough faster and the filling slower, and the dough gets thicker; do the opposite and it gets thinner. This is why an encrusting machine can go from 1:0 (dough only) to 0:1 (filling only); see How to Adjust the Encrusting Ratio.
• The two materials do not mix: The patent specifically notes that one design goal is to keep the dough and filling from being kneaded together during feeding.
• Each can be matched to its material: Dough needs gentle pushing that does not damage its gluten; bean paste or meat filling needs consistent metering. Separate feeding allows each to be adjusted independently. RHEON's models for Chinese-style steamed buns can even adjust the feed ratio of two separate stages, the "hopper cylinder" and the "flow regulator."
• More consistent weight: A metering pump delivers a fixed volume per revolution, so as long as it is fully filled, each piece weighs the same; see Weight Control.

Why Material Passages Should Be "Short and Wide"

The same patent also mentions that the connecting passages are made wider and shorter so the food is less likely to be damaged as it passes through. The longer a soft material is squeezed and rubbed inside a long, narrow passage, the more easily the dough's structure is broken down, and fillings may release water or lose their shape. A good encrusting machine keeps the distance between the hopper, pump, and nozzle as short as possible.

What Happens After Co-Extrusion

Once the two-layer rope leaves the nozzle, the shutter sits directly below it. When the shutter closes, it pinches off the rope and gathers the dough toward the center to cover the cut; at the same time, the conveyor below rises to support the bottom of the product, then descends together with the shutter. One open-close cycle completes one piece. RHEON's patents US6709256B2 and US8359973B2 both describe the synchronized control of the shutter and the receiving table during this step.

Variations Made Possible by Co-Extrusion

Because the dough and filling only meet at the nozzle, changing the nozzle or adding another feed opens up many more possibilities:

• Add a third material tube to make double fillings or "sauce inside the filling"; see Triple Encrusting.
• Skip cutting and let the rope come out continuously to make filled bars and rolled products; see Three Product Shapes.
• Switch to special nozzles to make two-color or patterned products.

FAQ

Q: How does the machine get the dough to wrap around the filling?
A: The dough and filling are fed separately and meet in a concentric double-layer nozzle, which extrudes a continuous rope with filling in the center and dough on the outside. A shutter then cuts and seals it.

Q: What is a concentric double-layer nozzle?
A: It has an inner nozzle in the center that delivers filling and a ring-shaped passage around it that delivers dough. The two share the same axis, and at the outlet the two streams combine into one filled rope.

Q: What are the benefits of feeding dough and filling separately?
A: The ratio can be adjusted independently, the two materials do not mix, the pushing method can be tuned to each material's properties, and the weight is more consistent.

Q: Do I need to change the nozzle to change product size?
A: A common approach is to adjust the feed volume and shutter timing, and only change the nozzle when the size difference is large. It depends on the machine design, so feel free to call us at 06-257-4644.