The Influence of Co-extrusion Process on the Quality of Co-extruded Profiles

The co-extrusion process of profiles is more complex and involves a wider range of factors compared to non-co-extruded profiles. The key aspects that need to be carefully controlled include the co-extruder’s process temperature, die head pressure, and the matching between the extrusion speeds of the co-extruder and the main extruder.

① Influence of process temperature and die head pressure

Figure 15-9 shows a schematic diagram of the confluence area in a two-layer co-extrusion flow channel. Because the two materials have different viscosities and flow velocities, the velocity distribution curves of the materials in the single flow channel and the confluence channel are different. When the two molten materials merge into the same flow channel, their viscosities will have a significant effect on the co-extrusion interface.

In practice, the surface co-extrusion of profiles is usually achieved by adding a co-extrusion flow channel to the mold used for producing ordinary profiles, allowing a thin layer of co-extrusion material to be extruded onto the surface. The thickness of the co-extrusion layer is typically only 0.3–0.5 mm.

If the co-extrusion material has high viscosity, poor flowability, or a low melt temperature, the thin co-extrusion layer, combined with a slow feed rate, leads to a long residence time in the co-extruder and mold, easily causing excessive die head pressure. As a result, the thickness and quality of the co-extrusion layer become difficult to control, and the co-extrusion material is prone to degradation, making production more difficult.

Therefore, to obtain co-extruded products of ideal quality, it is essential to reasonably control the extrusion temperature and die head pressure, ensuring that the melt viscosities of the co-extrusion and main materials are within a suitable range under good plasticization conditions.

② Matching between co-extruder and main extruder output speeds

In actual production, the output speed of the co-extruder should be adjusted based on the desired thickness of the co-extrusion layer and the line speed of the main profile. If the co-extruder speed is too high, it will cause excessive die head pressure, resulting in an overly thick co-extrusion layer that fails to meet product requirements. Conversely, if the co-extrusion output is too slow, the layer will be too thin, and in severe cases, material degradation may occur.

Additionally, during mold adjustment, only the main extruder should be started first, to ensure uniform extrusion of the main material on all sides before adjusting the co-extrusion output. However, whether in the mold adjustment or production stage, during startup, the co-extruder must be turned on first until co-extrusion material flows out of the die, and only then should the main extruder be started. Otherwise, because the main extruder’s output and die pressure are higher than those of the co-extruder, this can cause the co-extruder to stop feeding or even result in material degradation.