The process for co-extruded profiles is more complex and extensive than for non-co-extruded profiles. Among the key controls are the co-extruder process temperature, the head pressure and the matching of the co-extruder discharge speed to the main extruder discharge speed.

①Influence of process temperature and head pressure：

Figure 15-9 shows a diagram of a two-layer co-extruded flow channel at a confluence. The velocity profiles of the flowing material in the single and converging flow channels are different due to the different viscosities and flow rates between the two materials. When two molten materials converge in the same flow channel, their viscosity will have a great impact on the co-extrusion interface. In practice, it has been found that the co-extrusion of surface co-extruded profiles is based on the production of common profiles in the mould, and the co-extrusion flow channel is increased so that the surface of the common profile is extruded into a layer of co-extruded material. The thickness of the surface co-extrusion layer is generally only 0.3 to 0.5 mm.

If the coextruded material has a high viscosity, poor flowability and low melt temperature of the coextruded material, the thickness of the coextruded layer is small. Material to the rate of slow, in the co-extruder and die stay in the long time, easy to cause the head pressure is too large. On the one hand, the thickness and quality of the co-extruded layer is difficult to achieve the desired purpose, on the other hand, co-extruded material is also easy to appear paste material, to the production operation to bring trouble.

Therefore, to obtain the ideal quality of co-extruded products, is to reasonably control the extruder's process temperature and head pressure, under the premise of good material plasticization, so that the co-extruded material and the main material melt viscosity can be in a suitable range.

②Matching of co-extruder discharge speed with main extruder discharge speed

The actual production should be adjusted according to the thickness of the co-extruded layer required by the product and the production line speed of the main material profile, to adjust the co-extruder discharge speed. If the speed of the co-extruder is too fast, it will cause the co-extruder head pressure to be large and the co-extruded layer to be thicker, which will not meet the technical requirements of the product; on the contrary, if the co-extruder is slow, the co-extruded layer will be thin, which may lead to paste in serious cases.

In addition, the actual production should also pay attention to, in the mould adjustment stage, you can first open the mainframe, repair the mould to the main material on all sides of the same material, and then adjust the co-extrusion material extrusion. However, whether in the mould adjustment stage or in the production stage, in the opening of the machine, should first open the co-extruder until the co-extruded material extruded out of the die, and then open the main machine; otherwise, because the main machine extrusion and head pressure is greater than the co-extruder, easily lead to co-extrusion machine does not discharge material, and even occur paste material.