What are the possible reasons for the separation of the co-extrusion layer and the foam layer of the core foam profile, and how to solve it?

The reasons for the separation of the co-extruded layer and the foaming layer in the process mainly include: 
The improper selection of the raw materials for the co-extruded layer, the insufficient length of the flow channel section, and the mismatch between the design of the cooling system and the cooling mechanism of the raw materials all contribute to the problem.The large bubble defects in the co-extruded layer and the foaming layer can also lead to the separation of the co-extruded layer and the foaming layer. 
① The selection of the raw materials for the co-extruded layer is inappropriate. When the compatibility between the co-extruded layer raw materials and the core foaming materials is poor, and the melt bonding strength is low, it will cause the co-extruded layer and the foaming layer to separate. In severe cases, they can be easily peeled off with just a gentle tug. 
② The common extrusion outlet section is too short. When the common extrusion outlet section is too short and the convergence point is too far back, when the foaming material has completed foaming and begun surface cooling and contraction, the melt pressure of the common extrusion material will suddenly drop, resulting in a decrease in the bonding strength between the common extrusion material and the foaming material. 
③ The temperature control for cooling is unreasonable. When the shrinkage rate difference between the surface co-extruded material and the core foaming material is significant, especially when the surface co-extruded raw material is a crystalline polymer, if the inlet temperature of the cooling system is too low, the co-extruded layer material quickly reaches the crystallization point before the core foaming material starts to cool down, which will lead to the defect of separation between the co-extruded layer and the foaming layer. 
The solutions mainly include: 
① Alter the raw materials of the co-extrusion layer or adjust the formulation of the co-extruded materials to enhance their compatibility with the core foaming raw materials; 
② Increase the length of the flow convergence between the co-extruded material and the foaming material, and appropriately advance the flow convergence of the co-extruded material and the foaming material. 
③ Adjust the temperature of the cooling water to ensure that the cooling rate matches the characteristics of the materials.
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