What are the causes and solutions for the lifting and cracking of the co-extruded layer of core-layer foamed profiles?

What are the causes and solutions for the lifting and cracking of the co-extruded layer of core-layer foamed profiles?

The main reasons for defects such as warping and cracking of the co-extruded layer are as follows :
①The confluence point of the co-extrusion outlet die is not properly selected. If the confluence point is chosen too far forward and converges before the foaming material starts foaming, when the foaming material begins to foam, the cells in the foaming melt will rapidly expand, causing the surface area of the foaming material to also increase, and the composite part of the foaming material and the surface co-extruded material will be displaced Remove or burst the co-extruded layer on the surface; 
② The distance between the setting mold and the die and the cooling rate of the setting mold are not reasonably controlled. Due to the high temperature of the material, the gas in the core cells is still expanding. When the distance between the setting mold and the die is too far or the cooling rate of the setting mold is insufficient to cure the surface layer of the product, the foaming material in the core will burst the uncooled and unshaped co-extruded surface layer, resulting in the rupture of the co-extruded layer.
③ The influence of co-extrusion raw materials and formulas. This situation often occurs when the melt strength and ductility of the surface co-extruded material are relatively low, and the tension of the co-extruded material melt is insufficient to maintain the outward expansion of the foaming material, which will lead to the lifting and cracking of the co-extruded layer.

The main effective solutions are as follows :

- Move the convergence point of the co-extruded outlet die appropriately backward; 
- Shorten the distance between the shaping die and the die, and improve the cooling efficiency of the inlet section of the shaping die; 
- Adjust the formula of the co-extruded raw material or co-extruded material to increase the melt strength and ductility of the co-extruded material.
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