What are the principles for determining the extrusion process parameters of extruded profiles with surface skin layer and core layer micro-foaming?

What are the principles for determining the extrusion process parameters of extruded profiles with surface skin layer and core layer micro-foaming?

(1)The extrusion temperature lies between the plasticizing temperature of the resin and the decomposition temperature of the foaming agent

(2)The material should be fully plasticized in the extruder and no foam phase should be formed. In particular, it is necessary to prevent the foaming agent from generating gas prematurely, which could cause the gas to escape and the bubble diameter to increase, and avoid the occurrence of melt pulsation. The position of the foaming point (the junction of the melt phase and the foam phase) should be within the die head and be moved as far as possible towards the die head outlet, and then foam occurs at the die outlet.

(3)The concentration of the foaming agent should be strictly controlled. Generally, as the concentration of the foaming agent increases, the foaming ratio also increases, the density decreases, but it is not conducive to the off-mold foaming process. Moreover, the foaming point will be further away from the mold lip.

(4)The foaming point shifts towards the mold lip opening, which is beneficial for obtaining high-quality products.

(5)By adopting an appropriate low-temperature extrusion process, the bubble diameter is small and the number of bubbles is numerous, which can form an excellent cellular structure. If the extrusion temperature is too high, the gas distribution becomes uneven, resulting in unstable gas and the bubble diameter expands. If the extrusion temperature is too low, the material cannot be properly plasticized or the decomposition temperature of the foaming agent cannot be reached, thus no gas can be produced.

(6)Strictly control the temperature of the die cavity and adjust the melt pressure of the die cavity. Generally, the lower the die cavity temperature is, the higher the melt pressure becomes. The foaming point shifts towards the die cavity outlet, which is beneficial for off-mold foaming.

(7)As the extrusion pressure increases, the bubble diameter becomes smaller, the number of bubbles increases, and the foaming ratio increases. In actual operation, the screw rotation speed can usually be increased to enhance the extrusion pressure.