How should the raw materials be selected in the formula for PVC surface skin core layer microcellular foamed extruded profiles?

How should the raw materials be selected in the formula for PVC surface skin core layer microcellular foamed extruded profiles?

①Generally, low-viscosity PVC resins with a molecular weight of less than 58 to 65 (such as SG-7 and SG-8 type resins) should be selected to facilitate plasticization during the processing, and the melt has excellent fluidity and good bubble dispersion and expansion properties. 
②In low-foaming formulations, azodicarbonamide (AC) foaming agent is usually used. During the extrusion process, the melt containing AC, under the effect of lead salt stabilizers, causes the decomposition temperature of AC to drop from 200°C to around 170°C. 
③In the processing of surface-skin core-layer foamed profiles, generally, no foaming accelerator is needed. The heat stabilizers in the formula (such as lead salt stabilizers, rare earth stabilizers, etc.) can themselves adjust the decomposition temperature of AC. ④ To obtain a uniform, fine, and independent cell structure, a foaming regulator ZB-530 should be added appropriately, with a dosage of 6 to 8 parts by mass in surface-skin core-layer foamed profiles 
④No nucleating agent needs to be added; titanium dioxide and calcium carbonate can play the nucleating role.
⑥ The use of internal and external lubricants in the formula should be balanced to prevent the surface from rising before the melt enters the vacuum mold.
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