Simply speaking, there are three ways to make plastics foam by physical methods:
(1) The inert gas is first dissolved in the plastic melt or paste under pressure, and then the gas is released under reduced pressure to form pores in the plastic and foam;
(2) The low boiling point liquid dissolved in polymer melt is evaporated and foamed;
(3) Adding hollow spheres to plastics to form foams and foams, etc.
The cost of the physical foaming agent used in the physical foaming method is relatively low, especially the cost of carbon dioxide and nitrogen is low, and it can be flame-retardant and pollution-free, so the application value is high; moreover, the physical foaming agent has no residue after foaming, which has little impact on the properties of the foaming plastics. But it needs special injection molding machine and auxiliary equipment, so it is very difficult.
Chemical foaming
Chemical foaming method is to use chemical method to produce gas to make plastic foam: heat the chemical foaming agent added in plastic to decompose and release gas to foam; in addition, it can also use the gas released by chemical reaction between plastic components to foam.
The process of chemical foaming agent for injection molding of foamed plastics is basically the same as that of general injection molding process. The heating, mixing, plasticizing and most of the foaming and expansion of plastics are completed in the injection molding machine.
In a word, no matter which kind of plastic material is selected or which kind of foaming method is adopted, the foaming process generally goes through the stages of bubble core formation, bubble core expansion, bubble body solidification and finalization.
