Foamed plastic is a kind of plastic based on thermoplastic or thermosetting resin with numerous micro pores inside. Foaming is one of the important methods in plastics processing. The foamed plastics obtained from plastic foam contain two gases and solids - gas and solid. Gas exists in foam in the form of bubbles. The pores and bubbles are isolated from each other. They are called closed pores, and the connected ones are called openings, so that there is a separation between closed cell foam plastics and open cell foam plastics. The opening or obturator of foam structure is determined by the properties and processing technology of the raw materials.

　　The technology of plastic foaming has a long history. The earliest froth bakelite in the early 20s was made with similar methods of making foam rubber; rigid polyurethane foam and polystyrene foam appeared in 30s; polyethylene, polyvinyl chloride, epoxy resin and phenolic foam were produced in 40s; there were expandable polystyrene foam and soft polyurethane foam in 50s.

　　Now, basically all plastics, including thermoplastic and thermosetting, can be foamed into foam plastics. The industrial preparation methods include extrusion foaming, injection foaming, molding foaming, calendering foaming, powder foaming and spray foaming, etc. Among them, injection molding foaming is one of the most important molding methods. Here we focus on injection molding foaming.

　　Foam forming principle

　　The foaming methods of plastics can be divided into physical foaming method and chemical foaming method according to the different foaming agents used. Here is a brief introduction to foaming agent.

　　foaming agent

　　Foaming agent can be divided into physical foaming agent and chemical foaming agent. The requirements for the physical foaming agent are: non-toxic, odorless, non corrosive, non combustion, good thermal stability, no chemical reaction in the gas state, and the diffusion speed in the plastic melt in the gas state is lower than that in the air. The commonly used physical foaming agents are air, nitrogen, carbon dioxide, hydrocarbons, freon, etc;

　　Chemical foaming agent is a kind of substance that can release gas such as nitrogen and carbon dioxide when heated. The requirements for chemical foaming agent are: the gas released by its decomposition shall be non-toxic, non corrosive, non combustible, and have no influence on the molding, physical and chemical properties of products, the speed of releasing gas shall be controllable, and the foaming agent shall have good dispersion in plastics. Inorganic foaming agents such as sodium bicarbonate and ammonium carbonate, organic foaming agents such as azoformamide and azodiisobutyronitrile are widely used.

　　Physical foaming

　　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 injection molding with chemical foaming agent is basically the same as that of general injection molding. 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.

　　structural foam

　　Structural foaming is a chemical foaming method, which is a revolution in injection molding technology. It retains many advantages of traditional injection molding process and avoids some problems encountered in traditional injection molding process, such as insufficient product strength, too long production cycle and low molding rate.

　　In addition, the structural foaming technology can also mold large and complex products, use low-cost mold, multi-mode cavity can be operated at the same time, so as to reduce the production cost of products. The structural foaming product is a kind of conjoined foaming material with dense surface layer. Its unit weight strength and rigidity are 3 ~ 4 times higher than that of the same kind of non foaming material.

　　The biggest feature of structural foaming method is that it can be produced by ordinary injection molding machine without additional equipment. However, compared with ordinary injection molding machine, the high-pressure structural foaming injection molding machine using cavity expansion method has increased the secondary mold closing and pressure maintaining device.

　　In recent years, structural foaming injection molding technology has been widely developed, and there are many molding methods, but it can be divided into three types: low-pressure foaming method; high-pressure foaming method (Note: low pressure and high pressure here refer to the pressure in the mold cavity); two-component foaming method.

　　Low pressure foaming method

　　The difference between low-pressure foaming injection molding and ordinary injection molding is that the cavity pressure of the mold is low, about 2-7mpa, while the ordinary injection molding is between 30-60mpa. Under injection method is generally used in low-pressure foaming injection molding, that is, a certain amount of plastic melt (including foaming agent) is injected into the mold cavity, and the gas decomposed by the foaming agent makes the plastic expand