Mechanical method: Introduce large amounts of air or other gases into the liquid plastic with strong agitation. This method is mainly used in the industry to produce urea-formaldehyde foam, which can be used as a thermal insulation material or a set material in a movie (such as artificial snowflake).
Physical method: Low-boiling hydrocarbons or halogenated hydrocarbons are often dissolved in plastics, and the plastic softens when heated, while the dissolved liquid volatilizes and expands. For example, in the case of polystyrene foam, in the suspension polymerization of styrene, pentane is first dissolved in the monomer, or the polystyrene resin which has been polymerized into beads is treated with pentane under heat and pressure to obtain a so-called polystyrene foam. Expandable polystyrene beads. The beads are pre-expanded in hot water or steam, and then placed in a mold to pass steam, so that the pre-expanded particles are secondarily expanded and sintered to each other, and after cooling, a product having the same shape as the mold cavity is obtained (see Figure). They are widely used as shock-proof materials in insulation and packaging. An extrusion molding method may also be employed. In this case, the expandable beads may be used to be foamed and extruded into a sheet at one time; or ordinary polystyrene pellets may be used, and halogenated at the appropriate portion of the extruder. The hydrocarbon is mixed evenly with the plastic melt, and expands and foams as the material leaves the machine head. The extrusion method is commonly used for making sheets or sheets, and the sheets can be formed into food packaging boxes and trays by vacuum forming. Polyethylene can also be obtained in a similar manner to produce an extruded foamed article. Physical methods for introducing a gas include a dissolution method, a hollow microsphere method, and the like. In the dissolution method, a soluble substance such as salt, starch or the like is mixed with a resin to form a product, and then the product is repeatedly treated in water to dissolve the soluble substance, thereby obtaining an open-cell type foam product, which is often used as a filter material. The hollow microsphere method is to mix the hollow glass microspheres with high melting temperature with the plastic melt, and a special closed-cell foam can be obtained under the molding conditions that the glass beads are not broken.