The open pores of the micropores are called open-cell foams, and the closed ones are called closed-cell foams. The foam is both hard and soft. According to the American Society for Testing and Materials, at a temperature of 18 to 29 ° C, within a period of 5 s, around a 2.5 cm diameter round rod, if it does not break, the test sample belongs to a flexible foam; otherwise, it is a rigid foam. . Foam plastics can also be classified into low foaming and high foaming. Generally, the expansion ratio (the multiple after the expansion is increased by a multiple of the volume before foaming) of less than 5 is referred to as low foaming, and the ratio of more than 5 is referred to as high foaming.