during pre-expansion stage,the beads are heated to 90℃ by steam,the beads will soften;the blowing agent incorporated in the beads will vaporize.the pressure causes the EPS beads to expand to occupy 40~80 tims their original volume.it may reach its lowest and optimal pre-expansion density earlier using less steam than other competitive brands.if the operation is not controlled well,the over pre-expansion may occur which cause beads to wrinkle in order to increase density.