The volatile gas produced from the resin in the melt and the lubricating agent under processing temperature is stored in between the melt flow and surface wall of cavity and interferes the flow of melt,

 whereby the nebulous flow mark is formed. The treatment measures withal are: improving the exhausting condition of molds; properly increasing dimension of gate and reducing melt filling rate; properly reducing mold temperature and melt plastication temperature; replacing additives such as lubricating agent which can produce volatile gas.