Both outer layer and inner layer are extruded inside die head.
Each material flow channel inside die head is placed evenly. Each channel is after heat treatment and mirror
polishing to ensure material flow smoothly. Also die head provides compressed air between both layer. Calibration
sleeve is used to cool inner layer to form smooth and flat pipe inside. Pressure water flows inside calibration sleeve
to have good cooling effect. Vacuum is created on calibration sleeve surface when producing big diameter pipe,
