MDO related technologies

1. What is MDO

The full name of MDO is machine direction orientation, which is a post-extrusion process used to increase the orientation of the film in the MD direction, thereby improving the stiffness, optical, and barrier properties of the film. MDO stretching equipment is completed by four stages of preheating, stretching, annealing and cooling 8-12 rods.

♦ Preheating: Controlling the temperature at the softening temperature of the raw material can better help stretching without damaging the film; and because the film is not completely melted, smaller crystals can be formed during the stretching and orientation process to achieve better optics.

♦ Stretching: The orientation of the film is controlled by adjusting the stretch ratio. It should be noted that the stretching ratio is not as high as possible. At the critical point of stretching, the film performance will reach equilibrium or decrease.

♦ Annealing and cooling: affect the preheat shrinkage of the film, especially the flatness of the bag making; and the optical properties of the film.

There are two types of MDO machines, online and offline, and online machines are currently the mainstream choice. Because the film is stretched in the offline state, the film is thin in the middle and thick on both sides, which is more wasteful in use; the online machine can measure the thickness and feedback to the film making system, so that the film thickness after the stretch is more uniform and reduces cutting side.

2. The change of PE film performance after MDO

The following model diagram illustrates how the crystal structure changes as the film stretches before the critical stretch ratio. The unstretched film has lamella of different sizes and is freely oriented (Figure 2A); the stretched film forms smaller crystals and the molecular segments are oriented in the MD direction so that the MD direction crystallinity becomes higher( Figure 2 B).