This molding method relies on the vacuum force to make the sheet tensile deformation. Vacuum force is easy to be realized, mastered and controlled, so simple vacuum forming is an early and widely used hot forming method at present. After heating the sheet to the desired temperature, place it on the clamping ring, tighten it with the pressure ring, open the vacuum pump valve for vacuum pumping, control the vacuum valve through the photocell to adjust the vacuum degree, until the sheet reaches the required forming depth.

Due to the free vacuum forming method, the parts do not touch any mold surface, the surface of the parts is high gloss, without any defects. If the plastic itself is transparent, the piece can have low light absorption and transparency, so it can be used to make aircraft parts such as instrument covers and skylights.

The vacuum molding method can only change the stretching degree and outline shape of the parts in the forming process, so it cannot shape the parts with complicated appearance. In addition, in the forming process, the thickness of the deformation zone decreases with the increase of the tensile degree, so the tensile ratio in actual production should be less than75%。

For large thick plate blister forming, only one side of the heated material is in contact with the forming tool. Thus, the surface on which the material meets the mold has exactly the same surface profile as the forming mold. The profile and size of the non-contact surface of a molded piece depend only on the thickness of the material. According to the different contact surface of forming material and forming mold, forming process can be divided into male mold and female mold forming.