When the plastic sheet is formed, the sheet is fixed to the clamping device. Clamping devices that facilitate the fixing of sheets of various sizes are often used in thermoformed general-purpose and composite thermoforming machines. Some are equipped with a clamping frame for the entire molding machine.
Clamping devices can be divided into two categories: one is frame type and the other is split type. The frame clamp consists of two frames, upper and lower. The sheet is sandwiched between the two frames. When the frame is opened, the lower frame generally remains fixed. The lower portion of the upper frame of each type of single station forming machine is directly fixed to the molding chamber. Manual and semi-automatic molding machines for hand-loading parisons and finished products are equipped with safety operating devices within the open range of the frame when the frame size is large. For parisons with large form slippage, the clamping force is required to be adjusted over a relatively wide range. For this purpose, two rubberized rolls are used, which are pressed against each other by a spring and equipped with a pressure regulating device. The clamping of the continuous pull forming machine is a combination of the two side zippers and the front and rear brakes.