Power assisted roller-stretch apparatus and process

Abstract

A process and apparatus for pre-stretching a plastic film material above its yield point in a roller assembly comprising at least two rollers connected for rotation at different speeds during a load wrapping operation. The downstream roller is at least partially driven by the film web connected to a turntable while the upstream roller rotates at a predetermined speed slower than that of the downstream roller to induce film elongation between the rollers. The force on the film between the roller assembly and the load is reduced by a powered constant torque device connected to the rollers to bring the force to the load down to a level permitting the film elongation between the roller assembly and the load to be equal to (balanced) or less than the elongation incurred between the rollers. The force from the torque device required to bring elongation on the load in balance with the elongation between the rollers will be equal to the difference between the force required to elongate the film between closely spaced rollers and the force required to hold the elongation after the film leaves the rollers corrected for the mechanical advantage and friction of the pre-stretch device. Since the film has been stretched above its yield point, the relieving of stress from the film between the rollers and the load allows the film to retain a high degree of pre-stretch elongation.