Stretch films are essential in the packaging industry as they provide a versatile and high-quality solution for packing products safely and economically. The film is produced by a flat die extrusion process where AC motors and variable speed drives significantly ensure high-quality output. The process starts with the extruder, a pump that melts and transports high-viscosity fluids. The polymer enters the extruder via a gravimetric feed. The material is melted, mixed, and pushed through an extrusion head to give the desired shape through heat and mechanical stress combined. After exiting the extrusion head, the material enters a cooling unit. Here, water-cooled ‘chill rolls’ reduce the temperature of the film before it is finally wound onto the rolls.
Cast Film Extrusion Liners: Extruder
The extruder consists of a hollow cylinder that rotates a single or double screw driven by an electric motor; this is usually coupled to the plasticizing screw by a gearbox. The motor provides the torque required and rotates at the speed necessary to obtain the expected melt flow rate. Any failure in the precise control of the screw speed can cause changes in film thickness in the machine’s direction.
For the past 20 years, AC motors have typically taken over from DC motors to control the extruder screw in cast film extrusion line applications. They have delivered many advantages in terms of convenience, reliability, low maintenance, and a reduction in the overall dimensions of the system solution. AC motors are controlled by AC variable speed drives that guarantee stable rotation. Even at very low speeds (via a closed loop circuit through incremental encoders), they provide short circuit protection (low or high voltage) and incorporate EMC filters to eliminate electrical noise and interference.
In recent years, along with AC motors and drives, torque motors have also been utilized in screw extrusion control. These offer a complete direct drive solution that does not require the assembly of different elements, such as a gearbox, belts, and pulleys. Torque motors guarantee uniformity in motion, linearity, and constancy in the extrusion of the plastic material.
Cast Film Extrusion Lines: Cooling Section
As previously mentioned, when the material leaves the extrusion head, it is melted on chilling rolls that form the cooling section of the cast film extrusion production line. The cooling unit is comprised of a primary quenching roll that cools the film on one side and a secondary roll that cools the film on the opposite side. It also includes a motorized roll positioning system for correct vertical and cross-machine direction alignment of the rolls and, in many cases, a vacuum box or air knife.
The rolls must be perfectly aligned with the web to guarantee uniform tension and minimize thickness variations across the width of the film. In addition, the angular velocity of the rolls must be well controlled to prevent film thickness fluctuations in the machine direction.
Cast Film Extrusion Lines: Accumulator and Winding Section
Within the cooling and winding sections, we find the accumulator, which is used to allow splicing of the web being fed from an empty winder to a full winder at zero speed without stopping the line. At the end of the process, a further winder brings the extruder material onto rolls. The winding process has to preserve the film’s properties and dimensions when the rolls are unwound in other downstream processes. There are several different types of winders. A ‘turret’ or ‘center’ winder is typically used in cast film extrusion applications, where the web tension decreases as the roll diameter increases. All the movements performed in the cooling, accumulator, and winder sections are driven by AC motors and drives that govern the web speed and the correct web tensioning.
Cast Film Extrusion Lines: AC Drives and Motors
AC Drives with high-end control are very important for guaranteeing high-quality film throughout the process. Easy-to-configure software for the closed-loop control and optimum efficiency for many different types of material is a vital element of a cast film line system and process.
Parker’s AC30 series, with power ratings ranging from 0.75 to 450 kW coupled with the company’s Quicktool software with full IEC61131PLC functionality or Parker DSE Lite software, provides all the features needed to achieve optimum synchronization between all line sections. It allows customers to create, parameterize and configure user-defined applications using dedicated function blocks such as the winder, PID, and diameter calculator. The AC30 series provides access to an extensive library of application macros and worked examples.
Connectivity via EtherCAT, Profinet, Ethernet IP, and Modbus TCP IP through a dual Ethernet port enables the communication between individual drives simply and flexibly. It supports intelligent data analytics and connection to external servers. The line setpoint can be sent through a high-speed channel supported by 1588 time-synchronized peer-to-peer communication. Each part of the machine has its regulation, either within the drives or through communication protocol by the PLC. This animation from our partners at Parker shows how a high-performance drive solution supports the optimal control of a cast film extrusion line.
Please contact Sure Controls to learn more about our partners at Parker or cast film extrusion lines.
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