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Calendering is a specialty process for the manufacture of large-volume and high-quality products, mainly vinyl (PVC) film and sheet. Modified PE, PP, ABS, and other thermoplastics can also be calendered. An important area in the calendering of film is the ability of surface treatment, such as embossing films or enhancing the physical properties of the films by stretching.
In the calendering process, the polymer is melted and compounded with heat and pressure in an extruder/kneader. Then, the film is formed by the pressure of the calendering rolls. Special features of the film are defined by formulation and processing conditions. A typical calendering line comprises the following equipment.
Raw Material Bulk Handling & Mixing Unit
For the production of film, the ingredients can include both solid and liquid raw materials. Solids are conveyed pneumatically from silos into bins; weighed in batches; and dropped into a high-speed mixer. Liquids are metered into the mixer by piston pumps or other volumetric metering devices.
Compounding & Calender Feeding
Compounding is done in continuous mixers. The dry blend is force-fed into the extruder. Uniform compounding provides for minimal heat history and consistent feed rate, color, gauge, and surface.
Generally, the film industry distinguishes between two dominant calender types, “L” and “Inverted L,” with roll configurations from 4-7 rolls. These rolls are driven and temperature controlled. The temperature and speed of the rolls influence the properties of the calendered film.
Tempering Rolls/Embossing Unit
The film passes between an embossing roll and a cooled rubber roll. The embossing unit is used to imprint special features on the film. A variety of surfaces can be applied to the embossing roll to generate a desired surface effect.
The winding station includes cutting devices for edge trimming and in-line slitting of rolls.
The extrusion process converts plastic powder or pellets into a continuous uniform melt and forces this melt through a die, which upon cooling yields a desired shape. The end products are custom profiles, sheet, pipe, tubing, fibers, film, coatings, and pellets.
Raw material pellets or powder are conveyed pneumatically from silos to bins; weighed into batches; and dropped through a vertical opening in the feed section where they are introduced to a rotating screw with spiral flights. The material is conveyed along the screw and heated inside the barrel to reach the die system at an appropriate temperature and pressure. Temperatures of the various barrel zones are set according to the material, screw design, and processing goals.
Casting of film is a continuous process by which a thermoplastic material is melted and extruded through a flat die onto a chill roll, where it is quenched and re-solidified. The resulting film is stripped from the chill roll and wound for further processing or converting.
From the chill rolls, the film passes through a thickness scanner. Films are often corona treated to improve the surface properties for ink or laminate adhesion or silicone coated to improve de-nest properties. Finally the film passes into the winding station, which includes cutting devices for edge trimming and in-line slitting of rolls.
The cast co-extrusion process combines different polymers into one film in order to benefit from their different mechanical and physical strengths. This is accomplished by feeding multiple extruders into a single flat die.
Blown Film Extrusion
Blown films are normally extruded vertically in a tubular shape that cools, collapses, and winds up as a thin film. Upon leaving the circular blown film die, the hot tube is radially expanded and cooled by high-speed air from the air ring on the outside of the tube. Sometimes it is also cooled on the inside of the tube.
Once the polymer is cooled below its melting range, the tube is squeezed by two opposing flat surfaces to collapse the tube before it enters the primary nip rolls at the top of the tower structure. The now collapsed tube is transported on idler rolls down the tower by the secondary tension-controlled nip and slit into two individual sheets or webs. Then the film is wound onto cores to make film roll stock. The film can also be sent to an in-line sealing machine to make bags.
Blown co-extrusion is a growing field for products with more than one material, i.e. a multi-layered film. It requires multiple extruders and specialized die systems to bring these layers together.
Coating & Laminating
In the coating and laminating process, two or more materials are bonded together. The primary reasons for using coated and laminated films are: to apply a printed or decorated film to the top surface of another film; to apply a wear layer or surface sheet to protect a printed or decorated surface; and to combine materials of different physical and mechanical properties.
The basic methods used in film coating and laminating are fusion, wet combining, and heat reactivation. Fusion is the coating and laminating of two or more films layers using heat and pressure. Wet combining is the coating and laminating of two or more film layers using a tie-coat adhesive, which is applied wet, subsequently dried, and combined using pressure laminating in one continuous process. Heat reactivation is combining a pre-coated film by heating and reactivating the pre-coat adhesive so that it becomes receptive to bonding after later pressure laminating.
In addition to the process of lamination, special coatings can also be applied to obtain properties, such as moisture or gas barriers.
The stretching process is used for the production of shrink films. Films are stretched using two different methods – transverse-direction orientation or machine-direction orientation.
Through the transverse-direction orientation process, the film is uni-axially oriented, or stretched, in the transverse direction by utilizing high-performance tentering technology to achieve the required shrink properties. The machine used to stretch plastics in the transverse direction is called a tenter frame.
Through the machine-direction orientation process, the film is uni-axially oriented, or stretched, in the machine direction during the calendering process to achieve the required shrink properties. After leaving the calender, the film is fed to a system of differential draw rolls (i.e., rolls running at gradually increasing speeds). These rolls are heated sufficiently to bring the film to a suitable temperature. Under these conditions, the film is stretched in the machine direction. Then the film is rapidly cooled to set the orientation of the film.