Cast Film and Sheet

Computer-controlled automatic profile dies with electrical controlled sensors in closed-loop control systems have developed greater efficiency and accuracy to extrusion coating, cast film, and sheet lines. A scanner measures the web thickness and signals the computer, which then converts the readings to act on thermally actuated die bolts. The individual adjusting bolts expand or contract as ordered by the computer to control the profile. The more sophisticated systems measure adjusting bolt temperature and provide faster response time with less scrap and quicker startups. The scanner is typically an infrared, nuclear, or caliper-type gauge. 

The most common die configuration for production of cast film and sheet is a coat-hanger die (Fig. 7.7), named because of its resemblance to a common coat hanger. T-shaped dies are also used. The die opening is wider and thicker than the finished film. As the film is drawn down between the die and the chill roll, it contracts in width and thins, due to tension on the film being produced and polymer relaxation effects. 

Internals of coat hanger die for cast film and sheet... 

A direct activated carbon method is specified for determination of volatiles loss of calendered, extruded, and cast films and sheeting. The standard also contains a graph of maximum volatile loss vs. film thickness. 

Edge bead n. In some cast-film and sheet-extrusion operations, the narrow border at the edge of the sheet, usually somewhat thicker, that must be trimmed off prior to winding or stacking the product. 

Dartek, Cast film and sheet, nylon blown film, DuPont... 

Cellulose acetate (CA), commonly called simply acetate, is noted for its attractive appearance, toughness, and high impact strength. It is used in quality toys, appliance handles, eye-glass frames, pen barrels, caps, and electrical parts. Acetate extruded and cast film and sheet are thermoformed for packaging. Extruded acetate rods have found great popularity as tool handles. 

When the extrudate exits the die, it has to be quenched and possibly sized (drawn through a fixture) to maintain its final shape. Depending on the extrusion process, different methods are available to quench the final product. Cast film and sheet are quenched on rolls and in water baths, blown film is quenched by air in a blown film tower. Solid profiles, pipe, and tubing are quenched in calibration tanks filled with water and in some cases connected to a vacuum system. Polymer strands and monofilaments are quenched in air or water baths while wire coating is done horizontally in air or water. In small and large part blow molding, the melt is quenched in molds as the extruded tubular parison is inflated. 

Available as both film and sheet unless otherwise noted. All materials are available in PDA grades except for CTA, PVP, and PL EX = extrusion CL = calendering REG = regeneration CAST = casting BO = biaxial orientation. 

The basic methods for forming film or sheeting materials may be classified as follows melt extmsion, calendering, solution casting, and chemical regeneration. Of special note is the use of biaxial orientation as part of the critical manufacturing steps for many film and sheet products. 

Melt Extrusion. By far the most important method for producing film and sheeting materials reties on one or another of the various melt extmsion techniques (5). The main variations of melt extmsion are the slot (or flat) die-cast film process, the blown films process, and the flat die sheeting-stack process. These may be combined with one or more steps such as coextmsion wherein multilayer film or sheet is formed, biaxial orientation, and in-line coating (6). 

Solution Casting. The production of unsupported film and sheet by solution casting has generally passed from favor and is used only for special polymers not amenable to melt processes. The use of solvents was generally very hazardous because of their flammabiUty or toxic nature. The cost of recovery and disposal of solvents became prohibitive for many lower price film appHcations. The nature of the drying operations leads to problems with solvent migration and retention that are not problems with melt-processed polymers. 

Clear, water-soluble, oU-and grease-resistant films of moderate strength can be cast from hydroxyethylceUulose solutions. Elexible, nontacky, heat-sealable packaging films and sheets can be produced from hydroxypropylceUulose by conventional extmsion techniques. Both gums can be used in the formulation of coatings, and both can be used to form edible films and coatings. 

ISO 17557 2003 Plastics - Film and sheeting - Cast polypropylene (PP) films... 

Cellulose esters are used as plastics for the formation by extrusion of films and sheets and by injection molding of parts. They are thermoplastics and can be fabricated employing most of the usual techniques of (largely compression and injection) molding, extrusion, and casting. Cellulose ester plastics are noted for their toughness, smoothness, clarity, and surface gloss. 

Plastics film and sheet can be made in a number of different ways (including casting film from solvent or plastisols, pressing, and skiving from material in block form) but in commerce the most important processes for making these intermediates from thermoplastics are calendering and extrusion. 

Films and sheets are produced in several ways, including extrusion, calendering, and casting. Method used involves the properties required of the basic plastics and finished products as well as cost usually based on quantity. The following classification can be helpful as a guide to film and sheet thicknesses (1) film is generally less than 0.010 in. (0.003 mm) and (2) sheet at 0.010 in. or more. In turn sheet can be classified as ... 

Coextrusion is the process of forming an extru-date composed of more than one thermoplastic melt stream. The process came about because some service demands, particularly from the packaging industry, could not be satisfied by a single polymer although they could be met by a combination of polymers. Coextrusion was first practiced in the production of cast film and is now also used in blown film and sheet extrusion. The intention is normally to produce a laminar structure in which each layer contributes a key property to the overall product performance. Coextruded films may be very complex structures composed of many different functional layers, including tie layers whose purpose is to bond neighboring layers of limited compatibility. Five layers are not uncommon. However, side-by-side coextrusion is also possible. Fluoroplastics can be coextmded with other polymers such as ETFE and nylon. 

The main components of a die include the inlet channel, which delivers the plastic from the extruder to the die the manifold, which is designed to evenly distribute the melt within the die the approach or land, which carries the melt from the manifold to the die opening and the die lips, which perform the final shaping of the melt as it exits the die. For cast film or sheet, the die opening is slit-shaped, producing a thin, wide exiting stream of plastic. 

Multilayer structures are often used in packaging in order to take advantage of the properties of several polymers at once. Multilayer structures are made by lamination, coating, and/or coextrusion. Because of the multiple handling of materials and the thickness of the layers, laminations (see Chapter 8) may be relatively expensive materials. Coextrusion can be used to reduce the cost of multilayer plastic film and sheet. Coextruded packaging materials are multilayer plastic sheet or film constructions produced from more than one plastic resin in a single step, using either the cast or blown film process. In coextrusion, the materials never exist as separate webs. 

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