Sheet and Film Dies

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). 

Extrusion. Sheet, film, and profiled articles are made by extrusion (20). The resin is melted and forced through a die plate or head. Variations include multilayer and blown film applications. In multilayer coextrusions, different combinations of plastics are separately but concurrently extruded to form layered sheet or film. In the packaging industry, specialty resins such as high barrier ethylene—vinyl alcohol copolymers are combined with heat- and impact-resistant thermoplastics for food packages. The properties of each resin layer are additive, as opposed to the "averaging of property" in blends. Multilayers are also used for blow-molded containers, films, and sheet products (see also Film AND sheeting materials). 

The end-fed-sheeting die, as presented in Fig. 6.9, is a simple geometry that can be used to extrude films and sheets. To illustrate the complexities of die design, we will modify the die, as shown in the figure, in order to extrude a sheet or film with a uniform thickness. In order to achieve this we must determine the length of the approach zone or die land as a function of the manifold direction, as depicted in the model shown in Fig. 6.10. 

The following general classification may be helpful as a guide to film and sheet thickness selection for a die even though different groups within the different industries may have their own thickness definitions as well as their own terminology ... 

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. 

Extrusion dies vary in shape and complexity to meet the demands of the product being manufactured. There are five basic shapes of products made with extrusion dies, as illustrated in Table Film and sheet dies... 

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... 

Figure 2.25 illustrates a T-type die and a coat-hanger-type die, which are used for both film and sheet extrusion. The die must produce a smooth and uniform laminar flow of the plastic melt which has already been mixed thoroughly in the extruder. The internal shape of the die and the smoothness of the die surface are critical to this flow transition. The deckle rods illustrated in Figure 2.25 are used by the processor to adjust the width of the extruded sheet or film. 

Usually flat film and sheet dies are constructed of medium carbon alloy steels. Die flow surfaces are chrome-plated to provide corrosion resistance. The exterior of the die is usually flash chrome-plated to prevent rusting. Where chemical attack can be a severe problem (with PVC, etc.), various grades of stainless steel are used (Tables 2-4 and 2-8). 

To determine the source of a problem, it is necessary to understand the basics of a process and apply them to problem solution. For example, with film and sheet dies, tip adjustments only control the uniformity of the transverse thickness. To control the average thickness a proper relationship between the extruder pumping rate and the speed of roll-up is required. Closing the die lip opening has very little effect on the extrusion rate and does not make the entire sheet thinner. 

Dielectric High-frequency voltage applied to film or sheet causes material to melt at bonding surfaces. Material cools rapidly to effect a bond. Most widely used with vinyls. Fast seal with minimum heat applied. Only for film and sheet. Requires rf generator, dies, and press. Operation can range from hand-fed to semiautomatic with speeds depending on thickness and type of product being handled. Units of 3 to 25 kW are most common. 

Automatic profile control n. In film and sheet extrusion, a system for controlling the uniformity of thickness across the sheet. The main components are a traversing thickness sensor such as a beta-ray gauge, a computer and program that uses the sensor s signals to direct a mechanism that rotates the die-lip-adjusting bolts. 

Die-lip build-up n. In extruding film and sheet, the gradual accumulation of a bead of resin on the face of the die, parallel to the slit. Similar build-up can occur on dies for shapes. 

Flexible-lip die n. In film and sheet extrusion, a die in which a deep groove, reaching almost to the inside surface of the upper die body just behind the lip, has been machined. Adjusting bolts that can either push or pull on the lip pass through the gap from the upper body. The relative flexibility of the thin steel web from which the upper lip extends facilitates die adjustment to minimize sheet-thickness variations across the sheet. 

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