Uniform Melt Spreading in Sheet and Film Extrusion Dies

Extrusion of sheets and films present the challenge of uniformly spreading the melt out from the extruder over the width of the die. The goal is to have a uniform output over the entire span of the die at exit. The simplest die for this purpose is a T-die, as shown in Figure 3.64. A distribution chamber that is located after the transition from the extruder. It has a relatively large diameter so that the polymer melt fills this chamber with little resistance and pressure drop. Then the melt is forced under a large ΔP between die lips to form a sheet. This approach is adequate for some applications but the finite pressure drop in the distribution chamber does lead to a non uniform output that can be improved by changing the die geometry. A better design is called the coathanger die. This die has a specially shaped chamber and varying feed slot length to achieve more uniform output. As expected from the equations in Table 3.5, maintaining a uniform gap is also essential to controlling not only the product thickness, but also the output. Adjustable die lips are used to fine tune the gap and output.