Diehead pressure

As the polymer flows through the die, it adopts the shape of the flow channel of the die. Thus, as the polymer leaves the die, its shape will more or less correspond to the cross-sectional shape of the final portion of the die flow channel. Since the die exerts a resistance to flow, a pressure is required to force the material through the die. This is generally referred to as the diehead pressure. The diehead pressure is determined by the shape of the die (particularly the flow channel), the temperature... 

The roller die (B.F. Goodrich, 1933) is a combination of a standard sheet die and a calender. It allows high throughput by reducing the diehead pressure it reduces air entrapment and provides good gauge control. 

There are a number of extruders, which do not utilize an Archimedean screw for transport of the material, but still fall in the class of continuous extruders. Sometimes these machines are referred to as screwless extruders. These machines employ some kind of disk or drum to extrude the material. One can classify the disk extruders according to their conveying mechanism (see Table 2.1). Most of the disk extruders are based on viscous drag transport. One special disk extruder utilizes the elasticity of polymer melts to convey the material and to develop the necessary diehead pressure. 

It is important to notice that increased load, i.e., increased diehead pressure, reduces the bearing life by a power of three or more Also, the life L will be reached more quickly when the extruder runs at high speed. The predicted life, Ly, expressed in years, is obtained from the following expression ... 

The predicted B-10 life at any diehead pressure and/or screw can be found by the following relationship ... 

Diehead pressure before and after screen pack... 

The diehead pressure in the extruder determines the output from the extruder. It is the pressure necessary to overcome the resistance of the die. When the diehead pressure changes with time, the extruder output correspondingly changes and so do the dimensions of the extruded product see Fig. 4.1. As a result, when we monitor how the pressure varies with time, we can see exactly how stable or unstable the extrusion process is. 

A drawback of the stochastic identification technique is its complexity and the substantial computational requirements. Only a limited number of investigators have applied this technique to the extrusion process. Parnaby et al. [61-63] did the first work on stochastic identification of extrusion process models. A hierarchical automatic optimal control scheme was developed and evaluated on a laboratory extrusion line [63]. The only operator input required was the desired output rate and die inlet melt temperature. A variable die restriction was used to adjust diehead pressure and throughput. Considerable improvements in control were obtained, particularly in the control of diehead pressure. Other applications of stochastic identification to extrusion have been made by Patterson et al. (64,65], Costin [66, 67], and at the IKV in Aachen [81]. 

These two sets of curves indicate that an increase in screw speed can be offset by a reduction in the throttie ratio. The throttie ratio is determined by the iniet and outiet pressure of the meit conveying zone. It is ciear, therefore, that reducing barrei pressure will reduce polymer melt temperature. This can be achieved by using a less restrictive screen pack, a less restrictive extrusion die, higher die temperatures, or by using a melt pump to generate most of the diehead pressure. 

Even relatively small increases in the power law index, e.g., from 0.3 to 0.5, can reduce the critical screw speed significantly. This indicates that small increases in the power law index can cause significant increases in viscous heating and melt temperature. This is known in practice when we consider the extrusion characteristics of LLDPE relative to LDPE [325]. The power law index of LLDPE is considerably higher than that of LDPE. As a result, LLDPE tends to have more power consumption, higher melt temperatures, higher diehead pressures, and is more susceptible to melt fracture. 

This equation does not take into account the leakage flow or the effect of the flight flanks. The diehead pressure P is related to the total volumetric flow rate V by the die constant K ... 

If the flight pitch is constant and the polymer melt viscosity can be described by Eq. 8.78, the maximum diehead pressure for effective devolatilization can be written as [2] ... 

Thus, if the output increases approximately proportional to the screw speed, at some point the diehead pressure can exceed P iax, causing vent flow. This will happen more readily when the material is more shear thinning, i.e., when the power law index is closer to zero. 

If it is assumed that the diehead pressure is built up uniformly along the filled length of the extruder L, the drag-induced axial channel pressure (Eq. 10.56) can be superimposed on the linear pressure profile, as shown in Fig. 10.32. 

The drag-induced pressure generation in the C-shaped chamber plus the pressure rise due to the diehead pressure should equal the pressure drop through the calender gap ... 

In high speed TSEs, the fully filled length is relatively short, typically 20 to 40% of the length of the extruder. The fully filled regions occur where restrictive elements are placed along the screw and at the end of the screw where the diehead pressure has to he developed. Restrictive elements are often placed just upstream of a vent port to create a melt seal. In a starve fed extruder, a melt seal is necessary to be able to draw a vacuum at the vent port. This is illustrated in Fig. 10.65. 

A simple DAS is a chart recorder that can track important variables like screw speed, diehead pressure, melt temperature, motor amperage, etc. More useful is a computer-based DAS these come in two forms portable data collectors/machine analyzers and fixed-station data acquisition system. 

Obviously, the screw frequency pressure fluctuation will be problematic when the value of AP is large relative to the actual diehead pressure. This will occur when the diehead pressure is low, as pointed out by Wheeler [68], when the polymer melt viscosity is high, the screw diameter large, the screw speed high, the helix angle or pitch large, or when the channel depth is shallow. 

The next step is an increase in the diehead pressure to alter the pressure profile along the extruder and to achieve a more rapid compacting of the solid bed. The diehead pressure can be increased by simply adding screens in front of the breaker plate. Another possible solution is to starve feed the extruder however, this may reduce extruder output and requires additional hardware, i.e., an accurate feeding device. 

These patterns are often related to line tension, more specifically uneven line tension. In advanced cast film lines (BOPP or BOPET), there are tension-control systems that allow tension adjustment in specific regions of the film. In these lines, a gear pump is often necessary to minimize output variation. The diehead pressure in such extrusion operations can be very high (up to 600 bar). 

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