Die swell compensation

Die Swell Compensation. The polymer melt swells when it exits the die, as explained previously. This die swell is a function of the type of plastic material, the melt temperature, the melt pressure, and the die configuration. The die must be compensated for die swell so that the extruded part has the corrected shape and dimensions. Molds and dies for different fabrication processes will be described later in more detail when the processes are discussed. 

Die land plate provides a uniform cross-section passage that is typically 10 times longer than the thickness of the extrudate to relax the viscoelastic stresses in the melt before leaving the die (reduces die swell) and forms the shape of the extrudate leaving the die. The die land profile has the required shape to compensate for extrudate deformation after the die (die swell and drawdown) and yield the desired shape downstream. 

Die swell, where the extruded product increases in size as it leaves the die, may be compensated for by ... 

The TD variations are nearly totally due to die design. The first problem is to design a feed system that will distribute the melt uniformly to the shaping portion of the die. (See Fig. 7.4 for definition of parts of a die.) In the event this is not possible, then it must be possible to adjust the die lips in such a way that the fluid will leave the die with a uniform thickness. Part of the thickness variation in the TD is due to the inability to feed the die uniformly from the extmder, while the rest is due to the phenomenon of die swell. Since the degree of swell may vary nonuniformly over the cross section due to variations in the shear rate, the die lips (main shaping section) may have to be designed to compensate for this. 

FIGURE 7.4 Schematic of a typical extrusion die showing the various parts of the die (side view). The manifold distributes the melt from the extruder uniformly over the width of the die while the restrictor can be used to compensate for variations in flow rate across the width of the die. The lips give the melt stream the desired dimensions and shape. (Note Because of die swell the dimensions of the extrudate may be considerably different from those of the die lips.)... 

Metabolic acidosis can also result when a person is severely burned. Blood plasma leaks from the circulatory system into the injured area, producing edema (swelling) and reducing the blood volume. If the burned area is large, this loss of blood volume may be sufficient to reduce blood flow and oxygen supply to all the body s tissues. Lack of oxygen, in turn, causes the tissues to produce an excessive amount of lactic acid and leads to metabolic acidosis. To minimize the decrease in pH, the injured person breathes harder to eliminate the excess C02. However, if blood volume drops below levels for which the body can compensate, a vicious circle ensues in which blood flow decreases still further, blood pressure falls, C02 excretion diminishes, and acidosis becomes more severe. People in this state are said to be in shock and will die if not treated promptly. 

The above considerations explain why with the short dies of industrial practice, rubber extrudates frequently exhibit swelling variations which are actually compensated for by adjusting the takeaway conveyor speed. This is however not always satisfactory and the results presented here suggest that a suitable design of the entrance geometry could reduce post-extrusion swelling variations. 

It can be seen that the shear rates at the wall vary significantly. The wall shear rate in the corner is relatively low, while the highest shear rate occurs at the middle of the wall. Therefore, the elastic recovery in the middle will be larger than the elastic recovery at the corners. This results in bulged extrudate. It is not possible to obtain a perfectly square extrudate with a perfectly square flow channel. To eliminate this problem one has to modify the shape of the flow channel to compensate for the uneven swelling of the extrudate. A good die designer must anticipate the... 

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