Page 27 - Plastics News May 2021

FeAtures

Getting The Profile Die Design Just Right

Jim Frankland

That ‘Sliding’ effect has a considerable influence on the die design and it’s important to understand this when
troubleshooting die performance

alculating polymer flow is fairly straightforward when
Cdesigning a die for a simple round profile, but as the
shapes get more complicated these calculations can get
extremely challenging. Here’s what you need to consider.
Die forming typically occurs immediately after polymer
melt has exited an extruder. The most basic process
involves guiding the stream of molten polymer under
pressure through a die, which three distinct regions:
manifold, approach, and lip. The 'manifold' serves to
channel the polymer melt from its initial extrusion point
to a near-net-shape of the final product. The 'approach'
region further guides the melt into the final shape, and
begins to correct for any non-uniform flow. Finally, the additional surface area in the corners causes a reduction
'lip' forms the melt into the final desired cross section and in flow. This tends to round the corners. The shear stress
compensates for any remaining flow asymmetry. After in the polymer shape is always zero in its exact center
exiting the lip of the die, the polymer melt will undergo of flow, which may or may not be the center of the part
die swell before curing. Die swell is an expansion of the (except in the simplest shapes). The maximum shear
melt as the pressure is released, and is dependent on stress occurs at the die wall, and shear stress increases
polymer chemistry and die design. After curing, the solid, proportionally with the distance from the center point of
continuous part is drawn onto a take-up roller or cut into flow. That variation in shear stress alone develops a slight
transportable lengths, depending on the type of part. A difference in viscosity due to the changing shear. The
lot of experienced extrusion operators—especially those more complex the profile, the more complex the effects.
with limited background in polymer rheology—tend to Thus, as the die becomes more complicated, the variations
think the material “slides” through the die to form its final in flow become more complex and the die internals often
shape. In fact, the polymer should adhere firmly to the die do not look that much like the final part. Note the rounded
walls, as slips result in melt fracture. That effect has a corners in the relatively simple square profile in Fig. 1. For
considerable influence on the die design and it’s important that profile, the die had to be designed with additional
to understand this when troubleshooting die performance. space for polymer to flow into all the corners as the part
Once the polymer exits the extruder and enters the die it moves through the die.
sticks quite firmly to the die walls and is sheared in the Each surface of the die also needs to be uniformly heated
die openings as pressure from the screw or melt pump so that the viscosity along the walls provides for the same
forces it through the die. Calculating polymer flow is not pressure drop to match the other legs, so that the profile
all that difficult when designing a die for a simple round stays “square” exiting the die. Any unbalanced heating
profile, but as the shapes get more complicated these of the die can distort the shape by varying the viscosity
calculations can get extremely challenging. Just moving near the die wall. This occurs even in simple dies such
to a simple rectangle makes things more complex.
as one in Fig. 1, but it becomes much more pronounced
A circular profile tends to just get a little smaller than in more complex profile dies where the shape is more
the die cross-section (neglecting polymer swell) because irregular (see Fig. 2).
of the polymer adhering to the wall—but the shape does The sections labeled 1 to 4 show how the shape of the
not change. On the other hand, with a simple rectangular die channel would change from extruder exit on the left
profile the square corners will not fill out because the

27 May 2021 Plastics News