optimising extrusion die design on basis of resin rheology

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OPTIMISING EXTRUSION DIE DESIGN ON THE BASIS OF RESIN

RHEOLOGY

NATTI S. RAO

Plastics Solutions InternationalSchieferkopf 6

D-67434 Neustadt, Germany

ABSTRACT

Extrusion dies can be designed on the basis of empirical data gathered from experience and then optimised by means

of experiments. This kind of trial and error method can involve, particularly, in the case of large production units

huge costs.

This paper shows as to how a die can be dimensioned by utilising the knowledge of the rheological behavior of the

resin which the die is required to process, in order to obtain a melt at a low shear, low temperature and low pressure.

The design principles presented can also be used to avoid melt fracture at high throughputs and, in addition, to assess

the performance of an existing die. Examples of dies used in pelletizing, blow molding, blown film and extrusion

coating illustrate the design procedures treated. Comparisons between the results of modeling and practice have beendiscussed.

INTRODUCTION

Extrusion dies can be designed on the basis of experimental data gathered from experience and then optimised by

means of experiments. This kind of trial and error method can involve, particularly, in the case of large production

units huge costs. Furthermore it is difficult, if not impossible, to determine the combination of variables, which really

influence the efficient working of the die by this method alone.

This paper deals with the analytical methods of die design, which have been successfully employed in the practice.

To illustrate the design procedure involved, first the occurrence of melt fracture and means of avoiding it will be

briefly mentioned, followed then by the applications of the design model to the commonly used dies in various

extrusion processes.

Melt fracture can be defined as an instability of the melt flow leading to surface or volume distortions of theextrudate. Surface distortions [1] are usually created from instabilities near the die exit, while volume distortions [1]

originate from the vortex formation at the die entrance. Due to the occurrence of these phenomena melt fracture limits

the production of articles manufactured by extrusion processes. The use of processing additives to increase the

output has been dealt with in a number of publications given in [2]. However, processing aids are not desirable in

applications such as pelletizing and blow molding. This has led to examine the effect of die geometry on the onset of

melt fracture.

The onset of melt fracture with increasing die pressure is shown for LDPE and HDPE in Figure 1 [3]. As can be seen,

the distortions appear differently depending on the resin. The volume flow rate is plotted in Figure 2 [4] first as a

function of wall shear stress, and then as a function of pressure drop in the capillary for LDPE and HDPE. The

sudden increase in slope is evident for LDPE only in the plot flow rate against pressure, where as in the case of

HDPE it is just the reverse. Furthermore, for HDPE, the occurrence of melt fracture depends on the ratio length L to

diameter D of the capillary. The effect of temperature on the onset of melt fracture is shown in Figure 3 [5]. With

increasing temperature the onset of instability shifts to higher shear rates. This behavior is used in the practice toincrease the outputs. However, exceeding the optimum processing temperature can lead to a decrease in the quality of

the product in many processing operations. From these considerations it can be seen that designing a die by taking the

resin behavior into account is the easiest method to obtain quality products at high throughputs.

DESIGN PROCEDURE

Using the formulas given in the book [6] and in the paper [7] following design procedure has been developed to suit

the die dimensions to the melt flow properties of the resin which is to be processed with the die.



STEP 1: Calculation of the shear rate in the die channel

STEP 2: Fitting the measured viscosity plots with a rheological model

STEP 3: Calculation of the power law exponent

STEP 4: Calculation of the shear viscosity at the shear rate in Step 1

STEP 5: Calculation of the wall shear stress

STEP 6: Calculation of the factor of proportionality

STEP 7: Calculation of die constantSTEP 8: Calculation of pressure drop in the die channel and

STEP 9: Calculation of the residence time of the melt in the channel

APPLICATIONS

Based on the design procedure outlined above, computer programs have been developed for designing dies for

various processes. The principles of the design methods are illustrated below for each process by means of the

results of the simulation performed on the dies concerned. The designing principle consists basically in calculating

the shear rate, pressure drop, residence time of the melt during its flow in the die, and keeping these values below the

limits, at which melt fracture can occur. This is achieved by changing the die dimensions in the respective zones of

the die, where the calculated values may exceed the limits set by the resin rheology.

Pelletizer Dies

The aim here is to design a die for a given throughput or to calculate the maximum throughput possible without melt

fracture for a given die. These targets can be achieved by performing simulations on dies of different tube diameters,

flow rates and melt temperatures. Figure 4 shows the results of one such simulation.

Blow Molding Dies

Figure 5 shows the surface distortion on the parison used in blow molding , which occurs at a definite shear rate

depending on the resin. In order to obtain a smooth product surface, the die contour has been changed in such a way

that the shear rate lies in a range, which gives a smooth surface of the product (Figure 6). The redesigned die creates

lower extrusion pressures as well, as can be seen from Figure 6 [5].

Blown Film Dies

Following the procedure outlined above and using the relationships for the different shapes of the die channels

concerned, a blown film spider die was simulated (Figure 7). It can be examined on the basis of these results whether

these values exceed the boundary conditions, at which melt fracture occurs. By repeating the simulations, the die

contour can be changed to such an extent that shear rate, shear stress and pressure drop are within a region , in which

no melt fracture can occur. Figure 8 shows the shear rate and Figure 9 the residence time of the melt along the flow

path [8].

The results of simulation of a spiral die are presented in Figure 10 as an example. As in the former case, here again

the die gap and the geometry of the spiral channel can be optimized to fit them to the resin on the basis of shear rate

and pressure drop.

Extrusion coating dies

Taking the resin behavior and the process conditions into account, the flat dies used in extrusion coating can be

designed on similar lines as outlined above. Figure 11 shows the manifold radius required to attain uniform melt flow

out of the die exit as a function of the manifold length [8].

CONCLUSIONS

Melt fracture can be eliminated or postponed by using processing aids or by changing the temperature of the die.

However, these means are not desirable in many processes. This paper shows how die design can be suited to the

resin behavior, in order to avoid melt fracture. Examples of dies used in pelletizing, blow molding, blown film and



extrusion coating illustrate the design principles treated. The design procedure given can be applied to optimize any

die design and also to assess the performance of an existing die.

ACKNOWLEDGEMENT

The help of Dr. Günter Schumacher at the Forschungszentrum Informatik in Karlsruhe, Germany in preparing the

manuscript is thankfully acknowledged.

Figure 1: Irregularities of the extrudate observed at increasing extrusion pressure

with LDPE and HDPE [3]



Figure 2: Volume rate Q& vs wall shear stress wσ and volume flow rate Q& vs pressure drop

in capillary p∆ for LDPE and HDPE [4]



Figure 3: Effect of temperature on the melt fracture (region 2) for HDPE



Figure 5: Surface distortion on a parison used in the blow molding [5]

Figure 6: Die contour used for obtaining a smooth parison surface [5]



Figure 7: Calculated pressure drop p∆ in a spider die

used in blown film at different die gaps hr

Figure 8: Shear rate γ& along spider die



Figure 9: Residence time t of the melt as a function of the flow path l

Figure 10: Results of simulation of a spiral die used in blown film for LLDPE



References

1. Sammler,R.L., Koopmans,R.J., Magnus,M.A. and C.P.Bosnyak: Proc. ANTEC 1998, p.957 (1998)

2. Rosenbaum, E.E. et al.: Proc., ANTEC 1998, p.952 (1998)

3. BASF Brochure: Kunststoff Physik im Gespräch (1977)

4. Agassant,J.F., Avenas P., Sergent,J.Ph. and P.J.Carreau: Polymer Processing, Hanser, Munich (1991)

5. BASF Brochure: Blow molding (1992)

6. Rao, N.S.: Design Formulas for Plastics Engineers, Hanser, Munich (1991)

7. Rao, N.S.: Practical Computational Rheology Primer, Proc., TAPPI PLC (2000)

8. Rao,N.S. and K.O'Brien: Design Data for Plastics Engineers, Hanser, Munich (1998)

optimising extrusion die design on basis of resin rheology

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