Design and Manufacture of an Extrusion Die for Wood--Plastic Composite

Abstract

This paper presents a simulation study and experimental verification of a die design in extrusion processing wood-plastic composite profile. Flow balancing at the die exit is a major challenge in the profile extrusion die design as an unbalanced flow causes imperfect profile output. This is especially important when the final profile is complicated by having large dimensions and non-uniform thicknesses. In this study, an I-shaped profile of wood-plastic composite (WPC) has been considered for the die design. Finite element method has been used to design the die channel to yield a balanced output. Phases of die design and manufacture of the I-shape profile is presented. The results show that the prediction by the simulation could give a good insight to the die design which eventually could yield an acceptable profile.

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Composites
Journal of Reinforced Plastics and
DOI: 10.1177/0731684408089507
2008;
2009; 28; 1433 originally published online Jul 11,Journal of Reinforced Plastics and Composites
E. Soury, A.H. Behravesh, H.G. Nasrabadi and A. Zolfaghari
Design and Manufacture of an Extrusion Die for WoodPlastic Composite
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Design and Manufacture of an Extrusion Die
for Wood–Plastic Composite
E. SOURY,A.H.BEHRAVESH,* H. G. NASRABADI AND A. ZOLFAGHARI
Faculty of Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran
ABSTRACT: This paper presents a simulation study and experimental verification of a die design in
extrusion processing wood-plastic composite profile. Flow balancing at the die exit is a major
challenge in the profile extrusion die design as an unbalanced flow causes imperfect profile output.
This is especially important when the final profile is complicated by having large dimensions and
non-uniform thicknesses. In this study, an I-shaped profile of wood-plastic composite (WPC) has
been considered for the die design. Finite element method has been used to design the die channel to
yield a balanced output. Phases of die design and manufacture of the I-shape profile is presented.
The results show that the prediction by the simulation could give a good insight to the die design
which eventually could yield an acceptable profile.
KEY WORDS: wood-plastic composite (WPC), finite element, die balancing.
INTRODUCTION
E
XTRUSION PROCESS IS a major manufacturing method to manufacture two-
dimensional plastic parts such as tubes, pipes, sheets, rectangular or trapezoid
profiles, etc. In this process, an extrusion (of one or two screws) transfers molten materials
(often thermoplastics) through a barrel and pushing it through a die to produce uniform
shape of profiles [1].
Achieving the desired product dimensions is a complicated task and requires detailed
knowledge of material characteristics, flow analysis, and extensive experience in extrusion
processing. The most important issue in design of an extrusion die is balancing the flow at
the die channel. Failing to do so means the extruded product is likely to be imperfect,
twisted, and warped, or curved along its length. The major issue is that the molten material
may not exit the die at the same speed in the sections, or not at all some. This issue is
treated via extruding the profile with uniform exit velocity at all sections through a die
balance treatment. In practice, the exit flow control is achieved by experienced die
designers who may use empirical rules or simply a trial-and-error design procedure that is
usually time and material consuming. Currently, due to the development of software
packages for modeling the polymer flows, this procedure is being slowly changed from an
experimental to a numerical-based operation. For reducing the number of trials- and-
errors, a finite element method is highly useful [2].
The extruded profile may consist of sections of various thicknesses, for example an
I-shaped profile with the zones of dissimilar thicknesses. In an effort to obtain a balanced
*Author to whom correspondence should be addressed. E-mail: e_soury@modares.ac.ir
Figures 1 and 3–6 appear in color online: http://jrp.sagepub.com
Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 28, No. 12/2009 1433
0731-6844/09/12 1433–7 $10.00/0 DOI: 10.1177/0731684408089507
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flow at the die exit, each of these zones may involve flow regions of uneven thickness and
length. Generally, uneven cross sections lead to uneven shearing and resistance, thus
resulting in an unbalanced flow through the regions. Die balancing may be achieved by
adjusting the channel profile and length to control the local pressure [3].
Simply expressing theoretically, for two sections of different thicknesses H
1
and H
2
, the
length of each section must be adjusted so that the exit velocity at the two sections becomes
equal. Treating a simple non-Newtonian (power-law) flow for a slit die, the lengths of
sections must be adjusted so to satisfy the following equation:
L
1
L
2
¼
H
1
H
2

nþ1
ð1Þ
where n is the power-law coefficient.
However, because of the complexity of flow through the die channel and also the
interactions of flows at these sections, a more efficient procedure is to use software
packages (such as FLUENT) to achieve flow analysis with better accuracy [4,5].
In this study, the die balance is treated for extruding a composite of wood-plastic; a fast
growing composite material. A composite material is manufactured via a mixture or
compound of two or more materials resulting in a material having a combination of
the constituent materials properties; the compound type is physical (not chemical).
Wood-plastic composite (WPC) is produced by compounding plastics (often thermo-
plastics) with wood (e.g. sawdust) via an extrusion process. The product could be granules
or profiles. The granule can be either used in an extrusion process (to produce profile- a two
stage process) or in an injection molding process (to produce 3D objects) (Figure 1) [6].
GOAL OF RESEARCH
The objective of this research work was to design and manufacture a profile extrusion
die to produce an I-shape profile of wood–plastic composite (Figure 2). The first step was
to design a die channel with the main emphasis on an optimization procedure to achieve a
Wood flour
Wood
p
last
Plastic granules
Figure 1. Components of wood- plastic composite [6].
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uniform velocity at all sections of the die exit. The following step was to manufacture the
die to verify the first step.
GEOMETRICAL MODELING
The used extruder was twin-screw counter-rotating with screw diameter of 62.5 mm. The
extruder head had an output circular hole with a diameter of 52 mm. This gives a cross
section area of 2124 mm
2
. The desired dimensions of the output profile yields a total cross
section of 1535 mm
2
(Figure 2). In order to attain a smooth path, automatic lofting was
also further modified.
The channel cross sections at an equal distance of 25 mm are given in Figure 3. It is
observed that the thicker section (10 mm) remains unchanged at the further steps (5 to 8)
indicating a larger land length compared to the thinner section (5.5 mm).
Besides the above s trategy (smooth and constant transition at the channel),
considerations were taken to account to further smooth the flow path, such as: i) as a
low transition angle as possible, ii) avoiding sharp corners. The final shape of the die
channel (or cavity) is shown in Figure 4.
The final shape of die-cavity is shown in Figure 4.
SIMULATION RESULT
Due to complexity of the flow path from extruder head to die exit (Figure 4), a flow
analysis was performed using appropriate software (FLUENT 6.0) [7]. Although the
software does not support visco-elastic properties, it could give a good insight into the
design procedure. The material characteristics (here wood-plastic composite of 60% wood
50
5.5
10
110
Figure 2. Desired dimensions of the output profile.
Design and Manufacture of an Extrusion Die for Wood–Plastic Composite 1435
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content) is given in Table 1. It is important to note that, due to the nature of the material
that promotes a plug-flow, the power-law index is given a low value (0.01).
The boundary conditions of this case were considered as given in Table 2. The flow
type is laminar (due to low velocity) and because of the dual symmetrical form of the
profile die, a quarter of the profile was modeled. The results of the flow analysis is shown
in Figure 5. According to this figure the average velocity of the die exit is almost
homogeneous at all sections.
1234
5678
Figure 3. Diechannel cross sections at an equal distance of 25 mm.
Figure 4. Final design of the die channel (or cavity).
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Table 2. Boundary conditions of flow analysis.
Inlet velocity 0.01 m/s
Outlet gauge pressure 0 Pa
Wall temperature 430 K
Table 1. Characteristic of experimental material
(60% wood and 40% PP).
Density 920 kg/m
3
Melting temperature 773 K
Solidification temperature 403 K
CP 2500 J/kg K
Thermal conductivity 20 W/m K
Power-law index (n) 0.01
Consistency index ( k) 3000 kg s
n2
/m
50
Y velocity
0.025
0.015
0.01
0.00491763
0.02
40
40 60
30
20
20
x direction (mm)
z direction (mm)
10
−10
−20
−20
−30
−40
−50
0
0
Figure 5. Flow velocity at Y-direction for the I-shaped profile.
Design and Manufacture of an Extrusion Die for Wood–Plastic Composite 1437
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It is also noticed that a small difference is observed between the velocity profiles at
the thicker section with respect to the thinner section, so that the velocity seems smaller.
However, considering that the average velocity at the sections is a parameter of
importance, it is acceptable to have a little larger velocity profile at the thinner section.
EXPERIMENTATION
According to the designed model, a die was manufactured to produce the I-shape
profile. Via adjusting the processing parameters, appropriate output profile was obtained.
The temperatures at five zones (three for the extruder, one for adaptor, and one for the
die) were 190, 220, 215, and 1758C, respectively. The screw speed was maintained at
15 rpm.
The set temperatures and screw rpm prevented the wood burning that could promote
degradations resulting in discolored surfaces. The final output profile is shown in Figure 6.
It is interesting to note that the quality of output features (corners and fillets) were high.
The output speed was measured to be 10 cm/min.
RESULTS AND DISCUSSION
According to Figure 6, the manufacturing process of this case was successful and perfect
production of this part specifies that the die design procedure is acceptable and thus can be
introduced for similar parts.
Figure 6. The extruded WPC profile.
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CONCLUSIONS
The present research work attained the following conclusions:
1. A strategy is required to carry out the die design to attain a smooth and balanced flow.
2. As an objective in a die design, achieving a uniform velocity can sufficiently yield a
balanced die.
3. The flow analysis using appropriate software could help the designer to attain desirable
flow contours (uniform velocity). Any correction is required to be considered at the
design stage to avoid high expense of diemaking failure.
4. For WPC profile die design, it is essential to input appropriate values for material
characteristics as the flow would be much similar to a plug flow.
Experimentation with the manufactured die resulted in an acceptable output profile with
the desired material (wood-plastic composite).
REFERENCES
1. Ulysse, P. (2002). Extrusion Die Design for Flow Balance Using FE and Optimization Methods, International
Journal of Mechanical Sciences, 44: 319–341.
2. Alam, M. K., Tzoganakis, C. and Perdikoulias, J. (2005). Effect of Rheology and Die Design on
Flow Balancing of Profile Extrusion Dies, ANTEC, 1: 76–80.
3. Kostic, M. M. and Reifschneider, L. G. (2006). Design of Extrusion Dies, Encyclopedia of Chemical
Processing, DOI: 10.1081/E-ECHP-120039324, pp. 633–649.
4. Rauwendaal, C. (2001). Polymer Extrusion, Hanser Publication, Munich.
5. Johann Sienz, Bates, Stuart J. and John F.T Pittman (2006). Flow Restrictor Design for Extrusion Slit Dies
for a Range of Materials: Simulation and Comparison of Optimization Techniques, Finite Elements in
Analysis and Design, 42: 430–453.
6. http://www.pallmannpulverizers.com/wpc_industry.htm
7. FLUENT 6.0 User’s Guide S
g
.
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