op tim ization injection moulding process

8/13/2019 Op Tim Ization Injection Moulding Process

1/9

Dr.-Ing. Lothar Kallien. Sigma Engineering GmbH, Aachen.

Optimization of the Injection Moulding Process for Thermoplasts With 3D Simulation

Optimization of the

Injection Moulding Process

for Thermoplasts

With 3D Simulation

Dr.-Ing. Lothar Kallien

Sigma Engineering GmbH, Aachen


2/9



1

Optimization of the

Injection Moulding Process

for Thermoplasts

With 3D Simulation

1. Introduction

The use of innovative CAE technologies has made it possible

to drastically reduce the time required for process and product

development. A new simulation tool for optimization of ther-

moset components has recently become available, and can be

used for three-dimensional computation of the mould filling,

cross-linking and internal stress buildup on a monitor.In contrast to conventional programs, the described program

operates with volume elements deriving from the technology of

injection moulding of plastics. The frequently three-dimension-

al geometries of thermoset components are thus depicted in a

physically correct sense.

The flow algorithm for the filling simulation is based on the

Navier-Stokes equations, i.e. kinetic effects such as independ-

ent stream formation are predicted. Air inclusions that can re-

sult from turbulent mould filling are thus detected at an early

stage in mould design and eliminated by optimization.

The mould is three-dimensionally networked and the local tem-perature distribution calculated in the new 3D program. Thus,

inhomogeneous temperature zones such as corner effects and

their influence on the local cross-linking behavior are taken in-

to account. Multiple cycles can be simulated to determine the

temperature distribution in the mould during production start-

up up to the quasi-stationary state.

After stripping, thermally induced internal stresses, particular-

ly as a result of contraction constraints due to metallic inserts,

can lead to cracking. The buildup of such internal stress can be

calculated and the resultant cracking thus predicted.

2. 2D and 3D Simulation

In developing injection moulded components, simulation pro-

grams are used that based on empirical data and mathemat-

ical models - can compute mould filling, the holding and the

cooling phase to the point of stripping, and the extent of com-

ponent deformation /1,2/. Programs used to date for simula-

tion of injection moulding processes rely on geometric informa-

tion that approximately describes the upper, lower and middle

planes of the actual geometry. This method of calculation is

generally referred to as a 2?-dimensional shell model. The on-

ly approximate description of the component geometry by a

middle plane can have a negative effect on the quality of theresult. This particularly applies to calculation of components

with irregular wall thickness /3,4,5,6/. Furthermore, three-

dimensional flow effects cannot be resolved, since these

programs always assume parallel laminar flow. Figure 2.1

schematically illustrates zones in a plastic component in which

three-dimensional flow effects develop /4/.

Figure 2-1: Zones of three-dimensional melt flow in injection moulding /5/

3. CAD Transference and Cross-Linking in 3D

Including Mould and Inserts

Presently, volume-oriented CAD data exist for the product to be

calculated in most cases. These can be imported into SIGMA-

SOFT as STL files. All types of FEM volume networks can sim-

ilarly be transferred to SIGMASOFT. The geometry of the

moulding can be expanded to include the injection points or

the mould geometry using an integrated solid modeler.

Separation into a three-dimensional volume network is fully au-

tomatic. Depending on the complexity of the networking geom-

etry, this operation takes only a few seconds to a maximum of

2 3 minutes. In any case, the complete mould including all

cooling and heating channels is networked.

4. Simulation of a Thermoset Component

A water circulation system produced at LM Plast for a French

automobile was simulated in cooperation with the Vyncolit

Company. The component is used in PSA-group cars such as

the 106, 206 and 306 Peugeot models and the Saxo, Xsara

and Berlingo Citroen models (Figure 4-1).

Figure 4-1: Citroen Berlingo


3/9



2

This involved replacement of a die-cast aluminum component

by a thermoset part. One of the reasons for this substitution

was the major weight advantage: the aluminum component

weighs 614 g, and the plastic part only 344 g. Figure 4-2

shows a direct comparison of the aluminum component (left)

and the new thermoset part (right).

Figure 4-2: Comparison of an aluminum (left) andthermoset (right) component

The design geometry was retained essentially unchanged in

this simulation. The component is fabricated in a single-cavity

mould. The data were adopted from a CAD System, and sim-

ply imported into the preprocessor (Figure 4-3).

Figure 4-3: Transfer of geometry to the simulation

Networking in three-dimensional volume elements is fully au-

tomatic. Figure 4-4 shows the networking geometry including

the sprue.

Figure 4-4: SIGMASOFT volume model

After the geometry has been entered, the process and ma-

terial data must be added. These include the filling time, the

volume flow or filling pressure at the gate, the temperature of

the thermoset material when injected, the preliminary cross-

linking density, the mould temperature and information on the

thermal conductivity coefficients of the involved groups of

materials.

The density, thermal conductivity, heat capacity as a function

of the temperature and the cross-linking enthalpy of the ther-moset must be known. In order to calculate cross-linking as a

function of the time, cross-linking curves at a minimum of

three different temperatures must be entered into the program.

Figure 4-5 illustrates a cross-linking curve for a temperature of

160C. SIGMASOFT approximates this process fully automat-

ically using the Deng-Isayev model. The actual results and

the approximated cross-linking curve are directly compared in

the figure.

Figure 4-5: Cross-linking curve at a temperature of 160C

The cross-linking reaction during mould filling is also calculat-ed in SIGMASOFT. This is accomplished using a Cross-

Arrhenius formula that describes the viscosity as a function of

the shear rate, the temperature and the local cross-linking den-


4/9



3

sity. If the cross-linking density exceeds a critical value in spe-

cific zones, the viscosity rises sharply and flow is no longer

possible. The following formulas describe this relationship.

Alphagel is the cross-linking density at which flow is no longerpossible.

For gel the following term

describes the viscosity as a function of the cross-linking densi-

ty, the temperature and the shear rate.

In this case

and

If gel then

Tb: Reference temperature [K]

B: Arrhenius constant [Pas]

*: Material constant [Pa]

n: Cross exponent

gel: Cross-linking density

C1, C2: Constants

The process parameters for the component were as follows:

Filling time: 5s

Bulk temperature: 110C

Preliminary cross-linking density of the compound

during filling: 5%

Mould temperature: 170C

Mould filling, the cross-linking reaction, cooling to ambient

temperature and the resultant internal stresses were calculat-

ed. Figure 4-6 shows the temperature distribution at 85 %

mould fill. The compound enters the heated mould at 110C,

and the temperature rises further. The local shear rate is high-

est at the gate (Figure 4-7).

Figure 4-6: Temperature distribution at 85 % mould fill

Figure 4-7: Local shear rate distribution in the gate

The flow phenomena can also be visualized using tracer parti-

cles. The colors of these weightless particles in Figure 4-8

show the ingress age of the particles, whereas the vectors de-

scribe the directions and velocities of the particles. Figure 4-9

shows the temperature distribution at the conclusion of mould

filling. The coldest melt is located at the gate.


5/9



4

Figure 4-8: Tracer particles visualize the flows

Figure 4-9: Temperature distribution at 100% mould fill

The mould filling process can also be described by the local fill-

ing time (Figure 4-10). The pressure required for filling is illus-

trated in Figure 4-11. The value at the front flange (arrow)

could be verified by an internal pressure sensor in the mould.

A pressure of 230 bars was determined at this point.

Figure 4-10: Local filling times in seconds

Figure 4-11: Pressure distribution at conclusion of filling. The arrow shows

the position of the internal pressure sensor in the mould.

Since the compound is injected into the cavity at a preliminary

cross-linking density of 5 %, the local cross-linking density has

risen to 11% at the end of the filling operation (Figure

4-12). Particularly in the vicinity of ribs, where the melt can

no longer flow, the compound rapidly heats up and begins

to cross-link (arrow). The local cross-linking density has

increased to 55% after 16 seconds (Figure 4-13). The effect of

mould filling is clearly apparent, since the zones that are

farther away from the gate are more highly cross-linked by the

elevated temperature.


6/9



5

Figure 4-12: Local cross-linking density at conclusion of mould filling.Zones such as ribs, which are not exposed to a continuous flow, heat upand cross-link more rapidly.

Figure 4-13: Local cross-linking density after 16 seconds

After 40 seconds, 7 % of the total compound is cross-linked

(Figure 4-14). Zones with less than 70% cross-linking can be

visualized in an X-ray view (Figure 4-15).

Figure 4-14: Local cross-linking density after 40 seconds

Figure 4-15: X-ray view of zones with less than 70% cross-linking density

Following cross-linking in the mould, cooling in air to ambient

temperature is calculated. The thermally induced internal

stresses outside the mould can be calculated on the basis of a

plastic-elastic material model in order to obtain data on de-

formation. The stress calculation can be carried out for

both isotropic materials and anisotropic, fiber-reinforced

thermosets.

The stress calculation assumes that the material is completely

cross-linked when the mould is opened. The calculation is

based on the following temperature-dependent, thermome-

chanical parameters:


7/9



6

The isotropic elastic modulus or the elastic modulus length-

wise and transverse to the fiber direction

The isotropic coefficient of thermal expansion, or the param-

eter lengthwise and transverse to the fiber direction

The transverse contraction number

In this case, the plastic is a short glass fiber-reinforced mate-

rial. The elastic modulus in such a case depends very greatly

on the fiber orientation. The fiber orientation can be three-di-

mensionally calculated using SIGMASOF. Figure 4.16 shows

the fiber orientation in a rib foot. Figure 4-17 shows the local

distribution of the von-Mises internal stresses at ambient tem-

perature. Because of the previously mentioned limitation, the

results should be considered qualitative. Figure 4-18 shows

the main stresses along the y-axis. The local shifts can be cal-

culated on the basis of these values.

Figure 4-16: Three-dimensionally calculated fiber orientation in a ribtransition area. The vectors indicate the direction of the fibers, and the

colors show the degree of orientation in the pertinent element.

Figure 4-17: Thermally induced internal (von-Mises) stresses

Figure 4-18: Thermally induced main stresses along the y-axis

Figure 4-19 shows the shifts along the y-axis. The depicted de-

gree of deformation is exaggerated by a factor of 50.

Figure 4-19: Shifts along the y-axis

Figures 4.2x show a direct comparison of filling simulations.

4 Export of Results to FE Networks

Using a new interface, both the cross-linking reaction and fiber

orientation results can also be exported to finite element net-

works. Figure 4.20 shows the three-dimensionally calculated

fiber orientation along the xaxis for a test component. This

result can now be exported to a wide variety of finite elementnetworks. Figure 4.21 shows the fiber orientation after being

exported to a finite element network for further processing in

ABAQUS.


8/9



7

Figure 4-20: 3D-fiber orientation in SIGMASOFT

Figure 4-21: 3D-fiber orientation in a finite element network for further pro-cessing in ABAQUS

Figure 4.22 shows a direct comparison of the filling simulation

with actual filling studies that were later performed. The simu-lation in the top picture offers an excellent view of the connect-

ing seam in the component.

Figure 4-22: Direct comparison of the simulation and the result of an experi-mental filling test shows very good agreement even in details (arrow).

5. Thermoset Components With Inserts

The stress calculations are of particular interest when inserts

are to be embedded. Holec Holland NV in Hengelo manufac-

tures thermoset components for high-tension engineering that

are exposed to as much as 24 kV in later use. Metallic inserts

are embedded in the thermoset of these components. The dif-

ferent coefficients of thermal expansion of the thermoset and

the metal can lead to stress cracking that affects the operation

of the component. At Holec, the SIGMASOFT simulation pro-

gram is used to predict filling, cross-linking and stress buildup

during cooling. An optimized design for the component and the

mould can thus be developed prior to actual component and

mould construction. Figure 5-1 shows the thermoset compo-

nent, and Figure 5-2 the insert. Figure 5-3 shows the temper-

ature distribution at the conclusion of mould filling. The

effect of mould filling on the temperature distribution is alsoapparent in this component.


9/9



8

Figure 5-1: High-tension component from Holec

Figure 5-2: Metal insert

Figure 5-3: The temperature distribution at the conclusion of mould fillingshows the effect of this operation

6. Summary

Mould filling, cross-linking, cooling and internal stress buildup

during fabrication of thermoset components by casting or in-

jection moulding can be three-dimensionally calculated using

the new simulation tool. This permits optimization of the com-ponent design and mould prior to production. SIGMASOFT

operates on the basis of three-dimensional volume elements.

The advantages of this new 3D simulation method using volu-

metric elements for simulation of thermoset components may

be summarized as follows:

Model preparation costs are eliminated since CAD data can

be utilized and automatically networked.

Flow phenomena such as backwater areas in thick-walled

zones of mouldings or at points with different wall thickness

are described in physically exact terms.

Kinetic effects such as independent stream formation maybe predicted.

Calculation of the cross-linking reaction takes account of the

reaction enthalpy.

Local cross-linking has an effect on mould filling.

It is possible to take account of the back-pressure of air in

the mould.

The fiber orientation can be calculated in three dimensions,

and used for stress analysis.

The thermally induced buildup of internal stress during cool-

ing can be calculated. Thermal effects on the flow and cross-linking processes are

accounted for by the three-dimensionally coupled calcula-

tion of the moulding, inserts and mould.

The consideration of heating systems is three-dimensional;

the local effect on the mould wall temperature is calculated.

The cycle time can be predicted.

References

1. H. Bogensberger, Kunststoffe 85, 44 ff (1995).

2. P.F. Filz, Kunststoffe 88, 954 ff (1998).

3. B. Ohlsson, First International Thermoset Symposium in Iserlohn, Mrkische

Fachhochschule Iserlohn

4. P. Thienel, International Mould Construction Symposium 1999, Dr. Reinhold

Hagen Stiftung, Bonn

5. W. Michaeli, H. Findeisen, and T. Gossel, Kunststoffe 87, 462 ff (1997).

6. O. Altmann and H.J. Wirth, Kunststoffe 87, 1670 ff (1997).

7. A.J. van der Lelij, Kunststoffe 87, 51 ff (1997).

op tim ization injection moulding process

Documents