Observation of the polymer melt flow in injection molding process using co-injection molding technique

Pergamon International Communications in Heat and Mass Transfer, Vol. 21, No. 4, pp. 499-508, 1994

Copyright O 1994 Elsevier Science Lid Printed in the USA. All rights reserved

0735-1933/94 $6.00 + .00

0735-1933(94)00005-0

OBSERVATION OF THE POLYMER MELT FLOW IN INJECTION MOLDING PROCESS USING CO-INJECTION MOLDING TECHNIQUE

S. C. Chen, K. F. Hsu and J. S. Huang Mechanical Engineering Department

Chung Yuan University, Chung-Li 32023, Taiwan, ROC

(Communicated by J.P. Hartnett and W.J. Minkowycz)

ABSTRACT Studies of the polymer melt flow in injection molding process have been carried out by co-injection molding technique using alternating sequence of transparent and colored PMMA resin. Simulations are also developed to predict the melt front advancements for both skin and core melts. Fountain flow effect is evident in all case studies. During the packing process, the polymer melt flows significantly with the increased packing pressure due to the compressible nature of the melt and the flow concentrates around cavity location near gate area. That the polymer melt flows across the weld line around the gap center in the packing stage was also observed. Although numerical simulations show fair consistence with experimental results in both skin and core material distribution, edge effect remains to be taken into account to improve the simulation accuracy.

Introduction

Visualization techniques of polymer melt flow patterns inside an injection mold have

been developed in order to study molding dynamics [ 1-5]. Among these methods, dynamic

visualization with laser light or high speed video camera in a glass-inserted mold has been

implemented recently [3-5]. Although the dynamic visualization technique using transparent

mold can investigate a time-sequential molding phenomena in details, it is also subjected to

the limitation of the maximum allowable injection pressure, the observation area as well as

the complexity and the expense in mold construction. In view of this, we proposed an easier

way of studying polymer melt flow in injection molding process using co-injection molding

technique.

Co-injection molding process, also named as sandwich injection molding, is one of the

innovative multi-component injection molding processes recently developed. The co-

dOO

500 S.C. Chen, K.F. Hsu and J.S. Huang Vol. 21, No. 4

injection molding process involves simultaneous or sequential injection of a skin polymer

melt and a dissimilar but compatible core polymer melt into a mold cavity such that the core

material is embedded within the solidified layers of the skin material [6-10]. A schematic

diagram of the co-injection molding process is illustrated in FIG. 1o This process is

originally designed to provide a flexibility in part design and part manufacture by utilizing

the optimal properties of each material. For example, low-cost or recycled plastics can be

used as core material sandwiched within thin, expensive, rigid skin plastics or skin material

can be used with formed core material to reduce part weight and part residual stress.

Generally speaking, if co-injection molding process is properly designed, part weight, part

cost, injection pressure, residual stress and warpage can be reduced and the modification of

part properties may also be achieved. On the other hand, when a transparent skin material

and a colored core material of the same brand are used, flow information regarding to the

conventional injection molding process may be obtained directly from viewing the co-

injection molded parts.

In this paper, we present some of our recent experimental works on the study of the

polymer melt flow during injection molding process using co-injection molding technique.

Various factors such as the fountain flow effect, the non-uniform thickness and the edge

effect on the skin and core polymer melt front progression and their final distribution were

studied in details. Polymer melt flow during the packing process was also investigated using

skin-core-skin sequential injection. A simple algorithm based on the concept of control

volume/finite element method within each gapwise layer is implemented to simulate the core

and skin melt front advancements during the mold filling stage of the process. Some

interesting results are found and discussed.

Exoeriments and Simulations

One three-cavity mold as shown in FIG. 2 is designed to conduct the process studies.

In the first mold, cavity I consists three regions with a thickness varied form 4 mm to 2 mm

and back to 2 mm. It is designed to observe the fountain flow effect under abrupt

contraction and expansion geometry. Mold cavity II consists a block insert to form the weld

line and to introduce the asymmetric melt flow conditions. Its thickness also varied

according to the schematic described. Mold cavity III is a line-gated plate with uniform

thickness of 2 mm.

A 75 ton Battenfeld 750/750 co-injection molding machine was used for the present

experiments. Transparent PMMA resin was utilized as the skin materials whereas green-

colored PMMA of the same brand was used for the core layer such that the final distribution

of both materials can he seen easily by viewing the molded parts. The melt temperatures for

Vol. 21, No. 4 POLYMER MELT FLOW IN INJECTION MOLDING 501

FIG. 1 Schematic diagram of co-injection molding process. (courtesy of Battenfeld Operation

Manual)

Unit: m m

T- 4 7"2

T=4

I o T-2 T-3 T-4

80

160

' r - 2 o

o

FIG. 2 Geometry of the three-cavity mold.


both skin and core resin are 230 °C. Mold temperature is 30°C. The switch over in injection

sequence from the skin melt to the core melt as well as the switch over from core melt back

to skin melt injection is varied according to the geometry of each mold cavity. The packing

and holding pressure were also varied within 0 Mpa to 60 Mpa.

Simulations are carried out using control volume/finite element concept combined with

particle tracing algorithm on a gapwise layer basis. The relevant governing equations are

based on the Hele-Shaw type of flow model and are given previously [11-13]. However,

instead of using residual time approach [ 11-12,14] to trace the particle, each control volume

is divided into 20 layers in the gapwise direction and mass conversation is employed to each

layer in the present study. The melt front advancements in each layer are recorded in each

analysis interval. The detailed derivation and numerical algorithm can be found elsewhere

[15]. A modified-Cross viscosity model [12] is used. For PMMA, n = 0.2527, "r* =

1.193E+05 Pa, B = 1.157E-10 Pa.s, Tb = 15230°K. Density, specific heat and thermal

conductivity of PMMA are 1040 Kg/m 3, 2300J/Kg-°K and 0.2 W/m.°K, respectively. The

melt front advancements for both skin and core melt as well as the corresponding

distributions of both materials were obtained from the simulations.

Results and Discuss ions

The co-inject ion molded parts of plate II are shown in FIG. 3a, 3b and 3c,

respectively. These parts are co-injected using a skin-core-skin sequence. The sequence was

designed such that the last injected transparent skin material just enters cavity. During all

experiments, it is evident that the core melt front is catching up the skin melt front. This

indicates that the fountain flow effect does exit and dominate the melt front movement. To

study polymer melt flow in packing process, the applied packing pressures were set to be 0

Mpa, 30 Mpa and 60 Mpa, correspondingly. It was also clearly seen that the area of

transparent skin melt near gate region increases as the packing pressure increases. This is

evident that during packing process additional polymer melt was pushed into cavity due to

the compressible nature of the melt. On the opposite side of the gate where weld line was

formed, the first injected skin melt and the core melt were also pushed to flow as packing

pressure increases. However, the flows are less significant when compared with those near

gate location as seen from the movement of the skin-core interface at the core melt front. It is

also interesting to find out that the melt flows across the weld line around the gap center

during packing stage. The core melt front of the left hand side located just nearly behind the

weld line at the end of filling process when no packing pressure was applied. If 30 Mpa

packing pressure was applied, the green core melt front moved to the location just beneath

the weld line through gap center where melt temperature is still high. If 60 Mpa packing

pressure was employed, the core melt flows further ahead to cross the weld line slightly.

VoL 21, No. 4 POLYMER MELT FLOW IN INJECTION MOLDING 503

M ( i , r L g " , , r , I - -

S k i n t o C o t - - 5 w ~ , ; o ( ~ o e t

P a e k i l l g t ' r ~ ~ - z- . b a r

FIG. 3a Co-injected molded part of cavity II without applying packing pressure.

FIG. 3b Co-injected molded part of cavity II with an applied packing pressure of 30 Mpa.


FIG. 3c Co-injected molded part of cavity II with an applied packing pressure of 60 Mpa.

FIG. 4a The enlarged view on the weld line area of co-injected molded part of cavity II without

applying packing pressure.


FIG. 4b The enlarged view on the weld line area of co-injected molded part of cavity II with 60

Mpa packing pressure.

kl coro ~ ~ s k i o J FIG. 5

Core and skin material distribution in cross section along centerline for part I.

506 S.C. Chen, ICF. Hsu and J.S. Huang Vol. 21, No. 4

t t ~ n l r u t u r • : r U t u r • :

| C h T

S u r e :

FIG. 6a Co-injected molded part of cavity III without applying packing pressure.

FIG. 6b The experimental and simulated results for the location of the core melt front • (1)

experiment,(2) present simulation, (3) simulation result from C-Flow.


The enlarged pictures are shown in FIG, 4a and 4b, respectively. For non-uniform plate I,

fountain flow effect shows asymmetry under contraction and expansion situations as seen in

FIG. S. More detailed experiments are required for further investigation.

FIG. 6a and 6b, respectively, illustrate the experimental and simulated results for the

melt front advancements of both skin melt and core melt in a line-gated plate cavity. The

simulated location of the core melt front at the end of the mold filling stage is slightly ahead

of the observed one. The consistency between the predicted value and the experimental data

is reasonably good despite that the edge effect was not taken into account at this moment.

The simulation is also carried out using commercial software (named as C-Flow) developed

by Advanced CAE Technology [12]. The overprediction by C-flow in core melt front

location is even larger than that calculated by the present scheme developed from the control

volume method employed on each gapwise layer. This indicates that the present numerical

scheme is an acceptable method in the simulation of co-injection molding although more

detailed studies are required for further verification from both numerical and experimental

view points.

Summary

Studies of the polymer melt flow have been investigated via co-injection molding

technique using alternate sequence of transparent skin PMMA and colored core PMMA.

Computer simulations are also developed to predict melt front movement for both melts.

The following observations have been found:

(1) Fountain flow effect is clearly seen by the fact of the core melt front movement in

catching up the skin melt front. It shows asymmetric situation when the melt flows through

contraction and expansion geometry.

(2) During the packing process, the polymer melt flows significantly and the flow

concentrates around cavity location near gate area. The melt flow is much less distinguished

far away form gate.

(3) During the packing stage, polymer melt flow may flow across the weld line when

polymer melt at gap center is still hot.

(4) The edge effect is more significant when the part is thin.

(5) Although numerical simulations show fair consistence with the experimental results in

both core and skin material distribution, the accuracy remained to be verified and improved

by taking edge effect into account.

Acknowledgment~

This work was supported by National Science Council under NSC grant 83-0405-

E033-027 and a special group-research funding under Computer Integrated Manufacture of

Injection Molding Process Program of Chung Yuan University.


References

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Received March 9, 1994

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Observation of the polymer melt flow in injection molding process using co-injection molding technique