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TRANSCRIPT
Development of Advanced Structural
Foam Injection Molding
Kye Kim
A thesis submitted in partial
fulfillment of the requirements for
the degree of
BACHELOR OF APPLIED SCIENCE
Supervisor: Park, C.B.
Department of Mechanical and Industrial Engineering
University of Toronto
March, 2007
Abstract
The structural foam injection molding has a number of advantages over
the conventional injection molding including absence of sink marks on
part surface, weight reduction, low back pressure, faster cycle time, and
high stiffness-to-weight ratio. However, uniform cell structure and high
void fraction is unachievable due to nonuniform cell density distribution.
This non-uniformity is due to changes in pressure drop and pressure drop
rate, which is inevitable in injection molding process during mold filling
operation. In this study, processing parameters such as injection speed,
melt temperature, blowing agent content, and gate size are studied for
their influence towards void fraction and uniform cell structure using
advanced structural foam injection molding machine, which allows
gas/polymer one-phase solution in preparation stage.
i
Acknowledgements
I wish to thank all those who helped me. Without them, I could not have completed this
project.
Professor Park, C.B.
Ph. D. John W. S. Lee
ii
Table of Content
1. Introduction …………………………………………………………….. 1
2. Background ……………………………………………………………….……. 2
3. Fundamentals …………………………………………………………….. 3
3.1 Structure of the Advanced Structural Foam Molding Machine ……………… 3
3.2 Use of a Very Small Amount of Blowing Agent ……………………………... 5
3.3 Use of an Effective Nucleating Agent ……………………………………... 6
4. Experimental ……………………………………………………………... 7
4.1 Materials …………………………………………………….……………….. 7
4.2 Preparation of HDPE/Talc Compounds ……………………………... 8
4.3 Experimental Setup ……………………………………………………... 8
4.4 Mold Geometry …………………………………………………….. 9
5. Experiment ……………………………………………………………….……. 9
5.1 Processing Parameters Affecting High Void Fraction …………………….. 9
5.2 Injection Speed and Uniform Void Fraction Distribution …………….. 11
6. Results and Discussions …………………………………………………….. 13
6.1 Processing Parameters Affecting High Void Fraction …………………….. 13
6.1.1 Effect of Processing Parameters on the Degree of Mold Filling …………… 13
6.1.2 Effect of Gas Content on the Cell Density and Cell Size Uniformity ……... 15
6.1.3 Effect of Shot Size (Injection Stroke) on Cell Density and Cell Size
Uniformity …………………………………………………………….. 18
iii
6.1.4. Effect of Gate Size on Cell Nucleation in a Mold Cavity …………….. 20
6.2 Injection Speed and Uniform Void Fraction Distribution ……………... 21
6.2.1 Effect of Injection Speed on Flow Length of Foam Melt and Void Fraction
Uniformity …………………………………………………………….. 21
6.2.2 Effect of Variable Injection Speed Profile on Void Fraction …………….. 24
7. Conclusion …………………………………………………………….. 26
8. Reference …………………………………………………………….. 28
iv
Table of Figures
Figure 1. ………………………………………………………………………… 4
Foaming processes in conventional and advanced structural foam molding
Figure 2. ………………………………………………………………………… 6
Solubility pressure of N2 for 0.05, 0.10, and 0.15 wt%
Figure 3. ………………………………………………………………………… 7
Non-uniform cell size distribution due to varying pressure drop rate
Figure 4. ………………………………………………………………………… 8
Schematic of advanced structural foam molding machine [3]
Figure 5. ………………………………………………………………………… 9
Gate geometries and dimensions
Figure 6. ………………………………………………………………………… 12
Constant injection speed profile (A1-A9)
Figure 7. ………………………………………………………………………… 13
Variable injection speed profile (B1-B5)
Figure 8. ………………………………………………………………………… 14
Effect of processing parameters on the degree of mold filling
Figure 9. ………………………………………………………………………… 16
Effect of gas content and injection speed on the foam morphology
Figure 10. ………………………………………………………………………… 17
Cavity pressure profile near the gate area and the corresponding cell density
v
Figure 11. ………………………………………………………………………… 19
Effect of shot size on foam morphology
Figure 12. ………………………………………………………………………… 21
SEM pictures of the three gates
Figure 13. ………………………………………………………………………… 24
Effect of injection on (a) flow length of foam melt and (b) void fraction uniformity.
Figure 14. ………………………………………………………………………… 25
Effect of variable injection speed profile on void fraction uniformity.
Figure 15. ………………………………………………………………………… 26
Effect of speed change stroke on structural foams
vi
Table of Tables
Table 1. Summary of Experiments ……………………………………………. 10
Table 2. Summary of Experiments …………………………………………….. 12
i
Acknowledgements
I wish to thank all those who helped me. Without them, I could not have completed this
project.
Professor Park, C.B.
Ph. D. John W. S. Lee
1
1. Introduction
The conventional injection molding is the most widely used technology in polymer
production nowadays. Advanced from the conventional injection molding, foam injection
introduces gas into polymer, which derived several advantages and strengthens positive
characteristics of plastic. However, the conventional structural foam molding limits itself in
producing uniform cell structure with uneven void fractions.
Achieving uniformity is crucial since finer cell size and more uniform distribution
exhibit better mechanical and thermal properties. The nonuniform cell density distribution
along the melt flow direction comes from the change in pressure drop and pressure drop
rate, which is inevitable in injection molding process during mold filling operation. The
pressure drop and pressure drop rate at the beginning of the mold filling operation is the
largest and decreases over time. It leads to high cell density near the end of the part and
lowest near the gate. Accordingly, the void fraction is also highest near the end and lowest
near the gate [1] as it is a function of cell density in high-density foams [2]. In this study,
the experiments are carried out using an advanced structural foam molding technology was
developed in University of Toronto [3,4]. It allows achieving a one-phase polymer solution
in preparation stage, which is a starting to point to achieve a uniform cell structure in final
2
product.
The study is focused on understanding the key parameters of the processing
technologies, injection speed, blowing agent content, melt temperature and gate geometry,
and their effect on the quality of mold product.
2. Background
Low-pressure preplasticating-type structural foam molding machines are commonly
used because a small molding system, with low pressure in the cavity, is required for
producing large products [5]. Since the generated cells compensate for the shrinkage of
injection-molded parts during cooling, structural foams typically have outstanding
geometric accuracy. Advantages of foam injection molding include the absence of sink
marks on the part surface, low weight, low back pressure, faster cycle time, and a high
stiffness-to-weight ratio.
The advantages of foam injection molding initiated a number of researchers to
study and improve the structural foam molding technology. The representative cases will be
microcellular injection molding technology (MuCell Technology) by Trexel Inc [6,7], an
alternative microcellular foam process using preplasticating-type injection molding
machine by Shimbo [8], and another foam injection molding process in IKV, Germany [9].
3
In 2006, Park et al. presented an advanced structural foam molding technology based on a
preplasticating-type injection molding machine [3,4,10]. This technology was used for this
study and its mechanism is explained in the following section.
3. Fundamentals
3.1 Structure of the Advanced Structural Foam Molding Machine
In conventional structural foam molding, the main reason for large voids is from
uneven blowing agent concentration in the polymer matrix; a complete dissolve of blowing
agent is not possible for whole region of polymer melt. The existing conditions in
conventional structural foam molding such as low barrel pressure, fluctuation of barrel
pressure, inconsistent material flow rate, and screw stoppage during the injection stage are
the cause of uneven blowing agent concentration.
In order to overcome the problem, a new technology has been developed [3,4] at
University of Toronto. In a modified machine, an additional material accumulator (i.e., a
hydraulic piston) combined with a gear pump was installed between the extrusion barrel
and the shut-off valve (before the main accumulator) to completely decouple the gas
dissolution operation from the injection and molding operations. It is from the
understanding the behavior of conventional structural foam molding; the stop-and-flow
4
molding behavior causes inconsistent gas dosing.
The newly attached material accumulator allows nonstop screw rotation by
accommodating polymer/gas solution during the injection stage. Also, the gear pump
prevents backflow into the barrel; therefore, the pressure in the extrusion barrel can be
relatively well maintained and consistent gas dosing can be attained to achieve a uniform
polymer/gas mixture regardless of pressure fluctuations caused by the injection and
molding operations. Maintaining constant pressure in the barrel is critical in an accurate
control of blowing agent content because the injection of the blowing agent is driven by the
difference between the gas pump and barrel pressure.
2 Systems 2 phase gas- non-uniform
polymer mixture bubble structure
poor diffusion
and dissolution pressure drop
Conventional
Structural
Foam Molding
Advanced
Structural
Foam Molding
complete diffusion
and dissolution pressure drop
2 Systems 2 phase gas- non-uniform
polymer mixture bubble structure
poor diffusion
and dissolution pressure drop
Conventional
Structural
Foam Molding
Advanced
Structural
Foam Molding
complete diffusion
and dissolution pressure drop
complete diffusion
and dissolution pressure drop
Figure 1. Foaming processes in conventional and advanced structural foam molding
5
3.2 Use of a Very Small Amount of Blowing Agent
With existing foam molding technologies, it is difficult to completely eliminate the
swirl pattern on the surface of molded foam products. The swirl pattern is formed because
the bubble nucleated at the melt front is pushed and smeared on the mold cavity wall
because of the fountain effect. In order to improve the surface quality of foamed parts,
several processing technologies have been proposed. Gas counter pressure molding [14-17]
and co-injection molding [18-20] are used most commonly.
However, these technologies require additional cost for the installation of extra
devices. In this research, efforts will be made to improve surface quality by using a very
small amount of blowing agent. The perfect fit for this purpose is N2 because it can produce
high cell density with less gas, compared to other blowing agents such as carbon dioxide
(CO2), butane, etc. [21, 22]. Figure 2 shows the solubility pressure of N2 for 0.05, 0.10, and
0.15 wt%, which will be the amounts used for advanced structural foam molding. Since the
amount of N2 is very low, it is expected that the cell nucleation rate at the melt front will be
lower. In addition, because the pressure required to keep N2 inside the polymer matrix is
very low, the pressure increase in the mold cavity resulting from sudden injection is
expected to act like a gas counter pressure, thereby preventing cell nucleation at the melt
6
120 140 160 180 200 220 240 0
50
100
150
200
250
300 S
olu
bili
ty P
ress
ure
(psi
)
Temperature ( o C)
0.15 wt% N 2
0.10 wt% N 2
0.05 wt% N 2
front.
Figure 2. Solubility pressure of N2 for 0.05, 0.10, and 0.15 wt%
3.3 Use of an Effective Nucleating Agent
Unlike the case of extrusion where the pressure drop rate at the die exit is constant,
in injection molding, the pressure drop rate at the gate of the mold changes with time. The
pressure drop rate is highest in the beginning, and it tends to decrease with time. Therefore,
it would be very difficult to obtain uniform cell structures because cell nucleation is very
sensitive to the pressure drop rate [23, 24]. A higher number of cells are nucleated in the
beginning because of a higher pressure drop rate, but the driving force for cell nucleation
will decrease as the pressure drop rate decreases with time. Figure 3 demonstrates this
7
outcome.
One way of resolving this problem would be to use an effective nucleating agent
such as talc. Previously, Park et al. revealed that the sensitivity of cell density to pressure
drop rate decreases with increasing amount of talc [45]. The use of a nucleating agent is,
therefore, one of the strategies which will be employed in advanced structural foam
molding to produce fine-celled foams.
At t1
At t2
At t3
dP/dt1
dP/dt2
dP/dt3
dP/dt1 dP/dt2 dP/dt3> >
Non-Uniform Cell Size Distribution
Mold Cavity
At t1
At t2
At t3
dP/dt1
dP/dt2
dP/dt3
dP/dt1 dP/dt2 dP/dt3> >dP/dt1 dP/dt2 dP/dt3> >
Non-Uniform Cell Size Distribution
Mold Cavity
Figure 3. Non-uniform cell size distribution due to varying pressure drop rate
4. Experimental
4.1 Materials
The polymer material used in this study was HDPE of grade SCLAIR 2710, with
an average melt flow index of 17 dg/min and a density of 0.951 g/cm3. The talc was of
grade Cimpact CB710, with a density of 2.8 g/cm3 and an average particle size of 1.7 µm.
8
The blowing agent used in this study was N2 from BOC Gas.
4.2 Preparation of HDPE/Talc Compounds
A 20 wt% talc masterbatch was prepared using an intermeshing and co-rotating
twin-screw extruder with a screw diameter of 30 mm (Werner & Pfleiderer ZSK-30, L/D
38:1). The HDPE was then dry-blended with the talc masterbatch in a one-to-one ratio, to
produce HDPE/talc compounds with talc content of 10 wt%.
4.3 Experimental Setup
An 80-ton injection molding machine (TR80EH) from Sodick Plustech Inc. was
modified into the advanced structural foam molding machine [3] and [10]. Figure 4 shows a
schematic of the advanced structural foam molding machine.
5
21
3 4
13
12147 9
10
15 16
P1
P2
11
Extrusion BarrelScrew
Gas Cylinder Gas PumpGas Injection Port
Gear Pump
Shut-Off Valve
(or Non-Returnable
Check Valve)
Nozzle
Shut-Off
Valve
Accumulator
Molded
Part
Secondary
Accumulator
Hydraulic
Systems
Mold
Figure 4. Schematic of advanced structural foam molding machine [3]
9
4.4 Mold Geometry
The mold contained a rectangular cavity, and a fan gate was located at one end. The
cavity dimensions were 152.4 mm × 101.6 mm × 3.2 mm. The cavity and the gate
(including dimensions) are shown in Fig. 5.
1 mm
Gate 1 Gate 2 Gate 3
0.5 mm 1.5 mm 3.2 mm
dP/dt ≈4 MPa/s dP/dt ≈0.2 MPa/s dP/dt ≈0.04 MPa/s
3 Different
Gate Geometry
6” x 4” x 0.126” (3.2mm)
Figure 5. Gate geometries and dimensions
5. Experiment
5.1 Processing Parameters Affecting High Void Fraction
The following table summarizes the experiments carried out in this study. While
the talc content and mold temperature were fixed at 10 wt% and 30°C, respectively, factors
such as gate size, gas content, melt temperature, shot size and injection speed are varied.
Three different sets of experiments are conducted (A, B and C) while keeping some
parameters constant and varying others to investigate their effect on mold filling and cell
10
size uniformity.
Table 1. Summary of experiments
Run # Gate Gas Content [wt%] Melt Temperature [ C] Shot Size [cc] Injection Speed [mm/s]
A1 1 0.1 170 60 10~450
A2 1 0.1 200 60 10~450
A3 1 0.3 170 50 10~450
A4 1 0.3 200 50 10~450
A5 1 0.5 170 40 10~450
A6 1 0.5 200 40 10~450
B1 1 0.5 200 40 50, 200, 350
B2 1 0.5 200 45 50, 200, 350
B3 1 0.5 200 50 50, 200, 350
C1 1 0.5 170 40 400
C2 2 0.5 170 40 400
C3 3 0.5 170 40 400
Characterization of Foams
The void fractions were determined by the shot size (injection stroke) using Eq 1:
solid
foam
solidplungermelt
foamplungermelt
solid
foam
strokeinjection
strokeinjection
strokeinjectionr
strokeinjectionr
m
mFractionVoid
)(
)(1
)()(
)()(1
1
2
2
(1)
The fractured cross-section of a sample with platinum coating is examined under
scanning electron microscopy (SEM) for determining the cell density. With the aid of Eq 2
the cell density calculation can be completed [11]:
11
)1
1()( 2/3
2
FractionVoidA
nMDensityCell (2)
5.2 Injection Speed and Uniform Void Fraction Distribution
Another set of experiments is conducted to look into changing the injection speed
profile as a means of resolving the nonuniformity in void fraction along the melt flow
direction.
Throughout the experiments, N2 content, talc content, melt temperature, and mold
temperature were fixed at 0.2 wt%, 10 wt%, 200oC, and 30
oC, respectively. The summary
of experiments is described in Table 2. The first nine runs (A1-A9) are designed to observe
the effect of different injection speeds on flow length of the foam melts and the void
fraction distribution by filling the mold partially. The other five runs (Run B1-B5) used
variable injection speeds to completely fill the mold with foam expansion, using speed-
change strokes to optimize the void fraction uniformity. Figure 6 and 7 illustrates the
various speed profile conducted for each run in a graphical form.
12
Table 2. Summary of Experiments
Run
#
Injection
Stroke
[mm]
Injection
Speed
[mm/s]
speed-
change
stroke [mm]
A1 45 50 N/A
A2 45 100 N/A
A3 45 150 N/A
A4 45 200 N/A
A5 45 250 N/A
A6 45 300 N/A
A7 45 350 N/A
A8 45 400 N/A
A9 45 450 N/A
B1 60 200 N/A
B2 60 200 → 3 5
B3 60 200 → 3 10
B4 60 200 → 3 15
B5 60 200 → 3 20
time time
Injection
Speed
Plunger Position
(Injection Stroke)
0%
200 mm/s
50 mm/s
100 mm/s
150 mm/s
250 mm/s
300 mm/s
350 mm/s
400 mm/s
450 mm/s
time time
Injection
Speed
Plunger Position
(Injection Stroke)
0%
200 mm/s
50 mm/s
100 mm/s
150 mm/s
250 mm/s
300 mm/s
350 mm/s
400 mm/s
450 mm/s
Figure 6. Constant injection speed profile (A1-A9)
13
Injection
Speed
time
Plunger Position
(Injection Stroke)
time100%
33%25%17%8%0%
200 mm/s
3 mm/s
Injection
Speed
time
Plunger Position
(Injection Stroke)
time100%
33%25%17%8%0%
200 mm/s
3 mm/s
Figure 7. Variable injection speed profile (B1-B5)
Characterization of Foams
The void fraction was used to characterize the foam samples. The foam density was
determined by the water displacement method (ASTM D792-00). Samples were taken from
regions near the gate, at the center, and near the end of the injection-molded part. The
expansion ratio Φ is equal to the ratio of the bulk density of HDPE/talc compounds, ρo, to
the measured density of the foam sample, ρf. The void fraction was calculated as follows:
%1001
1FractionVoid (3)
6. Results and Discussions
6.1 Processing Parameters Affecting High Void Fraction
6.1.1 Effect of Processing Parameters on the Degree of Mold Filling
Figure 8 illustrates the results from experiment runs A1-A6; the effect of blowing
14
agent N2 and temperature on the degree of mold-filling by the foam melt. In this experiment,
for different amount of N2, different injection strokes were employed as follows: for
experiment runs with 0.1 wt% N2, 0.3 wt% N2, and 0.5 wt% N2 have 40 mm, 50 mm, and
60 mm injection strokes respectively. These injection strokes account for void fractions of
17%, 31%, and 45%, respectively.
<= Melt T = 170oC
<= Melt T = 200oC
<= Melt T = 170oC
<= Melt T = 200oC
<= Melt T = 170oC
<= Melt T = 200oC
(a) 0.1 wt% N2
(b) 0.3 wt% N2
(c) 0.5 wt% N2
10 50 100 150 200 250 300 350 400 450 mm/s
Figure 8. Effect of processing parameters on the degree of mold filling
The figure clearly indicates a critical role of injection speed; it is found that the
15
degree of mold filling increased as injection speed was increased. As a result, the foam
expansion has less space/distance to cover to fill the cavity and time for nucleation is
extended due to small temperature drop from reduced filling time period. An extended
investigation regarding injection speed will be conducted in the following section.
Unlike injection speed or blowing agent content, melt temperature does not
perform a significant role on the degree of mold filling. The difference between two
temperature setups, 170°C and 200°C, are not noticeable. However, it is premature to
confirm there is no relationship between melt temperature and mold filling since the
temperature range covered in the experiment was rather narrow.
6.1.2 Effect of Gas Content on the Cell Density and Cell Size Uniformity
In Figure 8, which is cross-sections of samples from run A1-A6, it is possible to
observe the influence of blowing agent content on the cell density. With 0.1 wt% N2 the
samples low cell density except for the end of the flow where the foaming occurs most due
to high-pressure drop and pressure drop rate. With the increasing amount of blowing agent,
cell density increases accordingly as seen in Figure 9.
16
(a) 0.1 wt% N2
(b) 0.3 wt% N2
(c) 0.5 wt% N2
10 mm/s
100 mm/s
200 mm/s
300 mm/s
400 mm/s
10 mm/s
100 mm/s
200 mm/s
300 mm/s
400 mm/s
10 mm/s
100 mm/s
200 mm/s
300 mm/s
400 mm/s
Gate End
Gate End
Gate End
Figure 9. Effect of gas content and injection speed on the foam morphology
Cell nucleation in a mold cavity is governed by two factors: (1) the pressure drop
rate at the gate when the cavity pressure is lower than the solubility pressure [12] and (2)
the cavity pressure drop rate when the cavity pressure is higher than the solubility pressure.
Gate pressure drop rate is determined by the gate geometry, while the cavity pressure drop
rate is determined mainly by the material’s rheology.
Our previous observations indicate that the gate pressure drop rate is usually much
higher than the cavity pressure drop rate. Therefore, in order to have a high cell density, it is
desirable to have a low cavity pressure so that the gate pressure drop rate governs the cell
nucleation.
Figure 10 shows the cavity pressure profiles recorded near the gate and the
17
corresponding solubility pressures. As shown in Figure 10(a), when 0.1 wt% N2 was used,
the cavity pressure was higher than the solubility pressure. Thus, cell nucleation was
mainly governed by the cavity pressure drop rate, resulting in a very low cell density. When
N2 content was increased to 0.3 wt% and 0.5 wt% [Figures 10(b) and (c), respectively], the
solubility pressure was higher than the cavity pressure in most cases; thus, cell nucleation
was governed in most cases by the gate pressure drop rate. Much higher cell density was,
therefore, observed for the samples with 0.3 wt% and 0.5 wt% N2, compared to those with
0.1 wt% N2 (Figure 10(d)).
0 2 4 6 8 10 12 140
100
200
300
400
500
600
700
800
900
1000
Pre
ssure
[psi]
Time [sec]
Injection Speed = 10 mm/s
Injection Speed = 100 mm/s
Injection Speed = 200 mm/s
Injection Speed = 300 mm/s
Injection Speed = 400 mm/s
Psolubility
0 2 4 6 8 10 12 140
100
200
300
400
500
600
700
800
900
1000
Pre
ssure
[psi]
Time [sec]
Injection Speed = 10 mm/s
Injection Speed = 100 mm/s
Injection Speed = 200 mm/s
Injection Speed = 300 mm/s
Injection Speed = 400 mm/s
Psolubility
Psolubility
0 2 4 6 8 10 12 140
100
200
300
400
500
600
700
800
900
1000
Pre
ssure
[psi]
Time [sec]
Injection Speed = 10 mm/s
Injection Speed = 100 mm/s
Injection Speed = 200 mm/s
Injection Speed = 300 mm/s
Injection Speed = 400 mm/s
(a) 0.1 wt% N2 (b) 0.3 wt% N2
(c) 0.5 wt% N2 (d) Cell Density
0 100 200 300 40010
0
101
102
103
104
105
106
107
108
109
Ce
ll D
en
sity
Injection Speed
0.1 wt% N2
0.3 wt% N2
0.5 wt% N2
Figure 10. Cavity pressure profile near the gate area and the corresponding cell density
18
However, some large bubbles were observed in the foam when 0.5 wt% N2 was
used. One possible reason for this observation is that it is difficult to dissolve a large
amount of N2 [13], which could result in large bubbles formed by undissolved gas pockets.
6.1.3 Effect of Shot Size (Injection Stroke) on Cell Density and Cell Size Uniformity
In Figure 9, it illustrated that the highest blowing agent content, 0.5 wt% N2, among
the three different amounts tested produces the highest cell density. However, it also
showed undesirable large bubbles in the cross-sections of the samples. In order to
investigate the cause of the large bubbles, the experiments (B1-B3) are conducted under
fixed N2 content with various injection stroke sizes (40 mm, 45 mm, and 50 mm).
As the Figure 11 illustrates, the large bubble disappears when the injection stroke
size increased from 40 mm to 45 mm. The injection speed seems it influences little for the
existence of the large voids for the case of injection stroke of 40 mm, which corresponds to
45% of void fraction. However, for the injection stroke of 45 mm, which corresponds to a
void fraction of 38%, low injection speed of 50 mm/s results in large voids. This
experiment results indicates that to achieve a uniform cell structure without any large voids,
an appropriate size of injection stroke should be selected.
However, in depth research should be conducted in choosing the size of the
19
injection stroke as larger sizes such as 50 mm (corresponding to a void fraction of 31%)
resulted only partial foam near the end of part even though the large voids disappeared. It is
assumed that due to the large shot size the foam melt experienced a pressure that was high
enough to prevent nucleation of the cells until it solidified; therefore, creating unfoamed
section at the end of the part.
Gate End
Gate End
Gate End
50 mm/s
200 mm/s
350 mm/s
50 mm/s
200 mm/s
350 mm/s
50 mm/s
200 mm/s
350 mm/s
(a) Injection Stroke = 40 mm
(b) Injection Stroke = 45 mm
(c) Injection Stroke = 50 mm
Figure 11. Effect of shot size on foam morphology
20
6.1.4. Effect of Gate Size on Cell Nucleation in a Mold Cavity
In order to achieve a uniform cell structure, the stage when cell nucleation starts
plays a significant role. The cells nucleated before they reach the gate will experience a
significant amount of shear stress when passing through the gate resulting in severe cell
coalescence. Therefore, cell nucleation should start only after the melt passes thought the
gate; otherwise, uniform cell structure will not be obtained.
The three different thicknesses of gate sizes are employed in the experiments (C1-
C3) to observe the changes of cell nucleation by locations. While Figure 4 shows the
geometry of the three different gate design, Figure 12 illustrates the cross-sections of the
samples from the experiments under fixed values of 0.5 wt% of gas content, 170 C of melt
temperature, shot size of 40 mm, and injection speed of 400 mm/s.
From Figure 12, it is obvious that Gate 1 has the smallest sizes of bubbles, which
are not severely elongated. It indicates that the bubbles are foamed at the last stage of
foaming. It is assumed that the thin gate size initiated high resistance, which led to high
pressure preventing cell nucleation during the injection stage.
In contrary, Gate 2 and 3 results in apparent cell coalescence with thick layer of
highly elongated cells. It indicates that the nucleation of cells occurred before the melt
21
reached the gate, experiencing shear stress while passing the gate, therefore resulting in
deformation of cells. It is likely due to the reduced flow resistance, which lowered the
pressure inside the gate accordingly. The results from this experiment suggest that the gate
design also plays a vital role in producing high-quality structural foam molding.
Gate 1 Gate 2 Gate 3
Figure 12. SEM pictures of the three gates
6.2 Injection Speed and Uniform Void Fraction Distribution
6.2.1 Effect of Injection Speed on Flow Length of Foam Melt and Void Fraction
Uniformity
It was found from the experiments that the flow length increased as higher
injection speeds were applied. Figure 13(a) illustrates the effect of different injection speeds
22
on the flow length of foam melts. This result can be explained by the relationship between
aped and momentum; higher injection speed results in a higher momentum. While low-
pressure structural foam molding relies on foam expansion to fill the mold cavity to
compensate its short-shot injection, higher injection speed decreases the distance that the
foam expansion must cover in order to completely fill the mold cavity.
Also, the reduced filling time from high injection speed allowed lowering the melt
temperature drop during mold filling, which eventually provided more time for cell
nucleation and growth. It should be noted that cell nucleation and growth in the mold cavity
will stop once the melt temperature decreases below the crystallization temperature.
Figure 13(b) shows the void fractions at three different positions (near the gate,
center, and end of the flow) in a mold. It illustrate that the influence of injection speed upon
the void fraction in different positions is rather negligible; the void fraction at the end of
flow was the highest and the void fraction near the gate was the lowest regardless of the
injection speed. However, this effect can be explained in terms of change in pressure drop
and pressure drop rate over filling time. The pressure drop and pressure drop rate are the
highest at the beginning of the process, and decrease over time. It means that the pressure at
the melt front is zero, whereas the region near the gate is under some pressure due to the
23
melt filling the cavity. Therefore, it is much easier for the foaming to occur at the melt front
than at the region near the gate.
Even though Figure 13(b) shows little effect from injection speed to void fraction
in different locations, it does display a relationship between injection speed and void
fraction at the end of flow. A linear relationship exists between injection speed and the void
fraction at the end of flow up to 250 ~ 300 mm/s, but it tends to level off at higher injection
speeds. This may be due to increasing shear force exerted on the foam melt resulted from
an increasing injection speed. Having a very high shear force might have caused active cell
coalescence and rupture, resulting in lower void fractions for very high injection speeds
(350 ~ 450 mm/s). On the other hand, the void fraction near the gate continued to increase
as the injection speed increased. This may have been caused by the reduced pressure near
the gate region due to the high momentum caused by high injection speeds.
24
0 100 200 300 400 5008
9
10
11
12
13
Flo
w L
ength
[cm
]
Injection Speed (mm/s)
0 50 100 150 200 250 300 350 400 450 5000
10
20
30
40
50
Vo
id F
ractio
n [
%]
Injection speed [mm/s]
Near Gate
Center
End of Flow(a) (b)
0 100 200 300 400 5008
9
10
11
12
13
Flo
w L
ength
[cm
]
Injection Speed (mm/s)
0 50 100 150 200 250 300 350 400 450 5000
10
20
30
40
50
Vo
id F
ractio
n [
%]
Injection speed [mm/s]
Near Gate
Center
End of Flow(a) (b)
Figure 13. Effect of injection on (a) flow length of foam melt and (b) void fraction
uniformity.
6.2.2 Effect of Variable Injection Speed Profile on Void Fraction
Beside the relationship between injection speed and other factors, Figure 13(b)
indicates the problem of a constant injection speed profile upon the void fraction
distribution; it shows that the nonuniform void fraction distribution along the melt flow
direction is inevitable.
As a result, a variable injection speed profile was introduced considering the
pressure and pressure rate drop at the beginning and the end. Figure 14 compares the void
fraction distributions obtained by constant injection speed profile and variable injection
speed profiles. As shown in the figure, a significant improvement in void fraction
25
uniformity was observed when variable injection speed profiles were applied.
Constant Injection Variable Injection0
10
20
30
40
50
Vo
id F
raction
[%
]
Injection Profile
Near Gate
Center
End of Flow
Near Gate
Center
End of FlowConstant Injection Variable Injection
0
10
20
30
40
50
Vo
id F
raction
[%
]
Injection Profile
Near Gate
Center
End of Flow
Near Gate
Center
End of Flow
Figure 14. Effect of variable injection speed profile on void fraction uniformity.
Figure 15 shows another crucial factor in void fraction uniformity; it depends on
when the speed change occurs. For example, a constant injection speed of 200mm/s results
in only 50% of the foam throughout the part; however, when the speed-change stroke (from
200mm/s to 3mm/s) is set to 33% from the end of injection stroke, foaming occurred
throughout the part. The experiments are conducted for speed changes occurring at 8%,
17%, 25% and 33% from the end of injection stroke; the speed- change stroke was further
increased, the foamed region started near the gate increased in size.
26
Figure 15. Effect of speed change stroke on structural foams
7. Conclusion
In the study of processing technology of advanced structural foam injection
molding, experiments were conducted to observe the effects from various processing
parameters. Injection speed was studied in depth by analyzing its effect in degree of filling
the mold as well as its profile’s influence on void fraction of the part. It is shown through
the experiment that the higher injection speed is, the higher degree of filling is achieved in
mold. Also, the variable injection speed profile effectively made the void fraction uniform
along the melt flow direction in structural foam molding.
Another processing parameter was the amount of blowing agent. Throughout the
experiments it was found that only the minimum amount was required (i.e., 0.3 wt% N2) in
achieving high cell density (over 107 cells/cm
3). It is due to its relationship with pressure
27
drop and pressure drop rate. When the content of N2 content was low (i.e., 0.1 wt%), the
pressure required to keep N2 inside polymer matrix is very low. Therefore, the pressure
increase in mold cavity resulted from sudden injection is expected to act like a gas counter
pressure, thereby preventing the cell nucleation at the melt front. On the other hand, when
N2 content was higher (i.e., 0.3 wt% and above), pressure drop rate at the gate governed
cell nucleation, resulting in a high cell density. Even though a specific geometry or sizes are
not determined, the results from the experiments were enough to understand that a proper
shot size and gate design is required in achieving uniform cell structure and high void
fractions. By optimizing all processing conditions, we achieved a uniform cell structure
with a very high void fraction (close to 40%).