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    Development of Process Monitoring Technologies for Polymer
                     Micro Moulding Process
                                    J. Zhao, G. Chen and Y. K. Juay


Abstract – As a key enabling technology, micro            rameter is not properly set since there is no melt
moulding is the process of choice for mass pro-           cushion as in the conventional moulding process
duction of various polymer micro components               [6]. If the metering is set too high, the pressure
with complex geometry. However, because of                built may be too high during plunger injection.
the micro dimensions of the moulded compo-                On the other hand, if the metering is set too low,
nents, it is difficult to assess the part quality         injection pressure may not build up properly,
without using a microscopic observation device.           resulting in short shots. Because of the micro
Proper process monitoring can therefore play an           dimensions of the moulded components, it is
important role in micro moulding process quality          difficult to assess the process status by looking
control. In the present study, a process monitor-         at the moulded components without using a mi-
ing system has been developed to monitor the              croscopic observation device. A process moni-
micro moulding process. Pressure sensors have             toring system is therefore necessary to maintain
been employed for plunger injection pressure              a constant and optimised process.
measurement. An on-line system has been built
up for on screen process parameter monitoring.
It was found that the measured injection pres-            2       OBJECTIVE
sure correlated well with part quality and can
therefore be used as a process optimisation and           The objective of the project is to develop poly-
quality control index for the micro moulding pro-         mer micro moulding monitoring technologies
cess.                                                     and to build a monitoring system to monitor the
                                                          process.
Keywords: Micro moulding, Process monitoring,
Process optimisation, Plastic gear, Design of
experiment                                                3       METHODOLOGY

                                                          3.1     Micro moulding machine
1       BACKGROUND
                                                          A Battenfeld Microsystem 50 micro moulding
In the injection moulding process, polymer mate-          machine was used in the experiment. This all-
rials undergo a phase transformation process              electric injection moulding machine is the inno-
from solid to melt and back to solid state again          vative answer from Battenfeld to the growing
at the end of the process. Continuous monitoring          demands in the manufacturing of micro-
of the process in terms of key process parame-            precision components in plastics [4]. The injec-
ters is very important for process optimisation           tion system of the machine is composed of a
and process control [1-2]. Process temperature            screw plastification barrel and a plunger/needle
and pressure are often selected as the parame-            injection system. By using the screw pre-
ters for process monitoring in the injection              plasticiser, a small diameter plunger/needle can
moulding process, particularly the cavity tem-            be used for melt injection to achieve precise
perature and cavity pressure.                             measurable strokes to control melt accuracy.
                                                          The plasticising screw of the injection moulding
In the micro moulding process, however, it is             machine has a diameter of 14 mm, and a 5 mm
usually difficult to measure the cavity pressure          diameter injection needle is employed for melt
due to the extremely small cavity sizes involved          injection. The maximum injection capacity of the
                                                                                             3
and the injection system designs adapted by               micro moulding machine was 1 cm .
micro moulding machines. In order to control
metering accuracy, several newly developed                3.2     Micro component and polymer mate-
micro injection moulding machines use a                           rial
plunger or needle injection system for melt injec-
tion and a screw extruder for plastic pellets plas-       A micro gear of 1 mm diameter with shaft struc-
tication [3-5]. By using such a needle/plunger            ture was selected in this study. Wide applica-
injection system, high injection pressure can be          tions can be found for micro gears such as in
achieved since there is no melt back flow in-             the watch industry, in micro pumps and micro
volved in the injection process. This, however,           delivery systems. The polymer material used for
may cause problems if the melt metering pa-               the gear was a polyoxymethylene (POM) resin


                                                      1
                              Development of Process Monitoring Technologies for Polymer Micro Moulding Process



trade named as Iupital F20-03 fabricated by Mit-
                                                                                                                                                   Pressure integration
subishi. The mould for the micro gear was a




                                                                 Pressure integration (bar s)
two-cavity mould. The mEDM method was used




                                                                                                   Injection pressure (bar)
                                                                                                                              Injection pressure
for the fabrication of the gear mould inserts. In-
jection gates of 300 mm were used for the micro
gears [7]. The weight of the moulded gears was
about 8 mg.

3.3       Monitoring system

A data acquisition and monitoring system was                                                                                                                Time (s)
built up to monitor the injection pressure, as
shown schematically in Fig. 1. In the system, a
force transducer was mounted at the back end                    Fig. 3. Injection pressure curve and injection pres-
                                                                sure integration recorded during micro moulding pro-
of the injection plunger to measure the injection
                                                                cess.
force. Pressure sensors available currently at
the market (Fig. 2) are generally too big to install
inside micro cavities for cavity pressure meas-
                                                                Shown in Fig. 3 are some typical injection pres-
urement. A force transducer was therefore used
                                                                sure curves recorded for the micro moulding
at the back end of the plunger where a trans-
                                                                process using the process monitoring system.
ducer of 5 mm diameter can be installed. Two
                                                                The data acquisition system starts to measure
proximity sensors were used to monitor the
                                                                the pressure when the injection plunge begins
plunger position and mould open/close informa-
                                                                melt injection and stops when the mould starts
tion respectively. The plunger position and
                                                                to open. Pressure curves for different moulding
mould open/close information was then used as
                                                                shots can therefore be recorded in the same
trigger signals to start or stop the measurement.
                                                                graph for the purpose of easy comparison. The
Signals obtained from the sensors were passed
                                                                data acquisition system can also conduct on-line
to a data acquisition system for analysis. Data
                                                                data analysis work. For example the integration
analysis work was carried out on line and the
                                                                of the injection pressure over the injection time
information acquired can be shown on a com-
                                                                can be carried out on-line and shown simultane-
puter screen and saved as data file.
                                                                ously on the recording graph.
              Injection plunger
  Mould
                                  Proximity sensor              3.4                             Experimental design
                                        Force transducer
                                                                In order to study how the injection pressure is
                                                Charge          correlated to the process parameters and part
                                                                              4-1
                                               Amplifier        quality, a 2      design of experiment was con-
                                                                ducted using four parameters: the metering size,
      Proximity                                                 the melt temperature, the holding pressure time
       sensor       Screen
                                                 Data           and the mould temperature. This is a half frac-
                                              Acquisition
                                                System
                                                                tional factorial design requires eight experimen-
                                                                tal runs. The purpose of the screen design was
                                                                to identify the process parameters that have sig-
Fig. 1. The process monitoring system setup for the             nificant effects on the process. The values of the
micro moulding process.
                                                                process parameters used in the design of ex-
                                                                periment studies are shown in Table 1. Four
                                                                centre points were added to the eight run design
                                                                to make a 12-run experiment.

                                                                   Table 1. Parameter values used in the experiment.
                                                                   Para-                          Mould                          Melt T    Metering        Hold P
                                                                                                     o                             o            3
                                                                   meters                         T ( C)                          ( C)      (mm )         Time (s)
                                                                    High
                                                                    level                          100                            230         210             2

                                                                                 Low
                                                                                level                60                           190         190             0

                                                                    Centre
Fig. 2. Different types of sensors available in the                  point                           80                           190         200             1
market.




                                                            2
                                                    Development of Process Monitoring Technologies for Polymer Micro Moulding Process



4                              RESULTS & DISCUSSION                                                                           This is expected as the gear diameter and injec-
                                                                                                                              tion pressure vary in a similar pattern in re-
In the micro moulding process, the micro feature                                                                              sponding to the change of process conditions.
details of a component are usually the most dif-
ficult areas to fill because of the micro dimen-                                                                              It is also noted in Fig. 5 that the interaction CD,
sions and the fact that the micro features are                                                                                i.e. the interaction between the holding pressure
usually located at or near the end of the flow                                                                                time and the metering size, is confounded with
path. For the micro gear studied in this paper the                                                                            the interaction AB between the mould tempera-
micro tips of the gear teeth were the last areas                                                                              ture and the melt temperature. This is because
to be filled during the moulding process. The                                                                                 the DOE design is a resolution III design where
filling status of the micro features can be re-                                                                               the second order inter actions are confounded
flected in variations of the part weight. The re-                                                                             with other second order interactions. Generally
sponses of part weight and gear tip diameter to                                                                               if the main effect of a factor is significant, its in-
the process conditions followed a similar pattern;                                                                            teractions with other factors are also likely to be
when the part weight was low, the tip diameter                                                                                significant. In this case, since both the holding
was small [8]. This provides an indirect way to                                                                               pressure time and the metering size are signifi-
control gear diameter since part weight is a pa-                                                                              cant, their interactions are also likely to play sig-
rameter that is more convenient to measure. It                                                                                nificant roles.
would be much easier if injection pressure could
be used as a process control parameter since it
                                                                                                                               D:Holding pressure time
can be easily monitored on-line.
                                                                                                                                       C:Metering size
                                                                                                                                               AB+CD
Fig. 4 shows the results of the design of experi-
ment studies on the gear diameter and the injec-                                                                                   B:Melt temperature

tion pressure. It is seen that the tip diameter of                                                                                             AC+BD
                                                                                                                                                                              (a)
the moulded gear varies with the change of the                                                                                                 AD+BC
                                                                                                                                                                            Diameter
process conditions in a similar pattern as the                                                                                     A:Mold temperature
injection pressure. When the injection pressure
                                                                                                                                                         0   1       2       3       4    5
is low, the gear diameter is smaller. This indi-                                                                                                                 Standardized effect
cates that the part quality is closely related to the
injection pressure.
                                                                                                                               D:Holding pressure time
Statistical analysis results for the gear diameter
                                                                                                                                       C:Metering size
and the injection pressure are shown in the
Pareto chart in Fig. 5 for the estimated effects                                                                                               AB+CD

and interactions in decreasing order of impor-                                                                                     B:Melt temperature
tance. It is clear that the holding pressure time                                                                                              AC+BD                          (b)
and the metering size are the two process pa-                                                                                                  AD+BC
                                                                                                                                                                           Inj Pressure
rameters that have significant effect on both the                                                                                  A:Mold temperature
tip gear diameter and the injection pressure. It is
                                                                                                                                                         0         1            2         3
also observed from the chart that the interaction                                                                                                                Standardized effect
between the metering size and the holding pres-
sure has also significant effects on the gear di-                                                                             Fig. 5. Pareto chart for (a) gear diameter and (b)
ameter and the injection pressure.                                                                                            injection pressure.

                         1.4                                                            2500


                         1.2
                                                       Diameter            Inj P                                              Fig. 6 shows microphotographs of two moulded
                                                                                        2000
                                                                                                                              gears at two different process conditions. The
                                                                                               Injection pressure (bar)
    Gear diameter (mm)




                         1.0
                                                                                        1500
                                                                                                                              gear shown in Fig. 6(a) was produced at a
                         0.8
                                                                                                                              condition with a combination of the low level
                                                                                        1000                                  holding pressure time and the low level metering
                         0.6                                                                                                  size. It can be observed that at this process
                         0.4
                                                                                        500                                   condition, the part was not fully filled, resulting in
                                                                                                                              a component with defects. While, at a process
                         0.2                                                            0
                                                                                                                              condition with a combination of high level of
                               1   2   3   4   5   6    7   8     9   10      11   12
                                               DOE Condistions
                                                                                                                              metering size and holding pressure time, a well-
                                                                                                                              formed component was produced. This clearly
Fig. 4. Gear diameter and injection pressure meas-                                                                            shows the significant effects of these two
urement results for each of the DOE run conditions.                                                                           processing parameters on the part quality.




                                                                                                                          3
                         Development of Process Monitoring Technologies for Polymer Micro Moulding Process




                                                                      Injection pressure (bar)
                                                                                                   (a)




                                      (a)


                                                                                                          Time (s)

                                      (b)

                                                                                                              (b)




                                                                        Injection pressure (bar)
                                                                                                   Time (s)
Fig. 6. Microphotograph of moulded gears at different
process conditions.
                                                            Fig. 7. Injection pressure curves recorded for the
                                                            moulding process of the components shown in Fig.
Fig. 7(a) and Fig. 7(b) show the injection pres-            6(a) pressure curves for low metering and holding
sure curves recorded for the moulding proc-                 pressure time; (b) well developed pressure curves.
esses of the two components shown in Fig. 6(a)
and 6(b) respectively. It is clear from these fig-
ures that the quality of the components is closely          In order to study the effects of the metering size
related to the injection pressure status during             setting and holding pressure time on the injec-
the moulding process.                                       tion pressure, further experiments were carried
                                                            out at different metering size and holding pres-
In Fig. 7(a), where a low metering size and hold-           sure conditions, while keeping the other process
ing pressure time were employed, the injection              conditions constant at the middle levels of the
pressure was not well developed because not                 DOE conditions shown in Table 1. In Fig. 8 the
enough metering was used. The pressure                      injection pressure curves for metering size set-
                                                                                        3           3
reached a maximum of about 750 bar and then                 tings varying from 180 mm to 200 mm are pre-
quickly reduced to zero without any pressure                sented. It can be observed from this figure that
                                                                                                           3
holding period. While for the curves in Fig. 7(b),          as the metering size increased from 180 mm to
                                                                     3                    3
the injection pressure built up was much high               200 mm at a step of 5 mm , the injection pres-
and a holding pressure stage was well main-                 sure built up gradually. At the low metering size
                                                                         3
tained during the moulding process. From the                of 180 mm , the holding pressure is not playing
injection pressure curves in two figures, one can           any role since there was not enough polymer
readily predict how the processes were carried              melt to fill the runner and cavity system. As the
                                                                                                        3
out without going through detailed product char-            metering size increased from 180 mm , the
acterization process. Since the injection pres-             maximum injection pressure as well as the
sure information is available on-line and can be            broadness of the curves increased. This again
recorded as data file, it is very useful for process        shows the strong interaction of the metering size
set up, process monitoring and process control.             and the holding pressure time on the moulding
                                                            process.




                                                        4
                                                              Development of Process Monitoring Technologies for Polymer Micro Moulding Process



                                                                                             in which mould cavity can be filled properly by



                            Injection pressure (bar)
                                                                                             applying an appropriate holding pressure. This
                                                                   Metering                  process setup and optimisation process can be
                                                                     size                    readily conducted with the help of the monitoring
                                                                                             of the injection pressure. Although the injection
                                                                                             pressure measured at the end of the injection
                                                                                             plunger does not provide as much mould filling
                                                                                             information as the cavity pressure measure-
                                                                                             ment, it is still a useful way for process monitor-
                                                                                             ing and optimisation in the micro moulding proc-
                                                                                             ess where applications of cavity pressure sen-
                                                                                             sors are normally not practical due to the micro
                                                                                Time (s)     cavities involved.




                                                                                                     Injection pressure (bar)
Fig. 8. Holding pressure profiles as the metering size
                       3            3
increased from 180 mm to 200 mm .


Since a strong interaction between the metering
size and the holding pressure time was ob-
served for the moulding process in our DOE
studies, the optimisation process of the metering
size can be strongly affected by the setting
status of the holding pressure. Fig. 9 shows                                                                                         Time (s)
some pressure curves recorded for a series of
holding pressure time settings conducted at a fix
                            3
metering size of 200 mm . It can be observed
that at this metering size, the pressure was well                                            Fig. 10. Hold pressure time effect on pressure curve
                                                                                                                                          3
built up, and the pressure curves were getting                                               at a condition of low metering size of 190 mm .
broader as the holding pressure time increases.
On the other hand, when the metering size was
                     3
reduced to 190 mm , the injection pressure built                                             5         CONCLUSION
up was very low as shown in Fig. 10, despite a
long hold pressure time being used.                                                          Micro moulding process monitoring and optimi-
                                                                                             sation studies have been carried out in this
                                                                                             study. The following conclusions can be drawn
                                                                                             from the experimental findings.
 Injection pressure (bar)




                                                                                             •   The injection pressure is found strongly af-
                                                                                                 fected by the metering and the holding pres-
                                                                                                 sure settings. If the metering size is set too
                                                                                                 low, injection pressure cannot be built up in
                                                                                                 the plunger injection process and the hold-
                                                                                                 ing pressure setting will not play any role.
                                                       Holding
                                                                                             •   Although the injection pressure measured at
                                                       pressure                                  the end of the injection plunger does not di-
                                                         time                                    rectly monitor the micro cavity filling and
                                                                              Time (s)           cooling status during the micro moulding
                                                                                                 process, the measured injection pressure
                                                                                                 curve is a good indication of the status of
Fig. 9. Effect of hold pressure time on injection pres-
                                                                                                 process setting and can therefore be used
sure at a metering size of 200 mm .
                                   3                                                             as a process optimisation and quality control
                                                                                                 index for the micro moulding process.

It is, therefore, clear that in the micro moulding
process, a good combination of the melt meter-                                               6         INDUSTRIAL SIGNIFICANCE
ing size and the hold pressure setting is essen-
tial to produce parts with good quality. To                                                  As more and more micro electronic mechanical
achieve such good combinations the metering                                                  system (MEMS) products are expected to enter
size setting must fall within an “optimum” range                                             into the market, there are increasing demands


                                                                                         5
                        Development of Process Monitoring Technologies for Polymer Micro Moulding Process



for fabrications of plastics micro components for              Control for Injection Molding”, Journal of In-
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ess it is difficult to access part quality without       [3]   N. Sparrow, “The Microtechnology Revolu-
using a microscopic observation device due to                  tion”, European Medical Device Manufac-
the micro features of the components fabricated.               turer, April, (1999).
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makes it difficult to setup and optimise the proc-             Process Engineering 99, P.D. Coates, IOM
ess. The process monitoring technique and the                  Communications Ltd, London, pp. 8-17,
monitoring system built up in this study is there-             (1999).
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process easier to optimise and control. It is                  Z.J. Xiong, “Polymer Micromould Design
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moulding industry to enter into the micro mould-               ber and Composites, Vol. 32(6), pp. 1-8,
ing area.                                                      (2003).
                                                         [7]   J. Zhao, R. Mayes, G. Chen and P.S. Chan,
                                                               “Micro Injection Moulding Process”, Pro-
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