Micro Injection Moulding Process

Technology Development

Dr Zhao Jianhong

Robert Mayes

Chen Ge

Chan Poh Seng

Xie Hong

Juay Yang Kay

N Ravi

(Forming Technology Group, 2002)

1 BACKGROUND

There has been considerable interest in recent years in microsystem technology, and it is expected to show continuing expansion over the next decade with the trend towards miniaturisation of components and increasing applications for micro-devices. Analysts predict that microsystem technology will have a far-reaching influence on device manufacture within the next few years [1].

Microsystem technology (MST) is also described as MEMS technology. MEMS (Micro-Electro-Mechanical Systems) is the name given to the combination of miniaturised mechanical and electronic structures in a system. Started in 1980’s, microsystem technology has become of growing importance over the past decades and is forecasted to be one of the main technologies of the 21st century [2]. The advantages of miniaturisation are many and the success of microelectronics is a proof of this [3].

The small size of microdevices makes smaller and more compact apparatus possible and this is promising for portable and hand-held systems [3]. The functionality of a device increases when various functions, such as sensing, readout and actuation, are integrated on one microdevice. Miniaturised mechanical sensors, for example, typically have a wide dynamic range and fast response times as a result of their high resonance frequencies. Heat transfer occurs more rapidly and effectively in micro heat exchangers compared to macro-scale devices.

MEMS is now at a stage where products are expected to enter the market at increasing paces. The total world market for MEMS is expected to surge to US$38 billion by 2002 [4], and will reach US$50 billion by the year 2005 [5]. The application area of micro system technology covers a wide range from electronic to optic, and from chemical to biotechnology and medical engineering [6-8]. However, MEMS are typically fabricated with batch-processing techniques similar

to those used for integrated circuits. Silicon is still the material that is used in most applications. One drawback of using silicon is high price due to its batch-process nature and high material cost.

The development of microsystem technology on a vast scale is dependent on manufacturing systems that can reliably and economically produce micro components. Polymer micro processing technology, e.g. micro moulding technology, is a key enabling technology for microsystems with the capability to provide disposable micro components at a low cost [9, 10].

Micro moulding is still very much in its infancy as a new branch of injection moulding. It is not just about scaling everything down; it is a specialised technique in its own right, with a different set of challenges. The moulding machine, tooling, material and process, as well as component handling and inspection need

to be specially addressed.

In order to meet the challenges towards miniaturisation, Gintic has embarked on micro moulding process development through this in-house program to develop core competency in polymer micro-fabrication.

2 OBJECTIVE

The objective of the project was to develop process technologies for plastics micro injection moulding process - permitting cost effective mass production

of micro-structures from a wide varieties of modern high-performance plastics.

3 METHODOLOGY

Micro injection moulding experimental studies were carried out using a micro moulding machine with a maximum melt injection capacity of 1 cm3. Micro components were designed and produced successfully using engineering plastics.3.1 Component Design and Material

Selection

Two types of micro components were studied in the project: components of relatively large volume with micro structures and “micro” components of very small volume.

The microstructure component studied was a lens array with nineteen “micro” lens surfaces designed on the top and bottom sides of the lens array. This type of lens array is widely used for laser beam coupling in the optical fibre communication industry. The polymer material selected for this component was polycarbonate (PC) because of its superior optical, mechanical and processing properties.

The micro parts studied were a serial of micro gears. Wide applications can be found for micro gears such as in watch industry, in micro pumps and micro delivery systems. Three gear designs with diameters from 1 mm and 3 mm were used in the micro part moulding studies. The polymer material used for all the three gears was polyoxymethylene (POM).

3.2 Micro Moulding Machine, Micro

Mould Design and Simulation

Studies

The micro injection moulding machine used in the present study was a Battenfeld Microsystem 50 moulding machine as shown in Figure 1. The injection system of the machine is composed of a screw plastication extruder and a plunger injection system. By using the screw pre-plasticiser, a small diameter plunger can be used for melt injection to achieve precise measurable strokes to control melt accuracy [11].

Two micro moulds were designed and fabricated for the lens array and the gear components respectively. The mould bases used for the two injection moulds were identical with an overall dimension of 120x160 mm. The micro lens array mould was a three-plate one-cavity mould to facilitate automatic de-gating, as shown in Figure 1. The mould for the micro gears was a family mould with inter-changeable mould inserts. An insert with two cavities can be used at one time to mould two identical gear components.

Figure 1 Micro moulding machine and mould Because of the characteristics of the micro cavities, the micro EDM wire cutting technique was used for the fabrication of the mould inserts. Electrodes with diameters down to 10 µm were used for the fine structures of the components such as the gear tooth.

Mould filling simulations have been carried out to study mould filling behaviour of the polymer resins in the micro moulding process.

3.3 Process Study by Using Design

of Experiment Method

In the micro injection moulding process there are some extra process parameters related to the control of plunger movement and melt storage barrel settings. This makes the process setup and optimisation more difficult to perform compared with the conventional injection moulding process. In order to optimise the process the effects of the important processparameters on the products need to be identified. In the present study the design

of experiment (DOE) method was used in

the process studies of the micro components.

The fractional factorial design was used in

the studies. Factorial designs at two levels require relatively few runs per factor studied. They are often of great value at

an early stage of an investigation for injection moulding process where a large number of process parameters are involved [12, 13].

3.4

Process Monitoring and

Optimisation

In the injection moulding process, continuous monitoring of the key process parameters is very important for process optimisation and process control. Process temperature and pressure are often selected as the parameters for process monitoring in the injection moulding process, especially the cavity temperature

and cavity pressure [14].

In the micro moulding process, however, it

is usually difficult to measure the cavity pressure due to the extremely small cavity sizes involved. In the present study, the injection pressure applied through the injection plunger was monitored and the injection pressure curve and its integration

over injection time were monitored and used for process monitoring and control.

A data acquisition and monitoring system

was built up to monitor the injection pressure, as shown schematically in Figure 2. Data analysis work was carried

out on line and the information acquired

was shown on a computer screen and also saved as data file in the data acquisition system.

3.5 Micro Component Characterisation

and Process Capability Study

Micro component inspection and characterisation were carried out using advanced analytical and microscopic techniques. The equipment used included

a coordinate measuring machine, a form talysurf stylus profilometer, a scanning electron microscope (SEM) and optical microscopes.

Screen

Proximity sensor

Proximity

sensor

Mould

Injection plunger

Figure 2 Process Monitoring and Control System

Process stability and capability studies were carried out for the micro moulding process. Components manufactured at optimised process conditions were characterised and statistically analysed to obtain information of process capabilities.

4 RESULTS

4.1 Injection Moulded Micro

Components

Figure 3 shows a photograph of the moulded plastics gears in comparison with

a paper clipper. Also shown in the figure is

a microphotograph of the moulded 3mm diameter plastic gear.

observed from these microphotographs that the plastic parts were well formed with clear structure definition and good surface smoothness. The tooth structure of the gears is in the dimension range of a few tens of microns – less than the diameter of a human hair.

1 mm diameter gear

1 mm diameter gear with shaft

Figure 4 Microphotographs of the 1 mm diameter gears

Figure 5 Moulded micro lens arrays using a polycarbonate resin

Shown in Figure 5 are photographs of the micro lens arrays moulded using a PC resin. The microphotographs taken by SEM show the lens front view at two different magnifications.

Part weight, dimension and surface quality of the moulded components were examined and analysed as quality indexes of the micro components. Listed in Table 1 are some physical features of the moulded components.

Table 1 Properties of the mould gears

Component 1 mm gear

1 mm gear with shaft

3 mm gear micro lens array Material

POM POM POM PC Part weight (mg)

0.6 8 16 90

4.2 Process Parameter Effects

For the 1 mm micro gear with shaft structure, a two-level factorial design 24-1 was used to study four process parameters: the metering size, the melt temperature, the hold pressure time and the mould temperature. Four centre points were added to the eight run design to make a 12 run experiment.

Analysis work was conducted for the experimental results. Part weight and diameter of the moulded gears were analysed as response parameters of the design of experiment.

The results of part weight and gear tip diameter measurement are presented in Figure 6 as a function of process conditions. It can be observed from this figure that the responses of the part weight and the gear tip diameter to the process conditions follow a similar pattern, i.e. when the part weight is low, the tip diameter is small. This indicates that the tips of the gear teeth were the last areas been filled during the moulding process. This is in agreement with the mould filling simulation results as shown in Figures 7 where the tips of the gear teeth are identified as the last filling part of the

component and also the potential places for air trap and shrinkage.

1

2

3

4

5

6

7

8

9

10

11

12

13

Part Number

W e i g h t (m g )

D i a m e t e r (m m )

Figure 6 Part weight and gear diameter measurement results for each of the test run conditions

Figure 7 Simulation results: melt filling pattern

Statistical DOE analysis results for the gear diameter and the part weight are shown in a format of Pareto in Figure 8 for the estimated effects and interactions in decreasing order of importance. It is clear from this chart that the holding pressure time and the metering size are the two process parameters that have significant effects on part quality. It is also observed from the chart that the interaction of the metering size and the holding pressure can affect the gear diameter significantly.

For the micro lens array, a half fraction factorial experiment design 26-1 was generated and conducted. The parameters studied included the mould temperature, the melt temperature, the cooling time, the injection speed, the metering size, and the pressure holding

time.

1

2

3

4

A:Mold temperature

AD+BC

AC+BD

B:Melt temperature

AB+CD

C:Metering size

D:Holding pressure time

Standardized effect

Figure 8 Pareto chart for gear diameter and part weight of the 1 mm gear with shaft structure

The statistical analysis results for the part weight are presented in Figure 9 as a Pareto chart. It is seen that the part weight of the micro lens was mainly affected by the metering size. However, apart from the metering size, the injection speed and the mould temperature also had observable effects on the part weight, i.e. the top three process parameters that significantly affect the part quality were the metering size, the injection speed, and the mould temperature.

Figure 9 Parameter effect on the micro lens array part weight

In lens moulding processes low injection speeds are generally used to reduce residual stress and orientation. In this DOE study the plunger injection speed used varied from 20 mm/s to 50 mm/s. Since the injection nozzle of the micro moulding machine is extended inside the mould without having any extra heating, the temperature at the nozzle tip lies in somewhere between the mould temperature and the barrel temperature. This temperature is normally lower than the barrel temperature because the mould temperature is usually lower than the

nozzle temperature. If a low injection speed is used, temperature drop at the front nozzle area may become considerable, and the viscosity of the melt may drop significantly, resulting in short shots, parts with high shrinkage and warpage. In such situations melt and mould temperature may play a more significant role.

4.3 Process Monitoring and

Optimisation

Process optimisation studies have been carried out for the micro moulding process. Since the main factors with significant effects on gear part quality were identified as the metering size and

the holding pressure time, optimisation studies were carried on the response surface of these two process parameters

for the gear moulding.

Shown in Figure 10 is the estimated response surface for the gear diameter of

the 1 mm gear with shaft structure, with

the metering size and the holding pressure time as the variables. This graph clearly indicates that the gear diameter is

not only affected by the metering size and

the hold pressure time, but also a function

of the combination of these two process parameters.

Figure 10 Statistic analysis response surface

for gear diameter

In order to optimise the process, experiments were carried out at various metering size and holding pressure time conditions to further understand their effects on part quality. The data acquisition and monitoring system was used to monitor and optimise the micro moulding process. Shown in Figure 11 are some pressure curves recorded for injection shots carried out at different metering sizes. It can be observed from the figure that as the metering size increases, the injection pressure is building up gradually. While at low metering sizes, the holding pressure is not playing any role since the material is not enough to fill the runner and cavity system.

Figure 11 Holding pressure profile increases as metering size increases from 180 to 200 mm3

Since there is a strong interaction between the metering size and the holding pressure time, the optimisation process of the metering size can be strongly affected by the setting status of the holding pressure. Shown in Figure 12 is the influence of the interaction of the holding pressure time and the metering size on part quality. With the holding pressure on, the cavity can be filled at a metering size of about 190 mm3, while without holding pressure, a metering size of about 210 mm3 is required to fill the cavity properly.

4.4 Process Stability and Capability

A stable process is a key prerequisite in mass production to produce parts with constant quality. Process stability studies were carried out in this project using the micro lens array mould. Part weight and injection pressure and its integration with respect to the process time were monitored as a function of the number of moulding shots.

Shown in Figures 13 and 14 are the recorded injection pressures associated with a number of moulding shots conducted at different stages of the moulding trails. It is observed that the injection pressure and its integration for the first few tens of moulding shots were very unstable, as shown in Figure 13.

170

180190200210

220

Metering Size (mm3)

D i a m e t e r (m m )

P a r t W e i g h t (m g )

Figure 12 Interaction between metering size and holding pressure on part weight and gear diameter

Figure 13 Injection pressure curve and its integration for shots No.25 to 30

Figure 14 Injection pressure curve and its integration for shots No. 175 to180

As the process went on, injection pressure was getting more uniform and remained at a very steady state after the process had stabilised, as shown in Figure 14. Process capability studies were carried out for the micro moulding process after the process was stabilised. The collected samples were characterised and statistical analysis work was carried out to obtain the process capability information. A process capability value of 1.33, which is usually considered as a good Cp value, can be obtained for the moulding processes.

5 CONCLUSIONS

Through this project capabilities have been built up in Gintic for the plastics micro injection moulding process. Listed below are some main achievements and findings of the project.

• Micro components down to 0.6 mg

made from a number of engineering plastics have been successfully produced.

• For different product designs and

polymer materials, different mould runner systems should be used for good melt flow and cavity filling.

• Process monitoring systems are very

useful for the micro injection moulding process. A monitoring system visualises the micro moulding process and makes the process parameter sensitive process easier to optimise and control.

• The metering size is one of the most

significant process factors that affect moulded part quality. There is a strong interaction between the metering size and the holding pressure.

• Although the stability is low at the

beginning of the process due to a small amount of polymer melt being used in each moulding shot, a good process repeatability can be achieved after the process has stabilised.

SIGNIFICANCE

The micro moulding process has great potential in MEMS applications. The areas where the microsystem technology can be expected to progress quickly include information and communication technologies – with specific emphasis on optical data communication, chemical micro reactor technology, biotechnology, environment sensors, electronic mounting

and connection technologies.

Biomedical branch is one of the areas that show the greatest growth and largest potential for further increases and it is expected to be an important segment for MEMS products.

The micro moulding capabilities and process technologies developed in this project have formed a technical base for further development of micro fabrication engineering in the local manufacturing industry to carve out a niche market.

REFERENCES

1. Sparrow, N. "The Microtechnology

Revolution," EMDM (European Medical

Device Manufacturer): March/April 1999:

Special Report on Microtechnology, 1999.

2. Gabriel, K. J., “Engineering Microscopic

Machines”, Scientific American, September 118 (1995).

3. Daniel, J. H., “Micromachining Silicon for

MEMS”, MTP, Cambridge CB2 1 TJ, UK,

1999.

4. “Market analysis for microsystems 1996 –

2002”, A NEXUS (The Network of

Excellence in Multifunctional Microsystems

- a body of the European Commission that

promotes MST within European industries)

Task Force Report, October 1998.

5. Wechsung, R., and Ayman El-Fatatry,

“The world-wide market for microsystems

– The NEXUS market study update,”

mstnews, 5/01, October 2001.

6. Niggemann, M., Ehrfeld, W., and Weber,

L., “Fabrication of miniaturized

biotechnical devices”, SPIE Processings,

Vol. 3511, Micromachining and

Microfabrication Process Technology IV,

Santa Clara, California, USA, 21-22

September 1998. 7. Konrad, R., Ehrfeld, W., Freimuth, H.,

Pommersheim, R., Schenk, R., and

Weber, L., “Miniaturization as a demand of

high-throughput synthesis and analysis of

biomolecules”, Proceedings of the 1st Int.

Conf. On Microreaction Technology

(1997).

8. Nighswonger, G., “Micromachining: A

bigfuture for small devices”, Medical

Devicelink, November 1999.

9. Kerkland, C., “The micromolding super-

express”, Injection Molding, December

1999, pp 98-102.

10. Weber, L., Ehrfeld, W., Freimuth, H.,

Lacher, M., Lehr, H., and Pech, B., “Micro

moulding – a powerful tool for the large

scale production of precise microstructures”, SPIE Proceedings, Vol.

2879, Micromaching and Microfabrication

Process Technology II, Austin, Texas,

USA, 14-15 October 1996.

11. Ganz, M., “Microsystem – the innovative

solution for microprecision parts”, in

Polymer Process Engineering 99 (Editor:

P. D. Coates), IOM Communications Ltd,

UK, 1999.

12. Box, G. E. P., Hunter, W. G., and Hunter,

J. S., Statistics for Experiments: An

Introduction to Design, Data Analysis, and

Model Building, John Wiley & Sons, New

York, 1978.

13. Zhao, J., Lu, X., Fong, L., and Chiang. H.

H., "Statistical Experiment Study of Gas-

Assisted Injection Molding Process,"

Society of Plastics Engineers ANTEC

Proceedings, Atlanta USA (1998) 454-458. 14. Zhao, J., R. H. Mayes, L. Dumoulin, Ng

Teck Chew and L. P. Tay, “Injection

Moulding Control Studies,” in Polymer

Process Engineering 01, Editor: P. D.

Coates, IOM Communications Ltd,

London, June 2001.下载本文