Polymer inserts tooling for prototyping of micro fluidic components in micro injection moulding

C. A. Griffithsa, S. Bigota, E. Brousseaua, M. Worgull b, M.Heckele b,

J. Nestler c and J. Auerswald d

a Manufacturing Engineering Centre, Cardiff University, Cardiff, UK, e-mail: bigots@cf.ac.uk b Institut fuer Mikrostrukturtechnik, Forschungszentrum, Karlsruhe, Germany

c Chemnitz University of Technology, ZfM, Chemnitz, Germany d CSEM Centre Suisse d'Electronique et de Microtechnique SA, Alpnach, Switzerland

Abstract An increasing number of microfluidic applications linked with a strong demand for low-cost solutions has

pushed research activities towards the development of polymer based microfluidic systems. This requires the use of micro injection moulding technology to replicate accurately polymer micro parts, however the tooling cost associated with this manufacturing route remains relatively high. The purpose of this paper is to investigate the feasibility of utilising polymer inserts for prototype tooling in micro injection moulding that can reduce product development time and cost associated with the design and testing of micro fluidic systems prior to mass fabrication.

Keywords: micro injection moulding, hot embossing, prototyping, polymer tool, polymer insert, microfluidic

1. Introduction

Microfluidic technologies have found a large variety of new applications in fields such as biotechnology, cytometry, medical diagnostics and micro chemistry. Products that offer point of care diagnostic abilities, for biomedical and pharmaceutical research, and portable devices such as micrometer scale total analysis systems (µTAS) or lab-on-chip systems are currently under investigation [1].

The development of new micro devices is highly dependent on manufacturing systems that can reliably and economically produce large quantities of micro components. Micro fluidic devices can incorporate the material properties of polymers such as optical transparency, low toxicity and high permeability to oxygen and carbon dioxide as required for cell research [2]. Also the current micro fabrication processes using polymers makes it possible to fabricate prototype devices that test new ideas and exploratory research in a time period much shorter than that using silicon technology. In this context micro injection moulding (IM) of polymer materials is one of the key technologies for micro manufacturing, where microfluidic components manufactured by micro IM fall into the Type B [3] category of having larger overall dimensions but incorporate micro features typically less than 200 µm.

Similar to the double hot embossing (HE) method [4], this paper investigates the use of a polymer on polymer process where the IM tool cavity itself is a high temperature polymer insert fabricated through HE.

The insert production can be done using this two-step replication method or using direct structuring micro manufacturing processes, bringing flexibility on achievable features based on cost effectiveness and process capabilities.

The purpose is to study the feasibility of using a prototype polymer insert tool in IM for part production quantities in the range of a hundred units within an acceptable time. Furthermore, the material replicated should ideally be the same as for the final product in order to allow appropriate testing of microfluidic devices

before potential mass fabrication. At first the complete insert manufacturing process

is described. Then, replications are performed within the range of four IM process factors with the idea of identifying polymer materials as a potentially economic tooling prototype and production alternative for future research in the study of fluids in micro/nano channels.

2. Manufacturing process description

2.1 Part design

The part adopted for the experiment is a 20x30x1mm micro-fluidic test structure designed for biomedical application (Figure 1). It is composed of a channel structure of varying channel depths, between 100µm and 400µm, and channel widths, between 100µm and 1000µm.

2.2. Insert fabrication

A positive copy of the final part to be produced was machined on a HE brass tool using micro milling with cutters down to 0.1mm. This tool was then integrated in a HE machine to produce IM inserts in PEEK. The HE process was optimised using a dozen of replications. Another dozen of complete parts were then produced using a force of 100KN and a tool temperature of up to 359°C. Figure 2 shows the temperature evolution together with the tool closure.

After all replication were made, the brass tool was clearly damaged (Figure 3) due to the high forces involved in the HE process. This resulted in dimensions variations between different PEEK insert. Brass was selected as tool material for rapid prototyping purposes. However, for future application another tool material, such as steel, should be considered for a longer tool life.

Fig. 1. Tool insert for the micro fluidics system

2.3. Insert integration

The PEEK inserts were integrated to a brass secondary tool, 1.0mm below the front face. A brass block was placed into the rear of the secondary tool behind the plastic insert, providing mechanical support against pressure and force from the moulding process. This assembled secondary tool once inspected for positional accuracy and parallelism was then fitted to the primary tool via a machined pocket (Figure 4) and bolted to the machine platens.

0

100

200

300

400

00:10 06:44 13:22 18:53 25:42 32:20 38:41 45:18

Time [mn]

Tem

pera

ture

[°C]

Substrate [°C]Tool [°C]

0

1

2

3

4

00:10 06:44 13:22 18:53 25:42 32:20 38:41 45:18

Time [mn]

Tool

Clo

sure

[mm

]

Fig. 2. Temperature evolution

a) Before replications b) After replications

Fig. 3. Hot embossing tool channels condition

Fig. 4. Injection moulding tool integration

Table 1. Material properties

Name PP COC ABS PC

Structure Semi-crystalline Amorphous Amorphous Amorphous

Moldflow viscosity index

VI(210)-0103 VI(260)-0058 VI(240)-0087 VI(300)-0269

Tt (°C) 150 135 90 170

3. Injection moulding experimental description

3.1 Materials

The polymers used for replication were PP, ABS, COC and PC (Table 1), all commonly used materials in IM. They were selected based on the rheology factors between the ranges of Viscosity (58-269 Pa·s) and transition temperature Tt (90-170 ºC). The viscosity considered is extracted from the Moldflow viscosity index used in Moldflow MPI 5.1 software.

One PEEK insert was used per material to perform the replications.

3.2 Process parameter

In micro IM, when considering features with high aspect and surface-to-volume ratios, process optimisation of melt/mould temperature, injection speed and holding pressure parameters are required for successfully replication. During this investigation the following four factors were investigated: • Melt Temperature (Tb)

Tb can have a range of control settings depending on the heating requirements and thermocouple elements used. The selected Tb should be correct for the material prior to leaving the nozzle and entering the mould. Each polymer has a recommended temperature range and the machine heating control ensures the set temperature provides the optimum flow characteristics. Selecting an optimum temperature window is important due to the effect on the melt flow and the possibility of preventing the occurrence of material degradation. • Mould Temperature (Tm)

For Tm, the low volume of polymer required, and the high surface-to-volume ratio that are typical in micro moulding mean that materials solidify much faster than conventional moulding. Therefore, the selection of tool temperature is important for preventing the premature solidification of the melt. By increasing the mould temperature the bulk temperature of the polymer should be kept sufficiently high to ensure the complete filling of the cavity. Previous studies in micro moulding have shown that moulds can be run continuously at temperatures even higher than the material’s glass transition temperature [5]. However, a balance between Tm and the transition temperature should be maintained at the injection stage in order to minimise the cycle time. • Injection speed (Vi)

The injection speed profile used for filling a cavity has two main effects on the selected temperature of the polymer. A high Vi can facilitate the filling of micro cavities, because the premature freezing of polymers in thin walled sections could be alleviated. At the same time it should be noted that such an increase of the polymer shear rate could result also in polymer degradation and cavity gassing. The presence of air in the cavity could also have a negative effect on the tool. In extreme cases, the build up of heat pressure and volatiles could result in etching some material out of the mould [6]. • Holding pressure (Ph).

From previous micro moulding experiments the holding pressure parameter has shown a variety of results for the filling performance. Considering a polymer insert is being used, the process effect of adding potential pressure to the cavity cannot be ignored and Ph was added to the experiment.

PEEK Insert

Secondary tool

Primary tool

Table 2. Design of experiments

PP ABS PC COC PP ABS PC COC

1 300 no 280 260 20 40 80 90

2 900 no 280 260 20 40 80 90

3 300 yes 280 260 20 40 80 90

4 900 yes 280 260 20 40 80 90

5 300 no 270 280 320 300 20 40 80 90

6 900 no 270 280 320 300 20 40 80 90

7 300 yes 270 280 320 300 20 40 80 90

8 900 yes 270 280 320 300 20 40 80 90

9 300 no 280 260 60 80 120 125

10 900 no 280 260 60 80 120 125

11 300 yes 280 260 60 80 120 125

12 900 yes 280 260 60 80 120 125

13 300 no 270 280 320 300 60 80 120 125

14 900 no 270 280 320 300 60 80 120 125

15 300 yes 270 280 320 300 60 80 120 125

16 900 yes 270 280 320 300 60 80 120 125

Tm (ºC)

220

220

220

220

220

220

220

Trial Vi Ph

220

Tb (ºC)

3.3 Plan of experiments

For each of the four materials, two levels of four process parameters were selected. The levels of Vi and Ph were the same for all materials, while Tb and Tm levels were different. The resulting experimental array is given in Table 2.

To investigate the effects of the process parameter variations on the PEEK tool life, each experiment was repeated 10 times, making a total of 160 parts for each tool.

3.4 Measurement methods

In order to ascertain the potential of the PEEK insert as a viable prototype/production tool, dedicated positions were selected on the microfluidic design (Figure 5) and the four tools were inspected in these areas before and after the experiments.

Nine variables have been selected as a guidance of tool life: 3 Wall Heights (h1, h2, and h3), 3 Wall Widths (w1, w2 and w3) and 3 Surface roughnesses (Ra1, Ra2 and Ra3).

At each position, the average roughness Ra was assessed with an optical surface profiler, using the White Light Interferometry (WLI) technique. The measurement of walls heights and channels depths was also performed using the WLI technique in close agreement with the ISO standard [7]. The width of the walls and channels was assessed with a Coordinate Measuring Machine (CMM) equipped with an optical probe, using its automatic edge detection capability.

In all three types of measurements described above, a reference coordinate systems was created on the first part measured and the different measuring positions were recorded. In order to take measurements in identical regions for the subsequent parts, these positions were set up either manually in the case of the optical profiler or automatically for the programmable and optically equipped CMM.

4. Results and discussion

The results of tool inspections and differences of measures before and after replications are presented in Table 3.

Taking into account the measuring equipments repeatability errors, only the significant measured differences are considered for discussion and are highlighted in the table (higher than; 5µm for width, 3µm for height and 20nm for roughness). In the case of insignificant differences, the tool is considered undamaged.

Ra2 Ra1

Ra3

w1,h1

w3,h3

w2,h2

Gate Runner

Ra2 Ra1

Ra3

w1,h1

w3,h3

w2,h2w2,h2

Gate Runner

Fig. 5. Measurement positions

Table 3. Measurements results Width (µm) Height (µm) Roughness (nm Ra)

W1 W2 W3 H1 H2 H3 Ra1 Ra2 Ra3PP Before 498.0 162.0 517.0 259.8 114.4 270.4 52.5 41.5 45.7

After 485.0 158.0 517.0 262.8 115.0 270.4 48.2 43.7 45.0diff -13.0 -4.0 0.0 3.0 0.6 0.0 -4.3 2.2 -0.7%diff -2.6% -2.5% 0.0% 1.2% 0.5% 0.0% -8.2% 5.3% -1.5%

PC Before 421.0 156.0 470.0 260.3 70.4 271.7 61.3 54.7 52.2After 405.0 164.0 485.0 258.3 70.3 273.0 92.2 99.5 130.5diff -16.0 8.0 15.0 -1.9 -0.1 1.3 30.9 44.8 78.3%diff -3.8% 5.1% 3.2% -0.7% -0.1% 0.5% 50.4% 81.9% 150.0%

ABS Before 493.0 156.0 521.0 261.4 127.1 263.1 76.3 107.0 90.3After 481.0 151.0 511.0 260.6 126.4 260.1 1013.3 78.0 86.8diff -12.0 -5.0 -10.0 -0.9 -0.7 -3.0 937.0 -29.0 -3.5%diff -2.4% -3.2% -1.9% -0.3% -0.6% -1.1% 1228.0% -27.1% -3.9%

COC Before 467.0 156.0 498.0 266.6 127.4 265.0 97.2 58.0 59.0After 429.0 155.0 502.0 261.9 127.4 260.0 71.5 150.0 64.0diff -38.0 -1.0 4.0 -4.6 0.0 -5.0 -25.7 92.0 5.0%diff -8.1% -0.6% 0.8% -1.7% 0.0% -1.9% -26.4% 158.6% 8.5%

4.1 Roughness measurement

For the roughness measurements there was no discernible variation in the insert after the PP replications. When processing PC, the Insert surface roughness increased at all three positions. And on inserts used to replicate ABS and COC materials variations of both lower and higher surface roughness were measured.

The increase in surface roughness could be due to surface deformation and polymer grafting then separation during injection and ejection respectively. The two identified improvements in surface roughness could be a product of abrasive smoothing from the polymer flow.

4.2 Geometry measurements

Only ABS has consistently shown significant decrease for all inspected insert dimensions, suggesting tool wear. Even though it was not run at the highest temperature out of all polymers, the ABS parts had a higher tendency to stick to the tool. ABS is a rubber modified glassy plastic made by the grafting of styrene polymers to acrylonitrile butadiene copolymer rubber. It was observed that parts were difficult to remove at the higher temperature settings.

Fig. 6. PEEK insert after ABS processing

This could be due to graft copolymers being more active at the site of the polymer molecule at higher temperatures and resulting in grafting to the PEEK polymer, thus removing or depositing material on the tool surface during part ejection [8]. This seems to be confirmed by the high variation of surface roughness measured at position 1 where ABS material appears to have been deposited on the PEEK insert (Figure 6).

Most width measurements were down, also suggesting tool wear. However, certain measurements, on inserts used to process PC and COC, showed that sizes actually increased after moulding. Though it is possible for the moulded polymer to leave a residue onto the tool surface the most likely reason is due to volumetric deformation. Moulding pressure and part ejection could cause directional forces to act in compression and tension on the tool. Elastic modulus is also different at elevated temperatures, as a result the forces are likely to increase the tools tendency to deform at such temperature. This seems to be supported by the fact the highest dimensional variations occurred when replicating the PC and COC materials, which were processed at the highest temperatures. In addition, the only significant height difference was that of h1 and h3 for the insert that processed COC. Both these heights were lower after moulding suggesting higher tool deformation due to higher processing tool temperature.

Finally, for all four materials the insert wall widths at position 1 were a lower size on average after moulding. This reduction could be due to the flow path of the polymer from the runner and gate position, and the potential for more volume of polymer to pass this area under high temperature and pressure.

4.3 Overall discussion

The PEEK insert structure did not appear to be affected when processing low temperature material such as PP, making it clearly a viable solution for prototyping. When processing PC and COC materials, the insert structures were affected with deformation of up to 8.1% and a surface roughness increasing up to 150nm Ra. When processing ABS material the tool insert appeared to be consistently worn, with up to 3.2% wall height reduction, and the roughness was highly increased in places.

Depending on the application, some of these levels of deformation could still be acceptable for prototyping purposes. In addition, some of the damage might have occurred only when using extreme process parameters and might be avoidable when using an optimised set of parameters.

Thus a more comprehensive analysis should be done, considering a comparison analysis after each test. This would provide a better understanding on the effect of each of the factors. Simulations should also be performed to show flow directions, localised heating, pressure areas and potential gassing at end of flow and weld lines.

Furthermore, some of the dimension variations were not easy to explain because the axis of deformation is difficult to predict with multiple forces. Future experimental research should incorporate a design with stand alone features where length height, width, volumetric deformation and wear can all be measured. Such an experiment should also be performed with a condition monitoring system to assess the polymer flow and condition in space and time for

each of the test conditions.

5. Conclusion

The experiment described in this paper demonstrated the potential of using polymer inserts in Injection moulding for prototyping of micro components in a wide range of materials.

Further work is required in order to better understand the effect of each processing factors and to identify the full process window in which this method can be applied. Further investigation will include flow simulations, complete condition monitoring and the use of a new part design allowing accurate geometrical, volumetric and wear measurements.

Acknowledgements

The authors would like to thank the European Commission, the Welsh Assembly Government and the UK Engineering and Physical Sciences Research Council for funding this research under the ERDF Programmes “Micro Tooling Centre” and “Supporting Innovative Product Engineering and Responsive Manufacture” and the EPSRC Programme “The Cardiff Innovative Manufacturing Research Centre”. Also, this work was carried out within the framework of the EC Networks of Excellence “Innovative Production Machines and Systems (I*PROMS)” and “Multi-Material Micro Manufacture: Technologies and Applications (4M)” and the EC funded project SEMOFS (IST-FP6-016768).

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