Medical Device Link.

MANUFACTURING

Factors affecting micro moulding

The development of new micro devices is dependent on manufacturing systems that can reliably and economically produce components in large numbers. In this context, micro injection moulding is an important technology for replicating micro components on a cost-effective, industrial scale. Components manufactured by micro injection moulding fall into two categories:

• Components that are genuinely micro in overall size, that is, less than 1 mm, but typically of the order of 100 to 500 microns.
• Components that are larger in size, but the functional features are micro, that is, usually up to 200 microns in size.

Nano moulding generally falls into the second category of larger components with functional features of less than 1 micron, that is, a few tens to a few hundreds of nanometres.

Currently, many micro components and devices are successfully moulded, including optical gratings and lenses, micro pumps, microfluidic devices and micro mechanical devices such as gears. All of these are used in many medical devices. In all of these examples, the replication of the micro features is an important issue that is determined by the reliability of the manufacturing process. The success of the replication process is dependent on the component’s size, aspect ratio and surface area.¹

Size of the component. This affects the moulding, especially if high aspect ratios are required. Also, the small size of the components means that several cavities can be included in one tool and filled with each shot. If this is the case, then the runner and gating systems become important considerations that can affect product quality.

Aspect ratio. This is the ratio of feature height or depth to width; a wall or trench with an aspect ratio of two would have a height or depth of two units to a wall or trench width of one unit. The aspect ratio that is required during a replication process is determined by the possible material and process parameters and is often limited by the combinations of characteristics that are available.² Micro structures on larger components are also limited by the achievable aspect ratio and an appreciation of this is required during the product design stage.

Surface area. The surface area to volume ratio of micro parts and micro features is large compared with large component mouldings. This results in rapid cooling and heat loss from the component material as the polymer is injected through the runner system and into the component. The runner system can be made to larger dimensions if necessary, but this consumes material from a limited shot size and adds to waste. The effect of large surface area is increased by the surface roughness of the tool surfaces. It has been shown that runner and cavity surface roughness affects the quality of moulding in any given combination of product size, material and moulding conditions.^3,4 Clearly, for extremely high aspect ratio micro features or components, surface roughness can have a critical effect on successful moulding.

Temperature and injection speed

Successful micro moulding demands the use of high injection speeds and high melt and mould temperatures.^5,6 High melt temperatures reduce the viscosity of the material, which aids the moulding process providing the temperature remains high. The melt temperature can be further affected by shear forces generated in the polymer melt that is in close proximity with the cavity and runner walls. High shear forces lead to a phenomenon know as shear heating whereby the creation of high shear forces during the moulding process leads to higher melt temperatures, which aids the flow of the bulk of the injected polymer in the mould. The schematic in Figure 1 shows the typical distribution of temperature and melt-flow speed during moulding in a narrow, high aspect ratio runner and cavity system.

Figure 1: Profile of polymer melt flow in a narrow, high aspect channel (generated using Moldflow Plastics Insight 5.1 software). (click image to enlarge)

In Figure 1, the "melt cross section" shows the effect of the temperature increase induced by the shear rate (the coloured portion with red being of higher shear rate and hence higher temperature) and the areas of high shear rate located close to the cavity walls. In Region 1, the polymer chains are randomly arranged, but as moulding continues and with the drag along the walls, the polymer chains become aligned, as seen in Region 2. The high shear rate also affects the viscosity of the material. The effect of reduced velocity close to the edges of the cavity walls is to be expected. However, this has a significant effect on mould filling when extremely small cross-section runners and high aspect micro features are involved, because the bulk material volume to surface area is small compared with large object moulding. It has been found that rough runner and mould surfaces can under certain circumstances improve mould fill by creating areas of high shear heating.^3,4

The above factors must be considered during the design of the product, but also during manufacture of the mould tool. Mould tool surfaces are more difficult to control when machining micro moulds or micro features because the surface roughness can be large relative to the feature size. This is especially the case when creating nano features and entirely new mould manufacturing methods are demanded.

Moulding large components with micro features

Figure 2a: Detail of microfluidic mould tool for creating micro channels.

One of the main challenges with moulding micro features on a larger part is the manufacture of the mould tool itself. Micro fluidic devices, for example, are typical components that require micro channels moulded on to a surface. The mould tool for making these components requires precision or even micro machining capability because micro fluidic channels are represented by free-standing walls on the mould tool. Figure 2a shows an example of a mould tool in which the smallest wall feature to create a micro channel is 20 microns in width and 200 microns in height. Making such a small wall in the mould cavity and then relying on it to remain intact during the moulding process is difficult to achieve. Successful moulding relies on the correct combination of injection speed, melt and barrel temperature, melt viscosity, mould temperature and even the location of the injection gate relative to the wall feature. Figure 2b shows the complete micro fluidic mould tool of which Figure 2a is part.

Figure 2b: Microfluidic device mould tool showing location of micro channels.

Injection moulding of large nano structures is routinely used in the DVD industry. The DVD mould tools consist of simple impressions in the order of 250–500 nm in size and depth in the surface of a polymer. These are created by bosses of corresponding size on the mould tool. But in the case of DVD and CD manufacture, the process is effectively a combination of injection moulding and hot embossing. The mould tool is an active tool in which the mould surfaces can be pressed together. The start of the process is similar to normal injection moulding, but after the mould is completely filled, the cavity walls are pressed together to further define the structure in the material. Figure 3 shows a small section of a typical moulded surface of a DVD where the digital tracks are approximately 450 nm in width with track spacing of 900 nm. Again, mould and melt temperatures and injection speed are critical, but so are the mould compression forces and mould cooling.

This moulding technology clearly demonstrates that nano features are reproducible in polymers in a large volume manufacturing environment and points the way for other photonic, optical, electrical and electronic components to be reliably reproduced.

Moulding discrete micro components

Figure 3: Digital track on the surface of a DVD. The track width is 450 nm and the track spacing is 900 nm as indicated by the white lines.

As with micro surface features, moulding of discrete micro components and manufacturing the mould tools are major challenges. True micro components have large surface area to volume ratios and this dictates the high injection speeds and high melt temperatures that are required. The high injection speeds and high melt temperatures coupled with shear heating can lead to degradation of the polymer and even burning of the material or dieseling of the trapped gasses. Dieseling is the phenomenon whereby the compressed gasses that are trapped in the mould spontaneously ignite and burn, thus creating high temperature and back pressure. This produces quality defects and short mouldings (incomplete filling of the mould), which increase manufacturing costs, but can also adversely affect the life expectancy of the mould tool. This can be overcome by venting the mould, but this in itself can present difficulties such as flashing or excessive witness lines that are relatively large compared to the size of the micro component.

Figure 4: Successful (lower) and excess temperature damaged (upper) moulded parts with high aspect ratio micro features.

Figure 4 shows a complex three-dimensional (3D) moulded health-care clip product. This is not strictly a micro part because the overall dimensions are larger than 1 mm, but the sub-features consist of high aspect ratio structures of dimensions much smaller than 1 mm. The top moulded part shows evidence of material degradation and burning, which resulted from process temperatures and injection speeds being too high. The lower moulded part is a much more successful moulding. The bars of the clip feature are effectively long narrow channels along which the polymer is required to flow. This demands low viscosity, high injection speeds and high melt temperatures with resulting mould flow profiles similar to that shown in Figure 1. Finding the optimum moulding conditions for this part is critical if the process is to be reliable in a large manufacturing volume moulding operation.

Applications

There are many possible medical applications for discrete micro components and components with micro features or structures. The main challenge is matching the mould manufacturing limitations with the onerous moulding-induced material conditions. If these can be addressed, then there is no reason why true micro components and complex 3D nano structures should not be successfully replicated. Complex surgical components, biosensors, microfluidic devices, implants and optical components are just a few regular applications of micro and nano moulding in polymers.

Dr Robert Hoyle is MicroBridge Operations Manager at The Manufacturing Engineering Centre MEC, Cardiff University, The Parade, Newport Road, Cardiff CF24 3AA, UK, tel. +44 29 2087 0018, e-mail: manufacturing@cardiff.ac.uk, www.microbridge.cf.ac.uk

The MicroBridge project is funded by the UK Department of Trade and Industry and the Welsh Assembly Government.