Replication integrity of micro features using variotherm and vacuum assisted microinjection moulding

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Abstract

Micro features on surfaces are critical for the functionality of a product, such as micro channels for a microfluidic chip and micro lens arrays for an artificial compound eye lens. In the present work, we design a series of microfluidic and microstructured surface patterns to evaluate the influence of variotherm and vacuum venting on replication integrity of micro features. By using a process monitoring system, including machine process and cavity pressure/temperature measurements, the influence of warm and cold circuit settings, and vacuum venting are characterised in terms of injection pressure, cavity pressure and mould–polymer interface temperature. These are then correlated with the replication integrity of micro features by assessing feature replication height, cross-section area and surface morphology. The performance of anti-stick coatings on a micro mould tool insert was also characterised in terms of moulding cycles. After a series experiments and optimisation, we have significantly enhanced feature replication with 100% replication of proposed features without either significant distortion or tearing, which cannot easily be achieved using conventional injection moulding. Additionally, we have successfully moulded features having good integrity and a maximum aspect ratio of up to 10 at a width of less than 8 μm over areas as large as 5.25 mm2.

Introduction

Surface micro features including channels, pillars, lenses and hybrid micro/nano structures, play a very important role for applications in microsystems and as a functional component. For example, the micro channels of a microfluidic device are critical to manipulate liquid samples. Large-area micro/nano features are key to maintaining functionality of a surface, such as its reflectivity or wettability. Besides the importance of manufacturing precision microstructured tools, precision replication of these features is also critical. To evaluate a workpiece surface, the concept of surface integrity” was first introduced by Field and Kahles in 1964. Surface integrity” was defined as the inherent or enhanced condition of a surface produced in a machining or other surface generation operation”. Microinjection moulding is one of the most commonly used mass production replication technologies and is capable of replicating micro/nano features with complex 3D geometries. Geometric replication integrity of micro/nano surface features is critical. In terms of injection moulding micro/nano features, when feature dimensions reduce to the micro/nano scale, its surface to volume ratio increases by 103–106 compared to millimetre scale features. This can lead to enhanced heat transfer and cause premature solidification, preventing polymer flow into micro scale cavities. Even when a micro pattern is fully filled, the consequence of bulk shrinkage of a substrate is that polymer micro features will shrink towards the substrate’s centre, causing intimate contact between the polymer and mould wall, increasing friction and adhesion. Additionally, adhesion of polymers to the mould could occur due to chemical reaction and physical inter-locking of polymer molecules with the mould surface. During demoulding, adhesion and friction at the polymer–mould interface can cause a feature to plastically deformation and break.

Variotherm mould temperature control and vacuum venting are considered to be promising auxiliary processes to assist filling of micro/nano features. Most work to date related to variotherm has focused on new concept development of heating/cooling methods and applied them to reduce flow-induced orientation, increase flow length and minimise weld lines, as seen from two review papers [1], [2]. Other works have applied variotherm to assist the filling of micro features. Shia-Chung et al. [3] developed electromagnetic induction heating combined with water-cooling to achieve rapid temperature control of a mould surface. The mould was designed to have a micro channel array of 30–50 μm in width and 120 and 600 μm in depth with a demoulding draught from 1/12 to 1/60. High aspect ratio features up to 12 were achieved using PMMA with a mould temperature of up to 120 °C. However, geometric integrity of the features was not examined and mould machining process can not be applied to either microfluidics or fuctionally surfaces. Keun and Sang-Ik [4] developed a selective high frequency induction heating by using different mould materials. Their results indicated that micro ridge features with widths from 40 μm to 100 μm can be filled with aspect ratios from 6 to 9 using ABS material. However, from the morphology of these micro features, we observed feature tilting and non-uniform thickness, which indicated that distortion from adhesion and friction from demoulding existed and needs to be studied for high aspect ratio features. Recently, Oh and Song [5] developed a local film based heating system for injection moulding of submicron features, where the heating area was divided into 9 zones. The progressive mould temperature setting from low to high away from the gate of the part showed better replication for 200–500 nm wide and 200 nm tall features, where feature replication was improved thanks to improved filling and more uniform shrinkage. Rytka et al. [6] compared conventional injection moulding, variotherm injection moulding, injection compression moulding and variotherm injection compression moulding for replication of a V-groove structure (height ∼123 μm, width 45 μm). It was found that variotherm injection moulding and variotherm injection compression moulding showed similar degrees of replication, where the variotherm is implemented with water heating and cooling system. Sorgato and Lucchetta [7] studied the effect of variotherm and venting on filling of 3 μm wide and 15 μm tall micro cavities. It was found that increasing mould temperature over the glass transition temperature can be significantly beneficial for feature filling, but vacuum venting has a slightly better effect on filling, which was observed to be because of rapid compression of the air ahead of the flow front and subsequent conduction of that heat into the mould surface [7], [8]. While these works show that good replication can be achieved for high aspect ratio features, they have not characterised the effect of heating and cooling of a variotherm system on feature replication integrity. Obviously, good filling without demoulding damage is critical for these micro features, which are closely related to variotherm heating and cooling. Vacuum venting can potentially remove entrapped air within the micro cavities of a tool insert. However, their effect on replication of micro/nano features are still controversial and the improvement is not significant [8]. Additionally, feature design represents local geometric confinement for a polymer flow, and is related to melt flow resistance and potential air entrapment. The previous works were all based on testing features rather than the real features with particular applications. It is important to consider these factors when designing a product.

In the present work, we design a series of microfluidic (micro wave and droplet arrays within a channel) and microstructured surface patterns (ellipse and square shaped) to evaluate the influence of variotherm and vacuum venting on replication integrity of micro features. By using a process monitoring system, including machine process measurement and cavity pressure/temperature measurements, it was possible to characterise the influence of warm and cold circuit settings, and vacuum venting in terms of injection pressure, cavity pressure and mould–polymer interface temperature. These were then correlated with replication integrity of micro features by assessing the replicated features in terms of height, cross-sectional area and surface morphology. Additionally, an anti-sticking coating was applied to the mould insert to reduce polymer adhesion to the mould. Its performance was also characterised in terms of the number of moulding cycles.

Section snippets

Injection mould, processes and materials

In the present work, a Fanuc Roboshot S-2000i 15B micro injection moulding machine is used to carry out all the experimental work. A microfluidic platform mould is used in this study: it has 22 mini-luer connectors in an area half the size of a credit card, as shown in Fig. 1. The mould tool insert is fixed to the stationary half of the mould. A water cooling and heating variotherm system is used in the present work, where both the warm and cold circuits share the same water flow channels. The

Tool insert

Fig. 7 shows the nickel insert fabricated using UV lithography and electroforming. The nominal designed depth of channels and features was 50 μm. However, because of the fabrication process of UV lithography, DRIE and electroforming, the feature depths were not uniform.

Optimisation of conventional injection moulding

We used the wave pattern designed inside of a microfluidic channel to evaluate replication integrity in the first experimental stage. Fig. 8(a and b) show the effect of selected control factors (injection velocity, holding

Conclusions

This paper studied the replication integrity of micro features, including wave, droplet, ellipse and square patterns based on variotherm and vacuum assisted micro injection moulding process. The studies show that an increase in warm circuit temperature of a variotherm system will cause an increase of cavity temperature and pressure. This is beneficial for feature replication. However, too high warm a circuit temperature can potentially decrease cavity pressure and cause adhesion related damage.

Acknowledgements

The authors gratefully acknowledge support from China Scholarship Council, Science Foundation Ireland (SFI) (Grant Number 15/RP/B3208 and 14/IFB/2719) and National Science Foundation of China (Grant Number 61675149, 51320105009 & 61635008).

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