Micro-injection moulding (micromoulding) is a new and rapidly evolving technology which allows the production of components with a scale and intricate level of detail not possible using conventional injection moulding techniques. The field has seen considerable growth in recent years as manufacturers become aware of the potential to produce components for such applications as medical implants and Micro Electro-Mechanical Systems (MEMS) for a fraction of the cost of current fabrication techniques. Introduction Process Data Measurement The machine used in this study is the Battenfeld Microsystem50, which is one of the first machines designed specifically to meet the demands of the micromoulding process. A number of innovations in the machine offer improvements over conventional injection moulding machines such as integrated clean room and handling modules, high precision shot dosing, minimal melt residence times and low material waste. The injection unit consists of a 14mm plasticising screw combined with twin 5mm diameter metering and injection plungers. The metering plunger offers very accurate dosing control and the injection plunger can attain injection velocities up to 800mm/s, which means the material is introduced into the cavity very rapidly and ensures a complete product despite the very short freezing times inherent in the process. Hopper Plasticating screw – volume ~6cm, low melt residence times Metering plunger – max shot volume 1375mm 150Bar back pressure Shut-off valve – prevents back flow of material into screw during transition from metering to injection chamber Injection plunger – max velocity 780 mm/s Nozzle – terminates at the split plane of the mould allowing sprue-less moulding and reducing material waste Rotating Moving Platen – twin mould operation allows handling/packaging during injection of subsequent shot, reducing cycle times Handling unit – incorporates machine vision optical assessment allowing in-process quality monitoring. Figure 3: Cut-away diagram showing features of Microsystem50 machine Figure 1: 0.008g Watch Gear (in production)Figure 2: 1µg 5-tooth gear on a rice grain (Accuromm/Juken) A data acquisition suite was installed on the machine to quantify the injection dynamics of the system and to monitor the process. The system incorporates the following sensors: - Temposonics R-series magnetostrictive displacement/velocity transducer attached to the injection plunger. Dynisco piezo force transducers mounted behind injection plunger and cavity ejector pin allowing injection/cavity pressure measurement. J-type thermocouples providing detailed mould temperature measurements. The sensor data was recorded using National Instruments E-series data acquisition hardware linked to a laptop PC running a custom-built application created using Labview software. Currently four 0-10V inputs and four thermocouple signals can be recorded at sampling rates up to 30kHz. Experimental A process run manufacturing stepped plaque components using a HDPE-based Hapex (Hydroxyapatite bone analogue supplied by Queen Mary and Westfield College, London) material was performed on the Microsystem. In order to assess the sensitivity of injection/cavity pressure measurements to variations in the process, three different mould set temperatures were used. The recorded data can be seen in Figure 4. Figure 4: Process data indicates cavity pressure readings much more sensitive than injection pressure readings to process variation Figure 5: Injection Pressure CurvesFigure 6: Cavity Pressure CurvesFigure 7: Product Masses Micromoulding Product Assessment One of the major challenges associated with micromoulded products is how to quantify the mechanical and morphological properties of the products. The physical size of these products negates the use of conventional weighing, mechanical property and dimensional testing methods and alternatives must be sought. Three methods are adopted at the University of Bradford: - Machine Vision, Atomic Force Microscopy and Nano-Indenting. Little work has been performed to date exploring the dynamics of the process and the influence that the high hear rates and rapid cooling may have on the properties of the end product. To address the knowledge deficit, a two year EPSRC-funded programme based at the University of Bradford has been completed which has studied many aspects of the technology including cavity manufacture, process characteristics, materials and product assessment. It is interesting to note that while the mould temperature variation has a clear influence on the cavity pressure data, there is no discernable effect on the peak injection pressure values. In conventional injection moulding, injection pressure data is often used as an indicator of variation in the process but it is apparent that cavity pressure measurement provides a much more sensitive tool for micromoulding process monitoring. Measurement of the product masses showed a distinct variation between the products moulded at each temperature. Figure 8: Handling unit and CCD camera installation; MV application; g ‘top hat’ product; CCD image Machine Vision Systems Recent years have seen a large increase in the use of machine vision applications to monitor manufacturing processes. The systems typically employ a high resolution CCD camera combined with specialised software applications which are able to identify geometric or surface quality variations of a product. The installation on the Microsystem50 interfaces with the control system and allows product quality assessment during the moulding cycle. Products which do not meet the specified quality criteria can be selectively placed in a reject receptacle for subsequent analysis. The technology is particularly well suited to the micromoulding process where the small product dimensions and rapid cycle time make visual inspection by an operator impractical and inefficient. However, the systems are only able to analyse dimensional properties and cannot provide information describing the morphology and mechanical performance of the product. Atomic Force Microscopy AFM is a surface measurement technique which has evolved since the mid-1980s. An AFM consists of a probe which holds a micro scale silicon device consisting of a flexible cantilever with a very fine tip at the unconstrained end. The tip is traversed across a sample material and the deflection of the tip is monitored using a laser and position sensitive detector. Nanoindenting Nanoindenting is a process in which a very small diamond point is driven into a sample surface at a constant rate to a specified depth. The force applied to the tip is recorded and the resulting force/displacement curve gives information which can be used to determine material properties such as modulus and hardness. Figure 9: Schematic of Atomic Force Microscope Figure 10: Microscope image of micro-milled cavity; product surface near gate; product surface 20mm downstream. Scan size – 75 µm x 75µm Surface roughness and feature replication are of great interest to the micromoulding community, particularly for medical implant applications where the surface has a large influence on the bioactivity of the product. The AFM has been used to assess the surface replication on a 25 x 2.5 x 0.25mm test plaque component. The cavity was created with a micro-milling technique using a 0.2mm cutter rotating at rpm. It was deliberately left in an unpolished state so that the transfer of cavity surface detail to the product could be assessed. Figure 10 indicates that tool scroll mark features of ~6µm were clearly replicated in the gate region, but were not as well defined downstream where the pressure was lower. The morphology of the products was examined by sectioning samples and etching the exposed surface with a potassium permanganate solution to remove amorphous material and expose the crystal structure. The resultant surface was then examined using AFM. Figure 11: Morphology variation of HDPE component ranging from the centre (0.125mm from cavity wall) to the outer surface The experiment shows that the rapid heat flow from the melt at the cavity wall prevents the formation of crystal structures but in the centre of the moulding the formation of banded spherulites is apparent. Figure 12: AFM nanoindenting tip Indenters are available as stand-alone devices, or as modules which can be installed on an AFM. Indentation cantilevers are thicker, wider, and longer than standard AFM cantilevers and are composed of stainless steel, as compared with silicon or silicon nitride, but are still able to operate as a standard AFM tip which allows an indent to be performed and then subsequently imaged by the system in scanning mode. Figure 13: Modulus and hardness values for 25 x 2.5 x 0.25mm test plaque; thermal image of fixed mould plate Analysis of both faces of a test plaque component was performed which showed a variation in mechanical properties along the length, attributed to the presence of a temperature gradient on the surface of the fixed half cavity plate, which was affecting the morphology of the product (Figure 13).