Medical Device Link.

MANUFACTURING

Improved accuracy and repeatability

Figure 1: Component featuring a 20-µm2 hole mesh moulded in a fully closed-loop multicavity mould tool.

High cavitation moulds have been used for many years to mould plastic medical components in high volumes. Typically, these include hot runner systems, which keep the polymer molten for a certain distance through the feed system, thus reducing pressure loss and economising on material use. More sophisticated systems also use pneumatically operated valve gates to open and close the gating to the mould cavities at a specific moment in time.

By using recently developed methods to close-loop the injection process within the mould tool, improvements in cavity-to-cavity repeatability in the order of 40% can be made. This is combined with a significant improvement in the accuracy of the moulded part. For example, in a 32-cavity mould, the thickness accuracy near the end-of-fill for a part, which is 40 mm long and 1.5 mm thick, could be improved from 100 μm to 30 µm, across all 32 cavities. This has valuable implications for the product designer and the producer. Figure 1 shows a medical device component moulded in a fully closed-loop multicavity mould tool.

The long-established use of sensors in precision moulds to feedback pressure and temperature information to the machine has been adapted to create the above technology. Usually these sensors have been used for in-process monitoring or for feedback to the injection machine to control the point of switchover to holding pressure. Recent developments allow information from temperature sensors placed near the end-of-fill in multicavity moulds to be used to adjust the temperature of the valve gates, which directly control injection into the mould cavity. The advantage of this is that the fill profile can be matched automatically for every cavity in the mould to within a fraction of a second. This results in a dramatic increase in cavity-to-cavity and shot-to-shot repeatability.

Combining dissimilar materials

Figure 2: Injection moulded micro-fibre optic connector. The part features a diameter of 0.6 mm, overmoulded steel pins and 0.2-mm diameter through holes that are 1.5-mm long.

The part shown in Figure 2 has been demonstrated running in an automated cell at recent trade exhibitions. Metal pins of 0.6-mm diameter are picked up by a robot and placed into the mould, which rotates on a turntable into the injection position. The molten polymer is injected around them, the turntable carries the moulded part out, the robot demoulds the parts and places more pins for overmoulding. This is done using two moving halves of the mould so that moulding and demoulding/insertion can be done simultaneously.

The difference between this and a conventional overmoulding process is the level of precision that is involved. Mould design and accuracy for this type of process is critical for efficient operation. Medical components such as overmoulded needles have been produced and minute amounts of polymer as small as fractions of a gram can be reliably injected.

The process can be extended to cater for overmoulding of other materials such as different polymers, filter membranes and pharmaceutical compounds. This opens up a range of possibilities for the designer in terms of combining materials in medical device or drug delivery manufacturing. Materials that can be processed by injection moulding include bioresorbable polymers, X-ray opaque materials and many grades of engineering plastics approved by the United States Food and Drug Administration. It should be noted that various factors need to be considered when combining materials in the process, including temperature compatibility, thermal expansion and processing window.

Creating microscale cored holes and injection gating

The 1.5-mm long through-holes in the micropart shown in Figure 2 are straight holes of 0.2 mm in diameter and square holes of 0.3 mm per side. It is practical to mould this type of feature in mass production because of advanced microelectrical discharge machining technology (microEDM), innovative mould design and the latest computer numerical control (CNC) machining technology. These allow the reliable operation of a micromould in which pins of 0.2 mm in diameter repeatedly locate into the opposing half of the mould for support and the parts are ejected off without damage.

Figure 3: Clip for fluid delivery. The part features a hole of 0.32 mm in diameter that runs through the component at a compound angle.

On larger parts, small diameter cored holes are still possible. The part illustrated in Figure 3 features a hole that is 0.32 mm in diameter and 7-mm long, which runs through the component at a compound angle. In microcomponents, moulded filters have been produced with filter holes as small as 40 µm.

Gates for micromoulds can be less than 60 µm in diameter. This is important to minimise the gate vestige left on microparts and to allow automatic degating in the process via three-plate mould designs. Examples of micro-parts moulded using auto-degating include bioresorbable staples and X-ray opaque probes with tip diameter of 0.15 mm.

Sometimes it is beneficial to leave microparts on the runner for assembly purposes, but direct gating has the additional advantage of minimising the feed system so that the amount of polymer material used can be reduced. Biocompatible materials are expensive, thus, this helps to keep part cost lower.

Surface structures of submicron accuracy

Single point diamond turning (SPDT) can be used to cut aspheric form accuracies on metals of less than 100 nm and achieve surface roughness of less than 1 nm Ra. This process has evolved into micromilling using diamond end mills, which allow the machining of highly accurate three-dimensional (3D) structures.

The SPDT process reaches a limit when surface structures below 10 µm in size need to be machined. At this point, more specialised processes such as electron beam and focussed ion beam machining need to be used, by which 3D submicron structures can be accurately produced. Nickel replicated surfaces taken from master patterns made by these processes can be taken and incorporated into injection moulds.

For moulding channels or upstands that require dimensions of a few microns or less, a number of micromoulding processes can be utilised. For example, the injection mould can be heated to the same temperature as the injected polymer and then allowed to cool to the ejection temperature of the material. This allows the microscopic features to be moulded without the distortion that may be caused in a conventional injection moulding process. This type of process can improve definition in microfluidics and diffractive optics.

Figure 4: Micromoulded fresnel lens with submicron surface structure. Atomic force microscopy image of a mould insert section; lenses pictured next to a mm scale, and a 3D image of the moulded surface by optical profiling.

The part shown in Figure 4 is a micro fresnel lens that has accurately profiled surface features of 0.2–0.8 µm in height. The original surface structure was made by electron beam machining, replicated in nickel, and incorporated into a mould for microinjection moulding in a four cavity tool with automatic degating. The cycle time for the moulding process was less than 6 s.

The increased potential of polymer optics

The advent of solid state lighting (light emitting diodes) is revolutionising the way that lighting systems are designed for almost every application. In parallel, miniaturisation of cameras using complementary metal oxide semiconductor (CMOS) imaging technology has enabled radically smaller imaging systems to be developed.

Glass optics still have important advantages in certain areas. Yet, in many cases polymer optics provide an ideal way to manage light output from solid state devices or to create miniature lenses for image sensors and laser diodes. The advantages of polymer optics include relatively easy replication of complex surfaces from a mould with a high level of accuracy; lower cost production for large quantities compared to glass; and increased design flexibility through the opportunity to integrate mechanical features into the same moulded part. The range of optical plastics available has increased considerably to give increased flexibility to designers. For example, polymers designed to transmit ultraviolet light without degradation are used in blue laser systems.

Well established software is now available to the designer that will predict the behaviour of the light through sequential and nonsequential ray tracing by taking into account the optical properties of the specific grade of plastic and the characteristics of the light source. In addition, mould filling software can now be used to predict birefringence in moulded components, which becomes important in certain imaging optics.

Complex “freeform” optics can now be directly machined by single-point diamond cutting onto nickel and nonferrous alloys. Special processes are also available allowing direct diamond cutting of steel. The machinery available allows the output from an optical design package to be directly fed into the machine control software.

Figure 5: Injection moulded optical lenses. The three small lenses are for application in CMOS imaging devices and are produced with an overall form error of less than 3 µm and less than 1 µm in the clear aperture region.

Metrology equipment for optics has also advanced. Microscale surface features can be measured by noncontact methods. Error measurements from profiling systems can be fed back to machinery to enable compensation cutting of mould inserts to offset the effects of shrinkage and distortion. Examples of optical parts made by injection moulding include mega-pixel camera lenses such as the small lenses shown in Figure 5. Form accuracy of the 3-mm diameter camera lenses is less than 2 µm.

Assess the expanding options

The designers of medical devices have access to a range of recently developed and continuously evolving processes for the manufacture of plastic components, high precision machined metal parts, and components containing multiple materials. Many of these processes can achieve excellent accuracy combined with a high level of production efficiency. In addition, with new levels of miniaturisation and advances in technologies such as optoelectronics, it is worthwhile for designers to look carefully at what is now possible and be open-minded about the opportunities provided.

Paul Glendenning is Business Development Manager at Micro Systems (UK) Ltd, 101 Golborne Enterprise Park, Warrington WA3 3GR, UK, tel. +44 1942 290 960, e-mail: paulg@microsystems.uk.com, www.microsystems.uk.com

Micro Systems (UK) Ltd will be exhibiting at Compamed in the IVAM High Tech for Medical Devices area, stand H29/2.