Machining of materials for medical applications has moved to a new level of precision. In parallel with this, molding technology has improved through the increased use of sensors in moulds, enhanced design simulation and processes such as micromolding. High cavitation moulds have been used for many years to mold plastic medical components in high volumes. Close-looping the injection process achieves improvements in cavity-to-cavity repeatability in the order of 40%. 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 �m to 30 µm, across all 32 cavities.
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 difference between this and a conventional overmolding 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 overmolded needles have been produced and minute amounts of polymer as small as fractions of a gram can be reliably injected.
Gates for micromoulds can be less than 60 µm in diameter. This is important to minimize 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.
For molding channels or upstands that require dimensions of a few microns or less, a number of micromolding processes can be utilized. The injection mold 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 molded without the distortion that may be caused in a conventional injection molding process. This type of process can improve definition in microfluidics and diffractive optics.
Glass optics has important advantages in certain areas. 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 mold 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 molded part. The range of optical plastics available has increased considerably to give increased flexibility to designers. Polymers designed to transmit UV light without degradation are used in blue laser systems.
Complex 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 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 miniaturization and advances in technologies such as optoelectronics, it is advisable for designers weigh all available possibilities.
(Source Courtsey : Paul Glendenning )