墻板加工機床總體及夾具設(shè)計

墻板加工機床總體及夾具設(shè)計,加工,機床,總體,整體,夾具,設(shè)計 Advances in Laser-Based Micromachining遼寧石油化工大學順滑能源學院 New micromachining techniques have been developed that provide greater accuracy in the production of metal and polymer devices such as stents.Wavelength and materials For the design engineer who wants to use lasers in a micromanufacturing process, the two main characteristics to consider are wavelength and pulse width. The wavelength of light emitted by a laser determines to a large extent the quality of the micromachining process. When light from a laser encounters a material, the light that is not reflected from the surface enters the material where it is absorbed or transmitted (Figure 1). The wavelengths emitted by common laser sources range from 157 nm in the ultraviolet (UV) end of the electromagnetic spectrum to 10 m in the infrared (IR) region. The most common lasers and the wavelengths they emit are shown in Figure 2.The majority of materials employed for medical devices are metal or polymer based. Metals are most efficiently processed by melting; historically, CO2 lasers, which operate at a wavelength of 10.6 m, have been used to process metal-based devices. However, because metals absorb more light at shorter wavelengths, lasers that operate in the 1-m range have become more prevalent. Initially YAG (yttrium aluminium garnet) lasers obtained a lot of usage, but more recently1-m wavelength fibre lasers, so called because the optical fibre that transmits the laser beam also acts as the gain medium, have almost monopolised the metal cutting market.Figure 1: Absorption of light in a material.Polymers can be processed by melting, but are most efficiently processed by direct breaking of the interatomic bonds that hold together the chains. This reduces the heat load into the material, which greatly improves process results. Ideally, the energy of a single photon will be strong enough to break an individual atomic bond. Given that the photon energy is inversely proportional to the wavelength, it follows that the lower the wavelength, the stronger the bonds that can be broken. Fluoropolymers are a good example of materials that are hard to machine without a low wavelength. Because of their high bond energy, an excimer laser operating at 193 nm is necessary to produce the required results.In the capability range, diode pumped solid state (DPSS) lasers lie between fibre lasers and excimer lasers and, as such, can be used to machine a wide range of materials. Although they never give the high throughput of a fibre-based melt process or the quality of cut in a fluoropolymer delivered by a 193-nm excimer, they can produce excellent results as shown in the image on the opening page of this article.Why pulse width mattersAll lasers heat up material during machining; there is no such thing as a cold laser process. Metals must be melted, which means temperatures of at least 1500 K are required. Polymers are machined by breaking the interatomic bonds, which leads to individual atoms with high kinetic energy. This kinetic energy can be deposited as heat into the polymer material. The area of heat damage in the bulk substrate is referred to as the heated affected zone (HAZ).Figure 2. Laser wavelengths and laser processing techniques matched to their sources.The laser is the heat source and for a pulsed laser the heat builds up during the pulse and decreases quickly after the pulse is finished. Therefore, one way to keep the HAZ to a minimum is to have the pulse width of the laser as short as possible. Figure 3 shows the pulse widths for a number of different laser sources. Because metals are machined primarily by heat, long pulse widths are required. Other materials such as polymers, which react badly to heat, require shorter pulses, usually in the nanosecond range.Laser processing methodsThe range of laser processing techniques are matched to particular laser sources for ease of reference in Figure 2. Different laser sources are more suited for different processing methods and this also determines the materials that can be processed. For interventional products such as coronary stents and associated delivery devices, stainless steel has historically been the material of choice. In the past decade other precious metals such as gold, and alloys such as nitinol have become popular because of their particular mechanical properties.Fusion cutting. Metals require melting, and a technique known as fusion cutting is the most common method of laser micromachining this type of medical device. In this process, gas is fed co-axially with the laser beam onto the surface of the material. The gas assist has a number of uses. First, it can be used to control the cutting process; for example, using oxygen to increase the speed ofstainless steel cutting and argon to prevent the oxidisation of nitinol. Second, it can reduce the build up of debris by blowing the melted dross away from the part. Third, it can cool the part and reduce the overall HAZ.Mask projection. Polymer materials, which require excimer lasers, have utilised a process of mask projection, whereby a particular shape is imaged by the laser onto the surface of the part and the feature machined. Fluid access ports for catheters and holes in embolic filtration devices are commonly machined this way. Although suitable for polymer processing, excimer lasers are expensive to run and require frequent servicing. The method in which the pattern is mask projected onto the device also limits the thickness of the materials that can be processed, which can lead to constraints for device designers.Figure 3: The pulse widths of different laser sources.Techniques such as gas-assisted fusion cutting and excimer mask projection have played an important role in the development of existing medical devices. However, as the next generation of laser sources reaches maturity and the range of materials open to designers extends, new methods of laser micromachining are being developed to increase the functionality of devices as well as lower the cost of manufacturing.Remote laser processingIn fusion cutting and excimer processing the beam is stationary and the part is moved. A computer-aided design file is fed to the control system, which moves in stages under the laser beam to achieve the desired cut shape. The high mass of the stages limits how fast the part can move and expensive control systems are necessary to ensure the quality of the cut part.Figure 4: Polymer and metal parts machined by remote cutting.Remote laser processing is a technique whereby the laser beam is moved over a stationary part. By using low mass steering mirrors, it is possible to move the laser beam at speeds up to 10 m/s, without any loss in positional accuracy. Historically, remote laser processing with galvanometers was utilised to cut shapes in flat sheet materials. Now a technique has been developed that allows remote processing to be used on tube devices to produce features such as the highly flexible, interlinked joints shown in Figure 4. Short-pulse pico- and femto-second systems are suitable for remote cutting; the technique can also be adapted for diode pumped, fibre and even CO2 laser processing.Off-axis cuttingStandard tube cutting systems are set up to translate and rotate the part under the laser and are usually limited to two axes. This restricts the design of devices because the laser is always cutting on axis towards the centre of the tube. The addition of a third axis allows the part to move away from the centre of the tube and off-axis laser cutting is possibleOff-axis cutting allows subtle differences in the cut profile to be obtained and these open up new design possibilities for medical devices. Figure 5 shows an example of a device, where the maximum bending angle has been increased by off-axis cutting. Although up to four axes have been a common feature in control systems for many years, it is only recently that this technique has gained interest in the medical device sector.Although off-axis cutting is compatible with standard fusion cutting machines and only minor upgrades are necessary to bring the extra axis into use, it is highly suitable for remote processing using a scanning galvanometer, where the mirrors can move the beam off-axis very quickly and without the need for an additional stage.Real-time part alignmentFigure 5: Cut profiles of on- and off-axis cutting, with an example of an off-axis part. The device shown here is reproduced with the kind permission of DEAM Corp., Netherlands.For fusion cutting, it is important to accurately control the position of the laser beam on the surface of the part. For devices such as stents, feature sizes are largely determined by the accuracy of mechanical stages and the dimensional variation in the tube stock. The cost of mechanical positioning stages increases substantially as stage positional accuracy and repeatability improve. High precision stages used for traditional stent cutting operations can cost approximately US$50,000.To provide improved positional accuracy at a lower cost, a real-time part alignment system has been developed, which replaces expensive stages with lower cost computing power. An optical-based system is used to track the position of the part relative to the laser beam and provide feedback to the control system to compensate for any positional errors.Future growthAlthough metals such as stainless steel currently make up a large part of the medical device market, future growth lies in the more difficult to machine materials such as speciality polymers and metal alloys. Traditional laser machining technology is not equipped to process these materials in a 24/7 production environmentnew methods are needed.Figure 6: Polymer stents machined using remote processing and a real-time alignment system.Combining techniques such as remote processing and real-time alignment allow the next generation of materials to be processed. One application of this combined approach is the machining of polymer stent devices, which require a high degree of accuracy without any reduction in quality. An example of a polymer stent, which was machined by a Blueacre Technology production tool, is shown in Figure 6. With an outside diameter of 1 mm and strut widths less than 50 m, these devices are a good demonstration of what can be achieved using these new techniques.Talk to expertsLaser micromachining is a complex area and it is important to obtain from experts in this field a full understanding of its benefits and how new techniques can be implemented in cost-effective ways. Discussions should start at the early stages of device development on how the laser will interact with different materials and the potential alternatives that are simpler and cheaper to process to thereby reduce the cost of the device and increase profit margins.7