Company insight
NiTi-stent cut flat from 0.4 mm NiTi-sheet. Here, a stainless steel sheet supports the structure while cutting.
and shape memory properties as well as biocompability. This means it can be “programmed” to retain a certain shape, or alternatively its super-elastic properties can be used where its resistance to deformation make it very useful for very fine structures that need a reliable spring back function. NiTi is difficult to machine by traditional methods. Laser can be used, but the region adjacent to the cut will have compromised mechanical properties that affect strength and cause fatigue. At the surface, a brittle recast layer of molten material can include micro cracks. Parts need time-consuming post-processing to remove these layers of material to ensure proper function of the component. Wire EDM basically share the same thermal problems. Femto-second lasers is an alternative method that operates with ultra-short pulses of very high power. This creates near instant removal by ablation where material is turned directly to plasma and no recast layers or heat-affected zones occur, which eliminates the above- mentioned post-processing. Manufacturers of NiTi consider waterjet technology to be excellent in terms of its ability to cut NiTi, but often see it as a rough process. This
Function principle of a micro abrasive waterjet cutting head generating the particle stream.
operation. Very different from the advanced metal alloys, piezoelectric materials like Lead Zirconate Titanate (PZT) are another interesting example where the MAW process can provide solutions. Fabricated from sintering methods, these materials present a hard but brittle polycrystalline layout which makes it difficult to cut with conventional methods. Diamond saws or core drills impose limitations on what geometries can be produced and laser can cause spallation and microcracks from thermal stresses. Micro abrasive waterjets, however, can obtain good surface finish and adds an ability to cut complex geometries. Sintered materials that include electrically isolated layers can be cut without smearing or melting at the surface maintaining the integrity of the separate layers.
can be helped with micro abrasive waterjet capabilities for fine mechanic manufacture. Surface integrity and precision can be obtained and the same machine tool can be powered up for thicker materials as well using larger nozzle sizes.
Micro abrasive waterjet applications
A cut surface on a PZT component including layered materials. The micro-sized removal process eliminates risk of electrical shortcut between layers.
An example of an advanced product is the vascular clip for laparoscopic surgery. The component was cut from 0.9mm thick NiTi-sheet. The clip has a spring- suspended toothed contact area on each jaw and features an interlocking latch mechanism that can keep it in a closed state. The geometry has thin spring sections that are 100µm wide and the part includes tolerances of +/-10µm. As the geometry has a very narrow opening and radii approaching 100mm, a 200-micron diameter jet was used (Finecut FAW200). The process is relatively swift and the total time to cut one clip was one minute and thirty one seconds, which sums up to a machine running cost of 50 cents per clip per piece in power, consumables and preventive maintenance. Stents are another NiTi application where the ability to cut fine features with no thermal impact can be used. As opposed to wire EDM, the many holes do not make the process more complex. The micro abrasive jet quickly pierces through the material and cuts the contours in one
Medical Device Developments /
www.nsmedicaldevices.com
Versatility and productivity The ability to cut almost any material to high tolerances makes the micro abrasive waterjet process ideal for prototyping. A wide range of options are available to facilitate advanced machining task. These include three, four or five- axis manipulation providing capability to produce complex geometries and smart fixturing solutions, sometimes using simultaneous axes to manipulate the part. On-board measuring devices can measure to a few microns in the machine. These are either tactile probe or video-based and are often used to verify setup locations of parts or fixtures. For some applications workpiece material challenges may mean that waterjet is the only – or at least most cost-effective way – to produce a part. Micro abrasive waterjets are also used in series production. Finepart currently has one US customer cutting NiTi components in six parallel Finecut machines, running in two shifts. These machines are equipped with automated spent garnet removal systems and chillers to keep constant temperature. The developments for production capability confirm that micro abrasive waterjets have become a mature manufacturing technology that should be taken into account by design and manufacturing engineers. ●
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