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This article was published in NS Medical Devices
Micro abrasive waterjet machining has become more widely used in the manufacturing of medical components. This method’s ability to process virtually all materials with a non-thermal system, that both preserves the material properties and produces fine surface finish with little or no need for finishing processes, offers qualitative and economic opportunities for medical device manufacturing. It is for these reasons that Finepart offer machines that utilise the technology.
Traditional abrasive waterjets (AWJ) have proved themselves to be very versatile tools with a widespread field of applications. Micro abrasive waterjets (MAW) are a more recent – and not yet widely known – development that have brought waterjet technology into the fine mechanic’s field. Both methods are based around the same process, but MAW is on a significantly smaller scale.
A fine high-speed waterjet, generated from ca 4000 bar (58ksi) accelerates hard particles (abrasives – typically from garnet or alumina) which are focused into a stream which, upon interaction with the target material, cause very localised material removal through particle erosion.
To obtain precision parts, the fine cutting jet must be installed in a machine tool capable of obtaining a very precise path motion. A state-of-the-art micro abrasive waterjet system has jets that offer a tool radius down to 0.1mm (jet diameter 0.2mm) and can produce parts to a tolerance down to +/-0.01mm (10μm).
Cut surfaces have no change in physical properties and are typically smooth and burr-free, with an Ra value down to ca 0.8μm depending on material.
Applications where this process may offer opportunities and solutions are diverse as the process can take on a vast variety of material, from soft biological tissue, rubber and plastic polymers, advanced metal alloys, fibre-reinforced materials, powder materials and ceramics. Also combined materials, such as sandwiched or compound materials, can be cut.
The erosion process provides the same jet ability to cut all these materials without changing its settings, adopting the cutting speed becomes the main factor to obtain the part surface quality and tolerance requirements. This fact provides an advantage in a relatively simple learning curve for new users.
NiTi component production could be a major market
Swedish micro waterjet manufacturer Finepart Sweden AB is a pioneering company in this market, dedicated to the development and marketing of very precise micro abrasive waterjets. Their Finecut machines have been offered worldwide since 2009 and are used in a wide range of applications, typically cutting exotic materials. A notable trend lately is Finecut-machines being employed specifically for NiTi applications, typically cutting from 0.3 to 5mm sheets. These applications account for more than 50% of the machines sold in 2021 and 2022. Most of these are installed in medical and dental applications, but the energy sector has applications as well.
What is Nitinol (NiTi)
Nitinol is an advanced alloy which offers interesting capabilities. Nitinol exhibits excellent super elastic 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 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
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 FAW200TM). 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 USD 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 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 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.
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.
BEYOND CUTTING EDGE
Finecut Micro Waterjet Series
Easy to operate 3, 4, and 5 axis micro waterjet machinery for high precision cutting advanced material parts with ultimate edge and surface finish in one single process.