APPLICATION FOCUS: POLISHING
POLISHING 3D PRINTED MEDICAL IMPLANTS
Laser polishing could serve as a resourceful alternative to traditional methods for finishing additively manufactured parts
Metal powder-based additive manufacturing has continually grown in popularity as a production process in recent years due to the flexibility and design freedom it offers. Such benefits have led to the process being used to produce medical implants. It enables implants suited to a particular patients’ needs to be designed and manufactured rapidly – vital if a short turnaround time is needed, should the patient require the implant for surgery. Unfortunately, this flexibility
comes with the disadvantage of having a rough, low-quality surface finish, leading to a number of post- processes having to be carried out before the implant is ready for use. Dental implants are often finished using electrochemical polishing. While this technique has the advantage of being high- throughput, especially considering the intricate design of dental implants, it does require hazardous chemicals – and therefore their storage and disposal. It is also a non-selective technique, meaning
the whole dental implant must be polished. This is not desirable, as while the abutment of the implant must be smooth to prevent bacterial growth on the gum, the crown- retaining part must be kept rough to enable bonding of the crown. This requires additional post-processing to reintroduce the roughness. Electrochemical polishing is also not suited to thin, smaller parts, as it can damage them. For cranial implants – large,
slightly rounded structures – due to their freeform nature and thinness, mechanical polishing is currently used to finish them. This is a semi- manual process that takes a skilled worker many hours and numerous process steps to complete. It’s also not suited to intricate parts. In collaboration with Renishaw,
researchers at Heriot-Watt University in Edinburgh, Scotland are developing laser polishing techniques (see figure 1) that negate the downsides of such polishing methods. They are targeting parts produced using selective laser melting built from either titanium alloy (Ti-6Al-4V) or cobalt chrome (Co-Cr). In recent work, the partners have developed optimised laser polishing parameters using these two materials to provide a high-quality surface finish at reasonable process rates. ‘Laser polishing works by using a high-power laser beam to focus onto a metal surface and create a localised melt pool,’ explained Mark
McDonald, a PhD student at Heriot- Watt’s Institute of Photonics and Quantum Sciences. ‘The surface tension of the melt pool pulls the surface together, and as the laser scans across the surface, the metal cools down again and resolidifies as a nice, smooth surface.’ As with other laser processes, one of the advantages of laser polishing is that it can be automated. In addition, with the polishing occurring via the melting and cooling of a metal surface, no waste products are produced that later require disposal. The process can also be used selectively to polish intricate shapes of different sizes, as only the parts touched by the beam are polished. Laser polishing also has minimal impact on the surface it is treating, introducing no structural defects.
Weaker focus, stronger results According to the researchers, the optimisation of laser polishing parameters for various materials has generally been carried out on planar surfaces, in contrast to the actual parts produced using additive manufacturing that are typically not flat and hence have significant height variation across the surface. Using a typical focusing lens and a scan head setup (figure 2b) the spot diameter (and hence the power density) of the incident beam on the surface will be strongly dependent on this height, unless active focus variation compensation is introduced.
‘To combat this we designed a
telescope system that creates a “weakly focussed” beam with low divergence, which provides a more consistent energy density,’ said McDonald. This system (figure 2a), which
used a 100W CW fibre laser, proved much more advantageous than a scan head and focussing lens system, enabling polishing without required additional focus control. To compare the performance of
this setup with the focusing lens plus scan head setup, a cobalt chrome cranial implant plate was split into four quadrants, as shown in figure 3. The scanning speed of the laser beam was 16.6mms-1
in
all cases, calculated to maintain an energy density of 1.5kJcm−2
.
A hatch spacing of 280μm was used to maintain a 30 per cent line overlap. For each laser polished region,
the surface height map was measured in two regions: bottom, corresponding to the section of the cranial implant where the scan head setup provides a 400μm beam diameter, and top, where the beam diameter from the scan head setup incident on the sample is smaller. Comparing the bottom and top regions for each quadrant shows that when the scan head is used, the top regions are less smooth compared to those polished with the telescope system.
Dental implant polishing To polish cylindrical parts such as
in association withIn association with
Figure 1. The laser polishing process 24 LASER SYSTEMS EUROPE SUMMER 2021
Figure 2. (a) the weakly focused telescope setup and (b) the scan head setup
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McDonald et al.
McDonald et al.
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