2019, May 20 ORTHOWORLD | BONEZONE
Posted in Research & Development
By Carolyn LaWell,  ORTHOWORLD’s Chief Content Officer.

Additive manufacturing (AM) requires that orthopedic device manufacturers choose from a host of technologies to post process and finish their parts to achieve the ideal surface. As the adoption of AM grows in orthopedics, we’ve noticed more supplier companies with cleaning and finishing capabilities focused on the technology. We asked three of them to speak to the successful techniques that they see in use, as well as the challenges that their OEM customers are asking them to solve related to finishing.

Our panel included:
Tommaso Beccuti, Additive Manufacturing Business Unit Manager, and Jonathan Slade, Global Medical Business Development Manager, Extrude Hone
Curtis Fox, Marketing Manager, Rösler
Francesco Robotti, Scientific Marketing Manager, Lincotek Medical (former Eurocoating)

What technology or technique do you recommend when it comes to surface finishing AM parts?

Beccuti and Slade: There is no universal answer to this question. There are many challenges, which differ from part to part. Sometimes components are less complex in shape and can be finished using more conventional techniques. Others are near net printed and conventionally machined to achieve the desired finish. However, when developing a true additive part, typically having been through a design for additive manufacture (DFAM) process, component features may not be reachable using conventional methods. Extrude Hone realizes this and has developed techniques using Abrasive Flow Machining (AFM) for internal channels, as well as non-contact electrochemical COOLPULSE for both internal and external finishing of additive components. Both of these techniques have been independently documented as exemplary methods of removing partially-sintered and bonded material from the surface.

Fox: We start with comprehensive fact-finding to help us understand the manufacturing process from start to finish, and then develop a combination of solutions with the customer. Often, the final finish on a 3D-printed part can be achieved by wet blasting the part after printing—this also removes any residual powder—and then vibratory finishing the part with specially formulated finishing media that reduces the surface roughness to the desired value.

Robotti: If the AM parts have a porous structure that favors bone integration, we recommend a cleaning method able to extract the residual metallic powder (loose beads) potentially entrapped in the lattice. We have developed and validated a specific process with high efficiency in depowdering parts (e.g. XClean®).

When it is requested to increase the osteointegrative potential of the porous structure, we advise the use of coatings like biomimetic calcium phosphates obtained by wet chemistry for infiltrating the lattice, avoiding the risk of clogging the interconnected porosity. Similar results can be gained by exploiting specific titanium oxides that can be obtained on the surface inside the pores (e.g. 3D growth®).

In contrast, if the functionality is related to smoothness, the finishing step must be good enough to transform an as-is AM surface (i.e. pretty rough) to a polished one. We have experience with tumbling, blasting, machining and polishing in combination and in the right sequence. The specific working strategy is basic. For example, the use of biocompatible lubricants or dry machining or cryogenic-machining may reduce the risk associated with post-process residuals.

What questions should orthopedic device companies ask themselves when considering their surface finishing process for their additive manufactured parts?

Beccuti and Slade: For most manufacturers, AM usually consists of turning their existing, conventionally manufactured component into an additive part, enabling them to profit from the well-known benefits of AM. When switching prints from conventional to AM production, thought should be put into whether the finishing requirements are truly needed. Most components we see usually have machining and surface finishing tolerances that mimic that of a CNC machined component. We, of course, understand that some of this is governed by standards and specifications, confidence, validations and predicate devices. However, the true benefits of an AM part become unlocked when you take a step back and really design the part with AM in mind. By combining unconventional manufacturing processes with AM, you can start to think outside the box. Do some of these external, non-critical faces really need a standardized machine finish or is this something inherited from previous iterations of the design? Are the geometrical features defined by how the part used to be conventionally machined?

Fox: Understanding the materials, production methods and final finish requirements of the part are excellent starting points. We focus on understanding the entire process. With AM parts, this is no different. Being able to understand how design and print parameters influence the printed part is key to providing solid advice to the engineer. Changing build parameters or the orientation in the build chamber can make it harder or easier to achieve the desired finish. So, bring in the expert early and don’t make post-processing an afterthought.

Robotti: A good source for questions is the combined approach between the essential requirements the surface must have to enable the device to accomplish its mission and the risks potentially associated with each of the post processes selected. For example, as long as AM is used for manufacturing a porous structure, one key question could be: How do the post processes that I need interact with the features of my porous structure? Will the processes change it? Will they damage or contaminate it? Conversely, if the functionality requested is smoothness or a tight tolerance exceeding the current capability of AM processes, the key question could be: How do you apply the post processes considering you insist on geometrically complex parts and have to be applied selectively on the surface, leaving untouched the rest?

What is the primary challenge that your orthopedic device customers struggle within surface finishing of AM parts? What advice would you offer on ways to overcome that challenge?

Beccuti and Slade: We have had many inquiries relating to the removal of partially-sintered/bonded material as well as the removal of surplus metallic powder. This is particularly relevant with osseointegration lattice structures, where partially sintered or trapped surplus powder can remain unseen. Foreign particles and particles that are bonded but could become detached over time are obviously not wanted with implantable devices. These could cause premature wear of bearing surfaces, as well as implant rejection in more serious cases.

The first steps to mitigating this are choosing the right powders, with the right quality and the best printing parameters. The better the quality of surface finish straight from the machine, the less finishing effort is required. There must also be a balance between build time vs. finishing time, because this ultimately affects the cost of production and may be important in time-critical components. There are several finishing solutions on the market to specifically combat partially sintered and free-roaming material removal. Extrude Hone has multiple technologies with well-documented results of removing this unwanted byproduct.

Fox: As exciting as rapid improvements of printing speeds, generative design and the adoption of metal AM as a production method are, producing AM components at high volumes becomes a challenge. As AM production volumes increase, companies will demand post-processing solutions that are optimized and automated. This will require bringing an experienced expert to the table early in the design and development phase. It will also require finishing solution providers to stay ahead of the curve to provide new, innovative solutions for AM parts.

Robotti: Customers realize that near net shape components, as made by 3D printing, very rarely match with the requirements of the ready to use components. The gap can be solved with an array of technologies; however, most of the time, the real issues remain in quality and logistics. To operate with validated post processes for AM parts is not trivial. Delivery time and costs can greatly increase moving the parts point to point to apply all of the finishing steps requested after 3D print.

One of the challenges is to decide if it is more efficient to prevent the contamination of your AM device or let it be contaminated during the work, having understood how to clean it after. Outsourcing some post-processing may result in convenience due to the number of options available. Flexibility, scale experience and delivery time have an important value in this business. If you outsource, we advise interfacing with vendors that are able to offer a one-stop shop solution (e.g. 3D print along with the following finishing steps), or that have the industrial flexibility for quickly tuning with the customer operations.

Another key post-process challenge is cleaning. There are many solutions available. A key point in our offering is the capability to select and adjust the cleaning for the specific process flow and for the specific part, and then validate.

Carolyn LaWell is ORTHOWORLD’s Chief Content Officer. 

Image courtesy of Rösler