Remedies for a fine part finish

In this article Colin Spellacy, head of sales at surface finishing specialist Rösler UK, analyses how mass finishing and shot blasting can be used when processing medical devices and components.

In the demanding field of medical device manufacturing, the use of mass finishing and shot blasting technologies are critical in ensuring the production of components that adhere to the strictest standards of safety, reliability, and operational efficiency.

These processes are not merely steps in the manufacturing sequence but are critical technologies that ensure each component achieves a level of surface integrity that is extremely important in medical applications.

Additionally, the use of an array of versatile mass finishing and shot blasting processes indicates the medical industry's commitment to excellence, underpinning the reliability and safety of medical devices that significantly impact patient care and treatment outcomes.

The mass finishing process is a mechanical-chemical technology for the surface finishing of a wide range of parts and components made from metal, ceramic, or plastics. Whether tumbling, vibratory finishing, or centrifugal finishing, mass finishing machines are filled with various abrasive media that work through mechanical forces and chemical reactions to achieve the desired surface quality on components.

This approach is particularly effective for refining small to medium-sized parts characterised by complex shapes and geometries. By ensuring uniform treatment across all surfaces, mass finishing techniques eliminate microscopic irregularities and potential bacterial harbourage sites, significantly reducing the risk of infection or discomfort upon patient contact.

Tailored processes

A range of different mass finishing processing methods can address the complex finishing requirements for medical devices and components. These different processing methods are tailored to the respective stage of manufacturing and the material being processed, which when looking for example at orthopaedic implants may include stainless steel alloys, cobalt and chromium alloys, titanium, or plastic.

However, when looking at prosthetic and orthotic devices, non-ferrous light metals and special plastic materials such as carbon fibre are typically used. It is because of this significant range of materials that the versatility of mass finishing is so important.

In most instances, medical devices will require a number of finishing operations. First of all, parts need to be deburred. This is accomplished through the use of specialist mass finishing machinery and grinding media, which work in unison to efficiently eliminate rough edges and protrusions.

Next, it is necessary to grind and smooth the surfaces. The process of eliminating surface imperfections from medical workpieces, which often result from various shaping or machining operations, is a critical step in ensuring the highest quality of the finished product. These imperfections, if left unaddressed, can significantly affect the performance and aesthetic appeal of the component or device.

To counteract these detrimental effects, mass finishing is employed to not only remove these flaws but also to drastically reduce the surface roughness and prepare it for any further enhancement processes.

Grinding and smoothing not only rectify errors; they lay the groundwork for more advanced finishing techniques such as dry polishing and anodising, which adds a durable, corrosion-resistant layer.

Dry polishing through the use of mass finishing can achieve exceptionally high gloss finishes on workpieces without leaving behind any micro scratches. This is made possible through the use of polishing media that is specifically tailored to match the material of the workpiece being polished.

This ensures that the abrasive action is both effective and gentle, preserving the integrity of the workpiece while elevating its aesthetic appeal. Dry polishing can produce Ra (Roughness Average) values of less than 0.02µm or Rz (Average Roughness Depth) values of less than 0.15µm, indicative of a surface that is virtually flawless.

Other mass finishing processes that can be used to affect the surface finish of medical components include ball burnishing and vibro peening. Ball burnishing is again used to smooth a workpiece's surface, enhancing its appearance and mechanical properties. It involves the use of small, hard balls – typically made of steel or carbon – that roll over the surface of the workpiece under pressure.

The pressure compresses the surface layers of the material, reducing surface roughness and imparting a high gloss finish. Ball burnishing can also work-harden the surface of the workpiece, improving its resistance to wear and fatigue. It can achieve a mirror-like finish without removing material from the workpiece, making it distinct from abrasive polishing techniques.

Finally, vibro peening is used to improve the fatigue resistance of metal components and subjects the workpiece to repetitive impacts from small media agitated by vibration. The impacts induce plastic deformation on the surface and near-surface layers of the material, introducing compressive residual stresses and work-hardening the surface. This treatment reduces the risk of crack initiation and significantly improves the workpiece's fatigue life.

Shot blasting

Shot blasting is a robust finishing method where abrasive particles are propelled at high velocities towards the surface of a component. It is a versatile process which is well suited to tasks ranging from surface cleaning and peening – which increases the material's strength – to texturing surfaces to enhance the bonding of subsequent layers, such as paint or coatings.

Shot blasting is critical not only for aesthetic improvements but also for functional enhancements of parts. In the medical device industry, it is employed to prepare surfaces of surgical tools and implants, ensuring they are free from contaminants and possess the necessary surface characteristics for safe and effective use.

The widespread adoption of shot blasting across diverse medical applications underscores its significant role in enhancing the quality, safety, and performance of products by providing a meticulously prepared surface for further processing and finishing steps.

For medical device and medical component manufacturers it can be used to achieve a variety of objectives. For example, it is a highly efficient way of rapidly removing residual sand and ceramic layers from workpieces post-casting, employing high-velocity steel or metallic pellets to clean and texture the surface for further finishing processes. This technique not only eliminates surface defects but also enhances the workpiece's structural integrity.

Shot blasting also eradicates forging scale and oxide layers that accumulate on workpieces during forging and subsequent heat treatment operations, ensuring a clean and uniform surface, and with the right blast media all kinds of surface imperfections can be eliminated.

Also shot blasting tends to leave a matt finish on shiny metallic surfaces, which is crucial for instruments and medical equipment used in operating rooms. This finish helps prevent glare, thereby enhancing the usability and safety of these tools under the bright lights of an operating environment.

Finally, shot blasting is perfect for surface texturing or roughening which significantly enhances the surface area of a workpiece, an essential preparatory step for both painting and gluing operations. This increase in surface area ensures that coatings and adhesives have a more durable bond. The specific roughness values imparted to the workpiece surface can vary widely, depending on the type of blast media employed during the texturing process. From fine sands to coarser grits, each type of media has the potential to create a unique surface profile, tailored to the requirements of further finishing operations.

Choosing a supplier

In the domain of medical device production, the selection of a supplier for mass finishing and shot blasting technologies is a critical for the effectiveness of the finished products. For manufacturers, it's imperative to partner with suppliers who not only possess advanced technological capabilities but also have a deep understanding of the industry's requirements, and a commitment to continuous innovation.

A supplier should ideally have a long pedigree in the provision of finishing technologies, and a comprehensive technical ability to process a diverse array of materials and component geometries, facilitated by an extensive assortment of machines and abrasive media designed to meet the specific finishing needs of a range of medical devices.

Also a commitment to research and development and a demonstrable forward-thinking approach allows for the provision of efficient, cost-effective, repeatable finishing solutions, vital when considering the stringent regulatory environment of the medical industry, where compliance with quality standards is non-negotiable and directly correlates with patient safety.

Rösler UK
www.rosler.com