The art of science16 May 2019
Developing a metal medical device requires balancing a budget, deadlines, design considerations and more. In light of rapid technological advances, deciding upon the best process to maximise manufacturing effi ciencies can be challenging. Louise Thomas explores the important factors to consider when making this selection.
Manufacturing all the components for tools and devices in the industry is an ongoing effort involving balancing precision and quality. With the growing needs of life sciences, metal components have become an integral part of building medical devices for the treatment of a range of diseases.
Biocompatibility is a constant challenge as devices need to be used internally and externally within a range of medical applications. For this reason, designs are continually changing and require rapid prototyping to ensure that they are fit for purpose. The ability to meet these demands in an efficient and effective way is the result of rapid advancements in metals and alloys for components that are small or complex.
When manufacturing a device that involves metal, a key decision lies between machining and stamping. These choices are determined by the fit, form and function of a particular component, along with the geometry of the part and material used.
If a medical device is designed for a large number of uses, then machining works best, as it produces complex parts that support durability. For products that are meant to be used just once, stamping is more suitable as it produces precision parts that tend to be less durable. Volume is another key driver in the decision-making process.
When millions of components are needed on a weekly basis, this can be easily achieved with stamping. Machining these quantities could require over 100 machining centres, making it unfeasible for this purpose.
Tooling costs are another important consideration. As machining is more versatile and precise, it is preferable for devices requiring tight tolerances that need to be produced in high volumes. As long as a suitable machining centre is available, it can be relatively easily programmed to accommodate the part geometry. In contrast, a progressive stamping die takes much longer to design and build, and is much more costly.
Part geometry is also an important variable to bear in mind as there are some components that are not able to be stamped. Certain diameters cannot be pierced using stamping. In such cases, machining is a better option, which can be combined with laser technology.
Material selection factors are also significant in making a decision between stamping and machining, particularly thickness and hardness. For example, stamping can be limiting because thickness must be uniform throughout the part and hardness needs to be carefully considered because of the potential for cracking during forming. In contrast, almost any hardness can be machined.
Most red metals, stainless steels, titanium and cold-rolled steel can be stamped. Titanium and stainless steel are the most commonly used materials for stamping. Both are popular for implantable medical devices due to their biocompatibility. However, titanium is often only available in wire, rod and sheet forms, rather than in continuous strips, the latter of which tends to be needed for the majority of stamping operations. The heavier the gauge and higher the hardness, the more difficult it is to use stamping.
There is also less room for error with stamping than with machining; it is thus hugely important that the designs are reviewed and approved by the manufacturer early in the design process. With machining, it is difficult to re-engineer the part using stamping to reduce costs without making fundamental design changes.
Meet in the middle
Metal injection moulding (MIM), or metal injection moulding plus machining, can be a useful middle ground to minimise costs while gaining the benefits of machining. MIM has been increasingly used in recent years by manufacturers to expand their design capabilities.
Increasingly complex applications mean that sometimes MIM is the only suitable process. With the inclusion of biocompatible and implantable materials, components that were routinely machined can now be ‘MIMed’ for a fraction of the cost. Some OEMs have brought this process in-house, but as it demands specialised skills and equipment, contract manufacturing can be an attractive alternative.
Orthoscopics was one of the first fields to adopt MIM. Graspers, blades, staplers and cauterising cutters are small, geometrically complex and must be manufactured in a biocompatible material. Early designs were machined, which meant each component was expensive. However, as MIM become more established, suppliers became more consistent, and more companies began using the process.
Today, MIM applications are broad, including bone drills, robotic arms for surgery, bone rasps, cutting jaws, biopsy jaws, needle guides, saw guides and hundreds of endoscopic instruments. Over time, moulding has become much more sophisticated, with pressure sensors in each cavity, as well as in runner systems.
MIM is essentially a four-step process, consisting of compounding, moulding, debind and sintering. The biggest disadvantage with MIM is the lead time. However, the choice of materials makes it a popular choice for OEMs. It is best suited to shaping complex components in high volumes and when higher material properties, or implantable materials, are needed.
Many OEMs have considered bringing MIM manufacturing in-house, but this has been met with mixed success. Start-up costs can be several million dollars for the equipment alone, which includes compounding, moulding, debind, sintering and secondary apparatus.
MIM has been proposed to be 85% science and 15% art. Understanding the dynamics of sintering, and how to minimise the effect of gravity and friction, is essential, and developing a consistent process often takes years of experience.
This can be minimised with the hiring of experienced personnel, but they are in high demand and thus can be hard to find. A further issue is how to design for the MIM process. Most specialists will help with redesigning components, but this can take time, as most also serve as programme managers.
In recent years, some companies have started to help OEMs design components to optimise the advantages of MIM. This can save time and money, as they can go directly into production without any expensive redesigns during the production process. Such organisations are also able to manage the entire journey from design, through production, and can teach the OEM about the process throughout.
A big decision
Finding the right supplier can be difficult, with hundreds of qualified contractors worldwide. Often newer suppliers will underestimate the amount of work needed to get a medical component into production. An established facility with substantial experience is thus likely to better meet required timelines and have a comprehensive understanding of the procedures required to meet medical specifications. However, the additional work created by increased quality requirements, along with the required documentation, means that established MIM facilities will be more expensive than newer suppliers.
Companies unfamiliar with requirements may initially be cheaper, but will soon discover they need additional resources or training to meet the needs of a particular device, so may not be cheaper over the long term. It is therefore essential for OEMs to do their homework and prequalify any potential supplier well in advance of making a decision. This can be assisted by a visit to a potential supplier’s manufacturing facility to discuss the device requirements and learn more about the processes that would be implemented.
There are a number of considerations once a supplier has been selected, whether for stamping, machining or MIM, to ensure that the process runs smoothly. Instigating a dialogue right from the start is key. An OEM cannot assume that the current contractor will work in an identical way to their previous contractor. Each organisation is different and has its own specific, potentially idiosyncratic, working practices. Communication might therefore involve confronting the contract manufacturer directly if there are any concerns or suspicions.
In situations where there is a clear mismatch between OEM and supplier, it can be tempting to finish what has been started to avoid unnecessary delays. However, in most cases, it is better to accept a setback and find a contractor who will be a more favourable fit over the long term. As in all outsourcing situations, cultural affinity is crucial, and both parties need to feel that they’re aligned in working towards a common goal.
Dialogue is of course a two-way street. It is crucial that the OEM understands the contractor’s manufacturing processes and capabilities, and allows its designers and engineers to provide recommendations about where those capabilities would be better served. In addition, the contract manufacturer should also feel able to provide suggestions and take a degree of ownership as part of the process. First and foremost, the OEM must pay close attention to its own design first, and there needs to be a balance between complete prescriptivism and an overly lax approach. When relying on multiple contract manufacturers, it remains absolutely crucial that all parties are on the same page throughout the process.
The MIM process
MIM brings together the shape-making capabilities of plastic injection moulding with the strength of metal. The four-step process begins with compounding, where a fine metal powder is mixed with a plastic binder.
This compounded material is then placed in a modified plastic-injection-moulding machine and injected into a mould. The component is called a ‘green’ part. This is then put into a chemical bath and most of the plastic binder is removed. Some binder remains in order to keep the metal powder in the moulded form. The process can take up to five hours, dependent on thickness and size.
The green part is subsequently put into an oven where the plastic binder is removed through evaporation. This process can last between hours and days, based on the part size. However, Most MIM companies are moving away from this process.
After debind, the green part becomes ‘brown’, and goes into a furnace, either vacuum or continuous, where any remaining binder is removed and the temperature increased to close to the metal’s melting point. The component then shrinks by around 20% to reach its final size.
Source: Powder Technology