Finding a suitable alternative material can be a daunting step into the unknown

For every engineer, specifying a new material for a product – especially if the product’s already in production – can be a daunting task. There is a worry of the unknown, that phone call to tell you about unforeseen production problems, or even worse, of a potential product recall. For that reason, material selection can be, somewhat, of a conservative process that’s slow going and largely driven by pragmatism. This is no truer than in the medical sector where a justifiably stringent amount of regulation restricts many materials and outlines all kinds of tests and requirements beyond simply strength or density, for example.

Sarah Egan, product manager of data product solutions and medical at materials experts Granta Design routinely comes across this problem in industry. “You’ve got to get the right balance of design, of the materials and the performance, so that we can start the project off on the right foot,” she says. “And, let’s assume that you haven’t just spoken to one of your colleagues and gone for something really familiar because you know it’s going to be ‘okay’.”

Like most modern engineering conundrums, it is all about getting reliable data upon which to base design assumptions and decisions, particularly during the initial concept design stage. In the healthcare industry, information about whether a material can be sterilised or wiped down can be difficult to find, especially if the material hasn’t previously been used for medical applications. And going direct to companies for more information can be a drawn out process.

“One of the problems is if a material has USP Class VI or ISO10993 certification,” says Egan. “If you’re trying to search in a database, which one of those do you choose? And if you choose one, you might eliminate a number of materials that would have been more appropriate.

“Another problem when specifying materials is over-constraining too early, because you can adjust the design to fit a material and adjust the material to fit a design.”

The key in choosing the most suitable material is understanding what needs to be achieved by making a product with an alternative, whether that’s making it smaller, lighter or cheaper. Perhaps another material is needed due to regulations restricting it or perhaps the raw material price has spiked. Whatever the need, the ultimate goal should be kept in mind and used as the basis for making material and design decisions.

Granta boasts one of the most in-depth material databases in the world, known as MaterialUniverse, and its CES Selector application is helping engineers select more suitable materials based on a number of criteria to open up new design opportunities and process improvements.

“What we aim to do at the outset with the database is not to cover every individual grade of material, but to fully understand the generic types,” says Egan.

Forcep redesign

Granta explored materials selection for medical forceps to help engineers determine if a better material was available compared to stainless steel. Using its CES Selector software, the team wanted to examine if it was possible to make the forceps lighter and more ergonomic, but also see if they could be injection moulded to reduce production costs. The material would have to be medical grade and sterilisable. Finally, it would also need sufficient mechanical properties to achieve the desirable performance.

“For every set of performance requirements, every mechanical loading condition and set of objectives, you generate [what’s known as] a ‘performance index’,” explains Egan. “For this application with the forceps, the relevant loading condition is a beam in bending.

“If we want to minimise the mass, you use an index of density over the flexural modulus to the half, and if you want to also minimise cost, you use the cost of material multiplied by density over the flexural modulus to the half. We also need to filter the materials by applying constraints, such as medical grade, sterilisable and injection mouldable.”

Users of the CES software don’t actually need to remember these indices, instead they choose a design scenario and the software selects the right performance index. This creates a quantitative and visual graph, known as an Ashby diagram, which is used to guide design decisions. To the left of the graph are the cheapest materials that meet the requirements and at the bottom are the lightest. It is possible to see that while initially unfilled PEEK was being considered, it is neither a particularly cheaper or lighter alternative to achieve the necessary stiffness, so immediately it becomes clear that this is a far from optimal choice.

An unfilled polypropylene copolymer is the cheapest option but the forcep handles would need to be redesigned to provide the necessary stiffness resulting in a 30% heavier component compared with stainless steel.

From the generated diagram,Polyacrylamide (50% glass fibre) appears to stand out, offering a more reasonable and straightforward trade-off between cost and performance. “From this generic material, there are several potential material grades [from different suppliers],” says Egan. “These can then be identified and compared using the specialist polymer datasets available.”

One impact from making the metal parts from plastic is that it will likely require some re-design, in this case plastic forceps will likely need to be made larger, meaning the nimble tool could become more bulky. By changing the performance index, it is possible to make a judgement on the impact early on, in this case the cross-sectional area of the forceps would need to be around three times larger to achieve the required stiffness.

Helpful comparison

Once a suitable material is identified the software pulls out a datasheet and compares it to any incumbent material and flags up any anomalies in comparative performance such missing properties or those below a predetermined threshold (for example 10%). It allows engineers to go through and see if there are any major problems, such as poor fracture toughness, for example, and essentially sanity check the alternative material early on. “CES Selector gives us the opportunity to have a look, ask questions, and explore,” says Egan. “At the end of the day it’s not just about the price per kilo, but how much of that material you’re actually going to use. You may well be surprised by what options are available to you.”

Justin Cunningham

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