Micro moulding and the future of medical device design

The device of tomorrow, today  - there’s an app for that!

The growing convergence between medical device and consumer electronics has not only brought some incredible innovation to the end user it has also brought an interesting kind of pressure to the design and manufacturing world. The general population of the modern world has come to expect that all of their devices, medical or not, be ‘smart’ in some way.

There is also a growing expectation that a devices built for a special purpose now function in multiple and sometimes in completely foreign ways. It wasn’t a stretch to accept that a hearing aid could double as Bluetooth connection between a mobile phone or television, but what if it also took your temperature, measured your heart-rate or counted your steps? It’s already connected to your body so why not? Think of all of the items worn by individuals that could have multiple functions. Any one of them could function as a medical device (or quasi-medical device) - many of which already do. So what’s next?

This demand is not only for at-home personal care devices, but for the doctor’s office as well. The “smart” diagnostic device, surgical tool or delivery apparatus is of equal interest among medical professionals. This interest is also compounded by the fact that there is need for these tools to be smaller, less invasive and in many cases disposable.

What do these demands and interests do for the already crowded and complex device? How can they even be manufacturable?

The demands on tomorrow’s medical device is stretching beyond the standard supply-chain capabilities in many ways and it has become necessary to enlist new technologies to push the limits of device design. One of the key technological innovations that enables the design and manufacture of next generation medical devices is micro moulding. This technology has allowed devices to shrink in size, become more complex, reduce manufacturing efforts and open design to new possibilities - with it, however, has come a series of common questions as to the best ways to understand and utilise micro moulding to its fullest. Understanding the answers to these important questions will allow for better and quicker device design efforts.

What is micro moulding?

While micro moulding as a technology isn’t necessarily new the interest level is at an all-time high - and so is the confusion of what it is and how to design for it. Part of the definition includes something related to the term ‘micro’ but there is not a standard textbook definition. In general, micro moulding can be looked at in one of three ways: moulded parts that are micro in size, micro in features or micro in tolerance. While in many cases a part design could include all three aspects any one of these can define true micro moulding.

Drawing the line between micro moulding and conventional moulding is not cut and dry. It may be safe to say moulded parts under 1 mm in size are for sure micro, it’s important to note there can be larger parts with features that are well under 1 mm, even approaching 1 µm or less that can also be considered micro moulding. While the overall dimensions of the part are “big” the features demand a skill in mould design and processing beyond most conventional approaches.

The same goes for tolerances. Everyday positional or geometric tolerances for micro moulding are around 25 µm and start to push the limits around 2 or 3 µm. Larger moulded parts with tolerance expectations near these numbers can also be considered micro moulding.

There is also some confusion that micro moulding is only defined by the press size being used to mould the parts. The press size does play a critical role in micro moulding but only for the same reasons one would pick a large press for larger parts. The moulding machine must match appropriate efficiencies to the part design so that less material is wasted and the tool can be balanced and controlled very precisely, especially when using expensive implantable materials and/or when the tolerances on the part design are very tight. In theory any sized moulding machine can make micro parts, however there are practical and physical challenges the larger the mismatch between the part size and press size.

One of the best ways to tell if a part design is approaching micro moulding is by the reaction of the supply chain. It will be fairly evident how difficult the part design might be by the number of ‘no-quotes’. Seek out those that consider themselves as experts in micro injection moulding and tooling to gauge efficacy of the project when unsure.

How is micro moulding different from conventional moulding?

In the most basic of comparisons there is no difference. Just like with conventional moulding, micro moulding is an injection moulding process with all of the standard requirements that any moulded project would require. For example, just as with the conventional method, any micro mould design will require a way to process and inject material into the mould cavity, the mould will require the ability to open and close, and the moulded part will still need to be ejected in some fashion. These requirements are most often overlooked when someone is new to micro mould design. There is nothing magical about micro moulding that allows the process to skip the conventional requirements.

That being said there are some key distinctions that set micro moulding apart from the conventional process. Despite some beliefs, micro moulding is not just large part moulding made smaller. The relationship between the part geometry, the material selection and tool design become more intertwined the smaller and more complex the part design becomes. Understanding how to build a mould for a micro part design so that the material will fill each feature to the desired tolerance, every time, is not as easy as it sounds.

The expertise is in knowing how to balance these relationships effectively given the delicate interaction between the geometry, the material and the processing. Moulding tools must be designed and crafted with artistry and skill in order to replicate the finest of features and maintain a robust and repeatable moulding process. Creating features under 1 mm or tuning a mould to achieve a few microns of tolerance necessitates a tool room operation dedicated to the art of micro moulding.

Are there any guidelines I can follow?

The most challenging aspect of design for micro moulding is knowing where and when the limits can be pushed. At the extreme end of the capability subtle changes can mean the difference between success and failure. This sensitivity to the design makes it very difficult to draw hard and fast rules. Setting standards that are too tight could possibly hinder creativity in the design and setting them too loose leads to frustration.

As a general rule, start with the ideal part design recognising where the model may be pushing the limits in terms of size and/or complexity. At the same time don’t forget the general mould design guidelines concerning gating, mould opening and ejection.

In addition, it is often thought that micro moulding produces no flash. This of course is not accurate, albeit the flash is measured in microns, it is still there. Be sure to keep flash requirements on the callout for areas like the parting line, gate vestige or any potential witness marks that may interfere with the functionality of the part. Other considerations are draft specifications, surface finish or critical design feature needs.

The best guideline however will be the Design for Manufacturability (DFM) discussion with your micro moulder. The experienced micro moulder often times can lend a hand with knowing what can be accomplished. It may be more important to start this discussion before your design is complete when designing for micro moulding. Don’t let the ideal part design suffer, it may be feasible as designed.

What should I know about material selection?

Material selection can be the most difficult part of any micro mould design process, especially when it comes to any regulated medical device or component. The relationship between the material selection and the feature performance is exaggerated at the micro level. For example, one part designed with a high-aspect ratio, thin-wall section at 75 µm thickness with material A (LCP) can run a 42:1 aspect ratio, material B (Nylon) runs 14:1 while material C (PEEK) only 3:1 - same geometry, extremely different results.

The difficulty is compounded by the fact that there is really nowhere standard to look for materials that will work with any given geometry. The material data sheets are good for basic details but don’t account for the potential at the micro level. On top of that, materials selection was probably based on the inherent material properties and/or regulation approvals, not its ability to flow in thin-wall sections. Matching the material requirements to the geometric requirements can be very challenging.

Often the only solution through this difficult part of the process is the DFM conversation with your trusted micro moulder. Knowing what materials can do, and in what situations, is based on years of experience working with the resins. Knowing where the limits can be pushed is an invaluable part of the micro moulder’s depth of knowledge.

The good news

This good news is that micro injection moulding can open up a whole new world of part design. This capability can be translated into new creativity allowing medical device OEMs to bring new and innovative product to the market. Not only will this technology allow for the manufacture of complex micro parts, but manufacturing in plastic in many cases reduces part cost, reduces weight, and allows for functioning products to be made using fewer overall components- a design feature desired by everyone.