Contract manufacturing Other types of injection moulding

Much like consumer products, medical device technology is becoming smaller as manufacturing becomes more advanced, largely thanks to the flexibility of injection moulding processes. Peter Littlejohns from Medical Device Developments’ online partner, NS Medical Devices, gives an overview of the technology’s other applications.

Thin wall moulding

Thin walls can be both a functional benefit and a way to improve patient comfort when it comes to certain medical devices. The term itself simply means creating the walls of a device in a way that makes them thin relative to the whole piece, but in practice this tends to mean walls thinner than 1mm. The technique requires regular injection moulding equipment, but in order for walls to retain their structural integrity as they are created thinner, the base materials used tend to be plastics like LCP, polypropylene and nylon – although silicone and metal can also be moulded this way.

Materials will depend on the device being produced and will have undergone testing at different levels of physical pressures and temperatures to ensure they can do the job. Thin walls are often found in wearable devices and micro surgical tools, but can also be used in invasive equipment like catheter ablation tools and endoscopes.

Gas-assisted injection moulding

When parts are created with regular injection moulding, there’s a risk that sink marks can occur, making the final product look unsightly and potentially weakening its structure. The reason this happens is because thick areas of a mould cool more slowly than thin ones when the resin is injected, and without enough pressure to pack these areas tightly against the walls, the uneven distribution can cause a sunken appearance.

Gas-assisted injection moulding is used to solve this issue by running gas (usually nitrogen) through channels built into the mould. The gas carves a hollow path through the middle of these thicker sections and creates the pressure needed to force the resin tightly against the walls of the mould, creating a smooth part that is structurally sound with no sink marks. This method is used to create complex parts without any visual blemishes, but because the pressure exerted by the gas lessens if it doesn’t flow in a relatively straight line, it isn’t recommended for parts with sharp corners in their design.

Liquid silicone injection moulding

While surface-borne infection is a general concern in hospitals, some medical devices have a greater need to remain hygienic than others, like tubes and respiratory masks.

Liquid silicone injection moulding is used for devices like these because of the chemical resistance of the rubber-like substance produced using this method. It also uses a clean production room so that no ambient air makes contact with the mould, preventing dust and moisture from getting into the mixture as it sets. Another benefit is that silicone is biologically inert, meaning it doesn’t react with biological tissue and can be implanted safely in the body. The basic raw material for silicone rubber is sand, or silicon dioxide, which is processed into pure silicon and reacted with methyl chloride, after which a range of processing steps are used depending on the properties desired for the silicone.

“Raw material specification and powder size is extremely important to developing and manufacturing high-quality MIM components.”

outstanding mechanical properties. It’s a green technology built around specialists with the ability and metallurgical expertise to formulate feedstocks in-house, which means it doesn’t rely on fragile global supply chains, greatly lowering risk for OEMs. Moreover, MIM is perfect for producing


single-use devices because of its ability to meet the higher volume requirements of complex geometries at a lower price point. That’s not all. MIM can support a wide variety of medical device applications. These include dental/orthodontic brackets and devices, surgical devices, orthopaedic components, delivery systems and robotic surgery tools, among others. Stainless steels are the most common alloys and they are often used to reduce component costs for single-use devices where contact with patients is limited. Advancements have also been made to utilise bio-implantable cobalt-chrome alloys such as MIM F75 (ASTM A2886) and MIM MP35N (ASTM F562) as a cost- effective alternative to titanium.

The MIM process So, how does MIM work? Well it begins with the compounding of feedstock, which is a mixture of powdered metal alloys and polymer binders. The feedstock is then injected into a mould that is designed very similarly to plastic injection moulding equipment. Once injected into the mould, a ‘green’ part is formed, which is, on average, 20% larger than the final component geometry due to the added binding agent.

The component then goes to the first debinding stage. The process utilised during this stage may change depending on the binding agent used. There are two common processes for first stage debinding: thermal and solvent.

After the first stage of debinding, the component is called a ‘brown’ part. The brown part is then prepped for the final sintering stage. To prep the component for sintering, flash is often cleaned and the components may be placed on sintering fixtures to combat distortion.

The MIM sintering stage is where the brown parts are placed in a high temperature furnace. The part is heated near its melting point, all the remaining binder is completely removed and the metal particles are bonded together. The part then shrinks and densifies until it reaches its final strength and geometry.

Depending on the component’s end-use and specification, secondary operations may be performed to improve the mechanical properties, dimensional control and machine features, as well as to add surface finishes.

The art of MIM

Whether they’re needed to minimise supply chain risk or provide the optimal medical device components, strong metallurgical expertise is a baseline for the top MIM manufacturers. This expertise is important for sourcing appropriate raw

Medical Device Developments /

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