PHOTO: Luer caps moulded with expandable cavities in the mould
MEDICAL parts have different requirements than other moulded articles and as a result tooling engineers over the years have pioneered new mould materials, coatings and mould maintenance practices.
For example, a typical luer fitting (component for making leak-free connections) is used for making a secure, leak-proof attachment with tubing connections in medical devices and other instruments. These small connectors frequently have undercuts and, when they do, can create considerable cost and risk implications for the moulding of these parts. However, practices are evolving and tooling engineers have begun using a single, expanding cavity component as an alternative to slide mould traditions of the past.
Containing the part’s undercuts within an expandable cavity, results in a mould that is smaller and simpler, with less moving parts. Design time is reduced and mould construction hours are less. With a smaller mould, lower hourly moulding machine rates result, and when a mould is less complex, it is easier and less costly to properly maintain.
Now these advanced mould components are available local from Mould & Die Solutions, the South African tooling equipment supplier which represents Roehr Tool Solutions of the USA.
Slide moulds require lubrication, introducing opportunities for part contamination, while an expandable cavity requires no such grease. And rather than nesting round parts within long slides, more conformal cooling and a more balanced runner system can be achieved by using a component that forms the part concentrically.
Expandable cavities are constructed in an expanded position, meaning that during moulding they are pulled back against a striker ring and collapsed inward. For ejection, the component is staged forward, which allows the segments of the expandable cavity to expand outward. This releases the undercut and the part is then ready for ejection. They are commonly staged forward during mould open to enable the fastest possible cycle time.
Success with expandable cavity system
Expandable cavities can sometimes be a project-saving solution, as it was for Mike Rockwell of Cape Fear Mould & Tool of North Carolina, USA. Cape Fear’s customer required two hot runner moulds with undercuts.
“One of the moulds was fairly simple except for some undercuts, but the second mould turned into a bear,” said Mike. “It had a long, thin core running through the centre, undercuts on the inside that required the expandable cavities, undercuts around the outside that required slides, and, to top it all off, it had a 0,04mm vent hole in the face.”
The complexity of the second mould wasn’t the only criterion that had to be met, but there were also space considerations and a tight delivery time to think about.
“The expandable cavity system was really the only option,” Mike notes. “We had worked with several collapsible cores in the past and have always been quite happy with those, so I was confident the expandable cavity system would work just as well. The beauty of the expandable cavities is that there is really no way to crash the mould.”
The project was a success and Mike says his customer was very satisfied.
In medical moulding, the stakes are high. The ramifications that are on every tooling engineer’s mind can cause one to default into building the next tool much like the last tool. However, beyond the cost savings of a smaller and more straightforward mould build, containing the part’s undercut details within a single component ensures part accuracy, optimal cooling, greaseless moulding and dramatically simplified mould maintenance. These advantages reduce risk and liability over the course of a mould’s lifetime.