MANUFACTURING
Torsional versus conventional
) |
Figure 1: Conventional ultrasonic configuration.
(click image to enlarge) |
The conventional longitudinal ultrasonic welding process has the converter, typically operating at between 20 kHz and 40 kHz mounted on top of a booster to increase the amplitude (Figure 1). The sonotrode or horn is coupled to the booster, which utilises the lower cross-sectional area of the sonotrode working face to magnify this amplitude still further. By using a pneumatic actuator in conjunction with the energy being delivered via the sonotrode, the process creates instantaneous local heat and melt. This passes directly into the polymer surface at the horn contact point with a contact swaging or spot welding action, or the ultrasonic energy is transmitted through the contacted product substrate and focussed into a joining bead or shear feature at the assembly interface. The continued vertical force from the pneumatic actuator force collapses the molten swage, spot or joint and holds it in position until the melt has solidified.
The characteristics of the conventional process, which produces amplitude in the vertical plane, has, however, limited the use of the technology for certain medical device applications until now. This has been mainly because of concerns over possible damage to fragile thin wall sections, perforation of micron thin seals or membranes, particulate generation from the joining process and fatigue of delicate electronics or mechanisms.
) |
Figure 2: Torsional ultrasonic configuration.
(click image to enlarge) |
) |
Figure 3: Amplitude delivered in a circumferential manner.
(click image to enlarge) |
Many of these problems can be eliminated by applying a torsional principle with ultrasonic energy, and this has increased the range of medical applications to which ultrasonics can be applied. With the torsional principle the converter is mounted tangentially to the booster axis (Figure 2), thus creating reciprocal circumferential amplitude (Figure 3) at the specially designed sonotrode. Using this principle, the process creates a reciprocating ultrasonic friction weld of the upper component (whether film, membrane or a rigid part) to the lower component. Frictional heat in the substrate joint area is developed in the horizontal plane as opposed to the vertical plane. The ultrasonic vibration is not pushed vertically into the product and this reduces acoustic stresses and fatigue in any internal assemblies. Other benefits of this approach are that no cavitation of the plastic, which can be an origin for particle separation, takes place in the vertical plane, and the shear force is not transmitted into the product beyond the liquid melt pool, which allows intricate forming operations in thin section components. The vertical force, derived from an electronic proportionally controlled pneumatic actuator, collapses the molten joint and maintains the required pressure until the weld joint has solidified. The small circumferential amplitudes used allow even the most delicate components and membranes to be joined, welded or sealed.
Advantages for joining and forming
The potential benefits of this process encouraged innovative engineers to evaluate the use of torsional welding and joining across a diverse range of applications. One of the first was the welding of thin films, membranes and fine filter media. These materials, which often have a typical thickness of 50 μm, are used to create a hermetic airtight seal on medical pots, devices, containers and drug delivery systems. It is essential, therefore, that any process used to weld the delicate film or membrane media does not induce any defects or damage.
With conventional ultrasonic welding techniques, the vertical amplitudes transferred into a product create a "diaphragming" effect when welding a thin film or membrane; the centre of the membrane acts like a drum skin oscillating at ultrasonic frequency. This, in turn, can result in perforation at the centre of the membrane, which effectively renders the conventional ultrasonic welding process unsuitable for this type of application.
As illustrated in Figure 3, torsional welding techniques produce small reciprocating amplitudes of typically 60 μm in the horizontal plane at the perimeter of the joint area of the parts, which need not necessarily be circular. The torsional welding process does not stretch the membrane during welding nor does it cause a diaphragm effect, thus it eliminates the risk of perforation when welding polymer based foils, films and membranes. Additional benefits of the technique include speed, whereby complete weld cycles can be achieved in less than one second, and the ability to be tolerant of product, liquid or powder contamination that may be present between the joining faces. The torsional welding technique also enables multiple membranes in close proximity to one another to be processed without causing any previously welded membrane to become dislodged or lose its hermetic seal. In comparison, the characteristics of the conventional longitudinal process could pose a risk of membrane separation or loss of seal integrity.
Application examples
 |
Figure 4: Example of encapsulation achieved using torsional techniques, an 8 mm diameter polypropylene tube with needle protection insert.
|
The benefits offered by the torsional principle coupled with ultrasonics mean that many welding and forming applications, previously deemed to be impractical or impossible with ultrasonics, can now be realised. Applications that are now possible include joining nonferrous metals and foils to ceramics or glass, with a molecular surface bonding effect created by the local high intensity friction heat. The technique also provides greater freedom for product designers when considering joining methods, especially for delicate or thin walled components.
The combination of ultrasonic energy and circumferential amplitudes can also be used to generate features or profiles in plastic components following the injection moulding process. This is especially useful for features that may be difficult to mould repeatably or that would be costly or impractical to incorporate within the injection mould tooling. The compact nature of the equipment coupled with its short cycle times make it suitable for integration within automated handling or assembly systems. Torsional forming techniques have been used to generate a mechanical stop feature within the internal diameter of a syringe. Plastic material is displaced and reformed using a specially designed sonotrode to tolerances that would normally be associated with precision machining operations. Other applications include the encapsulation of a miniature metal tube within a polypropylene plastic sleeve (Figure 4) by forming the end of the plastic sleeve using torsional techniques.
Ongoing innovation for packaging
) |
Figure 5: Moulded component prior to processing, left; Selective weakening feature, 0.4 mm wide groove, produced using torsional ultrasonics techniques, right.
(click image to enlarge) |
Examples of the innovative applications of this technology are its use to selectively weaken areas on moulded components to provide precision "break-off" features often used for tamper evident components or to allow easy opening of lids. Using the same circumferential amplitude principles described for welding or joining, these features are easily formed to fine tolerances (Figure 5) to give predictable "opening" or "break-off" forces.
Seal integrity on packaging applications within the medical and pharmaceutical industries is paramount. Continuous torsional ultrasonic principles are now being developed to enable the technology to be integrated within horizontal fill-form-seal machines to produce the centre fin seal. The ability to control the ultrasonic process to fine tolerances and the resultant seal integrity make this an attractive option compared with using traditional heat sealing.
Having found its way into numerous medical device operations, the torsional ultrasonic process looks set to continue as an innovative solution to many new and demanding applications for joining, forming and sealing.
Martin Frost is Sales Manager North at Telsonic UK Limited, Units 14 & 15 Birch Copse, Technology Road, Poole BH17 7FH, UK, tel. +44 1202 697 340, e-mail: sales@uk.telsonic.com www.telsonic.com
