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A demanding high-aspect ratio component for root canal filling produced by micro moulding

The “smartpoint”device was conceived to improve existing root canal filling treatments. The device used to fill the cavity created when the root and nerve of a tooth are removed, typically because of infection caused by decay or injury, is developed by UK’s DRFP Ltd. Traditionally, a thin plug and sealant are inserted after cleaning in a root canal procedure, but this procedure can leave voids in the canal and result in subsequent infection that requires the procedure to be repeated. The smartpoint device comprises a radiopaque core with a hydrophilic polymer coating, which expands laterally as it absorbs water from the tooth; this creates a tight seal that prevents infection to take place again after treatment.
One of the main challenges of manufacturing these devices is the production of the x-ray opaque central core, which requires micro scale dimensions, high precision tolerances and a high aspect ratio. This demanding high-aspect ratio component for dental surgery is being produced by micro moulding. Moulding of thin, high aspect ratio structures is very challenging and will normally require high-flow polymer material grades. However, to fulfil the specific requirements of this application, the component has to be moulded in a hygroscopic polymer filled to 60% by weight with x-ray opaque filler. The weight of the component is 0.06 gm, and the 43 mm long part has a tip diameter of 0.18 mm. The two cavity production mould for this component is of a three-plate design whereby the part can be directly injected on the axis at one end. The cavity form is constructed from a two piece “clam-shell” with the two halves precisely aligned to avoid any mismatch, including at the 0.18 mm diameter tip. The cavity form was produced in hardened steel by high-speed micro milling. On mould opening, the runner and gates are automatically separated from the two moulded components, and the mouldings are removed from the mould by a robot on the machine, to be inspected by a vision system. Two moving halves of the mould is used that rotate on a turntable on the machine. This allows one pair of cavities to be injected while another pair is simultaneously removed from the mould for downstream handling. Vacuum and air-blast functions are built into the mould for more effective and reliable operation. Whereas with some conventional polymers, it is relatively easy to fill the full length of the cavities, using a very highly filled material the process becomes much more demanding. In this application, the situation is further compounded by the unique material which has to be very closely controlled in terms of pre-processing as well as moulding. Precise humidity control is needed to allow reliable micro moulding of the polymer.
The solution was found through micro moulding process and tooling development in collaboration with the Micro & Nano Moulding Centre, University of Bradford, UK, and Micro Systems (UK) Ltd.
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