Presentation on theme: "Dr Ben Whiteside, Dr Mike Martyn, Prof Phil Coates,"— Presentation transcript:
1 Micromoulding: consideration of processing effects on medical materials Dr Ben Whiteside, Dr Mike Martyn, Prof Phil Coates,IRC in Polymer Engineering, University of Bradford, Bradford UK.P S Allan, G. Greenway and P Hornsby,Wolfson Centre for Materials Processing, Brunel University, Uxbridge, UK
3 Medical implant features Compatible materialsComplex 3-dimensional structuresTailored surface properties
4 Medical material issues Tight controlsProcess should not influence the integrity and structure of the materialTemperature sensitiveExposure of materials to high temperatures to be minimised
5 Conventional IM disadvantages Positional control of screw/ram not sufficientBarrel size causes high residence times of material at melt temperatureA high proportion of material is wasted in the sprue/runner system
7 Micromoulding benefits for medical applications Allows production of complex 3-dimensional products with dimensional tolerances <10umHighly repeatable process with little material wastageIncorporation of clean room conditions and sealing/ packaging systems
14 HypothesisThe high surface area to volume ratio of micro-moulded products allows rapid removal of heat from the product through the cavity wallMould temperatures should be higher than those used in conventional IM to prevent premature solidification and part-filled products
15 Step plaque mouldingMaterial: HAPEX (40% sintered hydroxyapatite HDPE matrix)Produced by IRC in Biomaterial ScienceQueen Mary and Westfield College, London
17 Product Mass – Hapex, step plaque 0.12% variation
18 Mould temperature - conclusions For products ~25mg recommended mould temperatures for standard injection moulding can be used with confidence for the Hapex materialFurther investigations to be performed at smaller length scales
20 Calculated wall shear rates 0.1 x 0.1mm0.2 x 0.2mm0.5 x 0.5mm1.0 x 1.0mm
21 Dynisco Pressure Transducer In-process rheometryDynisco Pressure Transducer435-30MCapillary dieinserts0.5 x 8.0 mm0.5 x 0.25 mm1.0 x 16 mm1.0 x 0.25 mmThermocoupleMeasurements performed on a 30 tonne Cincinnatti Milacron servo-electric injection moulding machine with a custom rheometric nozzle
23 Shear heating effectsSource: Anthony Bur, Steven Roth, NIST
24 ‘Top Hat’ Cavity Large diameter = 1.0mm Small diameter = 0.5mm Gate dimension 0.1 x 0.2mmMaterial BP Rigidex 5050 HDPE
25 Molecular weight measurement Sample material taken from runner system and cavityGel Permeation Chromatography (GPC) analysis performed by Rapra Technology Ltd on each sample to determine molecular weight distribution
26 Molecular weight distributions Source: RAPRA UK
27 High shear investigation - conclusions The process contains shear rates orders of magnitude higher than those encountered in conventional IMViscosity curves behave predictably in this regionShear heating will be a factorStable materials show no sign of degradationTemperature sensitive materials?
32 Surface feature replication -downstream CavityProductAFM scan size 75 µm x 75 µmPitch of scroll marks ~ 6µm
33 Surface feature replication - comments Mould features of the order of a few µm are accurately replicated on the product assuming pressure is sufficientFurther work to be performed to investigate the limit to which a feature is adequately moulded on a product
37 The Rondol Micro-Injection Compounder Advantages:Minimise residence time of polymer in plasticising screwExposure to single heating/cooling cyclePositive displacement allows use of low viscosity materials
39 Initial testing Pros Moulding trials successful Able to process low molecular weight materialsConsDosing control can fluctuateToggle clamp can result in flashing
40 Concluding CommentsMicromoulding offers many benefits which make it well suited for manufacture of medical componentsProcess conditions may cause problems when processing temperature sensitive materials but initial studies using HDPE show no signs of degradationMould surface features of length scale ~m are replicated on the productSurface finish can be engineered to influence biocompatibilityTwin screw compounding micromoulder offers a route for material blending and component manufacture in a single process
41 AcknowledgementsThe authors gratefully acknowledge the support of the UK Micromoulding Interest Group (www.ukmig.com), particularly Ultratools Ltd for their assistance with cavity manufacture.Thanks also to Queen Mary University for supply of Hapex material.
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