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Dr Ben Whiteside, Dr Mike Martyn, Prof Phil Coates,

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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

2 Outline Introduction Micromoulding technology Experimental
Mould temperature investigation High shear rate experiments Product surface measurements Moulding/compounding technology

3 Medical implant features
Compatible materials Complex 3-dimensional structures Tailored surface properties

4 Medical material issues
Tight controls Process should not influence the integrity and structure of the material Temperature sensitive Exposure of materials to high temperatures to be minimised

5 Conventional IM disadvantages
Positional control of screw/ram not sufficient Barrel size causes high residence times of material at melt temperature A high proportion of material is wasted in the sprue/runner system

6 Conventional injection moulding -material waste

7 Micromoulding benefits for medical applications
Allows production of complex 3-dimensional products with dimensional tolerances <10um Highly repeatable process with little material wastage Incorporation of clean room conditions and sealing/ packaging systems

8 Battenfeld Microsystem 50
Hopper Metering Piston Extrusion screw Heated Regions Shut off valve Injection piston

9 Battenfeld Microsystem 50

10 The Data Acquisition Setup
Dynisco PCI 4011 Piezo load transducer J-type thermocouples Temposonics R series displacement transducer Dynisco PCI 4006 piezo load transducer

11 1 Process Measurement – Data Capture
Injection Pressure Cavity Pressure Ram Displacement Ram Velocity 3 Temperature Channels Max sampling rate ~ Hz

12 Experimental Mould temperature influence
High shear rate investigations Surface feature replication

13 Mould temperature investigation

14 Hypothesis The high surface area to volume ratio of micro-moulded products allows rapid removal of heat from the product through the cavity wall Mould temperatures should be higher than those used in conventional IM to prevent premature solidification and part-filled products

15 Step plaque moulding Material: HAPEX (40% sintered hydroxyapatite HDPE matrix) Produced by IRC in Biomaterial Science Queen Mary and Westfield College, London

16 Cavity Pressure – Hapex, step plaque

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 material Further investigations to be performed at smaller length scales

19 High shear rate experiments

20 Calculated wall shear rates
0.1 x 0.1mm 0.2 x 0.2mm 0.5 x 0.5mm 1.0 x 1.0mm

21 Dynisco Pressure Transducer
In-process rheometry Dynisco Pressure Transducer 435-30M Capillary die inserts 0.5 x 8.0 mm 0.5 x 0.25 mm 1.0 x 16 mm 1.0 x 0.25 mm Thermocouple Measurements performed on a 30 tonne Cincinnatti Milacron servo-electric injection moulding machine with a custom rheometric nozzle

22 High-shear capillary rheometry test results

23 Shear heating effects Source: Anthony Bur, Steven Roth, NIST

24 ‘Top Hat’ Cavity Large diameter = 1.0mm Small diameter = 0.5mm
Gate dimension 0.1 x 0.2mm Material BP Rigidex 5050 HDPE

25 Molecular weight measurement
Sample material taken from runner system and cavity Gel 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 IM Viscosity curves behave predictably in this region Shear heating will be a factor Stable materials show no sign of degradation Temperature sensitive materials?

28 Surface feature replication

29 Surface feature replication
Plaque cavity 25 x 2.5 x 0.25 mm Fabricated using micro-milling technique Kern machine 0.2mm cutter at rpm. Left in an unpolished state.

30 Surface feature replication - gate
Cavity Product AFM scan size 75 µm x 75 µm Pitch of scroll marks ~ 6µm

31 Surface feature replication - gate
Cavity Product AFM scan size 75 µm x 75 µm

32 Surface feature replication -downstream
Cavity Product AFM scan size 75 µm x 75 µm Pitch 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 sufficient Further work to be performed to investigate the limit to which a feature is adequately moulded on a product

34 A single compounding/moulding process

35 The Rondol Micro-Injection Compounder

36 The Rondol Micro-Injection Compounder

37 The Rondol Micro-Injection Compounder
Advantages: Minimise residence time of polymer in plasticising screw Exposure to single heating/cooling cycle Positive displacement allows use of low viscosity materials

38 The Rondol Micro-Injection Compounder

39 Initial testing Pros Moulding trials successful
Able to process low molecular weight materials Cons Dosing control can fluctuate Toggle clamp can result in flashing

40 Concluding Comments Micromoulding offers many benefits which make it well suited for manufacture of medical components Process conditions may cause problems when processing temperature sensitive materials but initial studies using HDPE show no signs of degradation Mould surface features of length scale ~m are replicated on the product Surface finish can be engineered to influence biocompatibility Twin screw compounding micromoulder offers a route for material blending and component manufacture in a single process

41 Acknowledgements The 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.

42 Thank you


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