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 ( particularly Ultratools Ltd for their assistance with cavity manufacture.
Thanks also to Queen Mary University for supply of Hapex material.

42. Thank you