Transcript Micromoulding: consideration of processing effects on medical materials

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
Outline
• Introduction
• Micromoulding technology
• Experimental
• Mould temperature investigation
• High shear rate experiments
• Product surface measurements
• Moulding/compounding technology
Medical implant features
• Compatible materials
• Complex 3-dimensional structures
• Tailored surface properties
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
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
Conventional injection moulding material waste
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
Battenfeld Microsystem 50
Hopper
Metering Piston
Extrusion screw
Heated Regions
Shut off valve
Injection piston
Battenfeld Microsystem 50
The Data Acquisition Setup
Dynisco PCI 4011
Piezo load
transducer
Temposonics R
series
displacement
transducer
J-type
thermocouples
Dynisco
PCI 4006
piezo load
transducer
1 Process Measurement – Data
Capture
Injection Pressure
Cavity Pressure
Ram Displacement
Ram Velocity
3 Temperature
Channels
Max sampling rate ~ 30 000Hz
Experimental
• Mould temperature influence
• High shear rate investigations
• Surface feature replication
Mould temperature investigation
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
Step plaque moulding
Material: HAPEX (40% sintered hydroxyapatite
HDPE matrix)
Produced by IRC in Biomaterial Science
Queen Mary and Westfield College, London
Cavity Pressure – Hapex, step
plaque
80C
50C
20C
Product Mass – Hapex, step plaque
0.12% variation
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
High shear rate experiments
Calculated wall shear rates
0.1 x 0.1mm
0.2 x 0.2mm
0.5 x 0.5mm
1.0 x 1.0mm
In-process rheometry
Dynisco Pressure Transducer
435-30M
Capillary die
inserts
0.5 x 8.0 mm
0.5 x 0.25 mm
Thermocouple
•
1.0 x 16 mm
1.0 x 0.25 mm
Measurements performed on a 30 tonne Cincinnatti
Milacron servo-electric injection moulding machine
with a custom rheometric nozzle
High-shear capillary rheometry test results
Shear heating effects
Source: Anthony Bur, Steven Roth, NIST
‘Top Hat’ Cavity
•
•
•
•
Large diameter = 1.0mm
Small diameter = 0.5mm
Gate dimension 0.1 x 0.2mm
Material BP Rigidex 5050 HDPE
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
Molecular weight distributions
Source: RAPRA UK
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?
Surface feature replication
Surface feature replication
• Plaque cavity 25 x 2.5 x 0.25 mm
• Fabricated using micro-milling technique
• Kern machine
• 0.2mm cutter at 75 000 rpm.
• Left in an unpolished state.
Surface feature replication - gate
Cavity
AFM scan size 75 µm x 75 µm
Pitch of scroll marks ~ 6µm
Product
Surface feature replication - gate
Cavity
Product
AFM scan size 75 µm x 75 µm
Surface feature replication -downstream
Cavity
AFM scan size 75 µm x 75 µm
Pitch of scroll marks ~ 6µm
Product
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
A single compounding/moulding
process
The Rondol Micro-Injection
Compounder
The Rondol Micro-Injection
Compounder
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
The Rondol Micro-Injection
Compounder
Initial testing
• Pros
• Moulding trials successful
• Able to process low molecular weight materials
• Cons
•Dosing control can fluctuate
• Toggle clamp can result in flashing
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
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.
Thank you