Micro-Moulding of Polymers: Process and Product Assessments 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

Contents Micromoulding Results of studies at Bradford Process technology Results of studies at Bradford Process dynamics Process interaction Product properties

Battenfeld Microsystem 50 Purpose built micro injection process Servo-electric injection Automatic parts handling Clean room filtration Modular A purpose designed machine – started with a blank piece of paper in terms of specking the machine – has several features

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

Operation Cycle

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

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

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

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

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 Product AFM scan size 75 µm x 75 µm Pitch of scroll marks ~ 6µm

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

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

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

Morphology Measurement Surface following microtomy Surface following etching Component sectioned using glass-knife microtomy Surface is etched using potassium permanganate solution to produce representative surface Surface imaged using microscopic technique (TEM, AFM etc)

Morphology Measurements - Structure Nano-indent Crystal Structure

Morphology Measurements – Spherulite size

Morphology Variation 800 µm Line of indents 25µm separation

Morphology Measurements – Amorphous layer

The Rondol Micro-Injection Compounder

The Rondol Micro-Injection Compounder

The Rondol Micro-Injection Compounder Prism twin-screw extruder

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

Operation

The Rondol Micro-Injection Compounder

Initial testing It works! Able to process low molecular weight materials Dosing control can fluctuate Toggle clamp can result in flashing

Experimental Battenfeld Microsystem50 Stepped plaque cavity 60mg total shot mass HDPE 5050 Injection speed – 200, 400, 600 mm/s Screw speed 40revs/m Melt temp 200C Mould temp 50C