20. Dissolving the bubbles
Patm
Pi
ΔP = Pi - Patm = 2T/r
If melt is in diffusion equilibrium with atmosphere
gas in the bubble tend to flow outwards because
of pressure difference ΔP until radius r of the
bubble diminishes to zero.
25. Example of
Gas Permeable Polymers:
Silicone Rubber: high flexibility of
the silicon-oxygen chain in siloxane
provides “openings” which are free
volume and permit gas diffusion, see
Zhang and Cloud (2006).
44. What are micro-pellets?
• Small regular oval shaped
plastic granules with a
diameter between 500-600µm.
• Produced from the same base
material as powder, i.e. Qenos
711UV etc.
• Differ from powder by way of
the manufacturing process and
particle size distribution.
45. Process comparison
PELLETS POWDER
and
MICRO-PELLETS
Cyclone
Process Water PE pellets
Oversize
Centrifugal Dryer Particles
Sieves
Cutter Dust Collector
PDV
PE Powder Powder
190-210oC Grinding Blower
Water Head
Bath Bag 85-105oC
Extruder Bag Customer
Source: Practical Guide to Rotational Moulding
Die Plate Process Water
Source: Gala Industries by R Crawford & M Kearns
Pump
47. What are the benefits of micro pellets?
Manufacturing Process
• Consistent quality and size
• Less contamination
• Dust free during mould filling
Shape and Particle Size Distribution
• Potential for faster cook times
• Potential for lower peak internal air temperature
• Greater bulk density (0.51kg/m3 cf. 0.34 kg/m3)
• Improved flow (10 sec cf. 32 sec)
• Potential for improved mechanical properties
• Improved filling and definition of difficult to fill mould areas
• Moulded parts with more uniform wall thickness
• Maintained / improved surface finish
Other
• Cheaper raw material
IMPROVED ECONOMICS AND MORE OPTIONS FOR THE MOULDER
48. Micro Pellet Tank Trial
Oven machine, Tank Size = 2,250L, Shot Weight = 55kgs, Wall thickness = ~5mm, Colour = Merino.
50
Cycle Time (mins)
45
40
35
30
25 > 12%
20
Powder Tank - 1 Tank - 2 Tank - 3
Cool Time (min) 17 16 16 17
Cook Time (min) 28 24 25 24
Cook Time (min) Cool Time (min)
Meet requirements of tank standard impact testing with no ductile or brittle
failures using a 9.06kg drop dart @ 1.4m.
49. Rotomolding PPAs:
Silica Fume (SF): "Free-Flow"
Agent.
PEG+SF: Densification &
Processing Aid (D&PA).
PEG + Carboxylic Acids (CA):
Advanced D&PA.
Novel Processing Additives:
PEG + CA + SF.
50. Extrusion PPAs:
Silica Fume (SF): Thickening
Agent.
PEG+SF+Anti-Oxydants:
Processing Aid (PA).
PEG + Carboxylic Acids (CA):
Advanced PA.
Novel Processing Additives:
PEG + CA + SF.
51.
52.
53. Process comparison
MICRO-PELLETS MICRO-PELLETS
WATER COOLING AIR COOLING
Process Water
Centrifugal Dryer
Cutter Cutter
PDV PDV
Powder Powder
190-210oC 125-135oC
Water
Bath Bag Extruder
Extruder
Die Plate Die Plate
Source: Gala Industries Process Water
Pump Bag
Air Cooler