What Is Powder Grinding? - the Global Home of Rotational Moulding

Rotomould 2009 Sydney
What is Powder Grinding?
Ian Hansen
1
GRINDING OR PULVERISING BACKGROUND
Size reduction techniques have been used for thousands of
years. Early forms of size reduction were crushing or grinding.
In our industry we use an attrition (or abrasion) method. The
rubbing of a particle against a rigid force.
The technological advances over the years have developed a
wide range of varying mechanical size reduction methods.
A boy grinding ore with a
single grindstone
The grinding of polyethylene is difficult on most size reduction
machines because of its low softening and melting temperature.
As a result, the disc attrition mill has evolved as the most
successful mechanical device to grind polyethylene into powder
for the rotational moulding industry.
Millstones mechanised by
the circular movement of
an animal
2
THE PAST – BUHRSTONE MILL
The grinding action used on today’s plastic materials is described as a combination of impact,
shearing and attrition.
The disc or attrition mill used to pulverise polyethylene powder is a modern counterpart to the
early Buhrstone mill, which is considered the oldest form of grinding mill, and was the
“millstone” used for flour production.
Two horizontal circular stones (plates) were arranged so that one
rotates relative to the other. Grooves were cut into the stones and
were usually parallel to one radius of the segment.
The effect of these grooves, after
the grain was fed into the centre of
the upper stone, was to cause the
grain to gradually move out
towards the circumference.
The Pallmann company in Germany was founded in 1903, as a
flour mill company. In 1955 Pallmann developed a mill that was
able to pulverise plastics at ambient temperatures.
3
THE PRESENT – POLY GRINDING MILL
Start
Air
Cyclone
Powder
Hopper
Air Lock
Oversize
Bushmans
Coloured
pellets
Particles conveyed by air stream
Typical schematic arrangement
Vibrating
Sieves
Grinding
Chamber
Finish
Air
Blower
Powder
Packaging
4
Actual arrangement
Air
Cyclone
Air Lock
Vibrating
Sieves
Air
Blower
Particles conveyed by air stream
THE PRESENT – POLY GRINDING MILL
Grinding
Chamber
5
The present – Poly Grinding Mills or Pulverisers
The grinding plates are set with typical clearances of 0.3mm. The rotating plate can be
rotating at speeds of up to 7000rpm. The grinding tooth profile, clearance between the
plates, the speed of rotation and mesh size are selected depending on the material to be
ground and the required particle size.
.
feed
Grinding mill
housing
discharge
disc
6
Coloured pellets being feed into the grinding chamber
Feed In
DIS
GE
R
A
CH
7
Grinding Plates or Discs
The stones have been replaced by steel discs or plates arranged vertically or horizontally,
typically with one stationary plate and one rotating plate.
Vertical grinding plates
Horizontal grinding plates
8
Rotating Disc or Plate
Grinding
Discs
Grinding teeth
segments
The Discs have a series of
serrated teeth cut into them.
Fixed Disc or Plate
9
Grinding Plates Sharpening
The sharpness and angle of the cutting teeth needs to be managed to maintain output
rate, but also powder quality.
There is cost involved in shutting down
machines to sharpen the discs, which is
typically balanced, primarily, against the
output rate.
Discs have a finite life.
ICO Gainsborough, UK
Re-sharpening of grinding discs is an
important maintenance task. Most sharpening
is undertaken by 3rd party engineering
companies
10
Screening process
The old Buhrstone mill controlled the fineness of the product by pressure between the two
stones and by the grinding speed.
In today’s grinding machines, the grinding speed is fixed, and therefore the fineness of
powder is controlled by the selection of both the grinding teeth profiles and the size of the
screens used in the sifter.
The polyethylene particles that exit from the
grinding plates are conveyed to the sifter unit.
feed in
One material enters the stack of screens, and two
materials exit the screens.
Multi deck screens. The
oversize material (red) flows
over the entire screen (blue)
area. Wood frame – grey.
ize
e rs
Ov
Screen
Pe
llet
s
The correct size particles pass through the wire
mesh screens, while the OVERSIZE particles
eventually are returned back to the grinding
chamber, to pass through the grinding plates again
11
Sifter Units
feed in
Powder Feed
IN
discharge
mesh screens
Powder OUT
Sifter
discharge
Oversize
Powder OUT
Typical Sifter unit
12
Powder Screening
mesh
screens
Sifters (also called screeners) used today are typically the
gyratory type. These have multiple stacks of wooden
frames which have wire mesh screens stretched on them.
The powder is normally known after the wire cloth used in
the screens.
30 mesh = 95% of the powder is less than 600 microns
Wooden
frames
Stainless Steel screening
cloth (mesh)
13
Powder Quality
The grinding process has evolved over time, from the stones discs to metal discs. Despite
the advances in technology, variations still occur from batch to batch, and can result in
variable moulding outcomes.
Having the ability to identify significant variations in powder quality can minimise
unsatisfactory moulded products.
Dry flow is the measure of how a powder will tumble and flow
in the mould during the rotational moulding process. The dry
flow affects the heat transfer in the powder pool and how the
powder distributes itself over the mould surface.
Dry flow depends mostly on the particle shape, and to a
lesser extent particle size. Particles that have been ground
poorly will have ‘tails’ and tend to be ‘fluffy’. These two
characteristics will give the powder poor flow properties possible leading to uneven wall thickness, bridging across
narrow recesses and a high void content within the moulded
product.
Measuring Dry Flow is the single
most common method to
determine powder ‘quality’
14
www.plasticsconsulting.com
The Sales Pitch
At the 2008 Gold Coast conference we advised that the ARMA Materials committee
was working on an improved internationally recognised standard for Dry Flow funnels.
This was co-ordinating with our ARMO partners and ARMI
This specification has a number of improvements, with the most significant being
smaller tolerances on the outlet hole, to minimise the significant testing differences
between powder suppliers funnels, for “exactly” the same powder.
Current standard allows exit hole cross sectional area variation of +/- 20%
New standard allows exit hole of 9.99-10.01mm. An area variation of +/- 2%
ARMA are now able to offer the new dry flow funnel around the world for an
investment of around AUD350. Contact Leisa or Michelle for details.
This is best price/performance quality tool available to you.
15
Proposed new global standard for
Dry Flow Funnel fabrication
Surface roughness
of 16 micro inches
Measurements in mm
Material : Aluminium Alloy
Finish :Sulphuric Anodised
16
Incoming Quality Control
You don’t have AUD350 available within your
Quality Control budget?
Your powder is never going to be the cause of any
moulding problems in your factory?
If you rely on your powder supplier for incoming raw
material Quality testing, request a copy of their
testing certificate, for each batch delivered.
Ensure that your powder suppliers are using the
new global standard funnel in their testing of
powder quality.
17
What is the future for powder grinding?
Powder
Powder 200X
18
Contact details for information on sourcing dry flow funnel (and robust stand and density cup)
Email: [email protected]
What Is Powder Grinding?
By Ian Hansen
[email protected]
19
ITEMS AVAILABLE FOR PURCHASE FROM ARMA
A Practical Guide To Rotational Moulding
‐Roy J. Crawford and Mark P. Kearns This book – A Practical Guide to Rotational Moulding – describes the basic aspects of rotational moulding and includes information on the latest state of the art developments in the industry. A key feature of the approach is to use photographs wherever possible to illustrate the points that are being made. This book will be useful to those new to the industry, as well as those who are experienced in some aspects of the process. Rotational Moulding: Design, Materials, Tooling and Processing ‐Glenn L. Beall This book was written as a guide to the rotational moulding process. It informs on how to understand and use this manufacturing technique to its maximum capabilities. The reader will gain a broader understanding of this process and will be in a better position to design and develop low cost, high quality plastic parts which will work right on the first try. Rotational Molding: A Practical Guide ‐Paul Nugent This book is an in‐depth study of the rotational moulding process, bringing together much of the technical work performed in rotomoulding around the world over the last 30 years. Need practical tips on rotational moulding? Want to find out about the latest developments in technology and where they are happening? Want to see real‐world examples? Rotational Molding: A Practical Guide is an easy‐to‐use reference written in plain, readable English. Rotational Moulding : Theory & Practice ‐AISR & ARMA Hot off the press! This fantastic, easy to read complete guide to the process and design was originally authored by members of the Italian Association and it has recently been translated into English by ARMA. The book includes comprehensive chapters on the process, machines, moulds, materials, thermal cycles, designing, secondary operations and other applications for the process. Its the newest “must have” for your specialist library. Rotational Moulding Resource Manual ‐ ARMA Full of fantastic information, resources, templates and articles, the ARMA Resource Manual is free to members and updated regularly throughout the year. Like an extra copy? Just fill out the order form and we’ll get one through to all your plants and make sure they receive all the updates! ARMO Dry Flow Test Apparatus (2009 International Standard) ‐ARMA The ARMA materials committee proposed a new guideline for dry flow testing to the international industry in 2009, which has since been endorsed by the other international organisations, to allow “apples to apples” testing results, no matter where the tests are performed. The equipment has been specially engineered to withstand normal factory conditions. ARMA RESOURCES ORDER FORM Company: Contact: Address: Post Code: Telephone: QTY BOOK PRICE TOTAL
A Practical Guide to Rotational Moulding (Crawford and Kearns) @ $190.00 each Rotational Moulding: Design, Materials, Tooling and Processing (Beall) @ $150.00 each $...................................... Rotational Molding: A Practical Guide (Nugent) @ $450.00 each $...................................... Rotational Moulding : Theory & Practice (AISR & ARMA) @ $ 53.50 each $...................................... Dry Flow Funnel Only @ $350.00 each $..................................... Funnel Stand & Bulk Density Cup (Heavy Duty) @$450.00 each $...................................... **Prices shown include GST. *** Please note postage is not included in the above prices. For orders delivered outside Australia, prices will reduce! TOTAL COST $...................................... $.................................... PAYMENT TYPE OF CARD: NAME ON CARD: EXPIRY DATE:
CARD NUMBER: SIGNATURE:
Please FAX through your order form to : +61 (0) 7 3009 0600 or POST to PO Box 826, Ipswich, Qld, 4305, Australia. PO
OWDER DRY FLLOW FUNNEL
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T
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N
New
globa
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nel fabrica
ation
Materrial: Alumin
nium Alloy
Finish: Sulphuric Anodised