Powder Coatings--2. Manufacturing & Quality Control
This section describes the manufacturing and qualitycontrol process for powder coating materials.
The state of the art technology used for producing
industrial powder coatings consists of several distinct
stages, namely:
1) Weighing, premixing and size reduction of raw
materials
2) Extrusion of pre-mix, cooling and crushing of the
extrudate into chips
3) Micronising the chips into the final powder
4) Post mixing, packaging and storage.
At each stage of the production process the quality must
be checked because once the powder coating material
has been produced, it cannot be changed or adjusted in
any significant way. The formulation and the
manufacturing conditions are therefore critical.
Reworking of an ‘out of specification’ product is
difficult and costly. (See Figure 1 for a simplified flow
sheet of the powder coating material production
process).
Weighing, premixing and size reduction of raw
materials - Raw materials typically consist of resin,
curing agents, pigments, extenders and additives such
as flow and degassing aids. Each raw material must
pass their individually pre-set quality controls.
Each component is then weighed with the necessary
degree of accuracy (which may be to the nearest tenthousands
of a gram). All pre-weighed components are
placed in a mixing container according to the
formulation. The container is then attached to the
mixing drive and the raw materials are thoroughly
mixed by the specially designed premixer cutting
blades for a pre-set period of time. The raw materials
can also be reduced in size to improve the melt mixing
later in the process.
Quality control
A final sample of the raw material pre-mix is checked
for conformity and processed through a small
laboratory extruder and grinder. The resulting powder
is then applied onto a test panel, cured in the oven and
subjected to various tests:
1) Colour, surface flow and gloss
2) Mechanical performance (including curing)
3) Gel time.
If adjustments are required both the mixing process and
quality control procedures are repeated until the powder
achieves the specification.
No further modification to the powder can be made
after this stage in production.
Extrusion of the premix - The mix is fed into the
dosing system of the extruder. The extruder barrel is
maintained at a predetermined temperature (between 70
& 120ºC, depending on the product type). The barrel
temperature is set so that the resin is only just liquefied
and its contents are mixed using the screw in the barrel.
Consequently, the individual ingredients are dispersed
and wetted by the resin, which produces a
homogeneous composite. The feed rate of the dosing
equipment and the speed of the extruder screw are
balanced to ensure that the screw is kept loaded within
the extruder barrel.
The conditions of high shear and intimite mixing are
maintained within the extruder by precise adjustments
of these three parameters.
The molten mass produced in the extruder barrel is
forced to cool down via a cooling-transporting device.
The solidified material is then broken up and reduced in
size through a crusher into workable chips of 5 to
10mm in size.
Quality Control
At this stage in the process the product quality is tested
using a sample of the chips. The laboratory grind the
chips to a powder and prepare a test panel using the
material. The intermediate product is then checked for
quality against the following criteria:
1) Colour, gloss, appearance and flow
2) Mechanical and reactive properties
3) Application.
Too high a temperature in the extruder barrel will not
only result in a low melt viscosity, low shear forces and
poor pigment dispersion, but will also in turn produce a
low gloss coating. The resin and hardener in the
premix may also start to react in the extruder, which
will also have a detrimental effect on the product
performance.
It is not possible to make changes to the formulation at
this stage in the production process. It is also easier to
handle the extruded chip as a re-work raw material if
manufactured ‘out of specification’ than once the
powder has been micronised.
Micronising of the chip into the final powder - The
chips are ground to the required particle size in a
grinding mill. The chips are fed onto an enclosed
grinding wheel with stainless steel pins, which breaks
the chips down creating a powder. The powder is
carried through a classifier into a cyclone collection
system via a regulated air flow.
In order to achieve the optimal particle size distribution
(psd) further treatment may be needed which can
consist of cycloning, classifying, filtering or sieving.
In modern plants the rejected oversize from the sieving
operation is automatically fed back into the feedstream
of the micronising mill. The typical particle size range
for electrostatic application methods should be within
10 to 100 microns. Deviation from this psd can result
in poor performance and appearance of the powder.
Quality Control
The final powder coating is as rigorously quality
control tested as the extrudate to ensure it meets the
specification of the customer or market. As the particle
size distribution is a critical factor in the successful use
of the powder the particles are analysed for their precise
particle size distribution.
Post mixing, packaging and storage - In order to meet
the customer specification or special conditions of use
additives may have to be mixed through the final
product.
Powder packaging is provided in:
􀀹 carton boxes - up to 25kg
􀀹 bags - 400 to 900 kg
􀀹 metal/plastic containers (Durabins)
The powder can be safely stored if kept in its unopened
packaging in a dry, cool place (30ºC) for upto 12
months. Higher temperatures and longer storage
periods will result in absorption of moisture. Storage
conditions can vary for some powders so the product
data sheet should be referred to at all times.
Quality Control
It is advisable to check the powder after 6 months of
storage to ensure no quality problems have occurred.
Modern powder coating materials from Akzo Nobel can
achieve the same quality standards as liquid coatings
for appearance, chemical and mechanical resistance for
many applications. Once a product has completed the
development stage and its formulation and
manufacturing procedures have been approved, it will
be available for manufacture in any of Akzo Nobel’s
Powder Coating manufacturing plants worldwide. hope can help you 楼主吃了摇头丸？;P http://www.powdercoatingservices.co.nz/Our+Services/Powder+Coating/The+powder+process/Manufacturing++Quality+Control.html
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