Canadian Finishing & Coatings Manufacturing - July/August '24 - 15

INDUSTRIAL FINISHING: SURFACE PREPARATION
A manufacturer or job shop must ensure their washer is
constructed with enough stages to handle the most complex
pretreatment requirements, taking into consideration the
finish specifications of the parts being treated.
Pretreatment washers have significant advantages over
wash booths, most notably:
* Parts can be cleaned with more chemicals
* Pretreatment washers provide a more thorough clean, with
nozzles that blast parts from dozens of different angles
* A shop's low-temperature oven is not tied up for drying the
parts, preventing production bottlenecks
* Water costs are lower because of the efficiency of
pretreatment washers
* Additional labour is not required to manually wash the parts
Abrasive Blasting Methods
Abrasive grit blasting, also known as sandblast cleaning,
is a surface treatment process widely used in a variety of
industries, with many diverse purposes. If the surface you are
coating has any rust, a previous coating or scale on it, abrasive
blasting the product is critical to help achieve the highest
quality finish. Failure to remove rust, paint, or scale from the
surface prior to coating can cause the coating to not adhere
properly, often resulting in rework.
With abrasive blasting, an abrasive media is accelerated
through a blasting nozzle using compressed air. The abrasive
used varies based on the surface treatment required. Common
abrasives include steel shot, steel grit, glass bead, crushed
glass, aluminum oxide, silicon carbide, plastic, ceramic grit,
and copper slag. Media selection is a crucial decision in the
engineering of abrasive blasting processes. The different
media types have different hardness, shape and density, and
each is available in a wide range of particle sizes.
A form of abrasive blasting is wet blasting, in which water
moves the abrasives. The water traps the dust produced and
lubricates the surface, cushioning the impact on the surface
and reducing the removal of sound material. A wet machine
has the ability to eliminate the frictional heat that can damage
certain materials.
Most common in large-scale operations, shotblasting has
a different pressurizing system than sandblasting, using
spherical shot as an abrasive media and a centrifugal wheel
for propulsion. Shotblasting works by propelling round
materials, known as shot media, against a surface, which
removes the contaminants of the surface. Shotblasting is a
more aggressive technique than sandblasting, ideal for larger
and more difficult objects that need a strong application force
and denser media material to clean and prepare a surface.
The Society for Protective Coatings (SSPC) specifies four
standards of blasting:
* Brush-Off Cleaning: Cleaning of everything on a product,
other than tightly adhering residues of mill scale, rust and
old coatings, which exposes numerous evenly distributed
flecks of underlying metal. Acceptable in non-corrosive
environments where long-term coating life is not expected.
* Commercial Blast: Cleaning until at least two-thirds of
the surface is free of visible residues. Ideal for applications
where tightly adhering contaminants are allowable on the
surface, for products with lower quality standards and for
non-corrosive environments.
* Near-White Metal Blast: Removes at least 95 per cent of
visible residues. Used for harsh environments in which the
product is exposed to heavy usage.
* White Metal Blast: Removes all visible rust, mill scale,
paint and foreign matter. Used for conditions where
corrosion resistance is very important and the environment
is highly corrosive.
There are numerous environments to perform abrasive
blasting. Among the most common:
* Blast Booths: Provide a safe, efficient environment
July/August 2024
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Canadian Finishing & Coatings Manufacturing - July/August '24

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