ADVANCES IN MECHANICAL SURFACE PREPARATION
Kumar Balan, Global Product Specialist,
Wheelabrator Group
Burlington, Ontario (Canada)
Abstract
The coatings industry is very familiar with shot/grit blasting as a mechanical
means of surface preparation. Over the years, operators have worked with manual and
automatic blast equipment with its associated dust collection arrangements. As with every
manufacturing process, operating costs are very critical in this highly competitive
industry. Therefore, coaters are always on the lookout for ways to tune their process and
increase productivity.
Fortunately, blast industry professionals are also exposed to an allied process
called shot peening which promises to elevate blast cleaning to a more productive level.
Shot peening is a technique to strengthen production parts that relies on process
control. Application of these controls can greatly benefit the blast cleaning process.
Proper media selection and effective equipment operation and maintenance within
identified parameters can reduce costs and increase productivity. Though the pace of
process development and standardization has been relatively slower than more
sophisticated, computer controlled production machinery the future in this industry is
exciting and encouraging.
This discussion will also include green manufacturing and how it is more than just
a social cause for this mechanical surface preparation technique.
Blast Cleaning Basics
In 1932, the surface preparation industry saw the
introduction of its first airless centrifugal wheel. Until then
blast cleaning was being carried out only with airblast
nozzles. Today, both nozzles and centrifugal wheels are used
in the mechanical pre-treatment of steel. Assuming stability
of all other factors, productivity of the cleaning operation is
directly proportional to the amount of abrasive being
propelled on the part being cleaned.
Centrifugal wheels are more productive than blast
nozzles. In comparison, airblast nozzles (3/8” diameter at 60
psi) propelling metallic abrasive typically discharge 10% of
the blast media propelled by a centrifugal blast wheel (15”
1
diameter wheel, 20 HP motor). Centrifugal wheels are generally limited to ferrous blast
media, whereas blast nozzles can propel both ferrous and non-ferrous media. Centrifugal
blast wheels are either direct driven or belt driven through a bearing system. The
resulting line speed of the associated blast machine is directly proportional to the total
connected wheel horsepower.
SSPC: The Society for Protective Coatings identifies five initial rust conditions
before surface preparation. This covers the range from plain mill scale to rust and pitted
conditions. These are identified as Condition A, B, C, D and G (images available in the
SSPC-VIS 1)
The most common initial conditions of steel are Condition A and B. The resultant
expectation of surface finish is traditionally SSPC-SP 10 / Sa 2-1/2 / NACE 2 or NearWhite Blast Cleaning.
Cleaning Process Parameters
The purpose of cleaning is to remove rust, scale, sand or any such contaminant
from the surface of a metallic part. Blast cleaning may also impart a profile on the surface
to facilitate a downstream coating process such as painting or galvanizing. Given its
reach, it’s not uncommon for a metallic component to have been processed in a blast
machine atleast once during its manufacturing process.
The above described methods to evaluate cleaning quality and cleaning results are
largely visual. If a particular anchor profile or etch is desired on a part, the roughness
becomes a parameter to check using a profilometer. In certain cases, in order to remove
the element of subjectivity, a copper sulphate test is performed to determine whether a
part is properly descaled or not.
Blast cleaning doesn’t involve monitoring of media velocity, size or shape. In
fact, for certain cleaning applications, it is advantageous to have an operating mix of blast
media in the machine. This might also include a mixture of spherical shot and angular
grit. Let’s discuss peening and compare it to cleaning with relation to process variables
and determine if we can benefit from the quality of results.
Shot Peening:
Peening is a process that induces
compressive stress into the surface of the component.
Induced compressive stresses counter the tensile or
working stresses during the component’s work life,
neutralizing the detrimental effect and preventing
failure.
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When peening, the shot peener is required to achieve a particular Almen intensity
(measured by deflection on a representative strip of spring steel), based on demonstrating
saturation and 100% or higher visual coverage. This intensity directly translates to a
particular value of compressive stress that the component designer has specified to be a
requirement. In all cases, the size of blast media, i.e. steel shot, conditioned cut wire,
glass bead and ceramic is also dictated by the specification.
Both centrifugal wheels and compressed air nozzles are used for shot peening.
Consider the following to determine your choice:
•
•
•
•
Propelling blast media using centrifugal wheels is generally more
productive than using compressed air for the same purpose through a
single or multiple blast nozzles.
Blast nozzles are ideal when specific targets / areas on the part are
required to be processed.
Blast nozzles are also employed when the requirement is to peen with nonferrous media such as glass bead and ceramic.
Blast nozzles better adapt to automation (nozzle manipulation) than
centrifugal blast wheels.
In high-production environments such as in the auto industry, blast wheels are
most likely the only solution when peening parts within a prescribed time constraint.
Typical applications include peening connecting rods, gears, leaf and coil springs.
Independent of the type of media propulsion, shot peening requires monitoring
and closed loop feedback for critical process parameters such as media velocity and flow.
The “science” of peening rests in all such parameters and close adherence will result in a
final product that performs to or exceeds expectations. To summarize our discussions:
Cleaning
Peening
Media velocity
No monitoring, as long as
equal to or greater than a
critical value (usually 240
feet per second)
Measurement and
monitoring critical as it
affects part quality / life
Media size
Consistency not important
Consistency critical
Media shape
Semi-critical
Consistency and monitoring
critical
Measurement of results
Visual only
Quantitative, and needs to
be regularly done
For critical etching
applications
Specification driven and
required by most audits
Monitoring of results and
reporting inconsistencies
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Process Variables and Their Effects
In order to achieve repeatable and consistent peening results, the following
process variables are managed:
•
•
•
Media (shot) flow rate
Size of media (shot)
Velocity
•
Media flow rate is controlled using a
commercially available flow control valve
at the feed side of the blast wheel, usually
at the outlet of the media storage hopper,
or at the outlet of the blast tank upstream
to the hose leading to the blast nozzle)
•
Vibratory Classifier classifies the peening
media into oversize, right and
undersize/fines. A combination of two
sieves ensures that consistent size of
peening media is available for every
cycle. Size of peening media plays an
important role in determining coverage
and peening cycle time.
•
Mix
Waste
In a compressed air style peening
machine, the media velocity is determined
by the air pressure with which the media
is propelled, it follows a direct
Small, broken shot and dust
proportionality. Well designed peening
machines feature closed PID (Proportional – integral – derivative) loops for air
pressure which will maintain constant air pressure through a peening cycle. In a
centrifugal wheel type machine, the diameter of the blast wheel and its speed
determine the media velocity.
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To
hopper
Relevance to the Coating Process
Lower operating costs
Blast cleaning is defined by the energy imparted by the abrasive on to the part being
cleaned. This is a function of mass & velocity. If your blast machine is capable of altering
the media velocity (determined by wheel speed in a centrifugal wheel machine and air
pressure in an air type machine), you may find that cleaning a particular part does not
always require the maximum velocity that the wheel or the nozzle is capable of
delivering. Reduced wheel speeds and air pressure may provide you with equally clean
results. The resulting reduced energy dispensed means less energy consumed, reduced
wear on the machine components and media breakdown. This ultimately results in lower
operating costs.
Consistency of process – use of Almen strips
Blast cleaning results, even with visual comparators can be subjective. However,
there is nothing subjective about peening results. Operators are required to check Almen
strips at definite intervals (once a shift), or when changing from one part type to another.
Similarly, when cleaning parts, the same process can be followed at regular intervals,
maybe at the start of a shift, to determine whether the machine is performing as it was
when the previous batch of parts was being cleaned. This not only tests the health of your
machine, but also gives you documented proof of your operation. This provides a
credible explanation when your customer introduces a part with different material, scale
or contaminants and his expectations of cleaning quality are not being met by your
machine.
Our company conducted a case study on behalf of one of our customers. The
purpose was to assess the effect stand-off distance (distance from part to nozzle) had on
the intensity of blast for an airblast cleaning application. These results helped determine
the optimal distance given the constraints of surface roughness. This also led to use of
fewer nozzles and reduced compressed air, given that nozzle blast patterns flare out with
increased stand-off distances. It is important to note that such tests, as in this case, can
also be carried out using non-ferrous media such as aluminum oxide.
The relevance, of course, was reduced operating costs and better utilization of
resources.
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Stand-off
Distance
220 AlOx at 20 PSI
with N Strip
220 AlOx at 30 PSI
with N Strip
220 AlOx at 40 PSI
with N Strip
Arc
height
Ra Micro
inches
Arc
height
Ra Micro
inches
Arc
height
Ra Micro
inches
3”
0.011
85
0.0146
122.7
0.0153
135.5
5”
0.0088
57.2
0.01205
109.1
0.0127
112.4
7”
0.00755
57
0.01165
105.3
0.0111
88
9”
0.0064
56.1
0.0116
102
0.0107
84.5
0.0064
54.1
0.0109
99.3
0.01065
11”
Effect of stand-off distance on impact energy / peening intensity
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As seen in the tabulated data above, particularly at results at 40 PSI, the exponential drop
in intensity at the start, when stand-off distance is gradually increased. It continues to be
steady following that, until the part is almost 11” from the nozzle. This information is
useful in determining the capacity of a particular abrasive to impart energy, at different
distances from the part. This information helps in positioning the blast nozzles in an
automated blast machine and optimizing coverage with the distance from the part.
Other Inherent Benefits – Contaminant-free Blasting
Certain applications, particularly in the aerospace and medical industries, specify
contaminant-free blasting of critical components. Contamination may be potentially twofold, material and chemical. Chemical contamination can be avoided by using the
appropriate type of abrasive (stainless steel to prevent carbon steel contamination etc.).
Physical or material contamination manifests itself in the form of damage to the part by
way of a foreign object making its way into the blast stream. In working with critical
components such as aircraft engine parts, landing gear etc., foreign object damage is
unacceptable.
When working with such applications, it is advisable to complement a traditional
rotary screen and an airwash separator with a media screener such as a vibratory
classifier. Larger size contaminants such as nuts, bolts, etc. sometimes, accidentally find
their way into the media stream inspite of the presence of a rotary screen. In such cases,
with a second stage, these will be separated in the vibratory classifier and not clog the
pressure pot outlets in an airblast machine or damage wheel parts in a wheelblast
machine. Eventually, this prevents damage to the critical component being cleaned.
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Surface Preparation and Green Manufacturing
A cleaning process that generates dust hardly seems like the poster for green
manufacturing. However, consider the following aspects of blast cleaning and their
association with green manufacturing. A byproduct of green manufacturing initiatives is
the associated cost savings and increase in productivity.
A shot blasted part offers better bonding properties,
whether it is paint, adhesives or any such coating when
compared to a part that hasn’t been blasted. The profile
that is imparted on the part surface by the abrasive has a
direct impact on the paint consumption. This parameter
is closely controllable by monitoring process parameters
as discussed earlier. Paint savings are significant,
particularly in high volume production environments
such as metal processing facilities and shipyards.
Preservation lines, as they are commonly known,
consist of a pre-heater and blast machine upstream to the
paint booth. A uniform dry film thickness (DFT) on
plates and profiles is achieved by processing them through the blast machine and
cleaning to a near-white or white metal finish before painting.
The global railway industry utilizes blast cleaning techniques, both manual and
automated, in the manufacture of new and to re-condition railcars. The end goal is
consistent - minimizing paint usage for economical operation. Hot rolled steel strips are
cleaned in a series of blast machines for descaling purposes – a process that’s otherwise
carried out by acid pickling, with its effluent treatment environmental impact..
Summary:
This seemingly simple mechanical surface preparation technique does have a firm
technological footing to it. Lessons learnt from sophisticated applications in the
aerospace and medical implant fields are specifically applicable when processing more
conventional products as well. Such lessons lead to significant operating cost savings and
provide impetus that drives this industry to seek improved methods.
Another development that is worth mentioning is the emergence of newer designs of blast
wheels. Such designs have resulted in direct drives that eliminate maintenance prone
belts and sheaves. Blast wheel housings are now fabricated from wear resistant material
such as manganese steel instead of multiple sets of liners etc. Newer designs have also
reduced the number of parts, making it easier on maintenance.
The future of this industry will benefit from the use of advanced controls, and of course
the macro goals of better quality, lesser cost and faster delivery.
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