MIL-HDBK-1110
i) Vacuum Blasting. Although there are many designs
for vacuum blasting equipment, all systems have a head containing
a blast nozzle, surrounded by a shroud connected to a vacuum
system, and a collection chamber for debris.
j) Centrifugal Blasting. In centrifugal blasting,
abrasive is hurled by wheels instead of being air-driven. This
type of blasting is often used in shop work. Portable devices
have been developed for use on flat surfaces. Abrasive is
contained and usually recycled.
6.7.1.3
Abrasive Properties. The SSPC has a specification for
mineral and slag abrasive, SSPC AB 1, Mineral and Slag Abrasives.
Abrasives covered by the specification are intended primarily for
one-time use without recycling. The specification has
requirements for specific gravity, hardness, weight change on
ignition, water soluble contaminant, moisture content and oil
content. MIL-A-22262, Abrasive Blasting Media, Ship Hull Blast
Cleaning, a Navy Sea Systems specification for abrasives, also
limits the heavy metal content of abrasives. These and other
properties of abrasives are discussed below:
a) Size. Abrasive size is a dominant factor in
determining the rate of cleaning and the profile obtained. A
large abrasive particle will cut deeper than a small one of the
same shape and composition, however, a greater cleaning rate is
generally achieved with smaller-sized particles. Thus, a mix is
generally used.
b) Shape. The shape and size of abrasive particles
determine the surface profile obtained from blasting (Figure 4).
Round particles, such as shot, produce a shallow, wavy profile.
Grit, which is angular, produces a jagged finish. Usually a
jagged finish is preferred for coating adhesion. Round particles
are well suited for removal of brittle contaminants like mill
scale and are also used when little or no change in surface
configuration is permitted. Sand and slag, which are semi-
angular, produce a profile that is somewhere between that of shot
and grit. Currently, sand is used much less than other abrasives
because of health and breakdown factors.
c) Hardness. Hard abrasives usually cut deeper and
faster than soft abrasives. Hence, hard abrasives are best
suited for blast cleaning jobs where the objective is to remove
surface coatings. Soft abrasives, such as walnut hulls, can
remove light contaminants without disturbing a metal substrate
or, in some cases, the existing coating system.
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