Technical data

2.11 PMB REMOVAL METHOD.
PMB is an excellent and rapid method for finish system
removal, but it can cause severe damage to structure and
injury to personnel if not done properly with the right type of
equipment in the an approved facility. In addition, proper
waste management must be ensured for economic and envi-
ronmental reasons. Some types of materials and material
thicknesses cannot be plastic media blasted under any cir-
cumstances. Therefore, PMB is authorized at depot and field
level operations contingent upon SPD approval of facilities,
personnel training, and processes as specified in system or
item specific technical data. These instructions are not in-
tended to be all inclusive, but are general requirements to be
used in conjunction with additional instructions in applicable
system peculiar aircraft or equipment manuals. All PMB
finish removal operations shall conform to the following
requirements:
2.11.1 Media Type. Media used in PMB shall be
fabricated from plastic stocks which are free from high-
density particle contamination and other impurities. The
plastics used shall be of a specific, non-changing chemical
composition as specified in MIL-P-85891, Plastic Media, For
Removal of Organic Coatings, and paragraphs below. Media
used shall have a particle size of U.S. screen 20 to 40 mesh;
however, 12 to 16 mesh may be added as make up media. The
media shall have a particle shape which is irregular with
sharp, angular edges and corners. Plastic media is classified
by type which specifies the hardness and plastic and, there-
fore, the performance characteristics. The following are the
definitions of media types as specified by MIL-P-85891:
2.11.1.1 Type I. A polyester plastic with a 3.0 MOH (34
to 42 Barcol) hardness and the least damaging of all media
types. This media is recommended for use on aerospace
equipment and shall be used if a 1/2 square foot per minute
strip rate can be maintained under the aerospace stripping
parameters listed in Paragraph 2.11.3.
2.11.1.2 Type II.
The Type II media is more aggressive than the
Type I media and will induce greater residual
stresses in the underlying metals if used improp-
erly. Increased residual stresses can cause warp-
ing and increased crack growth rates in aircraft
skin materials and ground equipment enclosures.
A Urea Formaldehyde based plastic with a 3.5 MOH (54 to
62 Barcol) hardness. This media may be used on aerospace
equipment if and only if the Type I material produces less
than a 1/2 square foot per minute strip rate under the
parameters listed in Paragraph 2.11.3. Parameters for the
proper use of Type II media on aerospace structures are also
listed in Paragraph 2.11.3. Type II media is recommended for
use on non-aerospace equipment such as SE. For non-
aerospace use blast pressure shall not exceed 50 PSI on
materials at least 0.040 inches thick. The nozzle shall be held
at least 12 inches from the material to be stripped.
2.11.1.3 Type III. A Melamine Formaldehyde Plastic
with a 4.0 MOH (64 to 74 Barcol) hardness. This Type III
media is not authorized for use in stripping aerospace
structures. Type III media is very aggressive and is recom-
mended only for stripping iron based equipment where
residual stress problems will have no consequences. This
material is authorized for use on SE and other non-aerospace
applications where the metal is at least 0.0625 inches thick.
Blasting pressure should not exceed 40 PSI at the nozzle. The
nozzle shall be held at least 12 inches from the material to be
stripped.
2.11.1.4 Type IV. A Phenol Formaldehyde Plastic with a
3.5 MOH (54 to 62 Barcol) hardness. Type IV is not
authorized for blasting of aerospace structures. Type IV may
be used on SE and on other non-aerospace equipment. Blast
parameters are the same as for Type II.
2.11.1.5 Type V. An Acrylic Plastic with a hardness of
3.5 MOH (46 to 54 Barcol) hardness. This media is autho-
rized for use on aerospace systems as well as non-aerospace
applications. Blasting parameters: pressure 25-40 PSI; stand-
off distance 12 in - 24 in; angle (alclad and composites)0-60
degrees; angle (nonclad) 30 - 90 degrees.
2.11.1.6 Type VII. A Starch-g-Acrylic with a hardness
of 72 to 79 (Shore D hardness). This material is authorized for
use on aerospace systems and non-aerospace equipment
applications. Blasting parameters: nozzle pressure 45 PSI;
standoff distance 12 in - 24 in; angle 0 - 60. This media is
very moisture sensitive and the air flow shall be dry and oil
free.
2.11.2 Media Authorized for Air Force Use. Plastic
media (PMB), while meeting the military specification crite-
ria, can vary from manufacturer to manufacturer. The Air
Force Corrosion Prevention and Control Office (AFCPCO)
maintains a current list of manufacturers that have been tested
and meet Air Force requirements and first article require-
ments of MIL-P-85891. These materials are authorized for
use on aerospace and non-aerospace structures (where autho-
rized by the SPD). Contact the Air Force Corrosion Preven-
tion and Control Office (AFCPCO), 325 Richard Ray Blvd,
Bldg 165, Robins AFB, GA 31098, DSN 468-3284, email
[e-mail] to obtain a current list of qualified PMB
manufacturers.
2.11.2.1 All media shall be tested for contamination per
Paragraph 2.11.3.
2.11.3 Operational Parameters for Metallic Sur-
faces. All PMB operations on metallic surfaces shall
conform to the following parameters:
TO 1-1-8
Change 3 2-19