Impact Media For Mechanical Plating and Mechanical Galvanizing
Mechanical  plating  and  mechanical  galvanizing  utilize  the  energy  in  glass beads to 
"cold-weld"  the  plating  metal to the surface of  the part to be plated. The selection of the 
impact  media has an important effect on the quality of the plating obtained.
Mechanical   plating  was  developed  by  Erith  Clayton  of  The  Tainton  Co.,  Baltimore,  
Maryland, in the late 1940's and early 1950's. The Tainton Co. was involved in producing  
flaked  metals  from  metal  powders.   In  this  process,  metal  powders were tumbled with 
steelballs to produce a powder comprised of thin, shiny particles. Clayton noticed that the 
steel  balls  used  in  the  process did not rust, and hypothesized that this was the result of 
some   of   the   metal   powders   being   plated  on  to  the  steel  balls.   Clayton  felt  that  
modifications  of   the  chemistry  could  provide  a process for depositing metal on metal 
without  the  use  of  electricity.
Clayton  started  a  new  corporation,  Peen  Plate,  to  develop  the  chemistry  required to 
deposit   commercial   thicknesses   of  plating  metals.   After  numerous  experiments,  a 
process  was  developed  in  which  parts  were  tumbled with steel shot,  zinc dust, and the  
chemicals that Clayton had developed. The process generally took at least several hours,   
and  often  took over 8 hours to achieve the thicknesses required.  The steel shot required 
stripping  with  acid  after  each  run.
Peen Plate,  lacking  the  resources  to  achieve commercial development of the process,  
licensed   the  mechanical  plating  process  to  3M  of   St.  Paul,  Minnesota.    3M  made 
significant  improvements  in  the  process,  reducing  the  cycle time to approximately 90  
minutes  per  run.   One  of  the  most  important  improvements  made  by  3M  was  John 
Cutcliffe's  development  of  the  use of glass beads as the impact media in place of steel  
shot, an  invention  which  is  at  the  foundation  of  mechanical  plating today.  This was a 
useful  concatenation  of  3M's  position  in  mechanical  plating  and  their  position  as  a 
leading  developer  of  retroreflective  glass  beads  for  safety  purposes (Scotchlite signs 
and  Centerlite  'Road Mix').  Glass  beads  offer  the  advantages  of:
n      Chemical Inertness
n      Low Cost
n      Readily Available
n      Many Sizes in stock
n      Non- Toxic
n     Low Coefficient of Friction
n     High Crush Resistance
n     Non-absorbent   
n     Low Abrasive Wear
n Recyclable and Reusable
For  mechanical plating,  the usual "rule of thumb" is that for each cubic foot  (by volume) 
of  live  load  of  parts,  the  plater  uses one cubic foot of media. For plating cross recess 
screws,  the  ratio  of  media  to  parts  is  often  reduced, and  the  water  level raised.  For 
mechanical  galvanizing  (thicknesses over 0.001") the general rule is to use 2 cubic feet 
of  media  to  one  cubic foot of parts to provide additional cushioning to prevent chipping 
during  the  plating process.  If  the  part type is difficult,  the ratio of impact media to parts 
may be increased even more.

Media Mixes for Mechanical Plating and Mechanical Galvanizing
The media mix most commonly recommended is as follows: 
     n    4 volumes (50%) 4 mm (4 - 6 mesh) or 5 mm (3 - 4 mesh)
     n    2 volumes (25%) 8 - 10 mesh or 10 - 12 mesh beads
     n    1 volume (121/2%)  16 - 25 mesh beads 
     n    1 volume (121/2%)  mush or fine  beads - usually 50 mesh beads
This mixture is sometimes called a "4-ball" mix.  A "3-ball" mix is similar to the above but 
with one intermediate size removed.  A "2-ball" mix is usually large beads (3 - 5 mm) and 
mush beads.
On  some  machines,  this  preferred  media  mix  cannot  be  used.  The  most  common 
example  is  the  old  3M  "Metal Plating Centers"  which  (usually)  have 3/16" perforated 
holes in the separator unit which would trap the media with the parts. For these machines,  
we  recommend:  6  parts  8 - 10  mesh  beads;  2  parts 18 - 25  mesh  beads;  and 1 part 
50 - 70  mesh  beads.
On some part types,  such as cross-recess screws,  one media size will lodge in the cross 
recess.  Generally,  this is media in the 10 to 25 mesh range. If any media size is capable 
of  lodging  it  will  lodge.  Therefore,  the  plater  must select a media mix that contains no 
sizes that will lodge.
There  is  a  simple  test  for  lodging. Take  the  media  that  is being contemplated as the 
plating  medium  and  a few of the parts.  Place them in a pint  plastic bottle with water and 
shake  vigorously  by  hand  for  two  or  three minutes. If the media can lodge in the parts,  
it will be evident.
It  is  impossible  to  completely  separate  media  in  such  a way that 100% of the lodging 
size  is  eliminated;   media  in  the  sump,   in  cracks  or  crevices  in  the  barrel,   in   the  
piping -  all  these  contribute  to  the  problem.
For  some  part  types  the  only  alternative  is  to use straight  "mush"  media, which is 50 
mesh - 100 mesh (i.e.,  50 - 70 mesh [PS5070],  60 - 80 mesh [PS6080] or 70 - 100 mesh 
[PS7000])   with  no  larger  media.   This  media  mix  has  poor  flow  characteristics  and  
typically  plates  at  a  lower  efficiency  than  other  media  mixes.  However,    if  the  parts  
themselves  act  similarly  to  the  media,  this  will  work  acceptably.
Media should not contain an appreciable amount of broken media. Typical specifications  
are  under  5%.  Running  heavy parts at too high a speed will break down the media.  The 
'crush resistance'  of  glass  beads  is  about  31,000  to  36,000 psi.   This  is  significantly  
in  excess of the force needed to plastically deform the small  (3 - 7 micron)  zinc particles 
so  as  to  'cold  weld'  the  particles  to  the  substrate.   Thus,  broken  media  is  generally 
evidence  of  excessive mechanical energy being applied during the mechanical deposi-
tion process.
Another media mix that is worth evaluation is a mixture of only large beads (over 5 mesh) 
and  fine  media.  Typical mixtures are 50%  to 70%  large beads ( 3 to 8 mm) and 30% to 
50%  fine  beads  (50 to 100 mesh). The large beads are typically 3, 4  or 5mm beads but 
they can be even larger - such as 6mm,  7mm, or 8mm beads (available on special order 
from PS&T).The  larger beads are typically made by a molding process, and are typically  
both  durable  and  expensive.   A  media  mix  like  this  will  offer  both the impact energy 
associated  with  the  use  of  large  beads  and  the  "throw"  associated  with  fine  media.
For  some  part  types,  platers  have  developed  their  own  media  formulations.  A great  
deal of flexibility is possible in mechanical plating. The only  plating  formula PS&T does  
not  recommend  (unless  absolutely  necessary)   is   the use of  formulations  that do not 
include  a  fine  mesh  impact  media.  Without  the  fine media,  the deposit is rough,  the 
efficiency  is  low, and  the  throw  into  recesses  suffers.  The mechanical plating process 
relies  on  the  action  of  the fine beads to break  up agglomerates  of zinc that form in the 
(acidic) plating process.  Without the fine beads, the  agglomerates  remain  undispersed,  
resulting  in a coarse deposit or an 'orange peel' effect.
During  the  plating  process (including, in particular, the separation and media return) the 
fine  media  is  typically lost from the system due to dragout. The finer the fine beads,  the 
more  of   these  losses  are  encountered  (i.e., 100  mesh  is worse than 70 mesh and 70 
mesh  is  worse  than 50 mesh). This  must then be periodically replaced.  Alert operators  
can  tel  when  their  plating system is low in fine media by seeing how the process cleans  
in recessed areas such as thread roots  and  how well  the  process  plates in these areas.
Sampling  of  the  media  to  determine  the  relative amounts of each of the various sizes 
may  be  performed.  The  actual separation of the various sizes of media is performed by 
vibrating  a  stack  of   U. S.  Standard  Sieves (available  from  many  lab  supply  houses  
and  from  Gilson,  who  specializes  in  particle  testing).  The  most  common  difficulty  is 
obtaining  a  uniform  sample of the media since the media tends to stratify with the larger 
beads   rising  to the surface.   (Why? As particles bounce upward,  gaps open up beneath 
them - some large and some small.  The small gaps  are  more  common  than  the  large  
gaps,  so over time the small particles tend to movedownward and the large particles tend 
to move upward. [This explanation from Dr. Friedrich Prinz of Carnegie-Mellon University.])  
Dry  media  mixtures  may  be sampled with a tube or with a 'spinning riffler.'  Damp or wet 
media  may  be  sampled  with  a sampling probe such as those used to sample grain per 
ASTM C 183.  Slurries  may  be  tested  with  sample  cups  designed  with  a  long 'cutter' 
engineered to cut through the slurry  and  provide  a  uniform  sample.  Another  sampling
procedure  is  to  take  small  samples  continuously from the batch of impact media as is 
returned  to the plating barrel; this way, even  if  the  media  is  stratified,  a  representative  
sample will beobtained. Additional information on sampling procedures is available  from  
PS&T  Technical  Service  Department

Reference Materials
MIL-G-9954A (1 November 1966) "Glass Beads:  For Cleaning and Peening"  This is the 
Military  Specification  for  glass beads and many glass beads, even though not intended 
for military use, are sold by the MIL-SPEC sizing system.
ASTM E11-95 "Standard Specification for Wire Cloth and Sieves  for  Testing  Purposes" 
The  standard  reference  for  particle  sizes.
ASTM STP447B "Manual on Test Sieving Methods" More detailed information on types 
of  sieves, sampling  techniques  for particulate materials, and test sieving for a variety of  
industrial  products  with  some  useful  technical  background.
ASTM D1214-89 (1994) e1 "Standard Test Method for Sieve Analysis of Glass Spheres"   
How  to sieve  glass  beads  and  get  accurate  reproducible  results.
ASTM D1155-89   (Reapproved 1994)  "Standard  Test  Method  for Roundness of Glass 
Spheres"  In  this  test  method,  the  glass  beads  are  mechanically  separated  into true  
spheres  and  irregular  particles  on  a  glass  plate  fixed  at  a  predetermined  slope.
All   ASTM   specifications   are  available  from  the  American  Society  for  Testing  and 
Materials  by  mail,   fax,  or   web site  access.   ASTM,    100   Barr   Harbor  Drive,  West  
Conshohocken PA 19428. Phone 610-832-9585, fax 610-832-9555, 
"Particle Sizing and Sampling" (Catalog). Gilson Co., Inc. P.O. Box 677,  Worthington OH 
43085-0677;  800-444-1508   or  614-548-7298;    Fax:  800-255-5314  or  614-548-5314.   
This  company specializes  in  products for testing particulate materials - sample splitters,  
spinning rifflers,  testing screens,  sieves,  shakers,  riddles,  etc. 
McNichols  Master  Catalog - from  McNichols  Co.,   5505   W. Gray Street,  Tampa  FL 
33609-1017; 1-800-237-3820.  McNichols  is  a  source  of  perforated  metal,  wire cloth,
test  sieves,  and  similar products.
Physical and Chemical Properties of Glass Beads
Glass is one of the oldest industrial materials, dating back to about 2500 BC. Soda lime  
glass   (from  which  glass  beads  are  made)  is  an  amorphous    (i.e.,  non-crystalline) 
material  produced  from sand (Silicon Dioxide,  Sio2), Limestone (Calcium Carbonate,  
CaCO3)  and  Soda Ash (Sodium Carbonate, Na2CO3).
Typically glass will have the following physical characteristics:
n Specific Gravity 2.50			n Clear, colorless or slightly blue
n Crush Strength 31,000 - 36,000 psi	n No Free Silica
n Moh's Hardness 5.5			n Smooth, vitreous, non-absorbent surface

PS&T Size Designations for Impact Media for 
Mechanical Plating and Mechanical Galvanizing
Our  designation follow the 3M Industrial Mineral Product designations. The first two digits 
represent  the  smallest screen thought which most of the beads will pass and the next two 
digits  represent  the  largest  screen  upon  which  most of the beads will be retained.  For 
example,   the 3M  IM1625  was  a glass bead mixture  with  80%  of the beads larger than 
25  mesh  and  smaller  than 16  mesh.  Their  system  was  not  as  explicitly  accurate as 
intended - for example 3M's 'IM0405' beads were  molded 5 mm beads, so they probably 
should  have  been  'IM031/205' ;  the  IM1013 had  80%  of  the  beads  between 10 mesh 
and 14 mesh, so it should have been1014  (there is no13 mesh defined  by ASTM E-11);
and  obviously  'IM5050'  is  not really descriptive of the actual product which was 40 to 70  
mesh,  so  that  product  should  have  been  'IM4070.'
With the foregoing discussion complete, PS&T size designations are as follows:
A 5mm molded bead
A 4mm molded bead
A 2.85mm - 3.30mm screened bead mixture
A 2.00mm - 2.30mm screened bead mixture
A 1.70mm - 2.00mm screened bead mixture
A 1.55mm - 1.85mm screened bead mixture
A 1.25mm - 1.55mm screened bead mixture
A 1.00mm - 1.25mm screened bead mixture
A 0.75mm - 1.00mm screened bead mixture
A 20 - 30 mesh (80% range) bead mixture
A 30 - 40 mesh (80% range) bead mixture
A 40 - 50 mesh (80% range) bead mixture
A 50 - 70 mesh (80% range) bead mixture
A 60 - 80 mesh (80% range) bead mixture
A 70 - 100 mesh (80% range) bead mixture
We  can  also  "special order" any type of glass beads that you might require for any 
special application.
Beads  are  conventionally  given  a nominal size range.  However,  not all beads fall into 
that  size  range.  Up  to  20%  may,  by specification,  be above or below the nominal size 
range.  For  example, a typical 20 - 30 mesh bead may have as much as 5% as large  as 
14  mesh  (but  none  as  large as 12 mesh) and  as  much  as  15%  finer  than  30  mesh  
(but  not finer than 40 mesh).  If this represents a problem for a specific part type, then the 
plater  can  screen  out  the  offending  media sizes  using  appropriately  sized  screens.
Beads  that  are  very  fine  (100 mesh and above)  are  quickly  lost  in  most mechanical 
plating  processes  because  of  hydraulic  flow - the  lighter a bead is, the more likely that 
the flow will be strong enough to carry the bead into the waste treatment system.  For  that  
reason, all platers make up primarily with fine beads.  Some have practices in which they 
routinely  add  a fixed quantity of beads (often 50 pounds or one bag) per barrel per week.
Occasionally,  the  media  will  become  so  severely  contaminated  with tramp metal and  
metallic  fines  that  the  most economic means of recovering it is to remove all of the fine  
particles  -  glass  beads  and  contaminants - and  discard  them.
Plating  of  recessed  drive  screws  is  typically  accomplished  by using a ratio of about 3 
cubic  feet  of parts to be plated to 2 cubic feet  of media.  This allows the media to flush in  
and  out  of  the  head of the fastener, allowing at least some throw into the recessed drive. 
Normally  the only size of media used is 'mush' or about 50 mesh beads.  In essence, the  
head  of  the  fastener  is  the substitute for the large beads the plater would otherwise use.

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