Glass Beads For Mechanical Plating and Mechanical Galvanizing

Glass beads 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.  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 ,  60 - 80 mesh  or 70 - 100 mesh   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 .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
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.   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  samplingprocedure  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.

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,
web: http://www.astm.org.

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


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