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PLASTIC MEDIA--A LOOK AT
NON-ABRASIVE PARTS FINISHING
EtPts6-1
L
Richard A. Dotson
Vice President
Maxi-Blast,Inc.
South Bend, Indiana
Presented at:
FINISHING ‘95 CONFERENCE AND EXPOSITION
September 18-21, 1995
Cincinnati, Ohio
49 1
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492
INTRODUCTION
If you are one of the many who think of blast finishing in terms of
"sandblasting" and the many abrasive materials that are associated with this
process, then welcome to the world of non-abrasive finishing with plastic
media.
For many years any time blast finishing was discussed thoughts of steel shot
or grit, aluminum oxide, glass bead, sand, or some other abrasive material
always came t o mind. In the last few years the finishing industry has been
introduced to a number of non-abrasive materials especially designed for
finishing and cleaning parts, large and small, without causing damage or
tolerance changes.
Among these many new media are:
wheat starch, carbon dioxide pellets,
sodium bicarbonate and plastic media. Add to this group the more exotic
ways of non-abrasively finishing, including sponge blasting and lasers, and
the field becomes more exciting and varied than ever before. Parts of all
sizes and configurations, once relegated to hand finishing because of their
delicate nature or size, can now be non-abrasively finished if the right
process and media are chosen. Of this group of media this paper will
describe plastic media, the most versatile and widely used and the most
promising in terms of future applications and benefits to the user.
IN THE BEGINNING
Plastic media first appeared over 25 years ago as the answer to deflashing
and cleaning problems in the rubber, plastic, and electronic industries.
Plastic media is still widely used in those industries today to safely and
efficiently finish parts that have excessive flash and in light deburring
applications.
These thermoDlastic media
products are considered the
pioneers of plastic media types worldwide.
493
On the other end of the spectrum, a different type of plastic media has been
in wide use for the last 10-12 years for cleaning and "stripping" parts of
different coatings safely and non-abrasively. This type of media is a
granulated thermoset plastic media of different types and sizes used
internationally by small and large companies alike.
THERMOPLASTIC MEDIA
Media made from certain types of thermoplastic resins is still one of the
more popular methods of deflashing many types of parts. Thermoplastic media
is commonly found in three different types: polvcarbonate, which is usually
cylindrical in shape; nylon, normally cubical in shape; polvstvrene, which
is commonly found in a spherical or bead form (Figure 1).
Polvcarbonate media is the most common form of thermoplastic media and is
used for deflashing rubber parts in a cryogenic atmosphere. This media can
also be used t o deflash certain thermoset plastic parts in ambient blast
applications and offers excellent impact resistance when used in both
air-blast and wheel-blast machines.
When used cryogenically it is not
uncommon for polycarbonate media t o be used in temperatures as low as
-175°F, or lower, while still maintaining its effectiveness over many cycles
of use.
Nvlon media is the choice of most molders deflashing thermoset plastic
parts.
Nylon media is slightly softer than polycarbonate and a little more
forgiving t o the part surface. This is especially important if the part is
a glossy, cosmetic or "appearance" part. Nylon media is also an excellent
choice for both air-blast or wheel-blast equipment and maintains its
deflashing ability over many cycles of use.
Polvstvrene media is the lightest and smallest available media of all the
thermoplastic media choices.
Polystyrene
is especially effective in
deflashing applications where the flash is light and the parts are small or
delicate.
Polystyrene media is
commonly used to deflash electronic
components (diodes, connectors), small thermoset plastic parts, and some
rubber parts (small O-rings and seals).
Thermoset media, and its many uses and benefits, will be the focus for the
remainder of this paper.
Thermoset media first appeared in approximately
1984 as the answer to stripping aircraft for the U . S . Air Force and Navy.
Over the years, the uses for thermoset media in granulated form has grown
dramatically as illustrated by its overwhelming acceptance by industry.
Today, thermoset media is found commonly throughout industry in companies,
large and small, as the media of choice for salvaging rejected parts,
cleaning molds and tooling and stripping coatings from a variety of items
(Figure 2).
494
WHO
IS CURRENTLY USING GRANULATED MEDIA
e....
?
Thermoset media is commonly used by three distinct industry groups for these
purposes :
1.
2.
3.
Salvaging rejected parts (with paint or coating defects)
Cleaning molds and tooling (rubber and plastic molders)
Surface preparation (surface stripping of cars, trucks, planes,
etc. )
Salvaaina Rejected Parts
Industry has embraced the use of thermoset granulated media for salvaging a
variety of parts which have a defective paint or coating. In the past, it
has been commonplace for such parts to be thrown away because of the lack of
a fast and environmentally responsible way of salvaging rejects. Many
companies have discarded thousands of dollars of usable parts annually
because there was no alternative to labor intensive hand finishing or
chemical stripping which, because of the hazards involved, has come under
intensive scrutiny.
Thermoset media offers the opportunity to blast virtually any metal part,
and some composite parts, to remove a paint or coating and return that item
to production quickly. The key is selecting the correct plastic media so
that the grade selected is softer than the substrate of the part being
stripped.
Commonly, aluminum and steel parts are stripped with plastic
media and sent back to production in a matter of minutes. While this is a
versatile method for salvaging part rejects it is not the answer for every
company, especially if reject levels are low or parts are made from soft
substances, such as aluminum extrusions or lightweight plastic.
Companies facing these problems, and those producing parts that have
challenging configurations or are very small in size, may find blasting of
any type t o be prohibitive. These companies may have no alternative but to
use chemical dips or burn-off type processes.
The candidate for plastic media must take into consideration the number of
parts that must be stripped either on a daily, weekly, monthly, or annual
basis; the type and thickness of the coating that must be removed; the type
of equipment which is in use; the time allowed to salvage the part. With
these parameters in mind an appropriate plastic media can be selected to do
the work.
Cleanina Molds and Toolinq
Thermoset media is the favorite of many rubber and plastic molders for
cleaning molds and tooling. Molders are faced with the problems of cleaning
their tooling quickly and getting it back into production without excessive
down time
.
495
The three traditional choices for cleaning molds and tooling all have
built-in problems. Chemical cleaning, hand finishing and blasting with
glass bead have been used for years by molders as methods of cleaning molds
and tooling, but each is problematic in its own way.
Chemical cleaning can add t o the erosion of the mold surface if the mold is
over exposed t o the chemicals. Since this is a wet process, chemical
cleaning can promote corrosion during the rinsing process, a necessary step,
after the chemical soak. In addition, the use of chemicals may require
soaking of the tooling 7-8 hours, or more, depending on how much the mold
needs to be cleaned. After this excessive soak time, it is not uncommon for
the molder t o have to do touch up finishing work by hand since some areas
may still not be cleaned thoroughly.
Hand cleaning, the oldest and one of the more common methods of mold
cleaning is still the answer for low production, low tech molders. Many
companies devote one or more employees to the hand cleaning of molds which
are not used in high production activity. High volume production facilities
cannot afford the manpower necessary to clean many
molds that are being
rushed back and forth into production. Damage is also a factor when hand
cleaning is considered.
Since power tools, abrasive pads, solvents, and
other abrasive items are commonly used to clean molds, the process is very
"operator sensitive" and damage to the mold surface is not uncommon.
Notably, hand cleaning of molds and tooling is a labor intensive process
involving several hours to clean a mold thoroughly. In addition, many molds
are complex and have areas that are extremely hard to reach by hand, making
this procedure even more time-consuming and ineffective.
Abrasive blasting has been a long-time method of cleaning molds and tooling
and is still a favorite because of the bright, shiny surface left as a
by-product.
Glass bead is generally the media of choice and is a thorough
and fast way of cleaning molds and tooling. The problem with this process
is the damage done by blasting any tooling with an abrasive. The shiny
surface is an indication that a small amount of metal has been removed and
damage has begun.
This damage will eventually translate into increased
flash on the finished parts once the mold has been cleaned repeatedly with
the abrasive.
The resulting effect of repeated glass bead use on molds and tooling is the
rounding of corners and destruction of critical tolerances which must be
maintained in order for precision parts to be produced. While glass bead is
an inexpensive media to use, its deteriorating effect on expensive tooling
can be dramatic (Figure 3).
Surface Prenaration
For several years, plastic media has been used by a market segment that has
needed a non-abrasive media to strip paint and coatings from large items
including: cars, trucks, buses, planes, etc.
496
The basic use of plastic media in this instance is similar to the
application of salvaging part rejects only differing in the size of the
"part". Since plastic media was first introduced over 10 years ago for this
specialized use, this non-abrasive method has steadily gained recognition
and acceptance in stripping large items. The speed and safety involved in
disposing of waste material from plastic media stripping is much preferred
to the gallons of chemically polluted water that must be handled when a
large vehicle is chemically stripped.
Surface preparation with plastic media clearly emphasizes three distinct
advantages:
safety to the item being stripped; freedom from concerns of
corrosion; an increase in the speed with which most items can be stripped.
WHAT
. e . . .
ARE THE TYPES AND SIZES AVAILABLE?
Thermoset granulated media is traditionally available in six standard sizes
and four common types. Each type of material is crushed and sized according
t o the job t o be performed with the product (Figure 4 & 5).
Polvester media is the softest and least aggressive of the choices. While
polyester was once a popular choice of the military for stripping aircraft,
the slow performance was found to be unacceptable, especially with the
volume of stripping activity generated by the Air Force. Today, polyester
is used mainly for stripping paint and coatings from soft metals and some
composites surfaces safely but slowly.
Urea
is considered the most popular of the four types of granulated media.
This very versatile, medium hardness product is the favorite of both
military and industry worldwide.
Due t o its increased hardness, stripping
and cleaning becomes a faster activity and the media maintains an acceptable
longevity justifying urea as an economical choice.
Acrylic media is usually produced in thermoplastic form (a thermoset acrylic
media is available but is not widely used). Acrylic is the only one of the
four choices available as a thermoplastic, thus making it the longest
lasting of the four granulated media. Acrylic's performance is similar to
that of urea, but it is more forgiving to surfaces and is especially good
for stripping soft metals, airframes and composite materials.
Melamine media is the hardest and most aggressive of
to this quality, melamine strips surfaces quickly,
too harsh and aggressive for some parts.
Melamine
for stripping steel surfaces, but is not recommended
or composites (Figure 6).
WHERE
e . . . .
the four choices. Due
but is found t o be far
is an excellent choice
for use on soft metals
ELSE CAN THE PRODUCT BE USED?
It is not uncommon t o find thermoset media in use by manufacturers of sheet
metal products which must paint or coat those products before completion.
497
In many cases a part with a defective coating must either be thrown away or
salvaged through the use of some type of stripping method. Plastic media
quickly and efficiently removes coatings from these parts and puts them back
into the production process.
Painted automotive wheels represent another common use for plastic media.
Often these wheels must be stripped of a coating if there is a problem with
the paint line. Abrasive media is generally not a good choice for removing
coatings from such wheels which usually are made from soft metal. Plastic
media is an excellent choice for the quick and non-abrasive clean-up of this
problem.
Office furniture
represents another
common use
for plastic media.
Manufacturers who have open-blast rooms can strip large panels, and even
entire furniture pieces if there is a problem with the coating. This has
become a very popular application for plastic media because of the need to
replace time-consuming and laborious hand finishing.
A company involved in the removal of paint or coatings from automobiles,
especially antique and collector models, uses plastic media exclusively for
stripping work. Faster than hand finishing, safer than abrasive blasting
and more economical than other alternatives tried, plastic media has become
the standard for this unique business.
Other successful uses include coating removal on:
vending machines, pay
telephone housings, kitchen and bathroom fixtures, bicycle and motorcycle
frames, patio furniture, and many more (Figure 7). The list spans literally
hundreds of items and is growing constantly because of the advantages
enjoyed by the user.
In fact, almost any company with a need to remove paint or coatings is a
candidate for plastic media blasting. If the company possesses the correct
blast equipment, a media recommendation can be made which will potentially
solve a number of finishing problems without excessive cost.
WHEN
..... IS BLAST EOUIPMENT A FACTOR?
Plastic media is most commonly used as a dry-blast process and is more
effective when used dry than when mixed with water in a slurry. Although
plastic media is used in both air-blast and wheel-blast equipment, the
effect can be different according to the type of media used.
Thermoplastic. media can be used in both types of equipment in a number of
different applications. Thermoset media is best used in air-blast equipment
because of the higher breakdown rate experienced when this harder, more
brittle media is used in more aggressive wheel-blast equipment.
498
When companies are involved in the salvaging of rejected parts or mold
cleaning, the best equipment is an "enclosed" blast cabinet, air-blast
style.
When the application is the stripping and finishing of large items
or vehicles the best choice is "open-blast" equipment, also a form of
air-blasting, performed in a blast room.
When a user must choose a type of blast equipment for use with plastic media
it is wise t o consider the following factors:
1.
2.
3.
4.
5.
6.
7.
Substrate of the parts to be finished
Size of the parts to be finished
Configuration of the parts to be finished
Number of parts which require finishing in a particular time frame
Type of coating to be removed
Time allotted t o workers to strip parts
Type of media necessary to effectively strip the parts
Selecting the right blast equipment can make plastic media use very
efficient and cost effective or it can cause difficulties for the user.
Some air-blast equipment, particularly suction-blast style equipment, is not
recommended for use with most types of plastic media because of a lack of
air velocity generated. The best way to find out the type of equipment for
a particular application is to consult with a plastic media or blast
equipment specialist.
WHY
... IS THIS
PRODUCT SO WELL-RECEIVED?
Plastic media has been a well-received method of finishing
parts for several years for a number of distinct reasons.
advantages enjoyed by the user are these:
1.
2.
3.
4.
5.
6.
The opportunity t o reduce or eliminate chemical use from the
stripping process
Plastic media is a dry process which eliminates corrosion from parts
sometimes introduced by the use of a wet process
A faster method of stripping than offered by most alternatives
A method that is safe to parts when used appropriately
A method that is safe to personnel imposing no latent health hazards
An environmentally friendly finishing process
When plastic media is compared to other
offers several advantages:
1.
2.
3.
4.
a variety of
The primary
aon-abrasive stripping methods it
Faster finishing times
A less expensive method of finishing
A process that can be easily taught to most workers
A more aggressive process capable of removing a wider range of
coatings
499
CASE STUDY...
. .PAINT REMOVAL
The manufacturer of home and commercial appliances faced the problem of
stripping a l/2 mil, electrostatic deposition coating (E-coat) from a steel
"valve plate" without causing surface damage or roughness. For years, hand
sanding with abrasive disks had been used to strip this coating but too many
parts were damaged by this process.
The correct process would produce a
very smooth, unblemished surface, a result the sanding process could not
consistently produce.
Alternatives Considered
A chemical stripping process was briefly considered but corporate management
decided against pursuing this alternative because of cost and environmental
concerns.
Also briefly considered was glass bead blasting but the idea was
abandoned when this process proved to be at least as abrasive and
inconsistent as hand sanding.
The company then experimented with plastic media,
urea, in an enclosed blast cabinet.
The results
manufacturer realized benefits almost immediately.
testing a large sized
were positive and the
Process Advantases
This manufacturer realized a savings
counted most: time and monev.
in each
of the
two categories that
*
Before the change to plastic media all valve plates that were damaged
$16.00.
by sanding were scrapped. Cost per part
*
Parts that were successfully salvaged by sanding cost $8.65 each (all
expenses) t o strip. Time to sand each part was approximately 11
minutes.
*
The plastic media stripping process cost $3.95 per part (all expenses
and each valve plate could be completely blasted in less than 5
minutes.
*
All parts stripped with plastic media were left smooth and ready for
recoating; excessive surface roughness was never a problem.
-
Good News Travels Fast
The success of plastic media blasting for this manufacturer spread quickly
to other departments within the company. It was discovered that other parts
with defective coatings could also be stripped with plastic media further
reducing costs and finishing times €or the manufacturer.
The blast
equipment purchased by the company to strip the valves costing approximately
$17,000 was recouped in processing and labor savings within a matter of
months.
500
Although long established as an effective means of removing paint, only
within the past few years has plastic media become a popular alternative for
the removal of powder coating from various parts. As with those needing to
strip paint from rejected parts, powder coaters have found advantages to
using plastic media in place of alternative processes.
CASE STUDY
e...
POWDER COATING REMOVAL
A leading manufacturer of power tools for years had used
aluminum oxide, to strip paint from rejected tool housings.
an abrasive,
These steel housings could be completely stripped using aluminum oxide but
the resulting finish was often very rough leaving the part unacceptable for
recoating.
The blast cabinet used for the rework was frequently in
disrepair because of the damage done by repeated abrasive blasting. All
parts requiring stripping were coated with an epoxy hybrid powder with a
thickness of 2 mil.
Finishins Problems
In evaluating the problems
manufacturer cited these:
encountered by
use
of abrasive
blasting the
*
A rough, often unacceptable finish to the parts
*
High wear on blast equipment necessitating frequent purchase of
replacement parts
Increased cost t o the process because of the amount of replacement
parts purchased
A slower-than-desired strip rate frequently exaggerated by large
numbers of parts needing to be stripped during peak periods
A process which was deceivingly expensive, "hidden" by the relatively
low cost of the abrasive
*
*
*
speed and Cost Considerations
When the process was analyzed it was determined that only one (1) part per
hour could be stripped without causing irreparable damage to the part's
surface. This very slow finishing cycle only highlighted the need to change
to a faster process, one which could keep up with peak production periods
when rejected parts seemed to pile up even more.
Management further noted that when average media consumption per part was
calculated, cost t o strip each part was $7.50. The relatively low cost of
parts stripping ($7.50 per hour) was undermined by the extremely low strip
rates and costs of ongoing machine maintenance. Alternative methods had to
be investigated.
50 1
Plastic Media A8 An Alternative
Company officials considered several other stripping processes but decided
t o evaluate plastic media because the existing blast equipment could be used
for testing. A thorough purge of the blast system and new blast nozzle
easily accommodated a switch to a urea media, 16/20 sieve size. The results
were dramatic and advantages obvious even early in the testing.
The same parts could now be stripped at a rate of six (6) units per hour
without any damage to the housing. Although the plastic media was more
expensive per pound than the aluminum oxide, the consumption rate was lower
(4 lbs. per hour) and the resulting finishing cost per part now totaled
$1.23.
4 lbs. per hour x $1.85 per lb. = $7.40 (media cost)
6 parts per hour = $1.23 (per part)
$7.40
-
f->
A n 'additional bonus came
in the form of money saved on machine parts and
maintenance.
Plastic media was found to wear only the machine's blast
gloves
and blast hose and these were replaced very infrequently. Total
process savings were substantial especially during peak production periods
after the change t o plastic media use.
Calculated over the course of a
year, savings recognized were several thousand dollars.
It must be emphasized that plastic media is not the answer t o all finishing
problems. Like other processes, plastic media represents an alternative for
some but not a panacea for all.
The advantages enjoyed by hundreds of users worldwide are still significant
and deserve a final look. These advantages are:
*
*
*
*
*
*
*
Elimination of chemicals and problems associated with wet-blasting
A fast method of finishing
A finishing method safe to personnel and parts when used
appropriately
Frequent reductions in finishing costs
A versatile process recommended for use with many types of parts
A finishing method that is environmentally friendly
A dry process free from problems of corrosion
Plastic media use is still in its infancy and will gain
throughout the world as more companies are made aware of
versatility and benefits.
in popularity
the product's
Who wins? We all do. Anytime finishing technology advances to the point
where danger t o the environment is decreased while businesses enjoy an
increase in productivity and profit, that technology must be investigated.
502
FIGURE 1
Plastic Media -Thermoplastic
Polycarbonate
Nylon
Polystyrene
(Cylindrical)
(Cubical)
(Spherical)
FIGURE 2
Plastic Media -Thermoset
UI
0
P
Polyester
Urea
Melamine
Acrylic (Thermoplastic and Thermoset)
FIGURE 3
~~
Methods of Mold and Tool Cleaning
Chemical Soaks
Manual Cleaning
Abrasive Blasting (Dry or Wet)
Plastic Media Blasting
FIGURE 4
Types of Plastic Blast Media and Applications
For Paint and Coating Removal
UI
0
Q)
Granulated Polyester
Extremely sensitive or thin substrates; delicate parts
Granulated Urea
Aircraft propellers; aluminum automotive parts, automotive rims;
automotive vacuum-metallized parts
Granulated Melamine
Paint hooks and racks; office furniture; military ground transport
vehicles; industrial equipment
Granulated Acrylic
Automotive bodies (especially fiberglass); airframes; aircraft
wheels and r i m s
1
I
FIGURE 5
Thermoset Granulated Media Sizes
'
I
'
Sieve Size
Inches
Millimeters
12/16
.066/.047
1.6W1.19
16/20
.047/.033
1.19/0.84
20/30
.033/.023
0.84/0.58
3W40
.023/.017
0.58/0.42
40/60
.017/.010
0.42/0.25
601100
.010/.006
0.290.15
I
m
"
FIGUFE 7
Partial List of Item
Currently Stripped with Plastic Media
Refrigerators
0
Shock Absorbers
Computer Housings
Metal Desks
0
Tape Measures
Aluminum Shutters
0
Decal Removal
Telephones
Corvettes
0
Boats
Patio Furniture
Aluminum Wheels &
Covers
Rifle Housings
0
0
0
Golf ShaftsEquipment
Lamp Fixtures
Bicycle Frames
0
Motorcycle Components
Motorcycle Helmets
0
Bed Frames
0
cars
Football Helmets
Subway Cars
Aircraft Components
Satellite Dishes
Jet Skis
Vending Machines
Police Badges
Water Coolers
Van Seats
Sweeper Housings
Extruded Window
Tractors
Frames
Circuit Breaker Boards
KitchedBathroom
Snowmobile Skis
Metal Signs
Faucets
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