As electrocoat formulations have improved over the years, applicators
have been looking for new ways to use electrocoat to coat parts.
With the release of new pigment and resin systems, bulk application
of electrocoat, especially cathodic electrocoat, has come to the
forefront as a coating system for fasteners and small stamped parts.
Typically, fasteners have been coated in bulk using various application
methods. The most common method is a dip-spin application. This
process can achieve high throughput, but it does have some drawbacks,
such as transfer efficiency and the high possibility of recess head
fill or bridging on the threads. Electrocoat eliminates head fill
or thread bridging concerns while producing a film that is a consistent
thickness. This can all be achieved while parts are processed in
a high volume application, such as a barrel, to achieve excellent
production volume of pounds of parts produced per hour.
Finish Quality
Small stampings are usually racked in electrocoat systems. Although
you get the high quality and efficiency of the electrocoat system,
hanging small parts on a rack is time consuming and costly. A bulk
application of electrocoat can still give a quality finish and eliminate
racking.
To determine if a bulk application will fit, the first thing that
must be considered is the quality requirements of your customer.
If your customer requires a "Class A" finish, bulk application is
not for you. However, if your customer requires a continuous paint
film with only a few small blemishes, it could be a viable option.
Bulk electrocoat will also meet many automotive small parts specifications
for salt spray resistance, humidity and water immersion.
Bulk application of electrocoat on fasteners has made great strides
in quality improvement during the last few years. Small fasteners
are easy to coat in bulk and achieve excellent appearance. Small
fasteners also test very well in salt spray when coated 0.6-0.8
ml thick.
Bulk Coating for Automotive
In automotive circles, it is the larger fasteners that are difficult
to coat in any bulk application. The weight per sq ft of surface
area can cause touch marks on heavier fasteners. The touch marks,
although very small, can cause corrosion points. Often this is the
only corrosion on the part.
 |
| Figure
I: Barrel full of fasteners is transported from a black electrocoating
tank to the rinse tank. Bulk electrocoat offers high-quality,
high-throughputand lower overhead costs.
|
The goal for most automotive fasteners has been to meet the salt
spray requirements of USCAR 1. This specification has been drafted
as a reference test criteria for automotive fasteners. It is a difficult
specification to meet, but it does ensure that the finishes meeting
the requirements of USCAR 1 will serve the automobile industry well.
Bulk application of electrocoat can fit certain specifications for
fasteners right now. Work is being done to find a bulk electrocoat
finish to test to 600 hr salt spray and meet the USCAR 1 criteria
of no more than 0.1% red rust on the part. It will be a difficult
hurdle to overcome but a goal many feel is attainable.
Torque tension is another significant requirement of automotive
specifications, specifically on fasteners. This is to insure that
the fasteners will assemble consistently in the assembly plants
and make a good joint every time. Torque tension modifiers are added
to the product to achieve the necessary torque tension values. In
production runs of the cathodic electrocoat, fastener coating torque
tension values have been consistent with deviations well within
specifications.
It is important that you have a complete understanding of your
customer's test requirements to give you the basis for an evaluation
of bulk electrocoat as an application option. In many cases it can
work.
Part Shape and Size
Part configuration is another important aspect of determining if
bulk application of electrocoat can work for your organization.
Some parts lend themselves very well to bulk applications others
do not. When evaluating the parts you would like to run in bulk,
look at part size, part geometry, part weight and part final appearance.
Size of part in bulk application is important from an equipment
perspective. If the part is too large, you many not be able to ecoat
enough parts in bulk to make the investment worthwhile. If the part
is too small, special equipment maybe needed to process the pieces.
Typically, fasteners are relatively easy to process in bulk. Stampings
up to a size of 2-3 sq ft in surface area can be processed in a
bulk application. It is important to size your equipment for your
largest part.
Part geometry can restrict the types of parts processed in bulk.
Although parts with many sides are easy to process, flat parts are
not as easy. Part-to-part nesting needs to be evaluated when deciding
to bulk process parts. Parts that tend to nest will not coat as
uniformly as parts that break up in the load. This nesting will
cause part-to-part variance in film appearance and film thickness.
It will also increase the amount of part-to-part sticking in the
load. Be aware of the part geometry when evaluating a bulk application.
The weight of parts is another aspect of the process that needs
to be evaluated. Heavy parts with a high weight to square foot of
surface area may cause significantly more touch than lighter parts.
This happens because the part compresses the uncured film. As the
weight of the part and the load of parts increase, the more the
film compresses, until it actually causes a touch mark to form.
These voids then can cause a problem in corrosion testing.
Final Finish
The final part appearance is also critical in evaluating if bulk
electrocoat is acceptable as a finish option. The bulk operation
will produce a uniform film that has good aesthetics. It will rival
a part that has been coated on a rack line. It will not produce
a "Class A" finish. If your customer wants a "Class A" finish, bulk
applications are not for you. If your customer wants a finish that
is uniform in appearance, gloss and film thickness, then bulk application
of electrocoat is a viable option.
 |
| Figure
II: Copper-color electrocoating is also done on fasteners
and other parts.
|
Bulk barrel coating of fasteners has been done for many years using
anodic electrocoat. This worked well for barrel applications, but
it did not offer good corrosion resistance.
As development of cathodic formulations evolved, it was evident
that this type of chemistry offered many advantages compared to
older anodic technologies. Corrosion resistance, more resistance
and chip resistance were all advantages to anodic type formulations.
The biggest obstacle in using cathodic epoxy or acrylic chemistries
was parts sticking together and the high probability of the uncured
paint film being damaged during the coating and material handling
process.
Development work has centered on base cathodic epoxy formulations.
Proprietary resin systems in conjunction with advanced pigment feeds
have been developed for bulk processing of parts to yield acceptable
visual appearance and give enough corrosion protection to meet many
specification requirements for many industries. This technology
has been the backbone for most electrocoat applications in the world
today. It appears that with further development, it will allow electrocoaters
the option to bulk process parts that historically had been racked
coated.
Acrylic cathodic electrocoat has also been in development for bulk
application. These products give good corrosion resistance and are
excellent in outdoor exposure and resistance to chalking. Many of
the same issues that have been encountered with epoxy technology
have been evident with acrylic development. Application of acrylic
cathodic products is being done on a limited basis today. Many advancements
have been made to coat this technology in bulk. The lessons learned
from epoxy development have been transferred to acrylic development.
The biggest obstacle in bulk application of acrylics, especially
in a barrel, is sticking. This has been minimized with new resin
and pigment systems; however, it has not totally been solved.
The Bulk Coating Process
Parts are processed through a zinc phosphate system. Parts are cleaned
of all soils and pickled with an acid if necessary. The zinc phosphate
is then applied to the parts. Typical coating weights for the process
can range from 250-800 mg/ft2. Parts are then rinsed, and a final
seal (usually chrome-free) is applied. The final seal is rinsed
in RO or DI water. The parts are now ready for the electrocoat application.
In the electrocoat system, the parts can be immersed for 1-5 minutes
depending on application equipment and film thickness desired. Parts
are then rinsed in permeate rinses and a final DI water rinse. The
parts are then cured in an oven at the recommended temperature and
for the recommended length of time.
Coating thickness can be manipulated to meet the specification
of your customer. Film thickness of 0.6 – 0.8 mil is typically
deposited on fasteners and small parts to meet most specifications.
Higher film thicknesses are achievable by altering process variables.
Bulk Coating Equipment
Tanks for the system can be mild steel with a fiberglass reinforced
epoxy lining. This coating should be on all tank materials below
the solution level to prevent it from coating during the paint cycle.
Rinse stages can be plastic materials or stainless steel.
| Typical
"Inline" Process |
| Stage |
Process |
Time* |
Temp |
| 1 |
Alkaline Cleaner |
3 min |
145-155 |
| 2 |
Alkaline Cleaner |
3 min |
155-165 |
| 3 |
C/F H20 Rinse |
3 min |
Ambient |
| 4 |
C/F H20 Rinse |
3 min |
Ambient |
| 5 |
Fresh R-O H20
Rinse |
3 min |
Ambient |
| 6 |
Acid Pickle |
3 min |
Up to 160 |
| 7 |
Acid Pickle |
3 min |
Up to 160 |
| 8 |
C/F H20 Rinse |
3 min |
Ambient |
| 9 |
C/F H20 Rinse |
3 min |
Ambient |
| 10 |
Fresh R-O H20
Rinse |
3 min |
Ambient |
| 11 |
Zinc Phosphate |
3 min |
145-155 |
| 12 |
Zinc Phosphate |
3 min |
145-155 |
| 13 |
C/F H20 Rinse |
3 min |
Ambient |
| 14 |
C/F H20 Rinse |
3 min |
Ambient |
| 15 |
Fresh R-O H20
Rinse |
3 min |
Ambient |
| 16 |
Drain |
3 min |
Ambient |
| 17 |
Index to Paint |
3 min |
Ambient |
| *
Does not include transfer and drain times |
The pumping and ultrafiltration system is a standard system for
cathodic electrocoat. The system should sweep the bottom of the
tank to minimize any settling of the paint. The system should have
a heat exchanger and chiller system to be able to achieve recommended
bath temperatures.
The application equipment used in bulk electrocoat is basically
the same as in rack lines. The barrel used can be made from carbon
or stainless steel. Hole size for the mesh can be 1/8 to 1/4 of
an inch.
Material or part handling is the largest obstacle that must be
resolved to make this process foolproof. Touch marks and voids from
part-to-part touching have caused concerns for the overall corrosion
resistance capability of the film. Work is being done on two fronts
to eliminate these touch marks. Paint chemistry is being addressed
to make the uncured paint film more resistant so that when parts
touch each other, they do not cause a touch mark or void in the
coating.
Material handling engineering is also being researched to determine
the best way to apply electrocoat in bulk and eliminate part-to-part
and part-to-equipment touch marks. There are many interesting material
handling systems that can allow electrocoat to be applied in bulk
applications. Historically, it has been done in barrels or baskets.
New innovations in material handling systems may be applicable to
the bulk electrocoat process. Only time will tell if these innovations
fit the process.
Bulk electrocoat of fasteners and stampings can offer many advantages
to the application. High quality, high throughput and lower overhead
costs are a few of the advantages. These uses of electrocoat can
also allow end users some flexibility in specifying electrocoat
to meet ELV (end of life vehicle) requirements and finish consolidation
goals in the automotive industry for fasteners and small stampings.
Electrocoat can be an option to replace many finishes now being
used in industry today.
