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U.S. Steel Prepainted Coated Sheet can be fabricated by the same
processes that are used to fabricate bare metals, although it
requires additional care. It can be blanked, stamped, drawn,
rollformed, and fastened by all conventional fabricating methods
including, in some cases, welding. It can be embossed after
painting to give the surface an attractive patterned appearance.
Refurbishing, polishing or replacement of existing forming tools
may be necessary to prevent marring or damage to the paint surface.
Polished chrome-plated drawing dies or rollforming tools are
desirable.
Handling
As with
all coiled sheet stock, good handling practices are desirable for
best results in processing.
- Storage in
weatherproof buildings is preferable; otherwise, storage should be
under loose covers that will keep the material clean and free of
condensation.
- Care should be
taken to avoid damage to coil edges.
- Hooks and other
handling devices should have padding.
Forming
All paint films used on U.S. Steel Prepainted Coated
Sheet will withstand some degree of forming without crazing or
cracking; some types of paint coatings can withstand more
deformation than others, and a few will withstand more deformation
than the most ductile steel substrates. For best forming results,
both the formability of the steel sheet and the flexibility of the
paint film should be matched to the intended forming operation.
Proper matching will let users perform severe forming operations
such as deep drawing without destroying the integrity of the
paint.
Prepainted steel sheet can be formed with the same tooling and
presses used for uncoated steel, with minor adjustments in
fabricating practices.
As with bare stock, it is essential that equipment clearances
and tolerances be specified correctly.
Devices for feeding into processing lines can vary from highly
engineered coil ramps or cars, hydraulic rams, and expanding rams
in high productivity units to simple portable mandrels lifted by
crane in small production lines.
Forming should be done at temperatures above 65°F.
Preferably, even higher temperatures should be considered to
maximize the performance of the paint.
Whenever possible, multi-stage forming should be used for making
tight bends, deep draws, etc.
Although prepaint coatings are formulated to withstand the
rigors of forming, padding may be required at the equipment
locations where the painted surface could be damaged.
Non-chrome-plated, surface-hardened dies may be used but they
must be carefully maintained; plastic die surfaces are effective to
prevent damage or marring of the prepainted surfaces.
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Welding
Projection welding is the method recommended for welding
prepainted steel. This method allows the welds to be made from the
back side of the sheet without burning or blistering the paint on
the opposite or finished side. This method has been applied
successfully in commercial production operations. Like conventional
spot welding, this process can be used in rapid automatic or
semi-automatic operations; multiple heads may be used for
simultaneous welding.
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Fastening
Any style of fastener suitable for use with sheet metal can be
used to join prepainted coated sheet to itself or to other
materials, provided the fastener design is appropriate for the
structural requirements of the application. The list of acceptable
devices includes common fasteners like nuts and bolts; screws and
rivets of all types; and special devices like clamp fasteners,
clips and blind screws. Some fasteners have colored plastic-coated
heads to match the point.
The fastener
material should be carefully considered from two standpoints.
First, the fastener should exhibit a level of corrosion resistance
similar to the prepainted sheet to ensure long life of the
fabricated product. Second, the fastener material should be
compatible with the coating; that is, it should be selected to
avoid accelerated corrosion caused by intimate contact between
certain types of dissimilar metals.
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Adhesive Bonding
Adhesive bonding is a proven technique that can join steel
effectively to many materials without altering the properties of
the steel or its coating. it can also improve the product design
and performance. Prepainted sheet can be fastened readily by
adhesive bonding in the same way and with the same adhesives
as hot-dip galvanized sheet.
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Sealants
Sealants are often used between overlapping areas of
steel sheet to form watertight joints and to achieve superior
environmental durability. Neutral-cure silicone rubber
sealants are recommended for use with galvanized sheet. These
materials need no primer. They are flexible and non-corrosive as
well as resistant to heat, cold, water and ultraviolet rays.
Other types of sealants, like butyl rubber and styrene butadiene
rubber, may also be used successfully. For more information about
sealants for specific applications, consult a sealant
manufacturer.
Painting
Ordinarily, there is no need to apply paint to a prepainted sheet
surface. However, the prepainted finish does accept other paints
well, so prepainted sheet can be used in highway signs and other
applications where lettering and decorative designs must be
applied.
Prepainted sheets can normally be painted in the field when it
becomes necessarily or desirable to repaint. Recommended methods
for repainting are available from most paint stores. Typically, the
surface needs to be cleaned to remove dirt and loosely adhering
chalked paint. After drying, the surface can be coated with a
primer before applying the top coat for maximum corrosion
resistance.
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