page 238

-better properties than paint

-fast cure times

•Disadvantages,

-limited colors

-color changeovers limited

-oven curing required

-some materials damaged by UV

-deep recesses not well covered

43.3 COATINGS

•A coating can be applied to protect surfaces and/or improves appearances.

•Processes include,

Chrome plating - a reaction with chromic acid leaves a chrome plating on a part Phosphate plating - a reaction with phosphoric acid leaves a phosphoric coating on a part Electroplating - thin metal coatings are made by dissolving an anode in an electrolytic

solution with an electric current

Hot dip coating - parts are dipped in molten metal and get a new coating

Vacuum deposition (ion plating) - metals are vaporized in a vacuum chamber and these deposit in thin layers on a surface

43.4 MARKING

43.4.1 Laser Marking

•General problems with other methods are,

-mark permanence

-poor quality

•By contrast, contact ink printing,

-is efficient, inexpensive, and quality is high when ink adheres

-not well suited to many new plastics because of ink bonding problems

•Laser marking is generally,

-permanent

-high contrast

page 239

-user friendly

-fast

•A laser is used to melt or evaporate surface material to create visible difference on the marked surface.

•Two methods are commonly used,

scan - much like a television, the laser is vectored about the workpiece to create a complicated pattern.

micromachining - has a beam that is passed through a mask, then through a lens to focus, and finally to the work surface where the mark is burned.

mask

lens

laser beam

target

•Typical laser types used are, Nd:YAG

TEA CO2

•Marked areas with micromachining can be up to 1 cm2, or more with the scan method.

•Good applications,

-date coders/part numbers/customer info

-frequent setups

-mark permanency

•In volume the laser system cost become lower per unit than ink.

•Advantages over ink are,

-no downtime to change inks

-elimination of many quality problems found in inks (e.g., ink permanency)

-elimination of special printing plates, etc.