20. Aesthetic and Protective Finishing
• There are a number of operations which have very little impact on the engineering aspects of a product operation, but are important to the final user. These include,
color
markings and labels
20.1 Cleaning and Degreasing
• Various methods remove contaminants
Chemical degreasing: removes chemicals and prepares surfaces
Solvent degreasing: contaminants dissolve in a solvent bath
Vapor degreasing: the solvents are sprayed or vaporized to dissolve contaminants
Ultrasonic cleaning: a cleaning solution is used with ultrasonic vibrations
20.2 Painting
20.2.1 Powder Coating
• Basically a power is distributed over the surface of a part. Subsequent heat then melts the plastic to leave behind a high quality surface.
• Advantages,
energy and labor cost reductions
high efficiency
environmentally safe
good quality finish
• In contrast, paint uses solvents that dissolve whereas powder coatings are applied and then heat cured.
• The basic steps are,
1. apply a finely ground powder coating to a part.
2. Heat the part to melt and fuse the powder.
• The parts can be coated with a fluidized bed. The hot part is dipped in a fluidized vat of powder where it coats and hardens. A part curing process is also done.
• Parts can also be coated with electrostatically charged powder that is oven cured.
• Thermoplastic powders are commonly used.
They melt and reset at elevated temperatures, but do not change chemically
The molecular weight is high.
The materials are hard to grind into fine powders.
The thermoplastic coatings tend to be thicker (0.008” to 0.04”)and applied by the fluidized bed method.
Typical materials are,
thermoplastic polyester
primers can be used
• Thermosets are another common material,
in uncured state the components are low molecular weight solid resins. When heated the resins chemically bond to longer molecular chains.
typical coating thicknesses are 0.001” to 0.003”
typical materials are,
epoxy
hybrids
acrylic
TGIC polyester
urethane polyester
applied by a spray gun
• Pigments can be added to modify basic colors.
• Additives can be used for other properties,
hardness
salt spray resistance
strength
impact resistance
stain resistance
• Mechanical Surface Preparation,
the surface is mechanically worked to remove unwanted coatings, and roughen the surface to help the coating stick.
typical methods are,
air blasting with sand or slag abrasives in open or closed environment.
centrifugal wheel blasting.
• Chemical surface preparation,
often used on galvanized steel, steel and aluminum
typical cleaners include,
alkalines
acids
neutral
solvents
emulsions
• In mold powder coating,
powders are sprayed into mold cavities before the part is molded.
before/after/during molding the cavity is heated to 280-350°F and the coating chemically bonds to the part making better adhesion.
advantages of this method are,
chip and impact resistance
conductive primers can be applied with this method to permit electrostatic coatings.
other painting facilities can be eliminated
shelf lives for materials is over a year
good coverage, including complex geometries, uniform thickness.
The basic process is,
1. the mold is opened
2. spray guns (electrostatic) are moved into the mold and spray powder in to coat the empty mold
3. the powder cures on the surface that has been preheated to 280-350°F
A typical cycle time, including the coating, is less than half a minute
This technique is most often used with thermoset compression molding.
typical applications include,
machine and electrical housings
multiple molds can be coated at once
robotic coaters are available
limitations,
plastics can be hard to direct into the mold
parting lines of the mold build up extra material
the powder between the mold halves must be removed after the mold halves are brought together.
• Booths can be used to recover powder that is sprayed but does not adhere to the part.
gravity assisted booth
belt booths
self contained booth
• Systems can be used to remove most of the unbonded particles from the air. The extra components include,
ducts
a cyclone precipitator
filters
• Belt Booths -The booths have many of the components of a normal booth, except that the unbonded powder is drawn onto a conveyor belt: the circulating air is drawn though the belt, but the powder is filtered out.
• Self contained booth: an all in one unit that allows fast changeovers for new colors.
• Two types of ovens used,
convective ovens
infrared ovens
• Advantages,
no incinerators or air scrubbers (no exhaust)
no toxic by-products
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
20.3 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
20.4 Marking
20.4.1 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
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.
• 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.
• Typical setup time is 5-15 minutes for ink, but 1 minute for laser.
20.5 Problems
Problem 20.1 Lasers are good for marking objects when,
a) Markings have complex mixtures of color.
b) Cost is important.
c) Markings require a long life.
d) None of the above.
