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

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• 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,

polyethylene
polypropylene
nylon
polyvinyl chloride
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,

auto body panels
sanitary fixtures
sports equipment
bathroom fixtures
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

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• 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.

 

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