Reading Assignment

(Note: Reading assignment from multiple chapters)

35.2 Mechanical Cleaning and Finishing
35.3 Chemical Cleaning
- See Surface Preparation of Metals Prior to Plating
35.4 Coatings
35.5 Vaporized Metal Coatings
35.6 Clad Materials
35.7 Textured Surfaces
35.8 Coil-Coated Sheets
35.9 Edge Finishing and Burrs
35.10 Surface Integrity

Outline

Inorganic Coatings

Converted Surfaces and Coatings

Metal on metal
Metal on plastics
Ceramic on ceramic
Ceramic on metal

Phosphate Conversion Coatings

Creates a phosphate layer on the surface
Requires part to be clean first
Provides a base for painting, oiling, or waxing surface
Reduces friction (e.g., for processing)

Oxide Coatings (blackening and blueing)

Immersion of parts in hot caustic soda and nitrite or nitrate solution
See: UltraBlak

Anodizing

Used on Al, Mg, and Titanium
Creates a protective oxide coating

Chromate Coatings

Used on Aluminum, Zinc, Cadmium coated steel. Also used on Mg and Sn coated steel.
Provides a decorative appearance
Used as a base coat for paint
May contain a dye to color the converted coating

Zinc chromate – silver
Zinc dichromate – gold

Metallic Coatings

Electroplating
Electroless and Immersion Plating
Dipping
Metallizing
Diffusion Coatings

Electroplating

Provides attractive surface finish
Improves wear resistance
Increase thermal conductivity
Improve materials ability to be soldered
Build up worn areas

Electroless Plating

Metal is deposited by chemical means
Used to deposit nickel, gold, tin, copper, cobalt and zinc to
- Copper, aluminum, steel and plastic

Dipping

Base metal is cleaned
Dipped in flux
Molten metal bath
Excess metal drips off

Metallizing

Wire Metallizing or flame spraying
Plasma-arc spraying
Vacuum metalizing
- Vaporized coating material using tungsten filaments
- Vaporized metal floats in the vacuum and deposits on rotating parts
- Used to coat automotive trim, reflectors, hardware, jewelry, etc.
- Deposits high purity metal

Ceramic Coatings

Porcelain Enamels
- Glasslike material bonded to base metal
- Fired from 1-2000 deg. F.
- Durable, scratch resistant, easy to clean, resists chemicals and heat
Glazes
- Glasslike material applied to ceramic material
Refractory Ceramic Coatings
- High heat resistance

Organic Coatings

Coating Components

Solids

Pigments – small particles added to influence color a
Additives – special properties (thikenters, flow agente, fungicides, etc.)
Resins – unite pigments and binders

Solvents

Carry the solids, evaporate, release volatiles into the air (VOC’s – harmful interactions)

Powder coating

no solvents

These may used as:

primers
sealer
base coats
clear coats
top coats
etc.

Resins are most important component

Polymer resins (all paints, liquid and solid are either enamels or lacquers)

Enamels – form film thru evaporation, cross link (more popular than lacquers)

Lacquers – do not cross link
Fluidization of the solids

Solution fluidization
- Each molecule is dissolved in solvent
Dispersion fluidizaiton
- Clumps
Emulsion fluidization
- Large clusters of resin molecules surrounded with an emulsifier

Broad categories

Solvent born
Water born coatings
High solid coatings (low VOC) coatings
- Extremely popular bc of reduced solvent emmisions
- Alkyds, polyesters, epoxies, acrylics, silicons

Coating Sequences

Pretreatment and cleaning – critical to success.

chemical
ultrasonic
vapor degreasing
often more than one method necessary

Application

Proper method depends on:
Type of paint
Thickness of paint
Properties required
Size and number of finished parts
Transfer efficiency (waste)
Methods:
- Conventional air spraying
- Airless spraying
- Air-assisted airless spraying
- High volume, low pressure spraying HVLP, less than 10 psi
  - softer, more control spraying
- Electrostatic spraying
  - charged particles attracted to part with opposite polarity
- Good transfer efficiency
- Electrocoating
  - dip method of electrically applying paint

Curing

Room temterature
Heat Crosslinking
- used for coating too slow to cross link at room temp
Radiation Curing
- cured in seconds with UV or other radiation source.

Powder Coating

Applied dry, most cross link to form continuous film
Fine, homogenous plastic particles with additives
Electrostatically charged.
Melt when heated, tough film results
More E friendly (green)
Little waste, powder recoverable
Epoxy, polyseter, acrylic, silicon, hybrids
Sub-systems
- pre-treatment system
- application system
- booth with powder recovery system
- oven
- conveyor

Sequence

Mechanical cleaning
water brushing
abrasive blasting
grinding
vibratory processing
chemical cleaning
solution: alkaline, neutral, acid, or emulsion

application system
pneumatic spray, fluidized powder

spray booth
50% adheres, efficient recovery of overspray is important
Filter collects overspray

Curing
convection ovens
infrared ovens – transmits infrared radiation directly to surface cure faster, but even distribution is difficult.

Mechanical Cleaning and Finishing

Blast Cleaning

Barrel Finishing or Tumbling

Vibratory Finishing

Abrasive Flow (video)

Media

Natural abrasives

slag, cinders, sand, granite chips, limestone hardwood

Synthetic

Aluminum oxide, silicon carbide embeded in a binder

Compounds

Added to media and workpieces.

Liquid or dry
Abrasive or nonabrasive
Acid, alkaline, or neutral pH
Assist deburring, burnishing and abrasive cutting
Provide cleaning, descaling, or corrosion inhibition
Enhance color, and brightness

Belt Sanding

Wire Brushing

Buffing

Electropolishing

Reverse of electroplating

Removes material from raised surfaces first

Chemical Cleaning

Alkaline

Solvent

Vapor Degreasing

contaminants stay in the solvent bath

not environmentally friendly

Ultrasonic Cleaning

Acid Pickling

Sulfuric or Muriatic (hydrochloric) acid used to remove oxides acid

Passivation

Nitric acid passivation

Citric acid passivation

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Finishing Processes & Inorganic Coatings