© Mario S Pennisi 2003
Many activities are undertaken under the umbrella of manufacturing.
Each of these activities affects the surface finisher in some
way. Most materials can be used in manufacturing including
metals, non-metals and organic materials.
There is a large list of common manufacturing activities
and these will be discussed in groups as follows:
- Molten Activities
- Forming/shaping activities:
- Stretch forming
- Peen forming
- Deep drawing
- Rolling activities
- Roll forming
- Thread rolling
- Roll bonding
- Joining activities
- Adhesive bonding
- Diffusion bonding
- Cutting activities
- Powder metals
- Rotational moulding
- Blow moulding
All these activities affect the surface of the material as
well as the metallurgy. Surface finishes are, as the name
implies, finishes that are applied to the surface of metals
and materials so any activity that affects the surface of
the material to be coated is of interest to surface finishers.
Pressure shaping activities
- Wire drawing
- Before 4000BC: Pressure shaping activities were the
first ones used by man. Pure metals such as gold, copper
and meteoritic iron were hammered into shapes.
- 4000-3000BC: Jewellery was produced by stamping activities.
- 3000-2000BC: Wire was produced by cutting sheet and
drawing metal through dies.
- 1000-1BC: Coin stamping.
- AD1-1000: Steel developed, and coining, forging and
the production of steel swords.
- 1700-1800: Extrusion of lead pipe
- 1800-1900: Steam hammers, steel rolling, seamless
steel piercing, steel rail rolling and continuous rolling.
- 1900-1920: Hot extrusion
- 1940-1950: Extrusion of steel, swaging.
- 1950-1960: Cold extrusion of steel, explosive forming
- 1960-1970-Hydrostatic extrusion.
- 1970-1980: Precision forging, isothermal forging,
- 1980-1990: CAD to shaping equipment production
- 3000-2000BC: Bronze casting began as a method of shaping
- 2000-1000BC: Wrought iron and brass were cast and
- 1000-1500: Blast furnaces developed
- 1500-1600: Cast iron cannon produced and tinplate
- 1600-1700: Permanent mould casting of brass, copper
and metallic zinc.
- 1700-1800: Malleable cast iron and crucible steel
- 1800-1900: Centrifugal casting, Bessemer process,
open hearth steel was developed.
- 1920-1940: Die casting was developed.
- 1940-1950: Lost wax process developed for engineering
- 1950-1960: Continuous casting
- 1960-1970: Squeeze casting. Single crystal castings
- 1970-1980: Vacuum casting. Automation of casting and
pouring. Rapid solidification technology
- Roll forming
- Thread rolling
- Roll bonding
- 1600-1700: Rolling of lead, silver and gold was undertaken
and shape rolling of lead.
- 1700-1800: Iron bars and rods were produced by rolling.
- 1800-1900: Steel rolling, steel rail rolling and
continuous rolling developed.
- 1900-1920: Tube rolling developed.
- Stretch forming
- Peen forming
- Deep drawing
- 1700-1800: Deep drawing developed.
- 1800-1900: Power metallurgy developed
- 1920-1940: Tungsten wire produced from powder
- 1940-1950: Powder metals for engineering parts
- Adhesive bonding
- Diffusion bonding
- 4000-3000BC: Soldering
- 3000-2000BC: Riveting, brazing
- 1000-1BC: Forge welding of iron and steel. Adhesive
- 1900-1920: Oxyacetylene welding; arc, electrical resistance
and thermit welding.
- 1920-1940: Coated electrodes
- 1940-1950: Submerged arc welding
- 1950-1960: TIG, MIG and electroslag welding, explosive
- 1960-1970: Plasma arc and electron beam welding. Adhesive
- 1970-1980: Laser beam, diffusion bonding.
All these activities affects the surface of the material
and hence the coating.
What is it we are trying to coat?
Is it the surface the customer has sent us? Or is it the
surface below what the customer has sent? The exact composition
of the surface to be coated must be known if a quality coating
is to be applied. It is necessary to ensure the correct pretreatment
The surface that the customer sends you may not be the metal
surface to be coated; eg bright stainless steel is often coated
with a clear strippable coating to protect the finish. Also,
some steel tube is manufactured and marketed with a thin organic
Metals can be identified mainly by weight, colour and magnetism.
of some metals
Aluminium is a very light, malleable silvery metallic
material that is resistant to many forms of corrosion.
Halides eg chlorine found in salt water will attack aluminium.
The metal is non-magnetic.
It is sometimes difficult to distinguish some anodised
aluminium surfaces from mill finish. Anodised surfaces
are harder and more abrasion resistant than mill finished
aluminium surfaces. A drop of caustic soda or nitric acid
will react very quickly with a degreased mill finished
surface whereas the attack will be very much slower on
an anodised surface.
Anodised surfaces should not be powder coated. Remove
the oxide by etching if powder coating is required.
- Aluminium Castings
Molten aluminium is poured into a mould and allowed
to solidify to take the shape of the form.
Some die cast and sand cast alloys, particularly have
high silicon contents and smut will form very easily during
Die casting is usually done under pressure whereas in
an open mould casting eg sand casting the metal is simply
poured into the mould at atmospheric pressure. The die
castings (pressure injected) are less porous than the
open mould castings.
- Aluminium and Zinc Spray
In metallising a metal is sprayed while hot onto the
surface to be protected. The surface of a metallised surface
eg aluminium or zinc is relatively porous and looks a
little rough and white.
- Cold Rolled Steel
The most common metal to be coated with powder is steel
(iron), in one form or another. Iron and all mild steels
are magnetic. The colour of the surface can vary depending
on the condition eg rust, mill scale, abrasive blasted,
bead blasted, etc. It can come into the shop coated with
paint, grease, oils, etc.
- Hot Rolled Steel
The surface of hot rolled steel will be covered by mill
scale in various stages of adhesion. When continuous the
scale is a blue black colour, but often it has been damaged
and rust, etc will protrude through the mill scale. All
mill scale and other oxides must be removed before a coating
- Cast Iron
Cast irons are generally magnetic - (although some of
the cast steels may not be magnetic). Cast irons contain
high levels of carbon, which can provide cleaning problems.
Probably best to abrasive blast clean prior to phosphating.
The surface of cast irons can be porous and may require
the use of special powders for a good quality coating.
Copper is a reddish brown metal, which is non magnetic.
It conducts heat and electricity very well. It is sometimes
used for decoration.
Brass is a non magnetic yellow metal - an alloy of copper
and zinc. It is usually malleable and in the highly polished
state 70/30 brass resembles gold although it tarnishes
very quickly. Clear coatings are applied to reduce this
effect. The higher the copper content the more difficult
to maintain a good gold colour when applying a clear finish
to the metal.
Lead is a silvery white, soft, malleable metal used
for piping and in numerous alloys. Lead is extremely heavy
and non-magnetic. Lead is the main constituent in solders
which have a low melting point. This can pose problems
(such as disassembly) when soldered parts are to be powder
Zinc is a bluish white, non magnetic metal. Zinc may
arrive in the form of a die casting, a coating on steel
(galvanize or electrogalvanize). As a coating the colour
can vary from gold, green, yellow to blue/white to black.
The metal is also used in zincalume, zincanneal coatings
on steel sheet components.
- Stainless Steel
Stainless steel always contains iron and chromium and
may also contain appreciable amounts of nickel. Depending
on the composition and the condition of the metal it may
be magnetic or non magnetic. Much of the stainless steel
that comes to a powder coater is highly polished and coated
with a protective material.
The Generation of Shapes
Metal products may be produced either by casting or by
working metals. We hear the terms cast metals and wrought
Metal is heated until it melts and is then poured into
A mould is created of the shape to be produced. This
mould may be made from a metal or sand.
Metal moulds produce castings with good finishes.
Sand moulds tend to produce rougher castings. Sand moulds
are composed of clean silica sand, and organic materials
which help to bind the sand together to provide the strength
to resist fracture during the casting operation. The sand
is rammed into the desired shape of the part to be cast.
When molten metal is poured into a sand mould gases
are produced when the organic material burns. These gases
must escape and the routes available to them are:
- through the metal which is solidifying, or
- through the sand.
If the gases escape through the metal, porosity will be
formed in the part. These pores contain gases and pin holes
will be formed in the cured powder coating if they are not
If the gases are to pass through the sand, the compactness
of the sand has to be reduced so that the gases can pass
though. As the sand is compacted less and less it loses
strength and may crumble when the molten metal is poured
into the mould. If this occurs, sand will be entrapped in
the surface of the part that is cast and this will cause
roughness in the powder coating, or bad adhesion, or both.
A parting compound is applied to metal moulds to assist
the removal of the casting. This material must be free of
silicones if the casting is to be coated.
- Wrought Metals
Wrought metals come in many shapes and sizes.
Metal is cast into billets, which are homogenised so
that the constituents (parts) of the metal are evenly
distributed) and the billet forced through a die whose
shape is the final shape required in the extrusion. The
extrusion process may be carried out hot or cold. After
it is extruded the workpiece is then heat treated to remove
stresses and to achieve the required mechanical properties.
Finally the extrusion is straightened and cut to length.
There are four basic types of extrusion:
The surface of extrusion as received from the mill is
very heavily oxidised. Surface cracking can occur in aluminium,
magnesium and zinc alloys.
- Hot rolled mill finish
- Cold rolled, or
- Bright finish
Hot rolled material is heavily oxidised. The oxides on
cold rolled and bright material are thinner.
- Hollow Sections
Hollow sections can be extruded or produced by roll
forming strip to the desired shape and resistance butt
welding the seam. To improve the external appearance
of the tube the external weld is usually cleaned off
Extruded sections are heavily oxidised as is the weld
on butt seam welded tube. Rolled tube surfaces may contain
non-adherent metal chips from the rolling operation.
Metals can be fabricated using a vast array of techniques
for cutting, sawing, milling, machining, bending, rolling,
drawing, forming, punching, blanking and spinning.
Metals can be joined together to produce a fabricated
part by mechanical, chemical (adhesive) and metallurgical
Metals are bent using conventional presses, folding
machines, tangent benders, lock seamers, roll formers
and draw and stretch benders. To bend hollow sections
such as round, square or rectangular tubes, mandrel
or draw bending is recommended. For less onerous applications
wrap or compression bending, crush bending or roll
bending can be used. When metal is bent, the outer
fibres of the material are in tension and the inner
fibres in compression.
Bending operations can leave 'spalled' semi-attached
metal particles on the surface of the metal as well
as any lubricants that may have been used to assist
- Shape Rolling
Shape rolling was first developed in the late 1700's.
Various shapes such as bars of various cross-sections,
various open sections and railway tracks can be produced
by passing metal through a number of pairs of specially
The surface may be contaminated with rolling lubricants,
and/or non-adherent metal particles in the form of
- Deep Drawing/Forming
Deep drawing was first developed in the 1700's. In
forming, the metal is shaped into a component within
a die. In deep drawing, the metal is ironed and stretched
or compressed during the operation.
The surface can be contaminated with drawing lubricants
that generally are very adhesive and often chemically
bonded to the steel surface. Non adherent metal particles
in the form of scoring residues are common. These are
more likely to appear on highly oxidised surfaces such
as aluminium, etc.
In rod and wire drawing, the cross-sectional area of
a bar is reduced by pulling it through a converging
die - a little like the reverse of extrusion.
- Shearing, Punching and blanking
When punching and blanking two processes take place
- the metal is cut and then sheared. If you look at
a punched or blanked cut you will see a shiny part (the
cut) and a rough grainy part (the sheared part). If
the tools are correctly designed and maintained the
cut portion is about two thirds to three quarters of
the edge metal thickness. When punching or blanking
pre-galvanized sheet, the zinc on the surface is wiped
over the cut portion and provides some corrosion resistance
to the edge.
The rough, grainy portion of the edge gives rise to
Faraday Cage effects (in powder coating and highly varying
current density effects in electroplating) that producing
a specification coating is difficult at best and often
A rotating metal blank is pressed against a back up
chuck of the desired shape. It is often used for spinning
aluminium and aluminium alloys and 70/30 brass into
parabolic, spherical, conical, tapered or re-entrant
shapes. It is an economical process for small quantities.
The surface of the metal is highly worked and some
lubricant may be present. The surface is relatively
easy to prepare for coating.
- Cutting, sawing, drilling, machining and turning
Lathes were first developed in the 1800's. All these
operations may be termed material removal processes.
In these operations the machined surface should be free
of defects and may contain oils. However, if the tools
are not designed correctly nor properly maintained, then
it is possible to produce burrs and rough surfaces, which
can affect the coating process.
Cutting oils should be free of silicones. All silicones
are difficult to remove and interfere with all coating
- Powder Metals
Metal powders are compressed in a die and subsequently
sintered by heating at elevated temperatures. The process
was first developed in the 1840's. Powder metal components
are not usually coated.
Metals may be joined by mechanical, chemical or metallurgical