MANUFACTURING PROCESSES
© 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
  • Casting
• Forming/shaping activities:
• Stretch forming
• Peen forming
• Deep drawing
• Spinning
• Rolling activities
• Roll forming
• Thread rolling
• Roll bonding
• Joining activities
  • Welding
  • Soldering
  • Brazing
  • Adhesive bonding
  • Diffusion bonding
• Cutting activities
• Machining
• Punching
• Grinding
• Powder metals
• Plastics:
  • Thermoforming
  • Rotational moulding
  • Blow moulding
  • Moulding
  • Extrusion

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

• Coining
• Forging
• Extrusion
• Wire drawing
• Heading

  History

  • 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, superplastic forming
  • 1980-1990: CAD to shaping equipment production

Molten Activities

• Casting

  History

  • 3000-2000BC: Bronze casting began as a method of shaping metals.
  • 2000-1000BC: Wrought iron and brass were cast and worked.
  • 1000-1500: Blast furnaces developed
  • 1500-1600: Cast iron cannon produced and tinplate developed.
  • 1600-1700: Permanent mould casting of brass, copper and metallic zinc.
  • 1700-1800: Malleable cast iron and crucible steel developed.
  • 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 parts.
  • 1950-1960: Continuous casting
  • 1960-1970: Squeeze casting. Single crystal castings
  • 1970-1980: Vacuum casting. Automation of casting and pouring. Rapid solidification technology

Rolling activities

• Roll forming
• Thread rolling
• Roll bonding

  History

  • 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.

Forming/Shaping activities

• Stretch forming
• Peen forming
• Deep drawing
• Spinning

  History

  • 1700-1800: Deep drawing developed.

Powder metals

History

• 1800-1900: Power metallurgy developed
• 1920-1940: Tungsten wire produced from powder
• 1940-1950: Powder metals for engineering parts

Joining activities

• Welding
• Soldering
• Brazing
• Adhesive bonding
• Diffusion bonding
• Riveting

  History

  • 4000-3000BC: Soldering
  • 3000-2000BC: Riveting, brazing
  • 1000-1BC: Forge welding of iron and steel. Adhesive bonding.
  • 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 welding.
  • 1960-1970: Plasma arc and electron beam welding. Adhesive bonding.
  • 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 is applied.

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 coating applied.

Metals can be identified mainly by weight, colour and magnetism.

The Metals

• Aluminium

  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 pretreatment operations.

  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 is applied.

• 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

  Copper is a reddish brown metal, which is non magnetic. It conducts heat and electricity very well. It is sometimes used for decoration.

• Brass

  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

  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 coated.

• Zinc

  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 metals.

• Casting

  Metal is heated until it melts and is then poured into a mould.

  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 removed.

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.

• Extrusions

  This process was first developed in the late 1700's for producing lead pipe.

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:

• Direct
• Indirect
• Hydrostatic
• Impact


The surface of extrusion as received from the mill is very heavily oxidised. Surface cracking can occur in aluminium, magnesium and zinc alloys.

• Sheet and Strip

  Sheet is produced by rolling a billet of metal repeatability until the desired thickness is obtained. Between rolling operations the metal is annealed to re-soften it so that more rolling can be carried out. Rolling was first developed in the late 1500's. It is the process of reducing the thickness or changing the cross section of a workpiece by compressive forces exerted by a pair of rotating rolls.

• The surface of rolled sheet can be provided as:
• 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 mechanically.

  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.

• Fabrication

  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 processes.

  • Bending

    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 the process.

  • 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 designed rollers.

    The surface may be contaminated with rolling lubricants, and/or non-adherent metal particles in the form of scoring residues.

• 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 impossible.

• Spinning

  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 processes.

• 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.

• Joining

  Metals may be joined by mechanical, chemical or metallurgical techniques.

• Mechanical

  Mechanical techniques include riveting, threaded fasteners, stapling are used extensively to join metals.

  All these joining processes produce spaces between the joined metals and between the fastener and the metal being joined. Liquids will ooze out of these loose joints causing problems with all forms of coating processes, including staining and non-coverage. Most coating processes require the space between the sections to be at least as wide as the depth of the space, eg to effectively coat the bottom of a hole, the diameter of the hole should be at least the same dimension as the depth of the hole

• Adhesive

  Adhesive bonding using thermoplastic and thermosetting materials is used extensively to fabricate simple and complex assemblies. They provide sealing and insulating properties, reducing the opportunity for electrochemical corrosion between dissimilar metals. Adhesives can also reduce vibration through internal damping at the joints.

The problems for a coater are similar to mechanical joining techniques.

• Heating

  Heating joining techniques consist of 'mechanical' joints (soft solders and brazing where there is no melting of the parent materials) and welding which is a metallurgical bond (some of the parent metal as well as any filler material melt).

  • Soldering and brazing

    A solder is heated and melted and is used to fill crevices and joints.

    Low melting point solders are used to fill cavities or joints and for low strength mechanical joins. Aggressive chemical fluxes are required to remove the oxide film and expose the metal. These fluxes must be removed after soldering to prevent corrosion in service.

Low melting point solders generate a galvanic cell with aluminium leading to corrosion. Various techniques including ultrasonic soldering, reaction solders and friction solders are in use to reduce this problem.

Solders contain lead. Lead is difficult to prepare for coating because of its oxide film. Special chemicals are used. Also, it does not easily accept a chromate or a phosphate coating and thus is the weak link in a powder coating situation.

Brazing solders are high temperature solders consisting of various metal alloys depending on the metals to be joined. For aluminium, the brazing material contains high levels of silicon so that the galvanic corrosion problem associated with soft solders is reduced. Manual or torch brazing, furnace brazing and dip brazing are used. Highly reactive fluxes are required and these can lead to corrosion in service if not completely removed.

All solder fluxes must be completely removed or neutralised to prevent the onset of corrosion and to ensure that the coating adheres to the metal.

• Oxyacetylene welding

  A mixture of oxygen and acetylene is forced through a torch and ignited to produce a hot flame that is used for welding and cutting metals. For some metals it is more difficult to use than TIG and MIG processes. Oxyacetylene welding gives a broad heat source whereas intense localised heating is essential for welding metals such as aluminium. If Oxyacetylene welding is used for these metals, preheating is required.

  High levels of oxidation are produced with this technique as well as weld spatter and slags from the filler rods. All of these defects must be removed before coating as they interfere with the adhesion of the coating. In the case of weld spatter, coatings such as powder coating pull away from the tips of the spatter during curing leaving spatter peaks uncoated.

• Metallic arc welding

  Also known as stick welding, a rod is electrically energised to produce a high temperature arc (spark) against the metal to be welded. The 'stick' is consumed during the process and becomes the filler metal in the weld pool. These welds produce similar problems for coaters, as does Oxyacetylene welding.

• Resistance welding

  Two pieces of metal are forced together at the same time as a high current is passed between them. The resistance of the metals to electricity generates sufficient heat to melt the touching surfaces and join them together. Resistance welds produce a weld bead that is relatively clean except for the heavy oxides that are produced.

• TIG welding

  An inert electrode (tungsten) is completely shrouded in an inert gas such as argon, helium or a mixture of both. A filler wire may be used. TIG provides the conditions for high localised heat necessary for good aluminium and stainless steel welds. When correctly produced TIG welds should be clean with minimal oxides.

• MIG welding

  The Metal Inert Gas process provides heat through an arc between the part and a consumable electrode. As in TIG the arc zone is completely shrouded in an inert gas such as argon, helium or a mixture. MIG provides the conditions for the high localised heating that is required for good aluminium and stainless steel welds. The consumable electrode is fed into the weld pool automatically.

  When correctly produced MIG welds should be clean with minimal oxides. Unfortunately, we see many welds with porosity, holes, weld spatter and other defects.

• Weld cracking

  Cold cracking is due to the internal stresses set up during welding if the weld is not designed correctly. Preheating can help reduce the tendency. Hot cracking can occur in metals and alloys that have a large liquidus to solidus range when the metal is in a weak pasty condition.

  All cracks are difficult to coat. Fine cracks will not be covered by electroplating and may not be covered over with powder coating or wet organic coatings.

• Porosity

  Adverse conditions that give rise to hot cracking of some alloys also encourage porosity. Porosity is caused by gas in the weld pool not being able to escape before the metal solidifies. All material, particularly moisture that decomposes to form hydrogen must be removed from the weld zone. It is also essential to ensure the weld zone is fully shielded by the inert gas. The natural oxide coating on the aluminium absorbs water vapour while some of the intermetallic compounds formed in alloys containing magnesium and silicon react with water. Arc instability due to poor maintenance or inefficient wire feeding in MIG welding also contributes to porosity.

  Porosity in weld is a very difficult problem for a coater. The pores can be very fine and will 'show' though virtually all forms of coatings.