a) Heat Treatment
Hardening and tempering of engineering steels is performed to provide components with mechanical properties suitable for their intended service. Steels are heated to their appropriate hardening temperature {usually between 800-900°C), held at temperature, then "quenched" (rapidly cooled), often in oil or water.
If steel is heated until it glows red and is quenched in clean water immediately, it becomes very hard but also brittle. This means it is likely to break or snap if put under pressure. However, if the red hot steel is allowed to cool slowly, the resulting steel will be easier to cut, shape and file as it will be relatively soft. The industrial heat treatment of steel is a very complex and precise science.
In a school workshop most heat treatment of metals takes place on a brazing hearth. The fire bricks contain the intense heat of the metal being heated. Without the bricks, heat would escape and this would limit the temperature that could be reached.
Note: Mild steel and medium carbon steel do not have enough carbon to change their crystalline structure and consequently cannot be hardened and tempered. Medium carbon steel may become slightly tougher although it cannot be harden to the point where it cannot be filed or cut with a hacksaw (the classic test of whether steel has been hardened).
If steel is heated until it glows red and is quenched in clean water immediately, it becomes very hard but also brittle. This means it is likely to break or snap if put under pressure. However, if the red hot steel is allowed to cool slowly, the resulting steel will be easier to cut, shape and file as it will be relatively soft. The industrial heat treatment of steel is a very complex and precise science.
In a school workshop most heat treatment of metals takes place on a brazing hearth. The fire bricks contain the intense heat of the metal being heated. Without the bricks, heat would escape and this would limit the temperature that could be reached.
Note: Mild steel and medium carbon steel do not have enough carbon to change their crystalline structure and consequently cannot be hardened and tempered. Medium carbon steel may become slightly tougher although it cannot be harden to the point where it cannot be filed or cut with a hacksaw (the classic test of whether steel has been hardened).
Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.
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Case-hardening involves packing the low-carbon iron within a substance high in carbon, then heating this pack to encourage carbon migration into the surface of the iron. This forms a thin surface layer of higher carbon steel, with the carbon content gradually decreasing deeper from the surface.
Annealing. When metal is cold-worked it distorts the crystalline structure and makes it hard and difficult to work with. Annealing can be used to de-stress the crystalline structure making it less stressed internally. When a metal is annealed, it is heated to the ideal temperature (for that metal) and then kept at that temperature for a predetermined length of time. The cooling rate for annealing is much slower than tempering. For instance, if you were cooking something in the oven and instead of removing it at the end of the cook time you allowed it to cool inside the oven, that’s similar to annealing.
The primary reasons a metal product undergoes annealing is to reduce its hardness/make it softer. That way, it can be machined more easily. Additionally, some metals are annealed in order to increase electrical conductivity.
During the annealing process, metals–namely steels–are heated evenly to return them to as close to their pre-cold worked state as possible. As a process, annealing is necessary because materials tend to lose ductility after a certain amount of cold working. If cold working is needed continuously throughout the metal forming process, annealing becomes a necessary component of that process because it helps to restore the metal’s original properties.
During the standard annealing process, there are three stages: recovery, recrystallization, and grain growth. Here’s a rundown of these three stages:
Stage 1 – Recovery. In annealing, recovery is a process that acts to recover the physical properties of the metals such as thermal expansion, electrical conductivity, and internal energy. Essentially, it is this initial step that softens the metal. During recovery, dislocations or irregularities in the metal’s structure find their way into stress-free environments. As we’ll see in the recrystallization stage, it is these stress-free cells that help push the annealing process forward.
Stage 2 – Recrystallization. Recrystallization is a restorative process. In order for annealing to be effective, workpieces must be heated to a temperature that’s above its recrystallization temperature. During recrystallization, deformed grains of the metal’s crystal structure are replaced by the new stress-free grains that were developed during recovery. These undeformed, stress-free cells nucleate and grow until all of the original grains have been replaced. So, for example, low carbon steel should be annealed at around 900 degrees Celsius (or 1650 degrees F) because that is the temperature at which it’ll recrystallize. It is not always necessary to heat the metal into a critical temperature range. Mild steel products that need to be repeatedly cold worked can be softened by annealing at 500-650 degrees Celsius.
Stage 3 – Grain Growth. Grain growth is the third stage and this happens when annealing is allowed to continue after recrystallization is completed.In this stage, the microstructure of the metal becomes coarse and makes the workpiece lose some of its strength. Strength can typically be regained through a process called hardening. For steel, quenching and tempering is the hardening process you would use.
The primary reasons a metal product undergoes annealing is to reduce its hardness/make it softer. That way, it can be machined more easily. Additionally, some metals are annealed in order to increase electrical conductivity.
During the annealing process, metals–namely steels–are heated evenly to return them to as close to their pre-cold worked state as possible. As a process, annealing is necessary because materials tend to lose ductility after a certain amount of cold working. If cold working is needed continuously throughout the metal forming process, annealing becomes a necessary component of that process because it helps to restore the metal’s original properties.
During the standard annealing process, there are three stages: recovery, recrystallization, and grain growth. Here’s a rundown of these three stages:
Stage 1 – Recovery. In annealing, recovery is a process that acts to recover the physical properties of the metals such as thermal expansion, electrical conductivity, and internal energy. Essentially, it is this initial step that softens the metal. During recovery, dislocations or irregularities in the metal’s structure find their way into stress-free environments. As we’ll see in the recrystallization stage, it is these stress-free cells that help push the annealing process forward.
Stage 2 – Recrystallization. Recrystallization is a restorative process. In order for annealing to be effective, workpieces must be heated to a temperature that’s above its recrystallization temperature. During recrystallization, deformed grains of the metal’s crystal structure are replaced by the new stress-free grains that were developed during recovery. These undeformed, stress-free cells nucleate and grow until all of the original grains have been replaced. So, for example, low carbon steel should be annealed at around 900 degrees Celsius (or 1650 degrees F) because that is the temperature at which it’ll recrystallize. It is not always necessary to heat the metal into a critical temperature range. Mild steel products that need to be repeatedly cold worked can be softened by annealing at 500-650 degrees Celsius.
Stage 3 – Grain Growth. Grain growth is the third stage and this happens when annealing is allowed to continue after recrystallization is completed.In this stage, the microstructure of the metal becomes coarse and makes the workpiece lose some of its strength. Strength can typically be regained through a process called hardening. For steel, quenching and tempering is the hardening process you would use.
Normalising is used to improve the mechanical properties of, mainly, unalloyed and low-alloy structural steel and cast steel. This heat treatment creates a finer structure in the material, thereby bolstering both its tensile strength and its ductility, as well as minimising the internal stress. In essence, normalising the steel means returning processed steel to its ‘normal’ state. Normalising involves heating a material to an elevated temperature and then allowing it to cool back to room temperature by exposing it to room temperature air after it is heated. This heating and slow cooling alters the microstructure of the metal which in turn reduces its hardness and increases its ductility.
b) Alloying
Definition of an Alloy
An alloy is a metal that’s combined with other substances to create a new metal with superior properties. For example, the alloy may be stronger, harder, tougher, or more malleable than the original metal. Alloys are often thought to be a mixture of two or more metals. However, this is a misconception, as alloys can be composed of one metal and other non-metallic elements.
The predominant metal in the alloy is called the base metal. The other metals or elements added to the alloy are called alloying elements.
Examples of AlloysIn addition to increasing the strength of a metal, alloying may change other properties, including the resistance to heat, corrosion resistance, magnetic properties, or electrical conductivity.
- Steel is created from iron and carbon. Iron is a brittle metal, so it’s not suitable for use as a building material for constructing bridges and buildings. Structures created from iron would eventually collapse. Because it’s tough and has a high tensile strength, steel is an ideal construction material.
- Stainless steel, an alloy made from iron and chromium, is more resistant to corrosion and staining when it comes in contact with water as opposed to iron and carbon steel.
- Aluminum is soft and relatively weak. Its strength can be increased by adding other elements, including zinc, copper, magnesium, and manganese. When aluminum contains added elements, it’s known as an aluminum alloy.
c) Printing
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The history of printing starts as early as 3500 BCE, when the proto-Elamite and Sumerian civilizations used cylinder seals to certify documents written in clay. Other early forms include block seals, hammered coinage, pottery imprints, and cloth printing. Woodblock printing originated in China around 200 AD. It led to the development of movable type in the eleventh century and the spread of book production in East Asia. Woodblock printing was also used in Europe, but it was in the fifteenth century that European printers developed a process for mass-producing metal type to support an economical book publishing industry. This industry enabled the communication of ideas and sharing of knowledge on an unprecedented scale. Alongside the development of text printing, new and lower-cost methods of image reproduction were developed, including lithography, screen printing and photocopying.
Starting with the paper, There are two types of offset printing machines in common use for publication today: sheet-fed printing and web printing. In sheet-fed offset printing, individual pages of paper are fed into the machine. The pages can be pre-cut to the final publication size or trimmed after printing. In web offset printing, larger, higher-speed machines are used. These are fed with large rolls of paper and the individual pages are separated and trimmed afterwards. Sheet-fed printing is popular for small and medium-sized fixed jobs such as limited-edition books. Web offset printing is more cost-effective for high-volume publications whose content changes often, such as newspapers.
Now let's have a look at colour...
The CMYK process is used in most printing scenarios and allows us to 'create' a wide variety of colours when printing using only 4 inks.
For A level you need to be familiar with the following methods of printing:
For A level you need to be familiar with the following methods of printing:
Offset Lithography, also called offset printing, is a method of mass-production printing in which the images on metal plates are transferred (offset) to rubber blankets or rollers and then to the print media. The whole principle is based of the fact that water and oil can't mix.The print media, usually paper, does not come into direct contact with the metal plates and this prolongs the life of the plates. In addition, the flexible rubber conforms readily to the print media surface, allowing the process to be used effectively on rough-surfaced media such as canvas, cloth or wood. The main advantage of offset printing is its high and consistent image quality. The process can be used for small, medium or high-volume jobs.
Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress, evolved with high speed rotary functionality, which can be used for printing on almost any type of substrate, including plastic, metallic films, cellophane, and paper. It is widely used for printing on the non-porous substrates required for various types of food packaging (it is also well suited for printing large areas of solid colour)
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Screen-Printing has evolved from the ancient art of stencilling that took place in the late 1800’s and over time, with modifications, the method has evolved into an industry.In the 19th century it remained a simple process using fabrics like organdy stretched over wooden frames as a means to hold stencils in place during printing. Only in the twentieth century did the process become mechanised, usually for printing flat posters or packaging and fabrics.Screen printing is a technique that involves using a woven mesh screen to support an ink-blocking stencil to receive a desired image.
The screen stencil forms open areas of mesh that transfer ink or other printable materials, by pressing through the mesh as a sharp-edged image onto a substrate (the item that will receive the image).
A squeegee is moved across the screen stencil, forcing ink through the mesh openings to wet the substrate during the squeegee stroke. As the screen rebounds away from the substrate, the ink remains. Basically it is the process of using a mesh-based stencil to apply ink onto a substrate, whether it is t-shirts, posters, stickers, vinyl, wood, or other materials.
Screen Printing is most commonly associated with T-Shirts, lanyards, balloons, bags. It can often be used to print on to delicate materials like corrugated card. This process is also used when applying latex to promotional printed scratch cards or for a decorative print process called spot UV. All using the same process of squeegees and screens, the clear coating is then exposed to UV radiation lamps for the drying process; it is primarily used to enhance logos or depict certain text, spot UV is a great way to take your print to the next level.
Further reading
The screen stencil forms open areas of mesh that transfer ink or other printable materials, by pressing through the mesh as a sharp-edged image onto a substrate (the item that will receive the image).
A squeegee is moved across the screen stencil, forcing ink through the mesh openings to wet the substrate during the squeegee stroke. As the screen rebounds away from the substrate, the ink remains. Basically it is the process of using a mesh-based stencil to apply ink onto a substrate, whether it is t-shirts, posters, stickers, vinyl, wood, or other materials.
Screen Printing is most commonly associated with T-Shirts, lanyards, balloons, bags. It can often be used to print on to delicate materials like corrugated card. This process is also used when applying latex to promotional printed scratch cards or for a decorative print process called spot UV. All using the same process of squeegees and screens, the clear coating is then exposed to UV radiation lamps for the drying process; it is primarily used to enhance logos or depict certain text, spot UV is a great way to take your print to the next level.
Further reading
Gravure
Gravure printing is normally a reel-fed printing method although sheet-fed machines exist in small numbers. Gravure is used mainly for large runs for magazines and directories on thinner paper. However, a significant number of applications are run on paperboard for high volume packaging such as cigarette cartons and large volume confectionery/liquid packaging.
The presses used for printing packaging applications are different from the publication presses in two ways: they are narrower and print units are almost exclusively set up in a straight sequence horizontally (magazine printers can have print units set up vertically). The advantages of gravure printing lie in the high, consistent and continuously reproducible print result. Further, the advantage of fast ink drying (by evaporation) contributes to an immediate post-finishing of the printed goods. The process is ideally suited to situations where there is a range of print designs but a constant carton construction and size.
Gravure printing is normally a reel-fed printing method although sheet-fed machines exist in small numbers. Gravure is used mainly for large runs for magazines and directories on thinner paper. However, a significant number of applications are run on paperboard for high volume packaging such as cigarette cartons and large volume confectionery/liquid packaging.
The presses used for printing packaging applications are different from the publication presses in two ways: they are narrower and print units are almost exclusively set up in a straight sequence horizontally (magazine printers can have print units set up vertically). The advantages of gravure printing lie in the high, consistent and continuously reproducible print result. Further, the advantage of fast ink drying (by evaporation) contributes to an immediate post-finishing of the printed goods. The process is ideally suited to situations where there is a range of print designs but a constant carton construction and size.
Due to the high volumes and high speed in the gravure printing machines the operation is demanding when it comes to reel presentation. There must be tight tolerance in width, clean edges, well aligned cores and high quality joins (if any joins are present). Issues relating to web tension and dimensional stability are critical for achieving good register in printing and finishing. They therefore require consistent properties with regards to the tensile strength and stiffness of the paperboard. The choice of paperboard will affect the natural abilities for dimensional stability (see the section on flatness and dimensional stability).
The hardness of the cylinder also demands a smooth surface for best contact and ink transfer since the cylinder does not adapt to the surface structure in the same way as, for instance, a rubber blanket in offset printing. The thickness of the paperboard must be consistent. This also means that the gravure method is relatively more sensitive to surface defects like blade lines and indentations than other methods. The failure in ink transfer results in missing dots.
In food packaging, the paperboard’s solvent retention properties affect the speed at which the cartons can be wrapped without having to “air” the pallet to achieve acceptable levels of solvents trapped in the board. Factors governing the degree of solvent retention include the specific ink system used, plus well-managed and well-controlled dryers that are in good condition.
Freedom from visual defects is clearly of crucial importance and a high proportion of modern presses are equipped with a unit that will scan for print defects (including those caused by the board) and reject defective cartons. Clearly, the paperboard’s freedom from any surface defects, whether visual or physical is a key property.
The hardness of the cylinder also demands a smooth surface for best contact and ink transfer since the cylinder does not adapt to the surface structure in the same way as, for instance, a rubber blanket in offset printing. The thickness of the paperboard must be consistent. This also means that the gravure method is relatively more sensitive to surface defects like blade lines and indentations than other methods. The failure in ink transfer results in missing dots.
In food packaging, the paperboard’s solvent retention properties affect the speed at which the cartons can be wrapped without having to “air” the pallet to achieve acceptable levels of solvents trapped in the board. Factors governing the degree of solvent retention include the specific ink system used, plus well-managed and well-controlled dryers that are in good condition.
Freedom from visual defects is clearly of crucial importance and a high proportion of modern presses are equipped with a unit that will scan for print defects (including those caused by the board) and reject defective cartons. Clearly, the paperboard’s freedom from any surface defects, whether visual or physical is a key property.
d) Casting
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Sand casting is a metal casting process characterized by using sand as the mould material. The term "sand casting" can also refer to an object produced via the sand casting process. Sand castings are produced in specialized factories called foundries (this is the term used for a workplace where heavy metal work is undertaken). Over 60% of all metal castings are produced via sand casting process. Because of the texture of sand the finished product does not leave a high quality finish and it parts may need to be machined afterwards.
Moulds made of sand are relatively cheap even for steel foundry use. In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand (this is often referred to as green sand). The mixture is moistened, typically with water, but sometimes with other substances, to develop the strength and plasticity of tthe green sand. It is typically contained in a system of frames or mould boxes known as a the cope and drag. The mould cavities are created by compacting the sand around models (called patterns), by carving directly into the sand, or by 3D printing. Runners and risers are shaped into the mould to allow for the flow of metal and removal of air. Click here for more information
Even though sand casting is used for one off or batch work it is very labour intensive and therefore expensive. It is often used for railway carriage wheels, vices, motor housings, bollard and drain covers.
Moulds made of sand are relatively cheap even for steel foundry use. In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand (this is often referred to as green sand). The mixture is moistened, typically with water, but sometimes with other substances, to develop the strength and plasticity of tthe green sand. It is typically contained in a system of frames or mould boxes known as a the cope and drag. The mould cavities are created by compacting the sand around models (called patterns), by carving directly into the sand, or by 3D printing. Runners and risers are shaped into the mould to allow for the flow of metal and removal of air. Click here for more information
Even though sand casting is used for one off or batch work it is very labour intensive and therefore expensive. It is often used for railway carriage wheels, vices, motor housings, bollard and drain covers.
In Investment Casting, a wax or suitable polymer pattern is coated by dipping into the refractory material slurry. Once the refractory material coating is hardened then this dipping process is repeated several times to increase the coating thickness and its strength. Once the final coating is hardened the wax is melted out and molten metal is poured into the cavity created by the wax pattern. Once the metal solidifies within the mould, metal casting is removed by breaking the refractory mould. This methodis best used for intricate shapes such as jewellery and collectible figures. Hip replacement joints are also made this way.
Pressure Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mould cavity. The mould cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mould during the process. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter, and tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.
The casting equipment and the metal dies represent large capital costs and this tends to limit the process to high-volume production. Manufacture of parts using die casting is relatively simple, involving only four main steps, which keeps the incremental cost per item low. It is especially suited for a large quantity of small- to medium-sized castings, which is why die casting produces more castings than any other casting process. Die castings are characterized by a very good surface finish (by casting standards) and dimensional consistency.
This is a speedy process so is suited to mass manufacturing or large batch runs. Therefore it can be used to produce toy cars, collectible figures, decorative door knobs and handles.
The casting equipment and the metal dies represent large capital costs and this tends to limit the process to high-volume production. Manufacture of parts using die casting is relatively simple, involving only four main steps, which keeps the incremental cost per item low. It is especially suited for a large quantity of small- to medium-sized castings, which is why die casting produces more castings than any other casting process. Die castings are characterized by a very good surface finish (by casting standards) and dimensional consistency.
This is a speedy process so is suited to mass manufacturing or large batch runs. Therefore it can be used to produce toy cars, collectible figures, decorative door knobs and handles.
Gravity Die Casting employs cast iron moulds which allow aluminium & zinc castings to be produced more accurately and cheaply than with sand castings. Tooling costs of gravity die casting are a fraction of those needed for high pressure die castings. The rapid chilling gives excellent mechanical properties whilst non-turbulent filling ensures production of heat treatable gravity castings with minimal porosity. This method is best used for thicker mould sections and suited to large batch and mass production manufacture of Allow Wheels and engine components, door knobs and handles again.
Resin Casting
Plaster of Paris casting
e) Machining
f) Moulding
Blow moulding is used for making hollow thin walled components or products. It has a high initial cost because of the mould design and manufacture however, as it used for continuous production it can recoup the initial cost on large runs. It is used for the manufacture of drinks bottles, detergent bottles and shampoo bottles etc.
Injection moulding is a manufacturing process for producing parts by injecting molten material into a mould, or mould. Injection moulding can be performed with a host of materials mainly including metals, glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers
Vacuum Forming is a simple form of thermoforming, where a sheet of plastic is heated to a forming temperature, stretched onto a single-surface mould, and forced against the mould by a vacuum. This process can be used to form plastic into permanent objects such as sandwich packets and blister packs.
Extrusion is a manufacturing process used to make pipes, hoses, drinking straws, curtain tracks, rods, and fibre. The granules melt into a liquid which is forced through a die, forming a long 'tube like' shape. The shape of the die determines the shape of the tube. The extrusion is then cooled and forms a solid shape. The tube may be printed upon, and cut at equal intervals. The pieces may be rolled for storage or packed together. Shapes that can result from extrusion include T-sections, U-sections, square sections, I-sections, L-sections and circular sections.
Plastics extrusion is a high-volume manufacturing process in which plastic is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weatherstripping, fencing, deck railings, window frames, plastic films and sheeting, thermoplastic coatings , and wire insulation.
Rotational moulding, known also as rotomoulding or rotocasting, is a process for manufacturing hollow plastic products. Although there is competition from blow molding, thermoforming, and injection moulding for the manufacture of such products, rotational moulding has particular advantages in terms of relatively low levels of residual stresses, inexpensive moulds and economic viability for short runs. Rotational moulding is best known for the manufacture of tanks but it can also be used to make other innovative plastic products such as complex medical products, toys, leisure craft, and highly aesthetic point-of-sale products.
g) Lamination