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Manufacturing Contents |
Manufacturing is applicable in all areas of our lives, so much so that we often don't realize or think about it. From
the cars we drive, the containers our food comes in, the TV's, computers and other devices we use, power tools, heaters,
air conditioners, the pipes that deliver our water, and the list goes on and on to include just about everything defining our
modern society. These things are all manufactured or build from manufactured components.
The Fundamental Idea
The fundamental idea of manufacturing or production is to create, (or produce), something that has a useful form. This form
is most likely predetermined, calculated, with a certain physical geometry. Usually this geometry has certain tolerances
that it must meet in order to be considered acceptable. A tolerance outlines the geometric accuracy that must
be achieved in the manufacturing process. The "tightness" of the tolerances or in other words the allowed variance between
the manufactured product and the ideal product is a function of the particular application of the product.
Goals and Core Principles in Manufacturing
- Meeting performance requirements (ie. tolerances, strength, weight, ect.)
- Meeting cost of production requirements
- Ability to reproduce constant quality during mass production
- Large manufactured components should have uniform material properties throughout the component
The Primary Consideration in Manufacturing
When different manufacturing processes and methods are considered it is fundamental to develop
an understanding of the relationship between the process used and the properties of the finished product. For this it
is important to know what conditions a particular process will subject a material to, and how different manufacturing materials
respond to different conditions, (ie. stress, heat).
Manufacturing Materials
All manufactured products are made from some sort of material. Similar to the geometric tolerance, the properties of the
material of the final manufactured product are of utmost importance. Hence, those who are interested in manufacturing should
be very concerned with material selection. An extremely wide variety of materials are available to the manufacturer today. The
manufacturer must consider the properties of these materials with respect to the desired properties of your manufactured goods.
Simultaneously one must also consider manufacturing process. Although the properties of a material may be great, it may not be
able to effectively or economically be processed into a useful form. Also since the microscopic structure of materials are
often changed through different manufacturing processes -dependent upon the process- variations in manufacturing technique may yield
different results in the end product. Therefore a constant feedback must exist between manufacturing process and materials
optimization.
Basic Concepts:
Types of Materials
Materials can be classified into 3 basic types.
- Metals
- Ceramics
- Polymers
What are the properties of metals, the properties of ceramics, the properties of polymers?
Metals: Metals are hard, malleable, (meaning capable of being shaped), and somewhat
flexible materials. Metals are also very strong. Their combination of strength and flexibility makes them useful in structural
applications. When the surface of a metal is polished it has a lustrous appearance; although this surface luster is usually
obscured by the presence of dirt, grease, and salt. Metals are not transparent to visible light. Also metals are extremely good
conductors of electricity and heat.
Ceramics: Ceramics are very hard and strong, but lack flexibility making them brittle.
Ceramics are extremely resistant to high temperatures and chemicals. Ceramics can typically withstand more brutal environments
than metals or polymers. Ceramics are usually not good conductors of electricity or heat.
Polymers: Polymers are mostly soft and not as strong as metals or ceramics. Polymers
can be extremely flexible. Low density, and viscous behavior under elevated temperatures are typical polymer traits. Polymers
can be insulative to electricity.
What are metals made of, what are ceramics made of, what are polymers made of?
Or in other words, what is the basic microstructure of metals, what is the basic microstructure of ceramics, what is the basic
microstructure of polymers?
Metals: A metal is most likely a pure metallic element, (like iron), or an alloy, which
is a combination of two or more metallic elements, (like copper-nickel), the atoms of a metal, similar to the atoms of a ceramic
or polymer, are held together by electrical forces. The electrical bonding in metals is termed metallic bonding. The simplest
explanation for these types of bonding forces would be positively charged ion cores of the element, (nucleus's of the atoms and
all electrons not in the valence level), held together by a surrounding "sea" of electrons, (valence electrons from the atoms).
The electrons in the "sea" moving about not bound to any particular atom. This is what gives metals their properties such malleability
and high conductivity.
Figure 1
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Ceramics: Ceramics are compounds between metallic and non-metallic elements. The
atomic bonds are usually ionic, where one atom, (non-metal), holds the electrons from another, (metal). The non-metal is then
negatively charged and the metal positively charged. The opposite charge causes them to be bond together electrically. Sometimes
the forces are partially covalent. Covalent bonding means the electrons are shared by both atoms, in this case electrical forces
between the two atoms still result from the difference in charge, holding them together. To simplify think of a building framework
structure. This is what gives ceramics their properties such as strength and low flexibility.
Figure 2
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Polymers: Polymers are often composed of organic compounds and consist of long
hydro-carbon chains. Chains of carbon, hydrogen, and often other elements or compounds covalently bonded together.
Figure 3
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In figure 3, (a) represents a simple hydrocarbon chain, each group of hydrogen and carbon is called a mer, there are 13 mers
shown in the diagram the dotted lines indicate that the pattern is continuing indefinitely. Polymers chains often contain
thousands upon thousands of mers each. The [R] in (b) indicates a variable element or group of elements that could occupy a
certain position in the chain. The [X] in (c) also represents another variable element or group that could occupy another position,
this one being at the end or beginning of a polymer chain. The chains themselves bond to each other through secondary bonding
forces. To simplify polymer structure, think of a bowl of spaghetti.
Figure 4
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This is a polymer with a random or amorphous microstructure.
Figure 5
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This a polymer with a very high degree of order or a crystalline microstructure.
When heat is applied the weaker secondary bonds , (between the strands), begin to break and the chains start to slide easier
over one another. However, the stronger covalent bonds, (the strands themselves), stay intact until a much higher temperature.
This is what causes polymers to become increasingly viscous as temperature goes up.
Manufacturing Processes
This is a summary of the basic and most commonly used manufacturing processes
in industry today. Any of these processes can be used to produce a manufactured component. Also remember
when deciding how to produce manufactured items, a part may
require a combination of these processes to facilitate
its completion. For example, a cast part may require some machining before it becomes the final product. Or, a part may be made
through a powder metallurgy process, then undergo some kind of forming operation. The following describes the methods and
techniques involved in each of these manufacturing processes. Always keep in mind how material properties relate to
manufacturing process.
Casting: Definitely one of the oldest manufacturing processes. Castings have been
found dating back 6000 years. Fundamentally, casting involves filling a mold with molten material, which upon solidification takes
the shape of the mold. Castings can be made into the same shape as the final product, being the only process required. Or
sometimes a casting is the first process in the production of a multi-process manufactured part. Casting can be used to make
parts with complicated geometry both internal and external. With casting intricate parts can be made in a single piece. Casting
can produce very small parts like jewelry, or enormous parts weighing several hundred tons, like components for very large
machinery. Although careful influence of casting parameters and technique can help control material properties; a general
disadvantage to casting is that the final product tends to contain more flaws and has a lower strength and ductility compared to
that of other processes such as forming.
Forming: The category of manufacturing by forming includes a large group of processes,
that use force to induce a shape change in a material by mechanical working and plastic deformation. The most desirable quality
of a manufacturing material as a candidate for a forming process is high ductility and malleability, and a lower yield strength of
the material. When working with metals an increase in temperature will result in a higher ductility and a lower yield strength.
In manufacturing industry materials are often formed at elevated temperatures. In addition to shape change
the forming process will usually change the mechanical properties of the parts material. Forming can close up vacancies within
the material, break up and distribute impurities, and establish new, stronger grain boundaries. For these reasons the forming
process is known to produce parts with superior mechanical properties. With relation to temperature there are 3 types of forming.
Cold working, (room temperature), warm working, and hot working. Also with relation to the surface area-to-volume of a material there are
2 main categories, bulk deformation, and sheet forming.
Powder Processing: Powder processing is a manufacturing technique that produces parts
from the powder of a certain material. The powders are pressed into the desired shape, (called pressing), and heated
sufficiently to cause the particles to bond together into a solid component, (called sintering).
There are many advantages to powder processing.
With powder processing you can obtain excellent dimensional control of the product, keeping very tight tolerances, (+/- .005").
It also can produce parts with good surface finish. Parts can therefore be made into their final shape, requiring no further
processing. With powder processing there is very little waste of material. Since powder processing can be automated it minimizes
the need for skilled labor, and large numbers of complex parts can be produced at high speed. Metals that are difficult to work
with other processes can be shaped easily, (ie. tungsten). Also certain alloy combinations and cerments that can't be formed any
other way can be produced with this technique. Lastly, parts can be made with a controlled level of porosity, due to the nature
of the process. Powder processes also have a number of disadvantages. The first is high cost. Powders are expensive compared to
solid material, they are also difficult to store. Sintering furnaces and special presses are more complicated to construct than
conventional machinery. Tooling is also very expensive. Since powders do not easily flow laterally in a die when pressed, there
are geometric limitations to the parts that can be made. Powder parts may have inferior mechanical properties, (unless they
undergo a forging process). Finally variations in material density throughout the part may be a problem especially with more
intricate geometries.
Machining: In machining, a manufactured part is created to its desired geometric
dimensions by the removal of excess material from a workpiece via a shearing force exerted through a certain cutting tool.
Qualities of a desirable manufacturing material for this purpose would be:
1) Lower Shear Strength (to make cutting easier)
2) Shock Resistant (to withstand impact loading)
3) Material must not have a tendency to stick to the cutting tool
A materials relative ability to be machined is called machinability. Ceramics have high shear strengths making them difficult to
cut. Also they are not shock resistant, which causes them to fracture from the impact loading between the tool and workpiece.
Polymers although having low yield strengths, melt from the heat generated in the process, causing them to stick to the tool.
For these reasons ceramics and metals have poor machinability. Machining is generally applicable to metals. Machinability
varies among metals, hardened metals present a particular problem due to a very high shear strength. Often metals are machined
as close to their final shape as possible before hardened. That way the hardened material only has to undergo minimal finishing
operations. Machining has many advantages. Machining can produce extreme dimensional accuracy, often more so than any other
process alone, (tolerances of less than .001"). Machining can produce sharp corners and flatness on a part that may not be able
to be created through other processes. Machining accuracy allows it to produce surface finish and smoothness that can't be achieved
any other way. By combining different machining operations, very complex parts can be made. Machining does have disadvantages
Machining is a removal process and wastes material. Although economical if the number of parts to be made is small; labor, energy
equipment, and scrap cost are relatively high for large runs.
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