1-Martensite ( Group: Material)
Martensite is a form of ferrite that is
supersaturated with carbon. In other words, because of
the very fast cooling rate, the carbon atoms
do not have time to diffuse from their interstitial positions in the bcc
lattice to form cementite particles. Steel products produced with an as-quenched martensitic microstructure are very
hard and brittle, e.g., a razor blade. Most martensitic products are tempered by heating to
temperatures between about 350 and 650 C. The tempering process allows some of the
carbon to diffuse and form as a carbide phase from the supersaturated iron lattice. This
softens the steel and provides some ductility. The degree of softening is determined by the
tempering temperature and the time at the tempering temperature. The higher the temperature
and the longer the time, the softer the steel. Most steels with martensite are used in the
quenched and tempered condition.
Yet another microconstituent or phase called martensite is formed when
austenitized iron- carbon alloys are rapidly cooled (or quenched) to a
relatively low temperature (in the vicinity of the ambient). Martensite is a
nonequilibrium single-phase structure that results from a diffusionless
transformation of austenite. It may be thought of as a transformation product
that is competitive with pearlite and bainite.The martensitic transformation
occurs when the quenching rate is rapid enough to prevent carbon diffusion .Any
diffusion whatsoever will result in the formation of ferrite and cementite
phases.
The martensitic
transformation is not well understood. However, large numbers of atoms
experience cooperative movements, in that there is only a slight displacement
of each atom relative to its neighbors. This occurs in such a way that the FCC
austenite experiences a polymorphic transformation to a body-centered
tetragonal (BCT) martensite. A unit cell of this crystal structure (Figure
11.21) is simply a body-centered cube that has been elongated along one of its
dimensions; this structure is distinctly different from that for BCC ferrite.
All the carbon atoms remain as interstitial impurities in martensite; as such,
they constitute a supersaturated solid solution that is capable of rapidly
transforming to other structures if heated to temperatures at which diffusion
rates become appreciable. Many steels, however, retain their martensitic
structure almost indefinitely at room temperature.
Martensite grains take on a platelike or needlelike appearance, as
indicated in Figure 11.22. The white phase in the micrograph is austenite
(retained anstenite) that did not transform during the rapid quench. As already
mentioned, martensite as well as other microconstituents (e.g., pearlite) can
coexist.
2-Acetal(Delrin,
Celcon) (Group: Material)
These combine very high strength, good temperature and abrasion resistance,
exceptional dimensional stability, and low coefficient of thermal expansion.
They compete with nylon (but with many better properties) and with metal die
castings (but are lighter). Chemical resistance is good except for strong
acids. Typical applications are water-pump parts, pipe fittings, washing
machines, car instrument housings, bearings, and gears.
(Mechanical Engineering Handbook; Ed. Frank Kreith,p12-20)
New and better explanation
(Mechanical Engineering Handbook; Ed. Frank Kreith,p12-20)
New and better explanation
What Is It? POM was first marketed by DuPont in 1959 as Delrin. It is
similar to nylon but is stiffer, and has better fatigue and water resistance -
nylons, however, have better impact and abrasion resistance. It is rarely used
without modifications: most often filled with glass fiber, flame retardant
additives or blended with PTFE or PU. The last, POM/PU blend, has good
toughness. POM is used where requirements for good moldability, fatigue
resistance and stiffness justify its high price relative to mass polymers, like
polyethylene, which are polymerized from cheaper raw materials using lower
energy input.
Design Notes POM is easy to mold by blow molding, injection molding or sheet
molding, but shrinkage on cooling limits the minimum recommended wall thickness
for injection molding to 0.1 mm. As
manufactured, POM is gray but it can be colored. It can be extruded to produce
shapes of constant cross-section such as fibers and pipes. The high
crystallinity leads to increased shrinkage upon cooling. It must be processed
in the temperature range 190-230 C and may require drying before forming
because it is hygroscopic. Joining can be done using ultrasonic welding, but
POM’s low coefficient of friction requires welding methods that use high energy
and long ultrasonic exposure. Adhesive bonding is an alternative. POM is also
an electrical insulator. Without copolymerization or the addition of blocking
groups. POM degrades easily.
Typical Uses Automobile carburettors and door handles, videocassette parts,
gears and bearings, tool handles, plumbing parts, clothing zips.
Competing Materials Nylon, polyester, PTFE.
The Environment Acetal, like most thermoplastics, is an oil derivative, but
this poses no immediate threat to its use.
Technical Notes The repealing unit of POM is -(CH2O)n , and the resulting
molecule is linear and highly crystalline. Consequently, POM is easily
moldable, has good fatigue resistance and stiffness, and is water resistant. In
its pure form, POM degrades easily by de-polymerization from the ends of the
polymer chain by a process called “unzipping.” The addition of "blocking
groups" at the ends of the polymer chains or co-polymerization with cyclic
ethers such as ethylene oxide prevents unzipping and hence degradation.
( Ashby, M.F.,
Johnson, K. (2010). Polyoxymethylene (P0M), Acetal . Materials
and Design: The Art and Science of Material Selection in Product Design (p. 213). )
An amber-colored, transparent material made by the reaction of cellulose with acetic acid or acetic anhydride in the presence of sulfuric acid. In Germany it was made by treating beech-wood pulp with acetic acid in the presence of an excess of zinc chloride. It is employed for lacquers and coatings, molding plastics, rayon, and photographic film. Cellulose acetate may be the triacetate C6H7O2, but may be the tetracetate or the pentacetate, or mixture. It is made in different degrees of acetylation with varying properties.
Unlike nitrocellulose, it is not flammable,
and it has better light and heat stability. It has a refractive index of 1.47
to 1.50, and a sheet 0.125 in (0.32 cm) thick will transmit 90% of the light.
The specific gravity is 1.27 to 1.37, Brinell hardness 8 to 15, tensile
strength 3500 to 8000 lb/in^2 (24 to 55 MPa), compressive strength up to 20000
lb/in^2 (138 MPa), elongation 15 to 80%, dielectric strength 300 to 600 V/mil
(12*10^6 to 24*10^6 V/m), and softening point 122 to 205F (50 to 96C). It is
thermoplastic and is easily molded. The molded parts or sheets are tough,
easily machined, and resistent to oils and many chemicals. In coatings and
lacquers, the material is adhesive, tough and resilient, and it does not
discolor easily.
(Brady G.S., Clauser H.R., Vaccari J.A.,
Materials Handbook,15th Ed., pg. 204, Kayra Ermutlu)
New and
better explanation
Cellulose acetate is most useful of the cellulosic plastics. During
World War I, the British secured the aid Henry and Camille Dreyfus of Switzerland
to start large-scale production of cellulose acetate. Cellulose acetate
provided a fire-retardant lacquer for the fabric-covered airplanes used at that
time. By 1929, commercial grades of molding powder, fibers, sheets, and tubes
were being produced in the United States.
Basic methods for manufacturing this material resemble those used in
making cellulose nitrate. Acetylation of cellulose is carried out in a mixture
of acetic acid and acetic anhydride, using sulfuric acid as a catalyst.
The acetate or acetyl I
group or radical (CH3CO) is the source of chemical reaction with the hydroxyl
(OH) groups. The structure of cellulose triacetate is as follows:
Cellulose acetates exhibit poor heat, electrical, weathering, and chemical
resistance. They are fairly inexpensive and be transparent or colored. Their
main uses are as films and sheets in the packaging and display industries. They
are fabricated by nearly all the thermoplastic processes and molded into brush
handles, combs, and spectacle frames. Vacuum-formed display containers for
hardware or food products are common, and films that permit the passage of
moisture and gases are used in commertial packaging of fruits and vegetables.
Coated films are used in magnetic recording tapes and photographic film.
Cellulose acetate plastics are made into fibers for use in textiles. They are
also employed as lacquers in the coating industiy. Table E-4 gives some of the properties
of cellulose acetate.
(Lokensgard , E.(2010).Cellulose Acetate (CA). Industrial Plastics:
Theory and Applications (p. 439). )
4-Acrylic (Group: Material)
The acrylics are polymers derived from acrylic acid (C3H4O2) and compounds : originating from it. The most important thermoplastic in the acrylics group is polymethyl-methacrylate (PMMA) or Plexiglas (Rohm & Haas's trade name for PMMA). It is an amorphous linear polymer. Its outstanding property is excellent transparency, which makes it competitive with glass in optical applications, Examples include automotive tail-light lenses, optical instruments, and aircraft windows. Its limitation when compared with glass is a much lower scratch resistance. Other uses of PMMA include floor waxes and emulsion latex paints. Another important use of acrylics is in fibers for textiles; polyacrylonitrile (PAN) is an example that goes by the more familiar trade names Orion (DuPont) and Acrilan (Monsanto).
(P.Groover, Fundamentals of Modern Manufacturing ,page 158-159)
New and better explanation
The acrylics are polymers derived from acrylic acid (C3H4O2) and compounds : originating from it. The most important thermoplastic in the acrylics group is polymethyl-methacrylate (PMMA) or Plexiglas (Rohm & Haas's trade name for PMMA). It is an amorphous linear polymer. Its outstanding property is excellent transparency, which makes it competitive with glass in optical applications, Examples include automotive tail-light lenses, optical instruments, and aircraft windows. Its limitation when compared with glass is a much lower scratch resistance. Other uses of PMMA include floor waxes and emulsion latex paints. Another important use of acrylics is in fibers for textiles; polyacrylonitrile (PAN) is an example that goes by the more familiar trade names Orion (DuPont) and Acrilan (Monsanto).
(P.Groover, Fundamentals of Modern Manufacturing ,page 158-159)
New and better explanation
What
Is It?
When you think of PMMA, think transparency. Acrylic, or PMMA, is the thermoplastic that most closely
resembles glass in transparency and resistance to weathering. The material has
a long history: discovered in 1872, commercialized in 1933, its first major application
was as cockpit canopies for fighter aircraft during the Second World War.
Design
Notes
Acrylic, or PMMA, is hard and stiff as polymers
go, easy to polish but sensitive to stress concentrations. It shares
with glass a certain fragility, something that can be overcome by blending with
acrylic rubber to give a high impact alloy (HIPMMA). PVC can be blended with
PMMA to give tough, durable sheets. Acrylic is available as a sheet, rod or tube and can be shaped by
casting or extrusion. Cell casting uses plates of glass and gasket ng for a mold:
it allows clear and colored panels up to 4 inches thick to be cast. Extrusion
pushes melted polymer pellets through a die to give a wide variety of shapes,
up to 0.25 inches thick for sheet. Clear
and colored PMMA sheet lends itself to thermoforming, allowing inexpensive
processing. A hybrid sheet manufacturing process, continuous casting, combines
the physical benefits of cell casting and the cost efficiency of extrusion.
Extruded and continuous cast sheet have better thickness tolerance than cell-cast
sheet. PMMA can be joined with epoxy, alpha-cyanoacrylate, polyester or nitrile-phenolc
adhesives. It scratches much more easily than glass, but this can be partially
overcome with coatings.
Typical
Uses
Lenses of all types; cockpit canopies and aircraft windows; signs; domestic
baths; packaging; containers; electrical
components; drafting equipment; tool handles; safety spectacles; lighting, automotive tail lights, chairs,
contact lenses, Windows, advertising signs, static dissipation products; compact
disks.
Competing
Materials Polyearbonate, polystyrene, PVC, PET.
The
Environment Acrylics are non-toxic and recyclable.
Technical
Notes
Polymers are truly transparent only if they are completely amorphous that is, non-crystalline.. The lumpy shape of
the PMMA molecule ensures an amorphous structure, and
its stability gives good weathering resistance, PMMA is attacked by esters, ketones, acids and
hydro-carbons, and has poor resistance to strong acids or bases, solvents and
acetone.
( Ashby, M.F., Johnson, K. (2010).Polyrricthylniethacrylatc (PMMA),
Acrylic. Materials and Design: The Art and
Science of Material Selection in Product Design (p. 211). )
5-Pyramid Forecasting (Group: Management)
When developing the sales forecast, forecasters can derive
information for use in forecast
development from many sources. Qualitative techniques, such as
personal insight, sales force estimates, and the market research, can be used. Causal methods that attempt to factor
in economic trendsi changes in technology, or political forces can also be used. Perhaps the
best source, however is to roll-up the detailed forecasts located at the actual SKL level in
both units sold and revenues gained into their respective product families to provide a
summarized view. Once the aggregate total has been calculated, forecasters can then utilize
qualitative decisions to massage the forecast numbers, and then pass them back down to SKU
level where they will than be input into demand portion of the Master Schedule.
Second Edition, Ross D.F., Page: 199)
The model in Figure
2.6 is most appropriate for the discussions that we have been having, as it
differentiates the various levels of detail that can be forecast, and shows
that forecasts can be rolled up or forced down. The model that we have been
advocating so far would involve forecasting at the top levels of the pyramid
(total company, business unit, and product family), and forcing the agreed upon
forecast through the lower levels of detail. In its most advanced state, a user
can enter a forecast at any level of the pyramid and aggregate data up or
explode down through all of the layers of the pyramid.
( Kerber, B., B. J. Dreckshage, B.J. (2011).Pyramid Forecasting. Lean Supply Chain Management Essentials: A Framework for Materials Managers (pp.35,36). )
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