1)Slush Molding (Group: Production
Method)
Previous Answer
Slush molding is an excellent method of producing
hollow objects. A wide variety of products can be manufactured by this process,
including rainboots, shoes, hollow toys and dolls, and automative products such
as the protective skin coatings on arm rests, head rests, and crash pads. The
basic process of slush molding involves filling a hollow mold with plastisol,
exposing the mold to heat, gelling an inner layer or wall of plastisol in the
mold, inverting the mold to pour out the excess liquid plastisol, and heating
the mold to fuse the plastisol. The mold then is cooled and the finished part
removed. Slush molding can be a simple hand operation for limited production or
an elaborate conveyorized system for long runs. This process can be a one-pour
method, where finished or semi-finished products can be made by one slushing,
or a multiple-pour method, where two or more slushing are used. ( Michael L.
Berins, Plastics Enginnering Handbook of The Society of The Plastics
Industry, Kluwer Academic Pub. 2002, 5th Edition, p.491)
New/Better Answer
Slush molding produces
hollow products from vinyl plastisols. It is the reverse of plastisol dip
molding and an offshoot of open molding. Slush molding has been extensively
used for making dolls, balls, flexible toys of all sorts; fishing hip-boats,
automobile parts (gearshift boots, armrests, headrests, etc.), road-safety
cones, and others.
The mold may be split or
one-piece. The finished part is removed either by splitting the mold or, in the
case of a one-piece mold, by collapsing the part with a vacuum. This process
can be very labor intense. However it is also automated requiring relatively no
labor. Automatic systems fill molds with plastisol carried by conveyor belts through
an oven as it is being slushed (the mold is put into a control motion pattern).
The plastisol can gel repeatedly to a thickness of 0.06 in. (15.2 mm). The
excess plastisol is poured out of the mold and automatically returned to the
main tank for reprocessing. The molds proceed to another oven where curing is
completed.
The process is
temperature-time dependent. It goes through the following stages:
(a) Mold with a female
cavity is preheated,
(b) Mold cavity is tilled
with a measured amount of plastisol,
(c) Preheat sufficiently to
gel the required thickness of plastisol; mold is filled and held thr several
seconds before it is inverted and drained,
(d) Plastisol in the heated
mold is dispersed (slushed) evenly over the inner cavity surface by back-and-forth
motion of the mold, side-to-side motion, and/or rotation, usually an wild one
axis,
(e) Heated mold causes gel
to occur,
(f) Drain excess plastisol
out of the mold,
(g) Applied heat fuses the
plastisol,
(h) Cool, via water, the
mold that cools the vinyl
(i) Remove the flexible
product from the mold and trim if required.
By tilling a cold mold
followed with heat produces a gel or a skin on the mold cavity. This procedure
can improve the reproducibility of the mold texture or engraving on the cavity
wall. Care is required when using a cold-mold approach because the plastisol
can gel on the air entrapped in the cavity or the outside surface producing
poor drainage and thrilling lumps in the plastisol. The lumps can cause the
product to have uneven thickness and/or performance. The lumps have to be
screened out of the remaining plastisol that is recycled on the following slush
moldings, causing further problems.
(Dominick V. Rosato,Donald V. Rosato,Matthew V. Rosato , Plastic
Product Material and Process Selection Handbook, pp.501-502)
2)Spark
Erosion (Group: Machining Process)
Previous Answer
A gap
between two electrodes energized with electirc current causes sparking which
erodes both the electrodes. Control of the gap and passing current facilitates
control of the rate of spark erosion. One of the electrodes is subsitude with
the workpiece tobe eroded. The other electorde serves as an erodnig ,
Electrical Discharge Machining , tool.
The metal is
removed by the thermal effect of the current. The workpiece and the electrode ,
the discharge interface is submerged in dielectic liquid like paraffin or
transformer oil.
(Machine
tools handbook ; P H Joshi , pg. 84 2007)
New/Better Answer
Spark erosion machining or
EDM is a process that removes metal by utilizing an accurately controlled
electrical spark or discharge. These discharges are repeated many thousand
timesisec.in the selected area of workpiece.
The essential of the process
(involving basic relaxation circuit) are shown in Fig. 10.2. The cutting action
is actually the erosion of the metal. The control of this erosion is achieved
by rapid recurring spark discharges impinging against the surface of the
workpiece which must be an electrically conducting body. Usually a liquid
dielectric (kerosene, paraffin or light oil) is used to flush away the eroded
material. Any material irrespective of its hardness can be machined, provided
it conducts electricity. Semiconducting materials like germinium can also be
machined successfully by this method.
Basic Principle;
In spark erosion machining
process, the tool electrode is connected to the negative terminal of the special
electrical dc source and workpiece is connected to the positive terminal. When
two electrodes are separated by a dielectric and a suitable voltage is applied,
the dielectric breaks down. The strong electrostatic field produces emission of
electrons from the tool face and this causes ionization of the gap. An
avalanche of electrons and ions follows the resistance of the gap drops and the
electrical energy is discharged in the gap between tool electrode and workpiece.
This causes electrical breakdown of dielectric. This phenomenon occurs in few
microseconds, shock waves in the dielectric are created and the impact of
electrons on the work material causes transient pressure of about 1000 kg/cm2.
Due to high temperature of work material reached (1100C). the metal melts
instantaneously and part of it gets vaporized. This is then ejected into the
gap between the tool and workpiece.
(Anaheim,M Adithan, Manufacturing Technology, p.206)
Where customers require goods to be produced to their own particular specification. In such an enviroment, each order is a one-off, manufactured to customers order. Typically, only low stocks are held and production is organised in a manner calculated to achieve flexibility. That is, machines and personnel are arranged in a manner that allows production to be shifted quickly from one job to another.
CIMA Offical Learning System - Performance Operations, Scarlett R., p183,184)
3)Jobbing
Production (Group: Production)
Previous Answer
Where customers require goods to be produced to their own particular specification. In such an enviroment, each order is a one-off, manufactured to customers order. Typically, only low stocks are held and production is organised in a manner calculated to achieve flexibility. That is, machines and personnel are arranged in a manner that allows production to be shifted quickly from one job to another.
CIMA Offical Learning System - Performance Operations, Scarlett R., p183,184)
New/Better Answer
Jobbing
production is normally used when small numbers of varied types of elements are
to be produced. It requires a large number of diverse forms and molds. a high
standard of labor, and high attention to planning and supervision at all stages
of the process.
(C. Popescu,N. Ovararin,K.
Phaobunjong,
Estimating Building Costs, p.542)
4)Flow-line layout (Group: Production)
Previous Answer
This
layout implies that various operations on raw material are performed in a
sequence and the machines are placed along the product flow line, machines are
arranged in the sequence in which the raw material will operated upon. In this
type of layout all the machines are placed in a line according to the sequence
of operations, each following machine or section is arranged to perform the
next operation to that performed by its preceding machine or section.
(Introduction
to basic manufacturing processes and workshop technology, Rajender
Singh, p.22)
New/Better Answer
The
flow line layout uses the train or line system, which locates all the equipment
in the order in which it occurs on the flow sheet. This minimizes the length of
transfer lines and therefore reduces the energy needed to transport materials.
This system is used extensively in the pharmaceutical industry, where each
batch of a drug that is produced must be kept separate from all other hatches.
In other industries it is used mainly for small-volume products
Often,
instead of using the grouped or flow line layout exclusively, a combination
that best suits the specific situation is used.
(William D. Baasel, Preliminary
Chemical Engineering Plant Design, p.149)
5)High-Frequency Resistance Heating (Group: Surface
Hardening Process)
New Answer (No previous answer)
In this process (Fig. 8) for
noncircular components, a water-cooled conductor is placed in close proximity
to the surface to be heated and connected to the workpiece through a pair of
contacts at the outer edges and to a power source. When the high-frequency
current applied, heating occurs across the work surface to a depth dependent on
the frequency used, the duration of heating, and the power level, normally in
the range or 0.37 to 0.75 mm (0,014 to 0.030 in.). Once the heated strip
reaches the hardening temperature, the power is switched off, and
self-quenching occurs by beat dissipation to the bulk of the component. A significant
advantage of high-frequency resistance heating is that it does not require a closed
loop of current but can beat a path between two points, with the hardness between
two points being dictated by the shape of the conductor.
(Joseph R. Davis, Handbook of
thermal spray technology, pp.18-19)
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