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one hand the extruder manufacturer has already set the rate of production
for equipment and wants the die manufactured with as many holes as
possible. The pasta manufacturer, on the other hand, expects a
perfect product from the die. What are the problems? If the
die is designed with too many outlets, the following scenarios are
possible: 1) the die may be too weak, reach its yield point, and bend
under pressure; 2) the dough may not have the opportunity to properly
amalgamate before extrusion, which may result in a weak, low-density
product; 3) the operator may think that the extrusion rate is too great
and make the mixture a little harder (greater viscosity) with subsequent
damage to the die; and 4) the strands may overlap too much and
consequently be difficult to dry properly.
If the die is designed with too few outlets, too much
back pressure can develop with possible damage to either the die, the
extruder, or both. In this respect, collaboration with the pasta
manufacturer is essential so that the requirements may be satisfied.
The die with a pin presents additional problems, for
wall thickness must be considered. Shrinkage occurs on both the
outside and the inside diameters. The shrinkage factor is greater
for the outside diameter than it is for the inside diameter, and
extreme care must be exercises in drawing up the specifications.
This particular characteristic must be given close attention or wall
thickness will be either too thick or too thin, which will present
subsequent difficulties in drying , packaging, and cooking.
The trend today is toward more exotic shapes,
including those that appeal to children. Difficult configurations
are now designed using computer technology. Complicated
configurations must be given special attention--the extruding pressure
on each outlet must be adjusted to extrude a product that will have a
uniform thickness after cutting at the die.
On the subject of cereals and snacks, shrinkage or
expansion of the finished product cannot be determined empirically
because of the many different processes and raw materials used.
Factors Affecting Die Performance
One of our more serious concerns today is die
wear. Under normal circumstances, die wear becomes apparent
through the warning signals associated with packaging. Too heavy a
product yields less volume per unit weight, which results in too much
slack in packages. This applies predominantly to solid and tubular
products, in which gradual war can seldom be detected by visual
inspection of the product but instead must be determined by actual
measurement. The more elaborately shaped products generally give
some indication of wear by a change in physical appearance. For
example, sea shells tend to show greater curvature, mafalda shows a more
pronounced wave, and rotini and twists show a tighter curl.
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In
a sea shell production, the flow of dough is at its maximum at the
center of the shell, making this point more susceptible to wear than are
the ends. As wear increases, the dough flows faster at the center,
thereby increasing curvature. Today, the most common warning of
wear in shell dies comes in the form of checking either during or after
drying.
Checking can be described as minute cracks in the
finished product as a result of improper drying or dehydration of the
product. This may be a consequence of a heavy wall or improper
diameter product (possible result of die wear) with no correction in the
drying process. This checking can be attributed directly to die
wear and can be eliminated by reducing the thickness of the die outlet.
In the wavy products, such as mafalda and rippled
lasagna, die wear becomes evident visually by a more pronounced or
closer-curled wave. A cross section of these products should
present a flat, noodle-type appearance. The wave is the result of
a greater flow of dough on the ends of the slots in the die, making the
ends the points of greatest wear. An increase in wear is
accompanied by an increased flow of dough, resulting in a more
pronounced wave. A cross section of the product after wear made
with a die exhibiting signs of wear appears as a flat noodle in the
center with a spaghetti-like effect at the ends. This condition
presents both drying and packaging problems and can be eliminated by
proper die maintenance.
The rotini and twist products present cross sections
analogous to those of lasagna--a cross section of the product before
wear is a noodle type product whereas after wear, the ends (at the
circumference) develop a heavier, spaghetti-like appearance. The
increase in the flow of dough at these points results in a tighter curl
or a greater degree of twisting.
Elbow macaroni is tricky because wear occurs at
several points, and certain dimensional proportions must be maintained
to obtain a standard product curvature. Wear takes place at the
outlet, at the pin tip, at the base of the notch (where applicable),
and, in the case of brass pins, at the pin stem between the notch and
the tip of the pin. Many have been plagued with product splits on
short-cut products and splits or weird distortions on long
products. The cause, though not immediately detectable by visual
inspection, can generally be traced to grit in the raw material.
In the case of splits, the grit lodges between the pin and the outlet
(the grit is too large to be pushed out) and results in a definite split
in the extruded product. (A split is a term used in the pasta
industry to describe a break in a tubular product in which the cross
section looks like a split ring.) In the case of the weird
distortions of long tubular products, the grit is forced through the die
but in the process forces the pin to one side. Thus, off-center
pins--directly attributable to grit--are the base cause.
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A
rather mystifying condition is presented by uneven wall thickness in
short-cut products in which grit definitely does note enter picture.
When proper and standard operational procedures are
not carefully adhered to, the die yields during production. This
bending follows an elliptical pattern tending to distort the outlets and
disturb the concentricity of pin and outlet. The result is uneven
wall extrusions.
In these days of high-volume extrusion operations, a
major complaint is wear vs. number of hours production. Number of
production hours can no longer be used as a yardstick for wear.
Statistics must be based on tonnage pushed through the die, which will
give a more realistic basis for rate of wear.
How often should dies be returned for repairs and
reconditioning? Every three months? Every six months?
The answer depends on a number of production and handling factors.
A die in a continuous production system must be repaired more often than
must a die in limited production. The responsibility of setting
tolerances for the product sizes rests with the manufacturer. Once
these tolerances have been set for the various products, it is easy to
determine wear on the die. This task may be facilitated by the use
of gauges within the specified tolerances on the die or by enlisting the
aid of quality control on the product. In view of the many
variables that influence final product size, it may be beneficial to
work out some program involving die wear, although it most certainly
will be more practical to work on product tolerances, which may result
in a colossal headache.
All difficulties are not the result of die wear.
Improper maintenance will result in serious problems. For example,
a die not properly cleaned will have a thin crust of dough left on the
outlet. This will affect both product size and appearance.
Pressures may have a decided effect on the extruded product. One
problem that has gained prominence is the noodle with a slight
twist. A die extruding 1,000 lb per hour manufactured to identical
specifications as a die extruding 3,000 lb per hour will not give the
same twist as the latter die as a result of different extruding
pressures. The twist will be in direct proportion to
pressure. Similarly, pressure may affect the wall thickness of the
product. The same things apply to all products that curve or
twist, such as elbows, rotini, and sea shells.
Among the factors affecting the quality and appearance
of the extruded product are raw material, moisture content, pressure,
die outlet finish, number of outlets in die, material of die, and drying
procedures.
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