DRYING
Green ceramic parts are dried after forming, drying increases the green
strength and readies the part for sintering. When the interstitial phase is a
polymer or wax, such as in injection molding, the removal process is
called burnout.
Drying Conditions
Conditions of the body that are important are thermal conductivity,
permeability, and drying shrinkage. Conditions in the dryer that are
important are temperature and relative humidity.
Depending on particle packing, most parts will shrink when dried the
outside shrinks first since it is restrained from shrinking by the interior, it
may crack. This is particularly troublesome when the part is thick and the
interior may not even be warm. The problem then is to program drying so
that differential shrinkage between the surface and interior does not
exceed the tensile strength of the material.
In extreme cases, internal pressure can burst the part when the water
vapor is being generated faster than it can diffuse out of the body.
Permeability
Water evaporates during drying and has to escape from the interior
of the article. Remember that one mole of water (18 g) will create 22.4
liters of steam. Since it is hot, there is even a greater volume of gas that
has to escape. When permeability is low, internal pressures can buildup
and burst the part.
Fine-grained ceramics have low permeability and have to be dried .
Coarse-grained bodies have higher permeability and are not always
thoroughly dried as they can dry in the initial ramp of the firing curve.
Setting
The setting of the wet part is often critical. When set on an impervious
surface, the top and sides can dry but the bottom stays wet.
Always set the part on an absorbent structure so that air can reach all
surfaces of the piece. This is also true for oven drying.
Depending on the shape, one can turn the part from time to help attain
even drying on all surfaces.
Drying--- -------------Lecture(4)
14
Types of Dryers
Ceramic parts can be dried in many ways, depending on the properties of
the ceramic and the equipment available.
Air Drying
For this, one just sets the part to be dried on the bench. This is often a
suitable method. There are two problems with this method, however.
First, the part has to be set on an open mesh or absorbent material so that
it is not dries evenly. The second problem is that, if there is dust in the
air, the part should be loosely covered.
Oven Drying
This is the most common laboratory drying method, using an electric
oven. The oven has a temperature control, temperature indicator,
expanded metal shelves, and a vent. The expanded metal shelves allow
drying to take place on the bottom of the part they will dry evenly.
It is good practice to place thermometers at different places in the oven to
see how much variation occurs; there will be some.
Usually, the heaters are on the bottom so that the heated air can rise
through the setting. The temperature control is often at the top of the oven
some distance from the heating element. Some ovens have a blower or
fan to circulate the hot air. This is a good practice.
Humidity Driers
Humidity driers control both the relative humidity and the temperature.
By controlling both, the rate of drying of the parts is controlled. Humidity
is gradually reduced as the parts dry. Temperature can also be controlled
along with humidity. This makes humidity drying the preferred method if
parts warp or crack when dried by other methods.
The equipment is more expensive and not usually seen in the lab. This is
partially also true because lab parts are generally small. Large parts that
have low permeability are candidates for humidity drying.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
الرجوع الى لوحة التحكم
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