Operator`s manual

20
ProPress SP Furnace Operations Manual
Introduction to Sense Pressing
Introduction
Time, Temperature, and Pressure
Conventional pressings have classically been defined
by fixed combinations of time at a given temperature,
followed by a certain amount of pressure applied to
the ceramic to transfer it into the mold. All ceramic
manufacturers publish these parameters, usually a set for
100 gram molds, and a set for 200 and 300 gram molds.
The Hold time at the Hold Temperature insures the mold
is fully heat soaked and that the ceramic ingots have
reached their vitreous point so that when pressure is
applied the ceramic will flow into the mold pattern(s).
The Press Time is a worst case value in conventional
pressing furnaces which compensates for the number of
patterns, how they are sprued, and the wax weight of each
pattern. In many cases the ceramic has completely filled
the patterns significantly before the Press Time expires.
There was no way to determine when the patterns were
filled with conventional technology.
Conventional technology also uses a constant, unyielding,
pressure until the Press Time expires. Keeping this pressure
applied after the patterns are full only risks stressing the
investment material unnecessarily.
Types of Pressable Ceramics
Untilfairlyrecently,virtuallyallpress-ableceramicswere
feldspathic based ceramics, which tend to be more viscous
thanthenewerlithium-disilicatebasedceramics,asthey
flow into the mold. This higher viscosity requires a longer
Press Time at a given Hold Temperature and applied
pressuretollequivalentmolds.Thenewerlithium-disilicate
ceramics flow more freely at a lower Hold Temperature and
fill the patterns more rapidly. They also change state from a
solidtoanearliquidveryabruptly,overthespanofjustafew
degrees of temperature.
Conventional technology pressing ovens are not optimum
forpressinglithium-disilicateceramicsbecausethe
worst-casePressTimesettingskeeptheceramicmaterial
above the vitreous point longer than necessary, giving the
ceramic more opportunity to chemically react with the
surrounding investment material, forming a reaction layer
over the surface of the pattern which, once formed, must
be removed in a secondary operation once the mold has
been divested.
The SP Pressing Furnace senses the movement of the
ceramic in real time, and when the movement ceases
(the patterns are filled), the muffle heater is turned off, the
vacuum is released, and the table is lowered as quickly as
possible to drop the mold temperature below the vitreous
point of the ceramic before a significant reaction layer has
a chance to form. The thermal mass of the mold itself will
inherently control the rate of cooling, to prevent thermal
shock to the ceramic as it cools. It only requires ten to
twenty degrees Celsius drop to fall below the vitreous
point, (solidify) then the mold mass will allow the ceramic
to anneal as it cools in ambient air.
Mold Cracking
In addition to the ability of the SP Pressing Furnace to
sense the flow of the ceramic, it also simultaneously
employsauniquetechnologytominimizetheriskofover-
stressing the investment mold as pressure is applied.
As previously discussed, conventional pressing furnaces
apply a constant pressure throughout the Press Time. The
SPPressingFurnaceinitiallyappliesonlyabout62%ofthe
final pressure to bring the press rod up to, and in contact
with,themoldplunger.Thiseliminates40%oftheinertia,
which can “cleave” a mold as the investment material is
17. Start Vacuum
This parameter controls when the vacuum pump comes
on. The selections are made by rotating the Selector
Knob as in the example below. The range will be different
depending on the values already put into the program.
Correct Ranges
• Rate Rise (1) 400º C to 1200º C
• Hold Time (1) 00:00:00 to 18:10:00
• Rate Rise 2 (2-Stage Program) 400º C to 1200º C
• Hold Time 2 (2-Stage Program) 00:00:00 to 18:10:00
Rotate the Selector Knob to select the setting, then
push the Selector Knob OR press the Next button
to store the parameter.
18.ReleaseVacuum
This parameter controls when the vacuum is released,
and either air is allowed to enter the muffle. The
selections are made by rotating the Selector Knob
as in the example below. The range will be different
depending on the values already put into the program.
Correct Ranges
• Rate Rise (1) 400º C to 1200º C
• Hold Time (1) 00:00:00 to 18:10:00
• Rate Rise 2 (2-Stage Program) 400º C to 1200º C
• Hold Time 2 (2-Stage Program) 00:00:00 to 18:10:00
• When Table Opens
Rotate the Selector Knob to select the setting, then
push the Selector Knob OR press the Next button to
store the parameter. This is the final parameter. You
will automatically be returned to the Main Menu.
Add/Edit Program
Use Vacuum
>Yes