1-3
the furnace. Internal passages in the lid feed coolant between the inner and
outer sleeve of the view port housing where it is forced to flow around the
housing directly below the viewing glass. This technique keeps the housing
cool and prevents the sight glass from becoming excessively hot; because
borosilicate glass is a poor thermal conductor, the center of the glass will
remain hot even though the edges are cooled.
• Typically the tooling supplied by SST International for the Model 1200
contains a target plate or “boat” and a heat plate. Component locating features
machined directly into the boat together with pins or, in some applications,
inserts allows parts to be positioned and held in precise alignment during the
processing cycle. The plate is suspended above the heating element at each
corner on small sheet metal tabs that are welded directly to the radiation
shield. This unique arrangement allows radiant energy developed by the
heating element to act directly on the boat and at the same time keeps losses to
a minimum. A single thermocouple inserted directly into a hole on the edge
of the target plate monitors the process temperature and provides feedback to
the temperature control system.
1.2.2 Control System
The Model 1200 control system is intended to be used for basic process
development work and has been intentionally designed for simplicity with off-the-
shelf components and custom software. The system features a self-contained
intelligent ramping controller, two customized programmable logic controllers,
(PLCs), and discrete logic relays; this architecture supports and enhances fail-safe
/ fault tolerant operations.
System operation is accomplished with simple time-based incremental programs
that are keyed directly into the ramping controller. An array of panel lights
display the status of each function in real-time as the control system executes the
program. Three manual pushbuttons supply the necessary inputs for starting,
stopping, and operating the lid.
• Process profiles and temperatures are controlled directly with a Watlow Series
F4 ramping controller. This controller features a four-line high definition
LCD interface display with an internal 16 bit microprocessor. The system is
capable of storing 40 nameable profiles with a total of 256 steps.
• Two manually adjusted gas inputs are used together with programmable
selection valves for back-fill, pressurization, and cooling. Both inputs route
through pressure regulators and then through flowmeters for distribution to
either the cooling/pressurization jets located in the lid, or to the centrally
located purging/exhaust port located below the radiation shield.
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