HEATING SYSTEM
In the traditional presses, the plates heating system is made by electrical heaters or by flow
of oil or other liquid which are separately heated.
Since the heat exchange between the hot plates and the material to be pressed is made by
simple contact,it is understandable that not many sheets of it can be applied
simultaneously between the two plates.
The resin material, in fact, is a bad heat conductor, so the material sheets can have up to 30°C
of temperature difference in the various sheets levels.
This temperature gap reduces when the material reaches the final polymerization temperature.
In the
Adara press machine the material is heated by the Joule effect directly in the copper foils
which envelop the resin sheets. Each material sheet is heated then, by the copper foil which is in
intimate contact with it and plays as a heater.
By this system the temperature gap between two multilayers is not over ±3°C.
That means a great quality value even if the pre-preg has not constant performances.
Due to the controllable heating system in real time by electrical parameters, in the
Adara press there is
no needs to use Krafts or other tools to slow down the heating effect as per the traditional presses.
In the
Adara Press it is very easy to control the flow during pressing cycle by working
on the internal stack temperature and its rising rate.
A rise of 2°C/min. reduces the flow effect, while a rise of 4°C/min. generates a good flow effect.
By controlling all these process parameters it is possible to minimize voids in the circuits.
Many
Adara users have reduced the free spaces around the PCB, at less than 15
mm
on the multilayers with more than 4 layers, also at about 10
mm on multilayers with 4 layers.
The minimum flow allows to reduce the over dimension of prepregs from 50
mm
which is in standard press, to 30
mm with
Adara press.
PRESSURE CONSIDERATION
To facilitate the transfer of heat from the platens into the centre of the stack a very high
pressure
is applied by hydraulic pistons. In the past, pressures of more than 40
kg/cm were used.
With high pressure and non-uniform temperatures in the stack the material will move as a result of the stress.
This movement can be non-uniform and the result can be misregistration of the innerlayers
in the case of 6 or more layers.
Even using pin lamination this movement can happen and
can result in rejected product.
With
ADARA, there is no need for high pressure to help with the heat transfer because
the copper is the heater and
it is in intimate contact with the materials.
This could be called endothermic heating.
The pressure is applied by air instead of hydraulic, and typically 10 to 15
kg/cm² of pressure is applied to the stack.
So we can say
ADARA SYSTEM needs enough pressure to get the best result avoiding any stress
to the material pressed.
The air bag by applying the compressed air of 10
bar, pushes down a very large steel piston
to give a sufficient pressure on the material.
The piston is free-floating against the stack plane, self-leveling since it has an axis play.
In this way it remains always parallel to the stack plane.
Thus, it improves drastically the pressure distribution on the stack surface.
SETTING OF PROCESS PARAMETERS
The main process parameters to be written in the computer are:
- The temperature profile to be used for stack heating.
- The pressure profile to apply on the stack.
- The vacuum control.
A. THE TEMPERATURE PROFILE.
Before starting the press cycle, the operator can write in the computer the value of degree
centigrade, which must be reached in the stack versus cycle the time.
These values are shown as a graphic °C/
time on the computer monitor.
During the press cycle, the computer reads the stack temperature by 4 thermocouples.
Two of these are in the stack while the others are at top and bottom planes of the press.
The read temperature value is compared continuously with the preset temperature, so the electrical current
on the copper foil is adjusted for obtaining the minimum differences between these two parameters.
If the stack temperature is going over the set value, the current on the stack is reduced automatically.
Vice versa the current is increased when the stack temperature is going down the set value.
B. PRESSURE PROFILE.
The operator can set in the computer the desired value of specific pressure (
Kg/cm²) on the stack.
Based on the stack surface area, the computer can adjust the air bag pressure for reaching
the set specific pressure value V/S processing time or V/S stack temperature.
C. VACUUM CONTROL.
The vacuum can be set in the computer as function of stack temperature.
Its value is read by a vacuum probe.
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Adara 48
Adara 52
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