Regarding the hydraulic control modes of the HPHT Hydraulic Cubic Press, the sources provide a detailed analysis and discussion. With the widespread application of large-tonnage cubic hydraulic presses and large-cavity synthesis processes like 38mm and 40mm, the state of the pressure and temperature fields within the synthesis cavity has changed. Therefore, optimizing control modes, especially pressure control modes, has become a crucial task for diamond manufacturers and equipment suppliers.
Below are the main hydraulic (pressure) control modes for the HPHT Hydraulic Cubic Press:
Ideal Pressure Control Mode
To reduce the increased pressure gradient caused by pressure loss and phase change in large-cavity processes, and to meet the stable pressure conditions required for the growth of high-quality single-crystal diamonds, an ideal pressure control mode should have the following characteristics:
Controllable pressure increase curve: This allows for coordination with the heating curve to improve pressure transmission effects and reduce the generation of pressure gradients.
Gradually increasing pressure curve during the holding stage: This helps reduce the increased pressure gradient caused by the deterioration of pressure transmission performance due to pyrophyllite phase change.
Controllable pressure release speed: To accommodate different requirements for pressure release speed at high and low pressures.
Currently Applied Pressure Control Modes
The sources introduce several pressure control modes currently in use:
1. Traditional Pressure Control Mode
Characteristics: In this mode, pressure fluctuations are significant, representing a crude control method.
Applicability: It is not suitable for large-cavity synthesis processes.
2. Variable Frequency Pressure Holding Control Mode
Characteristics: This mode maintains constant pressure during the holding stage.
Drawback: It neglects to compensate for the pressure gradient caused by synthesis phase changes.
3. Passive Incremental Pressure Supplement Mode
Principle: This mode supplements pressure by a set increment after the holding pressure drops to a certain set value.
Nature: This is a passive pressure supplementation mode.
Drawback: In reality, the number of pressure supplements during the holding stage of the press is limited and related to the failure of the high-pressure seal of the press. Therefore, this mode does not truly achieve the goal of compensating for the pressure gradient through incremental pressure supplementation.
4. Active Incremental Pressure Holding Mode
Principle: This mode achieves incremental pressure supplementation by setting pressure increments and time intervals (i.e., number of pressure supplements).
Nature: This is an active incremental pressure supplementation mode.
Optimization: Using a variable frequency pressure holding method can minimize the pressure drop within each set time interval, although typically, press pressure-holding performance is good, and this pressure drop can be ignored.
Importance: Possessing active incremental pressure holding functionality is one of the important features for a cubic hydraulic press to effectively reduce pressure gradients and provide pressure conditions suitable for the growth of high-quality single-crystal diamonds in large cavities.
5. Pressure Control Mode Using Proportional Valves
Principle: Proportional valves control the thrust and displacement of electromagnets continuously and proportionally by controlling current or voltage according to a set curve, thereby achieving control over system pressure and flow.
Advantages:
Pressure holding stage: It can achieve continuous incremental system pressure during the pressure holding stage.
Pressure increase stage: It can achieve continuously controllable pressure increase speed during the pressure increase stage.
Pressure release stage: Through program-controlled pressure release actions, the pressure release speed is controllable.
Full-process control: Therefore, this mode achieves full-process curve control of pressure.
Assessment: The pressure control mode using proportional valves is considered the closest to the ideal control mode currently possible and is a relatively ideal hydraulic control system.
In summary, to provide pressure conditions suitable for the growth of high-quality single-crystal diamonds in large cavities, the cubic hydraulic press should at least possess active incremental pressure holding functionality to effectively reduce pressure gradients. Among these, the pressure control mode using proportional valves is a relatively ideal hydraulic control system. Additionally, the large-cavity synthesis process requires a synthesis tonnage of at least 1800T to meet its pressure conditions.