The synthesis process of cubic presses with large chambers (such as 38mm, 40mm, etc.)  places higher optimization requirements on the hydraulic control mode to address the changes in the pressure and temperature fields caused by the enlarged chamber, ensuring the growth of high-quality diamond single crystals.

This analysis details how various hydraulic control modes optimize pressure and temperature control to meet the requirements of large chamber processes.

I. Optimization of Pressure Control Modes
The primary challenge in large chamber processes is that the expansion of the chamber inevitably increases the pressure difference (i.e., pressure gradient) between the outer shell and the core of the synthetic rod, which is caused by pressure transmission loss. Simultaneously, during the diamond growth process, the transformation of the pressure medium pyrophyllite (to kyanite and coesite) and the transformation of graphite into diamond, coupled with the resulting volume contraction, causes the internal pressure of the synthetic chamber to drop. Furthermore, after phase transformation, the pyrophyllite becomes rigid due to increased friction coefficient and strength, affecting the efficiency of pressure transmission and pressure compensation. All these factors contribute to a greater pressure gradient within the cavity. Since the growth of high-quality diamond single crystals requires relatively stable pressure conditions, the control mode must be optimized to reduce this pressure loss and gradient.

The ideal pressure control mode should feature the following functions to optimize the pressure field:

1. Controllable pressure increase curve: This allows coordination with the heating curve to improve pressure transmission and reduce the generation of the pressure gradient.
2. Progressive pressure increase curve during the holding phase: This compensates for the pressure gradient increase caused by the deterioration of pressure transmission performance due to the phase transformation of pyrophyllite.
3. Controllable decompression speed: This accounts for the different requirements for decompression speed at both high and low pressures.

Various Control Modes and Their Optimization Effects:
Traditional Pressure Control Mode: Characterized by large pressure fluctuations, this is a crude control mode and is unsuitable for large chamber synthesis processes.
Variable Frequency Constant Pressure Holding Mode: This mode maintains constant pressure during the holding phase but neglects compensating for the pressure gradient resulting from the synthesis phase change.
Passive Progressive Pressure Compensation Mode: This is the original progressive compensation mode, achieved by compensating pressure by a set increment when the holding pressure drops to a set value. However, this mode is limited by the finite number of compensation cycles and the failure of high-pressure seals, meaning it does not truly achieve the goal of compensating for the pressure gradient.
Active Progressive Pressure Holding Mode: This mode achieves progressive pressure compensation by setting the pressure increment and time interval (number of compensation cycles). This mode provides the function to effectively reduce the pressure gradient.

The Optimal Pressure Control Mode – Proportional Valve Control:
The mode that most closely approximates the ideal control mode is the pressure control mode utilizing a proportional valve. A proportional valve controls the system's pressure and flow continuously by controlling the current or voltage according to a set curve.

The optimization benefits of this mode include:
1. Full Curve Control: It achieves full-process curve control of the pressure.
2. Continuous Progressive Pressure Holding: It allows the system pressure to continuously increase during the holding phase, effectively compensating for the pressure gradient caused by phase changes.
3. Controllable Speed: It enables the pressure increase speed to be continuously controllable during the boosting phase, and the decompression speed is made controllable through programmed decompression actions.
Therefore, the ideal hydraulic control system for a cubic press should possess the active progressive pressure holding function, and the proportional valve control mode is considered the more ideal system.

II. Optimization of Temperature Control Modes
The temperature field within the synthetic chamber is established through direct electric heating. The expansion of the chamber objectively provides conditions for forming a more balanced and stable temperature field. Consequently, the spatial proportion meeting the temperature requirements for high-quality diamond growth will be greater in large chambers.
The temperature gradient is one cause of the difference in diamond growth between the core and the exterior of the synthetic rod. The goal of temperature control optimization is to achieve a balance point between heating and heat dissipation, entering an insulation state to reduce temperature variation.
Current control modes based on heating power include Constant Power Mode and Variable Power Mode.

Optimization Strategy Requirement (Wide-Range Variable Heat Output):
Within the limited synthesis time, achieving or approaching the balance point between heating and heat dissipation (insulation state) requires timely and appropriate adjustment of the heating power. To provide this condition, the electric control system must have a wide-range variable heat output function.

III. Key Functional Requirements for the Overall Solution
Based on the analysis of pressure and temperature control, to provide the necessary conditions for high-quality diamond single crystal growth in large chambers, the overall solution for the cubic press should possess at least the following key functions:
1. The hydraulic control system must possess an active progressive pressure holding function to effectively reduce the pressure gradient. The proportional valve control mode is considered the more ideal hydraulic control system.
2. The electric heating system must have a wide-range variable heat output function to provide the conditions for achieving the insulation state (heating and heat dissipation balance).
3. The equipment must have a synthesis tonnage of at least 1800T to meet the pressure requirements of the 40-chamber synthesis process.