Single-cylinder direct pressure mold clamping device
The single-cylinder direct-pressure clamping mechanism is a core component for closing and locking the mold in injection molding machines. Using a single hydraulic cylinder to directly provide clamping force, it features a simple structure, rapid response, and high clamping accuracy. It is widely used in small and medium-sized injection molding machines and precision molding applications. Compared to toggle-type clamping mechanisms, the single-cylinder direct-pressure mechanism eliminates the complex linkage mechanism, reduces clearance errors between moving parts, and enables more precise control of mold opening and closing speed and position. It is particularly suitable for molding processes that require high clamping force stability.
The structure of a single-cylinder direct-pressure clamping unit is relatively simple, primarily consisting of a clamping cylinder, a movable platen, a tie rod, a locking piston, and a control system. The clamping cylinder is fixed to one side of the fixed platen, with its piston rod rigidly connected to the movable platen. Hydraulic oil pressure drives the movable platen back and forth along the tie rod. The tie rods, each fixed to the fixed and rear platens, guide and support the clamping force. Four tie rods are typically used, evenly distributed, to ensure balanced force. The locking piston, located within the clamping cylinder, controls the clamping and opening of the mold by changing the direction of hydraulic oil flow. Its diameter directly determines the maximum clamping force (clamping force = hydraulic pressure × piston area). For example, the clamping cylinder piston diameter of one injection molding machine is 200 mm, and operating pressure is 16 MPa. Its maximum clamping force can reach 500 kN, suitable for molding parts weighing up to 500 g.
The device’s operating process is divided into five stages: rapid mold closing, slow mold closing, high-pressure clamping, holding pressure, and mold opening. During the rapid mold closing stage, the hydraulic system supplies oil to the clamping cylinder via a high-flow pump, allowing the mobile platen to rapidly approach the fixed platen at a speed of 0.5-1m/s, shortening idle travel time. When the molds are about to make contact, the system switches to slow mold closing (reducing the speed to 0.05-0.1m/s) to prevent damage from impact. During the high-pressure clamping stage, the system pressure rises to a set value (typically 8-16MPa), and the clamping force reaches and remains at a preset value, ensuring that the mold is not stretched open by melt pressure during the injection process. After holding pressure is completed, hydraulic oil is reversed, driving the mobile platen backward and completing the mold opening. The entire process is closed-loop controlled by a proportional valve and displacement sensor, achieving a mold closing position accuracy of ±0.1mm, meeting the molding requirements of precision plastic parts.
The core advantage of a single-cylinder direct-pressure clamping device lies in its linear adjustment and stability of clamping force. Due to its direct pressurization, the clamping force is proportional to the hydraulic pressure. Adjusting the system pressure allows for stepless adjustment of the clamping force, typically ranging from 30% to 100% of the rated value, making it suitable for plastic materials with varying viscosities and molding pressures. For example, when molding thin-walled PP parts, the clamping force can be adjusted to 50% of the rated value to avoid mold deformation due to over-tightening. When molding high-viscosity PC parts, the clamping force should be adjusted to above 80% to prevent flash. Furthermore, this device offers excellent clamping rigidity, allowing for clamping force fluctuations to be controlled within ±2% during the holding phase, significantly lower than the ±5% achieved with a toggle-type device, helping to minimize dimensional fluctuations in the plastic parts.
In practice, single-cylinder direct-pressure clamping devices must be specifically designed according to the specifications of the injection molding machine. For small injection molding machines (50-200kN clamping force), a single-piston-rod clamping cylinder can be used to simplify the hydraulic circuit design. For medium-sized injection molding machines (200-1000kN), a dual-piston-rod or differential-cylinder structure is often used to improve the smoothness of the moving platen’s movement. The diameter and material of the tie rod must meet strength requirements. 40Cr steel is typically quenched and tempered, with a diameter ranging from 50-150mm depending on the clamping force. To extend the device’s service life, self-lubricating bearings are installed at the mating points between the moving platen and the tie rod, and the inner wall of the clamping cylinder is chrome-plated to reduce friction. Furthermore, buffer devices (such as a throttle valve and accumulator combination) are installed at both ends of the clamping cylinder to reduce shock and vibration during rapid movement, protecting the mold and equipment components.
The development trend of single-cylinder direct-pressure clamping units is toward intelligentization and energy efficiency. New units integrate pressure and temperature sensors to monitor clamping force and cylinder temperature in real time. Through a PLC control system, hydraulic parameters are automatically adjusted to prevent overload or overheating. A servo-hydraulic system replaces the traditional metering pump, providing high flow during rapid clamping and reducing flow during high-pressure clamping, resulting in energy savings of over 30%. For precision molding, some high-end units are also equipped with mold deformation monitoring systems. Using strain gauges mounted on the mold plate, they provide real-time feedback on mold deformation and automatically compensate for clamping force to ensure dimensional accuracy of the molded part. As the demand for efficiency and precision in injection molding increases, single-cylinder direct-pressure clamping units will replace traditional structures in more applications and become the mainstream choice for small and medium-sized injection molding machines.
