Hydraulic cylinder joint and piston stroke design
Hydraulic cylinder joints and piston stroke are key components of hydraulic core pulling systems, directly impacting the reliability of oil circuits and the precision of core pulling movements. Joint design must balance sealing performance and connection strength, while stroke design must balance core pulling requirements with movement safety. Optimizing both is fundamental to ensuring efficient hydraulic system operation. Improper design can easily lead to problems such as oil leakage, insufficient stroke, and piston sticking, which can, in severe cases, require mold downtime for repairs.
The selection of hydraulic cylinder connectors must match the oil circuit pressure and medium characteristics, while also considering installation space constraints. For high-pressure oil circuits (pressure ≥10 MPa), compression fittings or flared fittings are preferred. Their metal sealing structure can withstand repeated pressure shocks. The connecting pipe is made of cold-drawn seamless steel pipe with an outer diameter accuracy of ±0.1mm to ensure a tight seal with the compression fitting. For low-pressure oil circuits (pressure <10 MPa), plug-in connectors can be used. They are sealed with O-rings and are easy to install and cost-effective. The connector's thread specifications must match the cylinder interface, such as M16×1.5 or G1/2, and thread sealant (such as Loctite 574) must be applied during assembly to prevent loosening due to vibration. For molds with limited space, 90° elbow connectors can be used, but the bend radius must be ≥3 times the pipe diameter to prevent excessive oil circuit resistance.
The layout of the cylinder joints must avoid cross-interference of the oil circuits and facilitate installation and maintenance. Space for wrench operation must be reserved at the connection point between the joint and the cylinder, and the distance from the mold cavity wall must be no less than 50mm. The oil circuit should be arranged along the outside of the mold, away from high-temperature areas (such as near the gate) to prevent the increase in oil temperature from causing aging of the seals. The joints of multiple groups of cylinders must be numbered in the order of core pulling, and the corresponding oil circuits must be marked with different colors to avoid reverse core pulling due to wrong connection. In addition, pipe clamps must be used to fix the joints and the oil pipes, with a spacing of no more than 500mm to prevent vibration fatigue fracture of the oil pipes. Hexagon socket screws are used to fix the pipe clamps to the mold to avoid protruding surfaces interfering with other components.
The determination of the piston stroke requires a comprehensive consideration of the core pulling requirements and buffer design to ensure thorough core pulling and smooth movement. The basic stroke must cover the undercut depth of the plastic part and a safety margin. For example, if the undercut depth is 30mm, the core pulling distance must reach 35mm, and the piston stroke is designed to be 40mm, leaving 5mm as a buffer section before the piston reaches the end point. The buffer structure can use a tapered throttle groove at the end of the piston. When the piston is 5-10mm away from the end point, the hydraulic oil slows down through the throttle groove, reducing the movement speed from 0.1m/s to below 0.03m/s, avoiding rigid impact. For long-stroke core pulling (>100mm), a support ring must be installed in the middle of the piston rod, with a clearance of 0.03-0.06mm with the inner wall of the cylinder to prevent the piston rod from sagging due to its own weight and causing uneven wear.
The control accuracy of the piston stroke must be ensured by a limit device, the most common of which are mechanical and electronic limit devices. Mechanical limit devices place an adjustable stop at the end of the cylinder barrel, and use shims to adjust the stroke error to ±0.1mm. This is suitable for scenarios with high precision requirements. Electronic limit devices detect the piston position through a magnetic ring sensor, and the signal is fed back to the control system for automatic stopping. It has a fast response speed but requires regular calibration. Stroke control also needs to consider the impact of oil temperature changes. For every 10°C increase in hydraulic oil temperature, the volume expands by approximately 0.7%. Therefore, a compensation oil circuit is required in long-stroke cylinders to use an accumulator to balance the stroke fluctuations caused by temperature differences.
The compatibility of the cylinder joint and piston stroke must be designed throughout the mold development process. The joint must be installed below the piston’s trajectory to avoid interference. The piston rod end connects to the core using a floating joint, allowing for ±1° angular deviation to compensate for installation errors. A check valve must be installed at the end of the stroke to prevent the core from being pulled back due to the clamping force of the plastic part in the event of a power outage or loss of pressure, ensuring safe production. During the mold trial phase, the joint’s sealing performance (pressure drop of no more than 0.5 MPa during 30 minutes of pressure maintenance) and stroke accuracy (deviation of ≤0.2 mm for 100 consecutive movements) must be tested through multiple core pull-and-reset cycles. Only after meeting these standards can mass production begin.
