Injection reset and pre-reset mechanism
The injection molding reset and pre-reset mechanisms are key devices that ensure that the mold ejection system and core pulling mechanism can accurately return to their initial positions after mold opening. Their performance directly affects mold closing accuracy and production safety. The reset mechanism is responsible for returning the ejection components (such as the ejector pin and ejector plate) to their original positions after mold opening is completed, while the pre-reset mechanism, under special circumstances (such as when there is motion interference between the ejection mechanism and the core pulling mechanism), resets the ejection components in advance before the core pulling action to avoid damage to the components due to collision. For example, a mold with an inclined ejector core pulling did not have a pre-reset mechanism, resulting in a collision between the ejector pin and the inclined ejector during mold closing, causing the ejector pin to bend and the inclined ejector to wear. After the pre-reset mechanism was installed, the fault was completely eliminated. The design of the reset and pre-reset mechanisms must be determined according to the mold structure, ejection method, and core pulling type to ensure accurate movement and rapid response, thereby providing guarantees for the continuous and stable operation of the mold.
The reset mechanism’s basic components include a reset rod, spring, reset ejector plate, and guide. Its core function is to push the ejector system back to its initial position during mold closing. The reset rod is the most commonly used reset component, typically installed at the four corners of the ejector plate and mating with the reset hole in the fixed mold. During mold closing, the fixed mold plate pushes the reset rod, causing the ejector plate and ejector pin to retract. The diameter of the reset rod is determined by the weight of the ejector system and is generally 10-20mm. The length must ensure that there is still a 5-10mm margin after the ejector system is fully reset. Spring reset is suitable for small molds or applications with low ejection force. The spring is installed between the ejector fixing plate and the movable mold plate. The ejector system compresses the spring during mold opening and resets the ejector system during mold closing. The spring force is selected based on the weight of the ejector system, and the compression should be controlled at 30%-50% of the free length to avoid fatigue failure due to long-term use. The reset top plate cooperates with the guide column to ensure a smooth reset process without deviation. The clearance between the guide column and the guide sleeve is controlled at 0.02-0.05mm, and the surface roughness is below Ra0.8μm to reduce friction resistance.
A pre-reset mechanism is suitable for molds where the ejector system and core pulling mechanism intersect. Its function is to pre-reset the ejector components before the core pulling begins, preventing interference. Common pre-reset mechanisms include mechanically forced, hydraulically driven, and pneumatically controlled. Mechanically forced mechanisms use mechanical structures such as wedges and tie rods to achieve pre-reset. For example, a wedge is installed on the fixed mold. When the mold is closed, the wedge pushes the pre-reset rod, causing the ejector system to pre-reset. This simple and reliable structure is suitable for small and medium-sized molds. Hydraulically driven mechanisms use hydraulic cylinders to reset the ejector system. Solenoid valves control the oil flow, allowing precise control of reset time and speed. This makes them suitable for large molds or complex ejector systems. For example, a car bumper mold uses a hydraulic pre-reset mechanism. The reset time can be adjusted within 0.5-2 seconds to meet varying production schedules. Pneumatically controlled mechanisms are driven by air cylinders and offer fast response times (less than 0.1 seconds), but with lower thrust, making them suitable for light-load applications.
The design of the pre-reset mechanism requires precise control of the reset timing to ensure that the ejector system is fully reset before the core pull is activated. This typically requires the following: “reset stroke > core pull stroke” and “reset time < core pull activation time." For example, if a mold's core pull stroke is 50mm, the pre- reset mechanism's reset stroke must be ≥ 60mm to ensure the ejector system fully exits the core pull area. If the core pull activation time is 0.8 seconds, the pre-reset time must be controlled within 0.6 seconds to avoid timing overlap and interference. Mechanically forced pre-reset mechanisms require a reasonable wedge angle (typically 15°-30°). A too small angle increases the driving force, while a too large angle can cause reset shock. Finite element analysis software can be used to optimize the force distribution of the wedge to avoid excessive local stress. Hydraulic and pneumatic pre-reset mechanisms require pressure sensors and travel switches. Once the ejector system is fully reset, a core pull activation signal is issued, creating a closed-loop control loop and improving safety.
The installation, commissioning, and maintenance of the reset and pre-reset mechanisms are crucial to ensuring their performance. During installation, ensure the coaxiality between the reset rod and the reset hole is ≤0.05mm, and that the spring compression is uniform (<0.5mm) to prevent the ejector system from tilting during reset. During the commissioning phase, observe the reset process in slow motion to check for any sticking or unusual noise. Measure the flushness of the ejector pin end face with the mold cavity surface after reset (≤0.02mm) to ensure there is no gap when the mold is closed. For the pre-reset mechanism, verify the reset timing through multiple mold trials. Use a timer to record the reset time and the core pull start time to ensure there is no overlap. For example, by adjusting the flow rate of the hydraulic valve, one mold reduced the pre-reset time from 0.7 seconds to 0.5 seconds, completely eliminating any timing conflicts with the core pull. During routine maintenance, regularly clean the reset rod and guide column surfaces of oil and debris, refill lubricant monthly, check for spring elasticity loss, and ensure stable hydraulic system pressure. Replace components promptly if any problems are found. Through standardized installation, commissioning and maintenance, the service life of the reset and pre-reset mechanisms can reach more than one million mold cycles, providing reliable guarantee for the efficient and stable production of molds.
