Gear rack dethreading mechanism
The rack-and-pinion thread stripping mechanism is a key device in injection molds for processing products with internal or external threads. Through the meshing transmission of the gear and rack, it converts the mold opening and closing motion into the rotational motion of the threaded core or ring, enabling automatic demolding of threaded products and significantly improving production efficiency. Compared to manual thread stripping or hydraulic motor drive methods, the rack-and-pinion mechanism offers advantages such as compact structure, smooth transmission, and good synchronization. It is particularly suitable for mass-produced threaded products such as bottle caps, pipe fittings, and nuts. The design of this mechanism requires precise calculation of parameters such as the gear module, rack length, and transmission ratio to ensure that the rotation angle of the threaded core matches the withdrawal distance during the thread stripping process, preventing product slippage or deformation.
The rack and pinion dethreading mechanism consists of core components such as a rack, pinion, threaded core (or ring), guide mechanism, and limiter. The rack is typically fixed to the mold’s fixed or movable die base plate and moves linearly as the mold opens and closes. The pinion is rigidly connected to the threaded core, transmitting torque via a key or pin. When the rack and pinion mesh, the rack’s linear motion rotates the pinion, which in turn rotates the threaded core synchronously. To ensure transmission accuracy, the meshing clearance between the rack and pinion must be controlled within 0.02-0.05mm, and the tooth surface roughness should be below Ra0.8μm to minimize shock and noise during transmission. The guide mechanism often uses guide pins, guide bushings, or linear guides to ensure smooth, deflection-free rack movement. The limiter uses stops or travel switches to control the rack’s travel, preventing overtravel that could damage the rack and pinion or incomplete part demolding.
The transmission design of a rack-and-pinion threading mechanism requires determining key dimensions based on thread parameters. First, calculate the gear rotation angle based on the product’s thread pitch. For example, for an M10×1.5 thread with a thread length of 15mm, 10 rotations (15/1.5) are required, corresponding to a gear rotation angle of 3600°. Next, determine the gear diameter based on the gear module (typically 2-5mm). For a module m = 2mm, the gear pitch diameter d = m × z (z = the number of teeth). For a gear with 20 teeth, d = 40mm, resulting in a circumference of approximately 125.6mm. Each rotation requires 125.6mm of rack movement, so 10 rotations requires 1256mm of rack movement. This distance must match the mold opening and closing stroke. For multi-start threads, the lead (pitch × number of starts) must be calculated. For example, for a double-start M10×1.5 thread with a lead of 3mm and a thread length of 15mm, only 5 rotations are required, shortening the rack travel. In addition, the strength of the gear rack needs to be verified to ensure that the tooth root bending strength and tooth surface contact strength meet the torque transmission requirements to avoid tooth breakage during the transmission process.
The installation and commissioning of the rack and pinion stripping mechanism are crucial for effective demolding. During installation, ensure that the rack and pinion axes maintain parallelism within 0.05mm/m and perpendicularity within 0.03mm/m. Failure to do so can result in poor meshing, transmission jamming, and even component wear. During commissioning, manually push the rack to verify smooth gear rotation and that the thread core rotates in the same direction as the thread (right-hand threads require clockwise stripping, while left-hand threads rotate clockwise). During mold trials, gradually increase the mold opening and closing speed from 50mm/s to 150mm/s. Meanwhile, inspect the product’s threads for integrity, scratches, and deformation. If thread slippage occurs, it may indicate an incorrectly calculated rack and pinion ratio or excessive backlash, requiring parameter readjustment. If the product exhibits distortion, check the coaxiality of the thread core to ensure there is no radial runout during rotation.
Improvements and expansions in rack-and-pinion thread stripping mechanisms can accommodate the demolding needs of complex products. For products with helical threads or multiple sets of threads, a helical rack-and-pinion mechanism or a multi-set rack-and-pinion linkage design can be employed. Bevel gears or a reversing gearbox can be used to change the direction of power transmission. For example, in angled molds, bevel gears can be used to convert horizontal rack motion into vertical thread core rotation. To improve demolding efficiency, a dual-rack drive structure can be employed, symmetrically arranged on either side of the gear to reduce gear overload caused by unilateral force. For large threaded products, a gear reduction mechanism can be added, using a multi-stage gear transmission to reduce the driving force required for the rack and avoid rack bending and deformation. Furthermore, in high-temperature environments (such as engineering plastic molding), high-temperature-resistant rack-and-pinions (such as 40CrNiMoA material with a nitriding treatment) are required to ensure stable transmission performance even at temperatures between 150°C and 200°C.
