Comparison of Unscrewing Mold Structural Schemes for Threaded Plastic Parts
Threaded plastic parts are widely used in daily chemical packaging, pipeline connectors and electronic accessories. The internal and external threads cannot be demolded directly along the mold opening direction, so special unscrewing mechanisms are required to separate the thread structure from the plastic part. Different unscrewing schemes have obvious differences in structure complexity, mold cost, molding cycle, thread precision and applicable materials. It is necessary to combine the product material, thread specification, output and tolerance requirements to select the appropriate scheme in the early design stage. The core difficulty of thread demolding lies in the spiral matching relationship between the thread and the core. The plastic will shrink and wrap the threaded core after cooling. Improper demolding mode will cause thread scratching, tooth collapse and cross-threading. Blind internal threads cannot adopt side slide block structure and must rely on rotary unscrewing.
Forced demolding scheme and applicable scope
The forced demolding structure has no rotating transmission parts. The plastic part is pulled out from the threaded core by the elastic deformation of the material during ejection. The mold structure is simple, the cost is low and the cycle is short. It is only suitable for shallow short threads made of flexible PP and PE materials such as ordinary bottle caps. The disadvantage is that the thread is easy to deform and crack during demolding, the precision is poor, and it cannot be used for rigid materials, deep threads and high-precision assembly parts.
Rotary unscrewing: rack and pinion driven by hydraulic cylinder
This is the most widely used mature scheme. The hydraulic cylinder pushes the rack to convert linear motion into the rotation of the gear and threaded core. The structure is reliable, easy to maintain and low in cost, suitable for most conventional threaded products with medium and high output. The limitation is that the cylinder stroke limits the number of rotation turns. Long threads need a larger rack layout space, and the action rhythm needs to match the mold opening and ejection.
Worm and worm gear driven by hydraulic motor
The hydraulic motor drives the threaded core to rotate through the worm gear set, which can output large torque and run smoothly. It is suitable for large-diameter and long-thread products such as automotive pipeline joints. The disadvantage is that the worm gear has low transmission efficiency, easy wear, and the supporting oil circuit and sealing system increase the mold cost and maintenance difficulty.
Servo motor high-precision unscrewing scheme
The servo motor accurately controls the rotation speed, angle and stop position, and can synchronize with mold opening and robot picking signals. The thread stop position has high consistency, which is suitable for high-value precision medical and electronic threaded parts with high automation requirements. The defect is high mold manufacturing cost and complicated electrical debugging.
Slide block external thread scheme and comprehensive selection logic
Shallow external threads can adopt split half-cavity slide block structure. The two slide blocks separate sideways when the mold is opened, with simple structure and stable action. Internal threads mostly use rotary unscrewing. When selecting the scheme, forced demolding is preferred for low-precision flexible shallow threads, rack unscrewing is the first choice for conventional mass-produced threaded parts, worm gear transmission is used for large torque long threads, and servo unscrewing is adopted for high-precision automatic production. Reasonable scheme selection can avoid stuck thread and tooth damage in mass production.
