The demolding mechanism for threaded plastic parts is a key component in injection mold design. Its function is to smoothly remove plastic parts with internal or external threads from the mold cavity or core while maintaining the integrity and precision of the threads. Due to the helical nature of the thread structure, traditional linear ejection methods cannot be used for direct demolding. Rotary or forced demolding methods must be used. The specific choice is determined by a combination of factors such as the thread precision requirements, the properties of the plastic part material, and the production batch. For plastic parts with high precision requirements and complex thread profiles, a rotary demolding mechanism is generally used. For plastic parts with lower thread precision requirements and a certain degree of material elasticity, a forced demolding mechanism can be used to simplify the mold structure and reduce costs.
A rotary demolding mechanism is a common method for demolding threaded plastic parts. Its core principle is to mechanically generate rotational motion in the core or cavity, thereby separating it from the part’s threaded structure. This mechanism primarily consists of a transmission, a rotating core (or cavity), and a rotation-stop mechanism. Transmission mechanisms typically employ gears, chains, or worm gears, with gears being widely used due to their high transmission precision and compact structure. For example, when demolding internally threaded plastic parts, a gear connected to the core is installed inside the mold. During mold opening, the meshing of the rack and gear rotates the core, simultaneously cooperating with an ejection mechanism to eject the part. The rotation-stop mechanism prevents the part from rotating with the core. It typically utilizes a non-circular structure on the part (such as a flattened part or spline) that interacts with a rotation-stop pin inside the mold to ensure axial movement, preventing rotation.
Forced demolding mechanisms are suitable for plastic parts with large thread angles, coarse pitches, and highly elastic materials, such as bottle caps made of polyethylene and polypropylene. Their operating principle is to utilize the elastic deformation of the plastic part at the moment of demolding to forcibly eject the part from the threaded core using the axial force of the ejector mechanism. The forced demolding mechanism has a relatively simple structure, primarily consisting of an ejector plate, ejector pin, and threaded core. It eliminates the need for complex rotary transmission mechanisms, thus reducing mold manufacturing costs. However, forced demolding subjects the part’s threads to a certain radial force, potentially causing deformation or damage to the thread profile. Therefore, the magnitude and distribution of the ejection force must be strictly controlled. Typically, the ejection force per unit area is reduced by increasing the ejection area and optimizing the ejection location (such as setting an ejection point at the end of the thread). Polishing the core surface (to a roughness of Ra ≤ 0.8μm) also reduces demolding resistance.
The design of the demolding mechanism for threaded plastic parts requires consideration of the thread direction (left-hand or right-hand), pitch, and structural characteristics of the part. For multi-threaded parts, due to their larger pitch, the core rotation speed during demolding can be appropriately reduced to reduce the load on the transmission. For single-start fine threads, the core’s rotational accuracy must be improved to prevent thread damage due to rotational deviation. When a part has both internal and external threads, a dual-rotation demolding mechanism is required to control the rotation direction and speed of the inner core and outer cavity, ensuring coordinated rotation and avoiding shear forces on the part during demolding. Furthermore, for threaded parts with steps, a corresponding clearance structure must be incorporated into the demolding mechanism to prevent interference with the steps during ejection.
The reliability and stability of the demolding mechanism are crucial to production efficiency and part quality, and therefore require thorough verification and maintenance during design and use. During the mold trial phase, focus on inspecting part deformation, thread integrity, and demolding force during demolding. Demolding stability can be improved by adjusting transmission mechanism clearances (e.g., controlling gear mesh clearance to 0.02-0.05mm) and optimizing the precision of the anti-rotation mechanism. During daily production, the transmission components of the rotary demolding mechanism must be regularly lubricated (e.g., by filling the gearbox with specialized gear oil). The anti-rotation pins must be inspected for wear and replaced promptly if wear exceeds 0.1mm to prevent the part from rotating during demolding. For forced demolding mechanisms, the core surface must be regularly cleaned of residual plastic and oil to maintain a smooth surface. Furthermore, dimensional changes in the part threads must be monitored. If deformation exceeds tolerance, the demolding force must be adjusted or the demolding method changed.
