Fixed mold demoulding design
Fixed mold release design is a crucial step in injection mold design. It addresses the potential for plastic parts to adhere to the fixed mold after molding. Through rational mechanical design, the plastic part can be smoothly ejected from the fixed mold, avoiding deformation, scratches, or mold damage caused by poor demolding. Fixed mold release design requires comprehensive consideration of the part’s material properties, structural shape, and molding process. It’s commonly found in molds for thin-walled parts, deep-cavity parts, or parts with undercuts on the fixed mold side. Its design quality directly impacts production efficiency and product qualification rates.
There are various types of fixed mold ejection mechanisms, including push plate ejection, push rod ejection, ejector plate ejection, and pneumatic ejection. The push plate ejection mechanism consists of a fixed mold ejector plate, a push rod, and a return spring. When the mold is opened, a pull rod or cam drives the fixed mold ejector plate to eject the plastic part from the fixed mold cavity. This mechanism is suitable for large-area, thin-walled plastic parts, such as mobile phone cases and instrument panels. Its advantages include uniform ejection force and resistance to deformation of the plastic part. The push rod ejection mechanism uses a push rod on the fixed mold. When the mold is opened, the ejector rod or hydraulic cylinder drives the push rod to eject the plastic part. This mechanism has a simple structure and is suitable for small and medium-sized plastic parts. However, the placement of the push rod must be carefully considered to avoid affecting the appearance of the plastic part. The ejector plate ejection mechanism uses an ejector plate on one side of the fixed mold to eject the plastic part. It is commonly used for deep-cavity cylindrical plastic parts. The large contact area between the ejector plate and the plastic part effectively prevents wrinkles during ejection. The pneumatic demoulding mechanism uses compressed air to blow out from the tiny pores in the fixed mold cavity to separate the plastic part from the cavity. It is suitable for plastic parts with high surface quality requirements, such as optical lenses. However, it is necessary to set evenly distributed pores on the cavity surface with a pore diameter controlled at 0.1-0.3mm to prevent melt leakage.
The core parameters of mold release design include demolding force, demolding stroke, and demolding speed. The calculation of demolding force must take into account the holding force between the part and the mold, adhesion, and deformation resistance of the part. The formula is F = K × A × P, where K is the safety factor (range 1.2-1.5), A is the contact area between the part and the mold, and P is the holding force per unit area (generally 0.5-1.5 MPa). For example, for a part with a contact area of 500 cm², the demolding force must reach 3000-11250 N. The demolding stroke must be greater than the maximum height or depth of the part, typically 1.2 times the part height, to ensure that the part can completely separate from the mold. For a part with a height of 100 mm, the demolding stroke should be set to 120 mm. The demoulding speed should be controlled at 50-100mm/s. Too fast speed may easily cause vibration or collision of plastic parts, while too slow speed may affect production efficiency. The speed regulating valve can be used to achieve segmented control. The initial stage is slow to avoid damage to plastic parts, and the subsequent stage is fast to shorten the demoulding time.
The coordinated design of the fixed mold demoulding mechanism and other mold systems is crucial. The fixed mold demoulding mechanism needs to cooperate with the parting mechanism to ensure parting before demoulding to avoid interference. For example, the movement of the fixed mold push plate must be carried out after the fixed mold and the movable mold are parted, and sequence control can be achieved through the limit pull rod. In terms of coordination with the core pulling mechanism, if the plastic part has an undercut on the fixed mold side, the core pulling action must be completed first and then demoulding. At this time, a linkage mechanism can be used to perform the core pulling and demoulding actions in a preset order. The reset of the fixed mold demoulding mechanism must be reliable, usually using a reset spring or a return rod, and the reset accuracy must be controlled within ±0.1mm to prevent collision with the movable mold parts when the mold is closed. For hot runner molds, the design of the fixed mold demoulding mechanism must avoid the hot runner system to avoid affecting the heating elements and runner seals.
The optimization and verification of the fixed mold demolding design must be combined with actual production needs. For plastic parts that easily stick to the fixed mold (such as PVC and PMMA), the surface of the fixed mold cavity can be polished (Ra ≤ 0.08μm) or coated with a release agent to reduce adhesion. For large plastic parts, push rods or push plates evenly distributed at multiple points can be used to ensure uniform distribution of demolding force. During the mold trial phase, it is important to check the demolding of the plastic parts. If deformation occurs, it may be due to uneven distribution of demolding force, and the position of the push rods needs to be adjusted. If demolding is difficult, the demolding force needs to be increased or the surface roughness of the cavity needs to be optimized . In addition, the fixed mold demolding design needs to take into account automated production. Sensors can be installed on one side of the fixed mold to detect whether the plastic parts are successfully ejected to avoid missing parts or mold jams. Through continuous design optimization and mold trial verification, the fixed mold demolding mechanism can effectively improve plastic part quality and production stability.
