What is the ejection and reset mechanism of the mold?
Ejection and reset mechanism of mold
Ejection methods are influenced by factors such as product material and shape, and generally include ejector pins (straight pins, stepped pins), sleeves, push plates, and air ejection. Ejection methods can be used individually or in combination, depending on the mold life and the difficulty of mold processing.
Requirements for ejection mechanisms
① The ejection system of the mold should ensure smooth, complete, and deformation-free ejection of the plastic part. The mechanism should be as simple and reliable as possible, with stable ejection and complete return to the starting position.
② In general, a spring return method is used. This is mainly because when the part structure causes an imbalance in the demolding force, the mold needs a balancing and guiding device for ejection.
③ The plastic part remains on the half-mold with the ejection mechanism, generally in the male mold.
④ The plastic part must not be deformed or damaged. The ejection force should be applied to the part with the greatest rigidity, such as ribs, flanges, and shell sidewalls, with the action area as large as possible.
⑤ For a good appearance of the plastic part, the ejection mechanism should be located inside the product.
⑥ The ejection mechanism structure should be reliable.
Ejector pin type ejection mechanism
Advantages and disadvantages of ejector pin mechanisms
(1) Advantages of Ejector Pin-Type Ejection Mechanisms
① Ejector pins are relatively easy to machine. Even when hardness requirements are present, quenching and grinding are easier than other methods. They can be placed at any position on the product and are the most commonly used.
② Ejector pin holes are also easy to machine, and the required precision can be achieved. Sliding resistance is minimal, and jamming is rare.
③ Good interchangeability in case of damage, and easy maintenance.
④ The most commonly used ejection components include round ejector pins, ejector pins with supports, flat ejector pins, and ejector sleeves. The ejection position should be located where the demolding force is high, and it is not advisable to place it at the thinnest part of the product. Increasing the ejection area can improve stress distribution.
(2) Disadvantages of Ejector Pin-Type Ejection Mechanisms
Ejection occurs over a small area, and the ejection stress is concentrated in a localized area of the product. For cup-shaped and box-shaped products with small draft angles and high demolding forces, indentation and puncture may occur, making the use of ejector pins generally unsuitable.
Ejector Pin Arrangement Principles
① The ejector pin arrangement should balance the ejection force as much as possible. For structurally complex areas requiring greater demolding force, the number of ejector pins should be increased accordingly.
② Ejector pins should be placed in effective locations, such as ribs, pillars, steps, metal inserts, and areas with thick localized adhesive. Ejector pins on both sides of ribs and pillars should be arranged as symmetrically as possible. The distance between the ejector pins and the edges of the ribs and pillars is generally D = 1.5mm, as shown in Figure 7-16. Additionally, the line connecting the centers of the ejector pins on both sides of a pillar should ideally pass through the center of the pillar.
③ Avoid placing ejector pins across steps or on slopes. The top surface of the ejector pin should be as flat as possible. Ejector pins should be placed in structural parts of the plastic part where the stress is relatively good, as shown in Figure 7-17.
④ When there are deep ribs (depth ≥ 20mm) in the plastic part or when it is difficult to arrange round ejector pins, flat ejector pins should be used. When flat ejector pins are required, inserts should be used at the flat ejector pin locations to facilitate machining, see Figure 7-18 (a).
⑤ Avoid using pointed or thin steel, especially ensuring that the top surface of the ejector pin does not touch the front mold surface, see Figure 7-18 (b).
⑥ The ejector pin arrangement should consider the edge distance between the ejector pins and the water channels to avoid affecting the machining of the water channels and causing leakage.
⑦ Consider the venting function of the ejector pins. For venting during ejection, ejector pins should be placed in areas where vacuum can easily form. For example, in large planar areas of the cavity, although the clamping force of the part is small, vacuum can easily form, leading to increased demolding force.
⑧ For plastic parts with aesthetic requirements, ejector pins should not be placed on the visible surface; other ejection methods should be used.
⑨ For transparent parts, ejector pins should not be placed in areas where light needs to pass through.
⑩ Small ejector pins should not be used on large flat surfaces; ejector sleeves should be used at protruding holes.
⑪ Ejector pins should be placed where the product's strength is greatest for best results and to prevent whitening.
⑫ Ejector pins should not be placed under the slider. If this cannot be avoided, a slider return-to-position signal should be added for coordination.
Precautions for selecting ejector pins
① Select ejector pins with larger diameters. Given sufficient ejection space, choose ejector pins with larger diameters and prioritize larger sizes.
② Minimize the number of ejector pin sizes used. When selecting ejector pins, adjust their size to minimize the required dimensions, and prioritize preferred size series.
③ The selected ejector pins must meet ejection strength requirements. During ejection, the ejector pins must withstand significant pressure; avoid bending or deformation of small ejector pins.
④ For ejector pin diameters below 2.5mm, and with sufficient space, use supported ejector pins; for ejector sleeve wall thickness below 1mm or ejector sleeve diameter-to-wall ratio ≤0.1, use supported ejector sleeves, with the support length maximized.
⑤ The effective mating length of the ejector pin = (2.5～3)D, and the minimum must not be less than 8mm.
⑥ In general, the ejector pin surface should be 0.03～0.05mm higher than the core plane. For applications requiring a specific molded surface, consider adding a countersunk platform around the ejector pin.
⑦ For long rib molded areas with a height of 10mm or more, it is recommended to use a flat ejector pin.
⑧ When the ejector pin surface is located on an inclined plane, the ejector pin must be positioned, as shown in Figure 7-19.
Push plate ejection mechanism
Ejector plate ejection is a commonly used ejection mechanism in mold design. Ejector plate demolding mechanisms are suitable for demolding large cylindrical plastic parts, thin-walled containers, and various types of shell-shaped plastic parts. They are not suitable for plastic parts with complex parting surface shapes or difficult-to-machine ejector plate core holes. The characteristics of ejector plate demolding are uniform ejection, high force, smooth movement, and less deformation of the plastic part. The key design considerations for ejector plate demolding mechanisms are as follows:
① The mating structure between the ejector plate and the core should be conical, as shown in Figure 7-20; this reduces friction damage during movement and provides auxiliary guidance; the taper angle should be 3°–10°.
② The push plate should have a tapered fit with the core, and the inner hole of the push plate should be 0.2–0.3 mm larger than the forming part of the core, as shown in Figure 7-21, to prevent wear and jamming between the two.
③ The push plate and the return pin are connected by screws, as shown in Figure 7-22.
④ After demolding with the ejector plate, ensure that the plastic part does not remain on the ejector plate.
⑤ When ejecting large, deep-cavity shell-type plastic parts without through holes using the ejector plate, an air intake device must be added to the core.