(1) The ejector pins should be arranged to balance the ejection force as much as possible. For complex parts, the demolding force required is greater, and the number of ejector pins should be increased accordingly. (2) Ejector pins should be placed in effective parts, such as ribs, pillars, steps, metal inserts, and other complex parts with thick local plastic. 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 5.5.8. In addition, the center line connecting the ejector pins on both sides of the pillar should pass through the center of the pillar as much as possible. (3) Avoid setting ejector pins across steps or on inclined surfaces. The top surface of the ejector pins should be as flat as possible. Ejector pins should be placed in structural parts of the plastic part where the force is better. As shown in Figure 5.5.9. (4) Flat ejector pins should be used in deep ribs (depth ≥ 20mm) or when it is difficult to arrange round ejector pins. When flat ejector pins are required, inserts should be used as much as possible at the flat ejector pin location to facilitate processing. As shown in Figure 5.5.10 (5) Avoid sharp steel and thin steel, especially the top surface of the ejector pin should not touch the front mold surface. As shown in Figure 5.5.11 (6) The ejector pin arrangement should take into account the edge distance between the ejector pin and the water channel to avoid affecting the processing of the water channel and leakage. For specific requirements, see Section 10.2 of Chapter 10. (7) Consider the venting function of the ejector pin. In order to vent during ejection, ejector pins should be arranged in places where vacuum is easily formed. For example, in the large plane of the cavity, although the clamping force of the plastic part is small, it is easy to form a vacuum, which leads to an increase in demolding force. (8) For plastic parts with appearance requirements, ejector pins should not be arranged on the appearance surface, and other ejection methods should be used. (9) For transparent plastic parts, ejector pins should not be arranged in places where light needs to pass through.
B. Ejector Pin Selection Principles
(1) Select ejector pins with larger diameters. That is, when there are enough ejection positions, ejector pins with larger diameters and size priority should be selected. (2) The number of ejector pin specifications should be minimized. When selecting ejector pins, the size of the ejector pins should be adjusted to minimize the size specifications, and the preferred size series should be selected as much as possible. (3) The selected ejector pins should meet the ejection strength requirements. During ejection, the ejector pins have to withstand greater pressure. To avoid bending and deformation of small ejector pins, when the ejector pin diameter is less than 2.5mm, ejector pins with supports should be selected. After the product completes a molding cycle, the mold is opened, and the product will be wrapped around one side of the mold. It must be removed from the mold. This work must be done by the ejection system, which is an important part of the entire mold structure. It generally consists of three parts: ejection, reset, and ejection guide. 1. Design principles of the ejection system There are many forms of ejection systems, which are related to the shape, structure and plastic properties of the product. Generally, there are ejector pins, ejector tubes, push plates, ejector blocks, and air pressure compound ejection. Figure 8.1 Ejection system structure diagram The structure diagram of the ejection system is shown in Figure 8.1. Its design principles are as follows: ① When selecting the parting surface, try to keep the product on the side with the demolding mechanism. ② Balance the ejection force and position to ensure that the product does not deform or break. ③ Ejector pins must be placed in locations that do not affect the product's appearance and function. ④ Use standard parts whenever possible for safety, reliability, and ease of manufacturing and replacement. ⑤ Ejection points should be located where resistance is high, and should not be too close to inserts or cores. For deep-cavity molds such as box-shaped molds, where side resistance is greatest, a simultaneous top and side ejection method should be used to prevent product deformation and breakage. ⑥ When there are thin and deep reinforcing ribs, ejector pins are generally placed at their bottom. ⑦ Avoid placing ejector pins at the product's gate to prevent breakage. ⑧ For thin-walled products, ejector pins can be placed on the runner to carry the product out. ⑨ The fit between ejector pins and ejector pin holes is generally a clearance fit; too loose and burrs may form, too tight and jamming may occur. To facilitate machining and assembly and reduce friction surfaces, a 10-15mm fit length is generally reserved on the moving mold, and the remaining portion is enlarged by 0.5-1.0mm to form an escape hole. ⑩ To prevent the ejector pin from rotating during production, it must be fixed on the ejector plate. There are many different forms, and the specific determination must be made according to the size, shape and position of the ejector pin. 2. Selection principle of ejection type In the injection mold structure, the quality of the ejection mechanism design directly affects the quality of the finished plastic product. If the design is not good, a series of defects will occur in the plastic part, such as: warping deformation, cracks and whitening of the plastic part. The determination of the ejection type is the most important link in the ejection design. The ejector pin type, quantity and ejection position are optimized according to the ejection force and demolding resistance. (1) Ejector pin The ejector pin is the simplest and most common form of ejection mechanism. Because it is easy to manufacture, process and repair, and has a good ejection effect, it is the most widely used in production. However, the circular ejection area is relatively small, which can easily cause stress concentration, ejection through the product, product deformation and other defects. In tubular and box-shaped products with small demolding angle and large resistance, it should be avoided as much as possible. When the ejector pin is relatively thin and long, it is generally set as a stepped ejector pin to strengthen the rigidity and avoid bending and breaking [29]. Ejector pin structure, as shown in Figures 8.2, 8.3, and 8.4. (2) Ejector tube: The ejector tube, also called the ejector pin or ejector sleeve, is suitable for ring-shaped, cylindrical, or products with a center hole. Its ejection is uniform in force throughout the circumference, which will not deform the product and will not leave obvious ejection marks, thus improving the concentricity of the product. However, it should be avoided for products with thick or thin periphery to prevent processing difficulties and weakening of strength, which may cause damage. (3) Push plate: The push plate is suitable for various containers, box-shaped, cylindrical, and slender thin-walled products with a center hole. It ejects smoothly and evenly with a large ejection force and leaves no ejection marks. It is usually fixed to prevent the push plate from being pushed off during production or demolding. However, as long as the guide post is long enough and the demolding stroke is strictly controlled, the push plate may not be fixed.
Precautions for selecting ejector pins for plastic molds: The ejection system is one of the important functional structures of injection molds. It consists of a series of ejection parts and auxiliary parts, which can have different ejection actions. Ejector pins are the most commonly used ejection method. Ejector pins include round ejector pins, shouldered ejector pins, flat ejector pins, and push tubes. The following are considerations for selecting ejector pins: 1. To prevent deformation or damage to the plastic part, correctly analyze the magnitude and location of the adhesion force between the plastic part and the mold cavity, and select a suitable ejection device accordingly. This ensures the ejection force is applied to the part with the greatest rigidity and strength, i.e., as close to the wall, under the ribs or pillars as possible, and with the largest possible effective area (i.e., select ejector pins with the largest possible diameter) to prevent deformation or damage to the plastic part. 2. The structure should be reasonable and reliable. The ejection mechanism should operate reliably, move flexibly, be easy to manufacture and replace, and have sufficient strength and rigidity. 3. Shouldered ejector pins should be used when the ejector pin diameter is below φ2.5 and there is sufficient space; shouldered push tubes should be used when the push tube wall diameter is below 1mm or the push tube wall diameter ratio is ≤0.1, and the fixed part should have the largest possible value. The effective fitting length of the ejector pin = (2.5~3)D, and should not be less than 8mm. In the manufacturing process, we generally use 20-25mm. 4. Avoid placing ejector pins at insert joints.
5. For long, curved sections of plastic parts exceeding 10mm in height, flat ejector pins are recommended. The shorter the flat section, the better the strength and the easier the machining. The length of the cylindrical section should be specified in the design specifications. For cylindrical sections exceeding 10mm in height, push-tube ejection is recommended.
6. In cases with angled ejector pins, to prevent the product from sliding along with the angled ejector, the ejector pin surface near the angled ejector should be ground with a "+" groove.
Slides, Angled Ejectors
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When the sidewall of a plastic part has concave or convex shapes, side holes, or snap-fit parts, the side core must be extracted before the mold opens to eject the plastic part. This mechanism is called a slide. As shown in Figure 3.2.8, for the outer side hole of the plastic part, a rear mold slide is required for core extraction. As shown in Figure 3.2.9, for the inner groove of the plastic part, if an angled ejector is used, the top opening distance is insufficient; an inner slide must be used. Additionally, an ejection mechanism that utilizes angled ejection to simultaneously complete ejection and core pulling is called an angled ejector. For parts on plastic parts requiring core pulling, when the slide space is insufficient, an angled ejector mechanism can be used. In the angled ejector mechanism, the angled ejection distance should be greater than the core pulling distance (B > H), as shown in Figure 3.2.10, to prevent ejection interference. As shown in Figure 3.2.11, the inner and outer walls of the plastic part have concave shapes. Due to obstruction from the inner wall by ribs and insufficient height, the outer wall needs to be slide-mounted in the front mold, while the inner wall undergoes angled ejection.
