A.Principles of thimble arrangement
(1) The ejector pin should be arranged so that the ejection force is as balanced as possible. Parts with complex structures require greater demoulding force, and the number of ejector pins should be increased accordingly.
(2) The thimble should be set in effective parts, such as bone positions, column positions, steps, metal inserts, local thick glue and other structurally complex parts. The thimbles on both sides of the bone and column should be arranged as symmetrically as possible. The edge distance between the thimbles and the bone and column is generally D=1.5mm, as shown in Figure 5.5.8. In addition, the spacing of the thimbles on both sides of the column should be ensured as much as possible. The center line passes through the center of the column.
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(3) Avoid crossing steps or setting ejector pins on slopes. The top surface of the ejector pin should be as smooth as possible, and the ejector pin should be arranged in a structural part where the rubber part is better stressed. As shown in Figure 5.5.9.
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(4) Flat thimbles should be used in deep bone locations (depth ≥ 20mm) or when it is difficult to arrange dome pins. When it is necessary to use a flat ejector pin, try to use an insert at the flat ejector pin to facilitate processing. As shown in Figure 5.5.10
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(5) Avoid sharp steel and thin steel, especially the top surface of the ejector pin from touching the front mold surface. As shown in Figure 5.5.11
(6) The ejector pin layout should consider the edge distance between the ejector pin and the water conveyance channel to avoid affecting the processing and water leakage of the water conveyance channel. See Chapter 10, Section 10.2 for specific requirements.
(7) Consider the exhaust function of the ejector pin. In order to exhaust the ejector during ejection, the ejector pin should be arranged in the area where vacuum is easily formed. For example, in the larger plane of the mold cavity, although the tightening force of the plastic parts is small, it is easy to form a vacuum, resulting in an increase in the demoulding force.
(8) For plastic parts with appearance requirements, the ejection pin cannot be arranged on the appearance surface, and other ejection methods should be used.
(9) For transparent plastic parts, the ejector pin cannot be placed in the area that needs to be light-transmissive.
B. Principles for selecting thimbles
(1) Choose a thimble with a larger diameter. That is, if there is enough ejection position, an ejection pin with a larger diameter and size priority should be selected.
(2) The specifications of the thimble should be as small as possible. When selecting an ejector pin, the size of the ejector pin should be adjusted to minimize the size specifications, and at the same time, try to select the preferred size series.
(3) The selected ejector pin should meet the ejection strength requirements. When ejecting, the ejector pin must bear greater pressure. In order to avoid bending and deformation of the small ejector pin, when the diameter of the ejector pin is less than 2.5mm, a supported ejector pin should be used.
When the mold is opened after the product completes a forming cycle, the product will be wrapped around one side of the mold and must be removed from the mold. This work must be completed 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 ejection system
There are various forms of ejection systems, which are related to the shape, structure and plastic properties of the product. They generally include ejection rods, ejection tubes, push plates, ejection blocks, pneumatic composite ejection, etc.
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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 demoulding mechanism.
②The ejection force and position balance ensure that the product will not deform or break.
③ The ejector pin must be located where it does not affect the appearance and function of the product.
④ Try to use standard parts as safe and reliable as possible to facilitate manufacturing and replacement.
⑤ The ejection position should be set at a place with high resistance and should not be too close to the insert or core. For deep-cavity molds such as box-shaped molds, the side resistance is the greatest, and the top and side ejection method should be used to prevent the product from deforming and bursting.
⑥ When there are thin and deep ribs, a push rod is usually installed at the bottom.
⑦ At the glue inlet of the product, avoid setting an ejector pin to avoid breakage.
⑧ For thin meat products, set the ejector pin on the shunt to take the product out.
⑨ The fit between the ejector pin and the ejector pin hole is generally a clearance fit. If it is too loose, it will easily produce burrs, and if it is too tight, it will easily cause jamming. In order to facilitate processing and assembly and reduce the friction surface, a matching length of 10~15mm is generally reserved on the movable mold, and the remaining part is expanded by 0.5~1.0mm to form an escape hole.
⑩ In order to prevent the ejector pin from rotating during production, it must be fixed on the ejector plate. There are various forms, which must be specifically determined according to the size, shape and position of the ejector pin.
2. Principles for selecting ejection types
In the injection mold structure, the design of the ejection mechanism directly affects the quality of the finished plastic product. If the design is not good, the plastic part will produce a series of defects, such as warping deformation, cracks, and whitening of the plastic part. The determination of the ejection type is the most important step in the ejection design. According to the ejection force and demoulding resistance, the type, quantity and ejection position of the ejection pin are optimized.
(1) Push rod
The ejector rod is the simplest and most common form of ejector mechanism. Because of its convenient manufacturing, processing and repair, and good ejection effect, it is the most widely used in production. However, the circular ejection area is relatively small, and it is easy to cause stress concentration, product penetration, product deformation and other defects. Try to avoid using it in tubular and box-shaped products with small draft angle and high resistance. When the ejector pin is relatively slender, a stepped ejector pin is generally provided to enhance stiffness and avoid bending and breaking [29]. The push rod structure is shown in Figures 8.2, 8.3, and 8.4.
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(2) Pipe jacking
The jacking tube is also called the cylinder or cylinder needle. It is suitable for ring-shaped, cylindrical or products with a center hole. When ejected, the contact force is evenly distributed throughout the entire circumference, which will not deform the product and leave obvious ejection marks. It can improve the concentricity of the product. However, avoid using products with thicker and thinner surroundings to avoid damage caused by difficulty in processing and weakened strength.
(3) Push plate
The push plate is suitable for various containers, box-shaped, cylindrical and elongated thin products with center holes. It ejects smoothly and evenly, with strong ejection force and leaves no ejection marks. Generally, there is a fixed connection to prevent the push plate from being pushed down during production or during demoulding. However, as long as the guide pillar is long enough and the demoulding stroke is strictly controlled, the push plate does not need to be fixed.
Things to note when selecting plastic mold ejector pins
The ejection system is one of the important functional structures of the injection mold. It consists of a series of ejection parts and auxiliary parts, which can have different ejection actions. Ejector type is the most commonly used ejection method. Ejector components such as ejector pins include dome pins, shoulder ejector pins, flat ejector pins, and push tubes. Things to note when selecting thimbles are as follows:
1. To prevent plastic parts from being deformed or damaged, correctly analyze the adhesion of the plastic parts to the mold cavity and its location, and select the appropriate demoulding device in a targeted manner so that the ejection force is applied to the part with the highest rigidity and strength. The position should be as close as possible to the wall, under the bone and column, and the action area should be as large as possible (that is, a thimble with a larger diameter should be selected as much as possible) to prevent deformation or damage of the plastic part.
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2. The structure is reasonable and reliable. The ejection mechanism should work reliably, move flexibly, be easy to manufacture, easy to replace and have sufficient strength and stiffness.
3. When the diameter of the ejector pin is below φ2.5 and the position is sufficient, a shouldered ejector pin should be used; if the push tube wall is below 1mm or the push tube wall diameter ratio is ≤0.1, a shouldered push tube should be used, and the fixed part should be as large as possible. The effective matching length of the ejector pin = (2.5~3)D, the minimum shall not be less than 8mm, we generally take 20-25mm during the production process.
4. Try not to place the ejector pin at the splicing point of the inserts.
5. For long arc glue positions with a height of more than 10mm, it is recommended to use a flat ejector pin to eject. The shorter the flat part, the better the strength and easier to process. The length of the cylindrical part should be indicated in the design specifications. For pipes with a height of 10mm or more, it is recommended to use a push tube to eject.
6. For occasions with a slanted ejector pin, in order to prevent the product from sliding with the slanted ejector, the surface of the ejector pin near the slanted ejector pin must be ground with a "+" groove.
row position, sloping roof
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When the side wall of the plastic part has concave and convex shapes, side holes and buckles, the lateral core must be pulled out before the mold is opened to eject the plastic part. This mechanism is called a line position. As shown in Figure 3.2.8, the outer hole of the plastic part requires core pulling at the rear mold position. As shown in Figure 3.2.9, if the inner groove of the plastic part is ejected with an inclined top, the top opening distance is not enough, and the inside position must be used.
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In addition, the ejection mechanism that uses oblique ejection, ejection and core pulling at the same time is called oblique ejection. For parts of the plastic parts that require core pulling, when the row space is insufficient, the tilting mechanism can be used to complete the process. In the inclined ejection mechanism, the oblique 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. The inner side has bone obstruction and insufficient height. The front mold of the outer wall must be positioned and the inner wall of the mold must be pushed out obliquely.
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As shown in Figure 3.2.12, there should be no clamping lines around the side holes of the plastic parts. The side holes must be core-pulled in front of the mold and pushed out of the mold in a buckled position.
