(1) Ejector pin should be arranged so that ejection force is as balanced as possible. Parts with complex structures require greater demoulding force, and number of ejector pins should be increased accordingly.
(2) Ejector pin should be set in effective parts, such as bone positions, column positions, steps, metal inserts, local thick glue and other structurally complex parts. Ejector pins on both sides of bone and column should be arranged as symmetrically as possible. Edge distance between ejector pins and bone and column is generally D=1.5mm, as shown in Figure 5.5.8. In addition, try to ensure that center line of ejector pins on both sides of column passes through center of column.

 

Mold Design Guide-5. Mold Structural Design

(3). Avoid crossing steps or setting ejector pins on slopes. Top surface of ejector pin should be as smooth as possible, and ejector pin should be arranged in a structural part where rubber part is better stressed. As shown in Figure 5.5.9.

 

(4). Flat ejector pins 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 flat ejector pin to facilitate processing. As shown in Figure 5.5.10

 

(5) Avoid sharp steel and thin steel, especially top surface of ejector pin from touching front mold surface. As shown in Figure 5.5.11
(6) Ejector pin layout should consider edge distance between ejector pin and water conveyance channel to avoid affecting processing and water leakage of water conveyance channel.
(7) Consider exhaust function of ejector pin. In order to exhaust ejector during ejection, ejector pin should be arranged in area where vacuum is easily formed. For example, in larger plane of mold cavity, although tightening force of plastic parts is small, it is easy to form a vacuum, resulting in an increase in demoulding force.
(8) For plastic parts with appearance requirements, ejection pin cannot be arranged on the appearance surface, and other ejection methods should be used.
(9) For transparent plastic parts, ejector pin cannot be placed in area that needs to be light-transmissive.

(1) Choose a ejector pin 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) Specifications of ejector pin should be as small as possible. When selecting an ejector pin, size of ejector pin should be adjusted to minimize size specifications, and at the same time, try to select preferred size series.
(3) Selected ejector pin should meet ejection strength requirements. When ejecting, ejector pin must bear greater pressure. In order to avoid bending and deformation of small ejector pin, when diameter of ejector pin is less than 2.5mm, a supported ejector pin should be used.
When mold is opened after product completes a forming cycle, product will be wrapped around one side of mold and must be removed from mold. This work must be completed by ejection system, which is an important part of the entire mold structure. It generally consists of three parts: ejection, reset and ejection guide.

There are various forms of ejection systems, which are related to shape, structure and plastic properties of product. They generally include ejection rods, ejection tubes, push plates, ejection blocks, pneumatic composite ejection, etc.

 

Figure 8.1 Ejection system structure diagram
Structure diagram of ejection system is shown in Figure 8.1. Its design principles are as follows:
① When selecting parting surface, try to keep product on the side with demoulding mechanism.
② Ejection force and position balance ensure that product will not deform or break.
③ Ejector pin must be located where it does not affect appearance and function of product.
④ Try to use standard parts as safe and reliable as possible to facilitate manufacturing and replacement.
⑤ Ejection position should be set at a place with high resistance and should not be too close to insert or core. For deep-cavity molds such as box-shaped molds, side resistance is the greatest, top and side ejection method should be used to prevent product from deforming and bursting.
⑥ When there are thin and deep ribs, a push rod is usually installed at the bottom.
⑦ At glue inlet of product, avoid setting an ejector pin to avoid breakage.
⑧ For thin products, set ejector pin on shunt channel to take product out.
⑨ Fit between ejector pin and 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, reduce friction surface, a matching length of 10~15mm is generally reserved on movable mold, and remaining part is expanded by 0.5~1.0mm to form an escape hole.
⑩ In order to prevent ejector pin from rotating during production, it must be fixed on ejector plate. There are various forms, which must be specifically determined according to size, shape and position of ejector pin.

In injection mold structure, design of ejection mechanism directly affects quality of finished plastic product. If design is not good, plastic part will produce a series of defects, such as warping deformation, cracks, and whitening of plastic part. Determination of ejection type is the most important step in ejection design. According to ejection force and demoulding resistance, type, quantity and ejection position of ejection pin are optimized.
(1) Push rod
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, 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 ejector pin is relatively slender, a stepped ejector pin is generally provided to enhance stiffness and avoid bending and breaking. Push rod structure is shown in Figures 8.2, 8.3, and 8.4.

 

(2) Pipe jacking
Jacking tube is also called cylinder or cylinder needle. It is suitable for ring-shaped, cylindrical or products with a center hole. When ejected, contact force is evenly distributed throughout the entire circumference, which will not deform product and leave obvious ejection marks. It can improve concentricity of product. However, avoid using products with thicker and thinner surroundings to avoid damage caused by difficulty in processing and weakened strength.
(3) Push plate
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 push plate from being pushed down during production or during demoulding. However, as long as guide pillar is long enough and demoulding stroke is strictly controlled, push plate does not need to be fixed.

Ejection system is one of important functional structures of 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 ejector pins are as follows:
1. To prevent plastic parts from being deformed or damaged, correctly analyze adhesion of plastic parts to mold cavity and its location, select appropriate demoulding device in a targeted manner so that ejection force is applied to the most rigid and strong part of plastic part, that is, as close as possible to wall, under bone position, column position, and action area is as large as possible (that is, an ejector pin with a large diameter is selected as much as possible) to prevent deformation or damage of plastic part.
2. Structure is reasonable and reliable. Ejection mechanism should work reliably, move flexibly, be easy to manufacture, easy to replace, have sufficient strength and stiffness.
3. When diameter of ejector pin is below φ2.5 and position is sufficient, a shouldered ejector pin should be used; if push tube wall is below 1mm or push tube wall diameter ratio is ≤0.1, a shouldered push tube should be used, and fixed part should be as large as possible. Effective matching length of ejector pin = (2.5~3)D, minimum shall not be less than 8mm, we generally take 20-25mm during production process.
4. Try not to place ejector pin at splicing point of 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 flat part, the better strength and easier to process. Length of cylindrical part should be indicated in 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 product from sliding with slanted ejector, surface of ejector pin near slanted ejector pin must be ground with a "+" groove.
Slider, lifter
When side wall of plastic part has concave and convex shapes, side holes and buckles, lateral core must be pulled out before mold is opened to eject plastic part. This mechanism is called a slider. As shown in Figure 3.2.8, outer hole of plastic part requires core pulling at rear mold position. As shown in Figure 3.2.9, if inner groove of plastic part is ejected with an lifter, top opening distance is not enough, and inside position must be used.

 

 

In addition, ejection mechanism that uses oblique ejection, ejection and core pulling at the same time is called oblique ejection. For parts of plastic parts that require core pulling, when row space is insufficient, tilting mechanism can be used to complete the process. In inclined ejection mechanism, oblique ejection distance should be greater than core pulling distance (B > H) as shown in Figure 3.2.10 to prevent ejection interference.
As shown in Figure 3.2.11, inner and outer walls of plastic part have concave shapes. Inner side has bone obstruction and insufficient height. Front mold must be positioned on outer wall and inner wall must be ejected at an angle.

 

As shown in Figure 3.2.12, there should be no clamping lines around side holes of plastic parts. Side holes must be core-pulled in front of mold and pushed out of mold at a buckled position.