(1) Ejector pin should be arranged so that ejection force is as balanced as possible. Demolding force required for complex structures is large, and number of ejectors should be increased accordingly.
(2) Ejector pin should be set in effective parts, such as bone, column, steps, metal inserts, and some thick structural parts. Ejector pin on both sides of bone and column should be arranged as symmetrically as possible. Distance between ejector pin and bone, column is generally D = 1.5mm, as shown in Figure 5.5.8. In addition, center line of ejector pin on both sides of column should be as far as possible to pass through center of column.

 

(3) Avoid setting up ejector pins across steps or on inclined surfaces. Top surface of ejector pin should be as smooth as possible, and ejector pin should be layout at structural part where part is better stressed. As shown in Figure 5.5.9.

 

(4) A rectangular E.P. should be used when plastic parts is deeper (depth ≥20mm) or it is difficult to arrange a round needle. When a rectangular E.P. is required, rectangular E.P. should be inserted as much as possible to facilitate processing. Figure 5.5.10

 

(5) Avoid sharp steel and thin steel, especially top surface of ejector pin should not touch front mold surface. Figure 5.5.11
(6) Layout of ejector pin should consider side distance between ejector pins and water conveyance channel to avoid affecting processing and water leakage of water conveyance channel.
(7) Ejector function of ejector is taken into consideration. In order to exhaust ejector, ejector should be arranged at the place where vacuum is easy to form. For example, at larger plane of cavity, although rubber packing force is small, it is easy to form a vacuum, which results in an increased demolding force.
(8) For plastic parts with appearance requirements, ejector pin cannot be arranged on appearance surface, and other ejection methods shall be used.
(9) For transparent plastic parts, ejector pin cannot be placed on part that needs to transmit light.

(1) Use a large diameter ejector pin. That is, if there is enough ejection position, a larger diameter and size-first ejector should be selected.
(2) Size of ejector pin should be as small as possible. When selecting a ejector pin, size of ejector pin should be adjusted to minimize size specifications, and preferred size series should be selected as far as possible.
(3) Ejector used should meet ejection strength requirements. During ejection, ejector pin must withstand large pressure. To avoid bending and deformation of small ejector pin, when ejector pin diameter is less than 2.5mm, an ejector pin should be used.
After product completes a forming cycle, mold is opened, and product will be wrapped on one side of mold, which must be removed from mold. This work must be completed by ejection system, which is an important part of entire mold structure. Generally consists of ejection, reset and ejection guidance.

 

There are various ejection system, which are related to shape, structure and plastic properties of product. Generally there are ejectors, ejectors, push plates, ejection blocks, and pneumatic compound ejection.

 

Figure 8.1 Structure of ejector system

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 parting mechanism.
② Ejection force and position balance ensure that product will not deform or burst.
③ Ejector pin must be set at a place that 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 not too close to insert or core. For box-like deep cavity molds, side resistance is the largest, top and side ejection methods should be used at the same time to avoid product deformation and bursting.
⑥ When there are thin and deep stiffeners, pushers are usually set at the bottom.
7 At product inlet, avoid setting ejector pins to avoid cracking.
⑧ For thin products, set a ejector pin on manifold to take product out.
9 Fit between ejector pin and thimble hole is usually a clearance fit. If it is too loose, it will easily cause burrs. If it is too tight, it will cause seizure. In order to facilitate processing and assembly, reduce friction surface, generally, a mating length of 10 to 15 mm is reserved on movable mold, and remaining part is enlarged to 0.5 to 1.0 mm 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, and it must be determined according to ejector size, shape and position.

In the structure of injection mold, quality 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 link in ejection design. Ejection force and ejection resistance are used to optimize ejection type, number and ejection position.

Ejector rod is the simplest and most common form of ejector. Because of its convenient manufacturing, processing and repair, and good ejection effect, it is most widely used in production. However, circular ejection area is relatively small, which is prone to stress concentration, product penetration, and product deformation. Avoid using in tubular box products with small demolding gradient and high resistance. When ejector pin is relatively slender, generally a stepped ejector pin is provided to enhance rigidity, avoid bending and breaking [29]. Ejector rod structure, as shown in Figure 8.2, 8.3, 8.4.

 

Ejector tube is also called ejector sleeve, it is suitable for products with ring, tube or center hole. It is ejected with uniform contact force, which will not deform product and will not leave obvious ejection marks. It can improve concentricity of product. However, avoid using products with thick and thin thickness to avoid damage due to processing difficulties and weakened strength.

Push plate is suitable for a variety of containers, boxes, cylinders and thin thin products with a central hole. Ejection is smooth and uniform, ejection force is large, leaving no ejection marks. Generally there is a fixed connection to prevent push plate from being pushed down during production or demoulding, but as long as guide post is long enough and ejection stroke is strictly controlled, push plate may not be fixed.

When side wall of parts has a concave-convex shape, a side hole, and a buckle, side core must be pulled out before mold is ejected from mold. This mechanism is called slider. As shown in Figure 3.2.8, outer hole of parts requires core pulling of rear mold. As shown in Figure 3.2.9, if inner groove of parts is ejected obliquely and top opening distance is not enough, an inner slider must be used.

 

 

In addition, ejection mechanism using oblique ejection, ejection and core pulling to finish at the same time is called inclined ejection. For parts that need to be cored on parts, when space for slider is not enough, lifter mechanism can be used to complete it. In lifter 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 side walls of plastic parts are concave, there is a bone hindrance and insufficient height on inner side, front mold of outer wall and inner wall must be ejected obliquely.

 

As shown in Figure 3.2.12, there must not be any clamps around the side holes of parts. Side holes must be pulled out of front mold slider, and buckle should be ejected obliquely.

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 is the most common ejection method. Ejector-type ejection elements, including round ejectors, shouldered ejectors, flat ejectors, and push pins. Precautions for selection of ejectors are as follows:
1. Prevent deformation or damage of plastic parts, correctly analyze size and location of plastic part's adhesion to mold cavity, select a suitable demoulding device so that ejection force is applied to plastic part with the greatest rigidity and strength, that is, as far as possible against wall, bone, and column, area of action is as large as possible (that is, choose a large diameter ejector pin) to prevent deformation or damage of plastic parts.
2. Structure is reasonable and reliable. Ejection mechanism should work reliably, be flexible in movement, easy to manufacture, replace, have sufficient strength and rigidity.
3. If diameter of ejector pin is less than φ2.5 and position is sufficient, a shoulder ejector should be used; if pusher wall is below 1mm or pusher wall diameter ratio is ≤0.1, pusher with a shoulder should be used, fixed part should be as large as possible. Effective mating length of ejector pin = (2.5 ~ 3) D, minimum must not be less than 8mm, we generally take 20-25mm during production process.
4. Try not to place ejector pin on insert splicing place
5. It is recommended to use a flat ejector pin for long-arc glue position above 10mm. The shorter flat body part, the better strength and easier processing. Length of cylindrical part should be indicated in the design specifications; for pipe positions above 10mm, it is recommended to use a push tube to eject.
6. For occasion with oblique ejector pin, in order to prevent product from sliding with oblique ejector pin, surface of ejector pin near oblique ejector pin should be ground with a "+" slot.