Design of ejection mechanism of injection mold

 The design of the ejection mechanism of the injection mold. The ejection mechanism of the injection molded part is a very important mechanism in the injection mold. The structural form of the ejection mechanism of the injection molded part needs to be determined according to the shape, size, wall thickness, precision and six elements of the injection molded part. . Since the ejection mechanism of injection molded parts is installed on one side of the movable template of the injection machine, under the premise that the cavity of the fixed mold has been opened and the core pulling of the injection molded parts has been completed, its function is to eject the molded chilled hard injection molded parts out of the mold cavity or core. Due to the thermal expansion and contraction of the injection molded parts, the cold and hard injection molded parts will be tightly wrapped on the cores of the movable and fixed molds. Generally, it is required to keep the injection molded parts in the cavity of the moving model or on the core, so a demoulding mechanism is required to push the injection molded parts with a large demoulding force out of the cavity or the core of the moving model. In order to make the injection molded parts easier to demould and save effort, and also to prevent the injection molded parts from being cracked and deformed, it is generally required that the moving and fixed mold cavities and cores should be made with a certain slope, that is, made with a demoulding slope. Degree of surface and cavity. The demoulding of thread injection molded parts requires a corresponding thread demoulding mechanism. Before the melt fills the mold cavity, it must first fill the runners, runners and gates of the injection mold. After the melt is cooled and hardened, the condensate in the gating system needs to be removed before the next injection molding can be performed. The condensate in the sprue, runner and gate is made by the gate stripping mechanism.

The demoulding process after injection molding is that the movable mold retreats to a certain distance with the movable template of the injection machine, and the demoulding mechanism of the injection machine starts to push the push rod installed between the injection mold push plate and the push plate. Then the injection molded part is ejected from the cavity of the movable model or the core by the push rod. Usually, the action of ejecting injection molded parts is done on the moving mold. Since there is no ejection mechanism on the side of the fixed template of the injection machine, only under special circumstances can a demoulding mechanism be installed on the fixed mold to realize the ejection of the fixed mold of the injection molded parts. There are also demoulding mechanisms on both the movable and fixed molds, which can respectively demould the injection molded parts from the driven and fixed molds.

1. The selection principle of the demoulding mechanism

① When designing the demoulding mechanism, different forms of demoulding mechanisms must be adopted according to the geometric shape of the injection molded part, the appearance characteristics of the injection molded part, the wall thickness of the injection molded part, the deformation of the injection molded part, and the structure of the ejection mechanism of the injection machine. . When the injection molded part is demolded, only a slight elastic deformation is allowed, and the deformation of the injection molded part cannot be caused, let alone cracks in the injection molded part.

②The thrust distribution should be reasonably arranged according to the size of the demoulding resistance, the thrust distribution should be uniform, the thrust area should be as large as possible, and it should be close to the core.

③The thrust should be set at the position where the injection molded part bears a large force, such as the reinforcement rib, the flange, the shell wall, the vicinity of the metal insert, and the vicinity of the deep hole and deep groove.

④The force on the push rod should not be too large, so as not to cause partial top cracking of the injection molded part, and the traces of the push rod should not damage the appearance of the injection molded part as much as possible.

⑤The push rod should have sufficient strength and rigidity, and the push rod cannot produce elastic deformation during the ejection action.

⑥The movement of the demoulding mechanism should be flexible, reliable and free from wrong actions. The demoulding mechanism should be easy to manufacture and easy to replace. ⑦The demoulding motion cannot interfere with other motion forms.

2. Common demoulding structure

The design of the injection mold demoulding mechanism, the structure of the injection molded parts demoulding mechanism and the threading mechanism are many types, from the form of mechanism demoulding to manual demoulding form, mechanical demoulding form, pneumatic demoulding form and hydraulic demoulding form; from The structure of the demoulding mechanism is divided into push rod ejection mechanism, push tube ejection mechanism, oblique push rod ejection mechanism, stripping plate ejection mechanism and secondary ejection mechanism. The thread removing mechanism includes manual thread removing device; rack and pinion thread removing mechanism; hydraulic transmission gear and rack thread removing mechanism; screw rod and gear thread removing mechanism;

1. One-time demoulding structure

The one-time demoulding structure refers to the mechanism that can eject the injection molded part out of the mold cavity or core with one demoulding action after the mold is opened. There are the following types of structures.

⑦The demoulding motion cannot interfere with other motion forms.

2. Common demoulding structure

The design of the injection mold demoulding mechanism, the structure of the injection molded parts demoulding mechanism and the threading mechanism are many types, from the form of mechanism demoulding to manual demoulding form, mechanical demoulding form, pneumatic demoulding form and hydraulic demoulding form; from The structure of the demoulding mechanism is divided into push rod ejection mechanism, push tube ejection mechanism, oblique push rod ejection mechanism, stripping plate ejection mechanism and secondary ejection mechanism. The thread removing mechanism includes manual thread removing device; rack and pinion thread removing mechanism; hydraulic transmission gear and rack thread removing mechanism; screw rod and gear thread removing mechanism;

1. One-time demoulding structure

The one-time demoulding structure refers to the mechanism that can eject the injection molded part out of the mold cavity or core with one demoulding action after the mold is opened. There are the following types of structures. c. There should be a distance of ≥0.2~0.5mm between the outer circumference of the push rod and the core of the mold to prevent the push rod from reaching the core. In addition, when the push rod hole is worn out, there is room for replacement of a larger cross-section push rod.

d. In principle, the front end of the push rod should be 0.1~0.2mm higher than the surface of the core or cavity, so as to avoid traces of the push rod protrusion on the injection molded parts.

e. When the push rod may collide with the active side core when it is closed, a push rod reset mechanism should be added.

f. The position of the push rod should be set on the ribs and protrusions of the injection molded parts.

g. For the injection molded part of the rectangular shell, the push rod should not be installed in the corner part because of poor heat dissipation in the corner part.

h. When a latent gate is used, the end of the push rod of the runner condensate can be slightly lower than the bottom surface of the runner.

i. Since the internal stress at the gate of the injection molded part is large and cracks are likely to occur, so try not to install a push rod at the gate of the injection molded part.

j. The bilateral gap between the push rod and the push rod hole, one is not overflowing and the other is exhaust. The gap between the end of the push rod and the matching hole, for plastics with good fluidity, the matching length is 1.5d~2d, such as polyethylene, polypropylene, polystyrene and nylon, etc., is 0.03~0.04mm; other types of plastics It is 0.04~0.05mm. The gap between the push rod and the rest of the template holes should be 0.5mm.

2) Push tube demoulding mechanism: The transmission mode of the push tube is basically the same as that of the push rod. It is suitable for pushing out small-diameter tubular injection molded parts. The ejection force of the push tube should be uniform without damaging the injection molded parts. Commonly used push tube demoulding forms, the precautions for push tube demoulding design are as follows.

①The wall thickness of ejected injection molded parts is generally ≥1.5mm;

②The commonly used material for the push tube is T8A, hard to 50~54HRC, and the small hardened length should be greater than the sum of the depth of the cavity and the push distance;

③ When demoulding is fast, the injection molded part is easy to be squeezed, and its height dimension is difficult to guarantee.

(3) Stripping plate demoulding mechanism: For the demoulding of injection molded parts with thin walls, deep cavities and no traces of push rod ejection, such as large shell injection molded parts or small shells with multiple cavities in one mold, stripping can be used The form of the demoulding of the piece board. The stripping plate has a better demoulding effect on the shape of the injection molded part. The stripping plate is used to prevent deformation of the injection molded part due to its large ejection area, uniform thrust and stable ejection. But when the shape of the moving mold core is complicated, since there is only a unilateral gap of 0.01~0.03mm between the hole of the stripping plate and the moving mold core, the processing of the hole of the stripping plate will become more difficult. There are the following points for attention in the demoulding design of the stripping plate:

① The mating surface of the stripping plate and the moving model core should be an inclined surface, and the inclination angle should be 5°. If a straight surface is used, it is not conducive to guiding, and it is easy to wear and cause the melt to overflow and produce flash.

② The structural form of the stripping plate, when the stripping plate and the push rod are combined to form a floating movement, the push rods participating in the ejection should be of the same height. ③ The large-area stripping plate should be equipped with a guide device.

(4) Core and molding insert demoulding mechanism: The structure that uses the core and molding insert of the injection mold to demould the injection molded part is generally used when the injection molded part is not allowed to have traces of the push rod. The design considerations for the core and molding insert demoulding mechanism are as follows.

① The mating surface of the core and the cavity of the middle model should be a bevel, and the bevel angle is 5°, which can reduce wear. If a straight face is used, it is not conducive to guiding, and it is easy to wear and cause the melt to overflow and produce flash.

②The structural form of the core molding insert. In order to improve the wear resistance of the middle mold cavity, inserts are made of hardenable steel with good wear resistance.

③In order to achieve a smooth parting between the middle template and the fixed template, a mold opening mechanism should be used, and it should be noted that after the moving and fixed molds are parted, the mold opening mechanism can open the middle template to prevent cracks in the injection molded parts.

④ The opened fixed and middle mold parts should be provided with a limit device to prevent the middle mold part from detaching from the support of the guide pillar.

(5) Compressed air demoulding mechanism: For deep cylindrical injection molded parts, a high degree of vacuum will be formed between the mold cavity or core. The mold form is the main form, and the demoulding of the stripping plate is assisted. (6) Combined ejection mechanism: For large and medium-sized shell injection molded parts, it is difficult to ensure that the injection molded parts will not be deformed and damaged when the push rod, push tube, loose block, stripping plate and pneumatic demoulding are simply used. No phenomenon of cracking. At this time, push rods, push tubes, loose blocks, stripping plates and pneumatic joint stripping mechanisms are often used to demould injection molded parts. The commonly used combined release mechanism is mostly used in the case of complex shapes of injection molded parts, large release force, and easy deformation during release. The so-called combined demoulding mechanism refers to the demoulding of injection molded parts by any two or more forms of push rod, push tube, loose block, stripping plate and pneumatic demoulding mechanism.

(7) Spiral demoulding mechanism: The design of the demoulding mechanism of the injection mold. Parts such as helical gears and worm gears are often formed in injection molded parts. The oblique teeth or helical teeth on such injection molded parts, if the conventional demoulding method It is difficult to demould, and the plastic gear has the characteristics of few teeth and small modulus, and often has the property of displacement. When the teeth of the gear are demolded, the ejection force required is relatively large, which often causes deformation, cracking and damage of the injection molded part, and the phenomenon that the oblique tooth profile is cut. In order to avoid such phenomena, Dongguan Machike Injection Mold Processing Factory needs to adopt a spiral demoulding mechanism. The so-called spiral demoulding movement is the push rod or push tube for demoulding. On the one hand, it needs a linear demoulding movement; on the other hand On the one hand, there is a need for rotational movement at the same time. Under the guidance of the helical core or the helical cavity, it is compounded into a helical demoulding movement that is consistent with the rotation direction and lead of the helical teeth of the injection molded part.