24.0 EJECTION SYSTEM IN DIE CASTING DIES
24.1 Introduction:-
The ejector system consists of
1. Ejector grid
2. Ejector plate assembly
3. Ejector.
24.2 Ejector grid
The ejector grid is that part of the mould which supports the mould plate and provides a space in to which the ejector plate assembly can be fitted and operated. The grid normally consists of a back or bottom plate on to which a number of conveniently shaped support blocks are mounted. There are three alternative designs
a) The in-line ejector grid.
b) The frame type ejector grid.
a) In-line ejector grid
This consists of two rectangular support blocks mounted on a back plate. The ejector plate assembly is accommodated in the parallel space between two support blocks. The design is quit suitable for small types of mould where the overall size of the ejector plate assembly does not necessitate the support blocks being fitted a great distance apart. In this design, to avoid the possibility of the mould plate being distorted by the injection force, a thick mould plate or extra blocks are often used.
b) Frame type ejector grid
The most common type is the rectangular frame type ejector grid. It is constructed by four support blocks mounted on a back plate. This type is commonly used for the following reasons.
a. It is simple and cheap to manufacture.
b. It provides good support to the mould plate on a small mould.
c. A rectangular type ejector plate can be used.
d. The ejector plate assembly is completely enclosed thereby preventing foreign bodies entering the system. When the outside of the mould is plate is circular, it is often convenient to design a corresponding shaped ejector grid. A circular support frame is slightly more expensive to produce than a rectangular type. As the size of the mould plate increases, the effective support provided by the above type of ejector grid decreases. This situation can be improved by providing additional local support block.
24.3 Ejector plate assembly
The ejector plate assembly is that part of the mould to which the ejector element is attached. The assembly is contained in a pocket formed by the ejector grid, directly behind the mould plate. The assembly consists of an ejector plate, a retainer plate, and an ejector rod. One end of the ejector rood is threaded and it is screwed in to the ejector plate. The ejector rod is guided in an ejector bush fitted in the back plate. Now in new machines, the rods are directly connected to the machine itself. The back plate is relieved in place of bush.
The moving half or ejector half of the mould is mounted on the moving platen of the machine. The actuating rod can be adjusted for various alternative ejector strokes. When the mould opens, as the ejector rod as some point of the stroke, strikes the actuating rod. The entire ejector assembly continues to move to left until the opening stroke is complete. This relative movement between the ejector plate assembly and the mould plate is necessary to operate the ejector element. In normal arrangement for smaller type of ejection system, the machine’s actuator rod passes through the center of the moving platen. On larger machines, several actuator rods are used so that a balanced force can be applied to the ejector plate. In this case, the actuator rods push directly on to the ejector plate. A separate method of guiding and supporting the ejector plate assembly is necessary when ejector rod bush are not incorporated.
a) Ejector plate
The purpose is to transmit the ejector force from the actuating system of the die cast machine to the component through an ejector element. The force required to eject a component is appreciable, particularly when the component is deep and incorporate little draft. Ejector plate must be sufficiently thick not to deflect to any significant extant. It may fail in operation if it is too thin. Deflection tends to occur at the beginning of the ejector stroke when there is maximum sticking between the component and the core. If the ejector plate deflects, side forces are applied to the ejector elements which results in increased wear in the mould plate holes, bend ejector pins and in extreme cases in the core plate seizure of the system.
b) Retainer plate
The purpose of the retainer plate is to retain the ejector elements. Thickness of the plate is governed by the depth of the head of the ejector element it retains. Retainer plates are normally made of mild steel (MS).
c) Guiding and supporting ejector plate assembly
The type of guide system used will depend largely up on the size of the Die. For small dies, the ejector rod is guided in a ejector rod bush securely fitted in to the back plate, which maintains alignment and provides support for the ejector plate assembly. An alternate method for aligning and supporting the ejector assembly is by providing guide bushes incorporated with in the assembly, and guide pillars attached to the back plate. These guide pillars can be used as local support blocks also.
d) Ejector plate assembly return system
In a stripper plate mould, the stripper plate is directly connected to the ejector plate by means of tie – rods. When the mould closes, the stripper plate strikes the fixed die plate thereby causing the stripper plate and the ejector plate to be returned to the rear position.
Mainly two types of systems are used.
a. Bumper return system.
b. Retractive return system.
The bumper return system
Push back pins are large diameter ejector pins which are fitted to the ejector plate assembly. In the moulding position, the push back pins are flesh with the mould plate surface. In the ejected position, the push back pins protrude beyond the mould plate surface. Thus, when the mould is in the process of being closed, the push back pins strike the fixed mould plate (cavity plate) and progressively return the ejector plate assembly to the rear position.
Retractive return system
In this system, an ejector rod is fitted to the ejector plate by means of thread. With this method, the return and forward movement of ejector assembly is positive. The machine will push the system and pull the system.
For small moulds, where the ejector assembly is of rigid construction, a spring can be used to return the ejector plate assembly. The spring is fitted on the ejector rod with a cap to keep in position.
e) Stop pins (stop pad)
With a large ejector plate or large ejector bar system, it is preferable to incorporate stop pins on the under side of the ejector plate, just below the return pins. The heads of the stop pins should be relatively larger to prevent the possibility of them being hopped in to the back plate. Stop pins reduce the effective seating area, thus preventing the possibility of the ejector elements remaining slightly above due to the foreign materials being trapped behind the ejector plate.
24.4 Ejectors
The basic ejection technique are as follows.
a. Pin ejection
b. Sleeve ejection
c. Stripper bar ejection
d. Blade ejection
Pin ejection
This is the most common and simple type of ejection. The moulding is ejected by the application of a force by ejector pins. In operation, the ejector plate assembly to which the ejector pins are attached is moved forward relative to the mould plate. Thus the ejector pin pushes the moulding from the core. The working diameter of the ejector pin must be a good slide fit in it’s mating hole in the mould plate in order to avoid plastic material creeping through the clearance. The ejector pin is shouldered and proper seating is provided must allow the ejector pin to float. The reason for this arrangement is as follows.
The direction of movement of the ejector pin is controlled by the hole in the core insert. If the hole is not at right angles to the core insert face, then the arrangement allows a relative lateral movement between the ejector pin and retainer plate, thus avoiding the chance of bending or breaking.
In it’s rear position, the top surface of the moulding face pin should be flush with the top of the core. If the pin face is projecting above, an indentation will be formed on the moulding. If it is sunk below the core, a boss on the moulding is formed. The top surface of the pin must have the same finish as the rest of the impression as it froms the part of the moulding surface.
The location and the number of ejector pins used depends on the component size and shape. Aim is to eject the moulding with as little distortion as possible. The ejector pin should be located such that the moulding is pushed off evenly from the core. Sudden change in shape tends to unbalance ejection, therefore, the ejector pins should be located adjacent to these points. It is better to use maximum number of ejector pins in order to avoid distortion. When a small diameter ejector pin is required, it is desirable to keep the working length to minimum. This is called a stepped ejector pin.
Pin ejection is the cheapest of the mechanical ejection methods. Ejector pins are hardened to obtain a wear resisting external surface.
Sleeve ejection
In this method, the moulding is ejected by means of a hollow ejector pin which is called sleeve. It is used in the following circumstances.
1. For the ejection of certain type of circular mouldings.
2. For the ejection of circular bosses on a moulding of any shape.
3. To provide positive ejection around a local core forming a round hole in the moulding.
The sleeve is a sliding fit in the core insert and on the core pin and rear end is fitted to the ejector retainer. Large diameter core pins can be fitted directly on to the back plate. The sleeve and the core pin must be allowed to float because the outside diameter of the sleeve is a slide fit in the core insert. When the ejector assembly is activated, the sleeve is moved relatively to the core and to the impression and the moulding is ejected. This is an effective method because the ejector force is applied to a relatively large surface area. This design is restricted to circular type.
Stripper bar ejection
This method is an extension of butting face ejector pin principle. The ejector element is caused to push against the bottom edge of the moulding. This method of ejection is suitable for thin wall box type mouldings because of the effective ejection area obtainable is greater. A single bar is used along each wall of the moulding. This reduces marks left on the surface of the moulding. The stripper bar is coupled to the ejector plate by a tie rod.
Blade ejection
The main purpose of the blade ejector is for the ejection of very slender parts such as ribs and other projections which can not be satisfactorily ejected by the standard type of ejector pin. A blade is basically a rectangular ejector pin.
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