EJECTION

EJECTION

7.1    Introduction:-

All thermoplastic materials contract as they solidity which means that the moulding will shrink on to the core which forms it.  This shrinkage makes the moulding difficult to remove.  The amount of shrinkage depends on the material used. It is normal practice, therefore, to provide some means by which the moulded part can be positively taken out (ejected) from the core.  Facilities are provided on the injection machine for automatic actuation of ejector system and this is situated behind the moving platen. Because of this, the mould’s ejector system will be most effectively operated if placed in the moving half of the mould.  Thereby the core always most satisfactorily is located in the moving half.  

The ejector system consists of

1. Ejector grid
2. Ejector plate assembly
3. Ejector.

7.2    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.

3    The circular support block 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.

1. It is simple and cheap to manufacture.
2. It provides good support to the mould plate on a small mould.
3. A rectangular type ejector plate can be used.
4. 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.

c)    Circular support block grid

In this design, circular support block or pillars are used to support the mould plate only.  This system is used for large moulds when no extra support would be gained by including rectangular blocks as well.

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 continue 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.  

Ejector plate

The purpose is to transmit the ejector force from the actuating system of the injection machine to the moulding through an ejector element.  The force required to eject a moulding is appreciable, particularly when the moulding 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 moulding 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.

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).

7.4    Guiding and supporting ejector plate assembly

The type of guide system used will depend largely up on the size of the mould.  For small moulds, 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.  

 

7.5    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 mould plate thereby causing the stripper plate and the ejector plate to be returned to the rear position.

Mainly two types of systems are used.

1. The push back pin return system.
2. The spring return system.

The push back pin 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.

Spring return 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.

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 they 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. 

                

7.6    Ejectors

The basic ejection technique are as follows.

• Pin ejection
• Sleeve ejection
• Stripper bar ejection
• Blade ejection
• Valve ejection
• Air ejection
• Stripper ring ejection.
• Stripper plate 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 a 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.

Valve ejection

A valve ejector is basically a large diameter ejector pin.  It is used for the ejection of large components.  This ejector applies the ejection force on the inside surface of the moulding.  

Air ejection

This is the simplest to install.  This is just easy to connect to the fixed or moving half.  The medium of actuation is compressed air.

Stripper ring ejection

The stripper ring is a local stripper plate and is used on moulds with one or two impressions only.  The stripper ring is made to seat in the mould plate.  This is actuated by Tie-rods

Stripper plate ejection

This ejection technique is used mainly for the ejection of circular box type mouldings and for mouldings with thin wall sections.  The core is a sliding fit in the cavity of stripper plate.  When mould starts to open, the stripper plate moves back with core plate once the moulding is clear of the cavity, the movement of the stripper plate is arrested, while the core plate continues the rearward movement.  The core is there by withdrawn through the stripper plate and the moulding ejected.  A stripper plate mould consists of three parts.  Core plate, Cavity plate, and Stripper plate.  When the mould is opened, there are two spaces (at X and Y) and these are termed as Day light.  The stripper plate is mounted on guide pillars.  The stripper plate must be sufficiently thick to resist the bending force applied to the plate as the moulding is ejected.  The length of the guide pillar is longer.  The reason is that the guide pillar must support the stripper plate over the complete stroke and in addition, it must still project enough to enter the Stripper plate is mounted between the cavity plate and core plate.  It is coupled to the moving mould by either bolts or chains.  When the mould is opened, the stripper plate is retained on the fixed half side until the free movement allowed by the length has been taken up.  Further movement actuates the stripper plate and the moulding is ejected.  

7.7    Method usually used for actuating the stripper plate

1. Tie-rod action

A conventional ejector plate and ejector grid system is adopted in this design.  The stripper plate is coupled to the ejector plate by tie-rods.  As the moving half moves backwards, the ejector plate and the stripper plate is arrested and the moulding is stripped from the core.  In this design, the ejector rod bush is not included as the stripper plate is guided on the main guide pillars.

2. Length – bolt actuation

In this design, the ejector plate is arrested by length bolts suitably situated with in the mould.  The fixed mould plate is recessed to accommodate the head of the length bolt and a clearance hole is provided in the moving mould plate to accommodate the nut and locknut.  This design is cheaper to produce and the mould is lighter to handle.  

3. Chain actuation

In this case, chains are used to arrest the motion of the stripper plate, instead of length bolt.  One end of the chain is connected to the stripper plate and the other end to the fixed mould plate.  When the mould is in the closed position, the chain hang down in a loop.  As the mould is opened, the chain is progressively straightened until they arrest the stripper plate movement.  This design has certain disadvantages.

• If the chain goes in between the mould faces while it is being closed, sever damage will occur.
• If one of the chains break while the mould is opening, an offset load will be applied to the stripper plate which will tend to distort the member.

4. Direct actuation

This method is possible on medium and large size injection mouldings machines where several actuator rods are used.

7.8        Ejection from the fixed half

In some cases, it become necessary to mount the core and ejector system on the fixed half.  This mould half must be sufficiently thick to incorporate an ejector grid and ejector assembly behind the fixed mould plate.  The actuating mechanism must be incorporated in the mould since the facilities are not provided on the machine for actuating from the fixed half side.

7.9    Sprue pullers

When the mould is opened, it is essential that the sprue is pulled positively from the sprue bush.  In the case of single impression, the sprue is pulled at the same time as the moulding is pulled from the cavity.  But in multi impression mould, the sprue is left in the sprue bush each time the mould is opened.  The common sprue pulling methods utilizes an undercut pin or an undercut recess situated directly opposite to the sprue entry.  The plastic material which flows into the undercut upon solidifying provides sufficient adhesion to pull the sprue as the mould is opened.  The undercut may be of the reverse taper type or the cold slug well can have straight sided and grooves cut into it.  A sprue pin is positioned behind the cold slug well so that when ejection occurs, the slug is ejected.