11.0 SPLITS
11.1 Introduction:
A moulding which has a recess or projection is termed as an under cut moulding. An undercut moulding necessitates the removal of that of the impression which form the undercut prior to ejection.
11.2 External undercut components
Any recess or projection on the outside surface of the component which prevents its removal from cavity is termed an external undercut.
There are two forms of undercut:
- The undercut which is local , in that The recess or projection occurs in one position only. Ex: The clip on a pen cap.
- The undercut which is a continuous recess or projection on the periphery of the component. Ex: the hose connector.
In either case it is necessary to split the cavity insert into two parts and open these at right angles to the line of draw, to relieve the undercut before the moulding is removed. Since the cavity is in two pieces, a joint line will be visible on the finished product.
11.3 Position of a joint line
The joint line can be positioned on any center line for a symmetrical component (Figure 11-1). For unsymmetrical components, there is only one possible position. For example the joint line for pen cap can occur on the center of the projection. An incorrect joint line will restrict the free opening movement of the split which will result in scored or cracked components. The joint line should be as invisible as possible on the moulding.
11.4 Splits
It is necessary to make the cavity insert in two parts for mouldings which incorporate an undercut. In (Fig.11-2a.) the cavity form is machined into the cavity plate. When the mould is opened, the core can be withdrawn but the undercut make its removal impossible. In (Fig.11-2b..) the same cavity form is machined into two separate blocks. These split cavity blocks are called splits. The moulding can be extracted by opening the splits. There are two basic designs where the splits are retained on the mould plate and actuated automatically. Sliding splits and angled lift splits. In both there are moving parts and it is necessary to guide the splits in desired direction. Actuate the splits and securely lock the split in position before the material is injected into the mould.
Sliding split
The splits are mounted in guides on a flat mould plate and are actuated by mechanical or hydraulic means. The splits are positively locked in their closed position by heels which project from the other half. Sliding splits can be mounted on either the moving or the fixed mould plate. The principle of the sliding action is shown in Figure 11-3.
Guiding and retention of splits
There are three main factors.
1. Side movements must be prevented to ensure that the split halves always come together in the same place.
2. All part of the guiding system must be of adequate strength to support the weight of the split and to withstand the force applied to the splits by the operating mechanism.
3. The two halves must have smooth, uninterrupted movements.
The guiding function is accomplished by providing a T-shaped slot on the
mould plate and using a shouldered split.
Mould plate design (Figure 11-4)
A T-shape can be produced by one of the following methods.
1. By using a T-slot milling cutter
2. By attaching separate guide strips to a flat mould plate.
3. By machining a U shaped slot on mould plate and the attaching flat steel strips
Type I
(Fig.11-5) shows the two types of basic shouldered split design. The two part construction is advantageous for larger splits.
Type II
In this design, a male T shaped projection is provided below the base of the split. The guiding arrangement is directly below the split, which permits a narrower mould. As the side of the split is in line with the edge of mould plate, a cam plate can be fitted close to the split for actuation (Fig.11-6)
Type III
Splits are provided with projection from the base which slides in the slot on mould plate. General splits retention is achieved by splits shoulder and T slots on the mould plate
Methods of operation
The splits are actuated by various types of cams like finger cam, dog-leg cam. And cam track method. As the mould is opened, the cam, attached to the fixed half, cause the splits to slide across the moving plate and when the mould closes, the splits are progressively closed. The cam generally lose contact with the splits as the mould opens and should either split be moved out of position prior to the mould closing serious damage will occur. To avoid this, some safety features are incorporated in the design. Another method actuating the splits is by the use of compression springs, but as these can be used only to open the split, the locking heels are used to close the splits. This system, while simple and cheap, has limitation as only relatively small movement can be obtained. The splits can also be actuated hydraulically. The main advantage of this system is that large splits movements are practicable. Various actuating methods will now be discussed.
11.5 Split Actuation:
Finger cam actuation
Finger cams are hardened pillars mounted at an angle in the fixed mould plate. The splits have corresponding angled circular holes to accommodate these finger cams. The split has shown in closed position in Figure 11-8. As the mould opens, finger cams force the splits to move outward. Splits movement eases as the contact with the finger cam is lost. Further movement of moving half causes the ejector system to operate. On closing,
the finger cam re enter the holes in the split and forces the split to move inwards. The final closing is done by the looking heals. The traverse by each split across the face of the mould plate is determined by the length and angle of the finger cam. Figure 8-9)
The movement M = (L sin Q) - (Cl cos Q)
Finger cam Length = (M/Sin Q) + (2c) I Sin 2Q
Where
M = Split movement
Q = Angle of finger cam
L = Working length of finger cam
C = Clearance
The split movement must be kept to a maximum at the same time ensuring that the moulded part can be easily and quickly removed from the mould The clearance C has two purposes
- It ensure that the force which is applied to the split during the injection is not transferred to the relatively weak cam
- It permits the mould to open a small amount before the split are actuated In certain cases this movement can be used to withdraw the core from the moulding The amount of delay movement “D” can be determined by the relationship
D = Cl Sin Q
An angle of 10 degrees is suitable for finger cam A maximum of 25 degrees is permissible for excessively longer cams The diameter increases with the angle The lead angle at the front is normally (0 + 5 deg) One or two finger cams are used to operate each split depends on the split size
One method of fitting the finger cam into the mould plate is shown in Figure11-8. A backing plate is not required as the finger cam is supported by the machine platen when the mould is fitted on to the injection moulding machine
Dog-leg cam actuation
This method of actuation is used where a greater split delay is required. The dog-leg cam is of a general rectangular section mounted in the fixed mould plate. The split is incorporates a rectangular hole, the operating face of which has a corresponding angle to that of cam.
In this design, the split do not start to open immediately when the mould is opened because of the straight portion of the dog-leg cams (Fig.11-10). The moulding which is encased in the splits will be pulled from the stationary core. Further movement of the moving half causes. The actuation of the splits theory by releasing the moulding. A typical mould design is given in Figure 11-11.
The relevant formula for calculating the opening movement, the length of cam and the delay period are given below.
M = La tan Q-c
La = (M+c) /tanQ
D = (Ls-e)+(c/tan Q)
where
M = movement of each split
La = angled length of cam
Ls = straight length of cam
Q = cam angle,
c = clearance
D = delay
e = length of straight portion of the hole.
This method of actuation utilize a cam track machined into a steel plate attached to the fixed mould half. A boss fitted to both sides of the split runs in this track. As the mould opens, the bosses follow the cam track and open the split.
The relevant formulae for calculating the distance traversed by each split, the length of cam track, and the delay period (Figure 11-12) are given below.
M = La Tan Q-C
Angled length
Of Tan Q
C 1 1
Delay period D = La+ + R x Tan Q (Tan Q Sine Q
Where
Q = Cam track angle
C = Clearance
R = Radius of the boss
LS = Straight length of cam
Details of the operation are given in Figure 11-12. The bottom drawing shows a typical cross section through this type of mould. The splits are mounted on a mould plate. The bosses , screwed into the side faces of the split, protrude in to the cam track plates. The latter are securely attached to the side faces of the fixed mould plate. There should be a small clearance between the cam track plate and the moving mould half. The figure (11-12a) the mould is shown in the closed position. As the mould opens, the bosses follow the cam track and thereby cause the split to open As the mould closed, the bosses reenters the cam track and the splits are progressively closed
Spring actuation
Compression springs are used to force the splits apart and angled faces of the chase Bolster is used to close them The design is limited to mouldings with shallow undercuts. The splits open immediately as the mould parts and no delay is possible. The moulding must be in the moving half in order to eject positively. A typical basic design is shown in Figure 11-13. The splits are mounted on the mount plate and retained by guide strips. Studs project from the base of the split into a slot machined in the mould plate. The outward movement of each split is therefore controlled by the length of the slot. A compression spring is fitted between the studs in a link-shaped pocket situated in the lower mould plate. The sequence of operation is shown in Figure 11-14.
Hydraulic actuation
In this design, the splits are actuated hydraulically and is not dependent on the opening movement of the mould. The splits can be operated at any specific time. Figure 11-15 and 11-16
11.6 Split
It is essential to hold the splits rigidly during the actual injection phase, as the high pressures developed with in the impression will tend to force them apart. Sliding splits can be conveniently locked in position by the use of a chase bolster. Each split will have a sloping or angled face accurately notching the angled face of chase - bolster. Two basic designs of chase bolster are:
1. Open channel type
2. Enclosed channel type
Open-channel chase bolster
Simplest of the designs is by machining a channel with angled sides across the width of a plate The projections formed are called locking heels. Wear plates are used, to resists wear. The height of the locking heel should be at least ¾ the depth of the split. Thus the gap between the chase bolster and the moving mould plate depends on the height of the split. Open-channel chase-bolster can be made as an assembly by attaching individual heel block to a plate. Refer Figure 11-17. And 11-18
Enclosed chase bolster
An enclosed chase bolster is preferred for deep splits as it results in a more rigid structure. The chase bolster is made by machining a pocket which can be of tapered circular or tapered rectangular form (Figure 11-19). The circular form is less expensive to manufacture but it is easy to fit wear strips in a rectangular form.
11.7 Split safety arrangements
It is necessary to provide certain safety features in moulds with cam method of actuation. When the mould is fully open the splits are not in contact with the cams. Therefore the splits may be moved out of alignment by shock, vibration or even gravity. To safeguard against opening by gravity, the splits should operate horizontally with respect to the machines. To prevent the movement of the splits by shock or vibration, following method can be used.
a) Spring detent method
A spring loaded plunger is fitted below the surface of the split When the split is open the plunger is engaged in small central depression which retain the split nominally in that position. The distance between the plunger and the depression is equal to the movement of the split (Fig.11-20)
b) Spring loaded method
The splits are spring loaded so that after they are actuated they remain in the open position. A stud fitted to the underside of the splits is free to move in a recess in the mould plate. A spring is fitted between the stud and the end of the slot (Fig.11-21)
An alterative spring loaded method which is suitable for large type of split is shown in Figure 11-22. A bolt is fitted to the locking angle of the split. The bolt extends through a plate securely attached to the mould plate. A spring is mounted between the plate and the head of the bolt.
11.8 Angled - lift splits
In this design, the split are mounted in the chase bolster which forms part of the moving half of the mould. The splits are caused to move out with an angular motion , the outward component of which relieves the undercut portion of the moulding. The splits are normally actuated by the ejector system. A typical design is shown in Figure 11-23. A substantial chase bolster which may be the enclosed or open channel design, locks the splits against the applied injection force. When the open channel design is used, it is necessary to provide some means of alignment between the two splits halves as the ends of the splits are not located.
The alternative system with respect to particular actuating methods are as follows
1 Angled guide dowel actuating system
2 Cam track actuating system
3 Spring actuating system.
Angled guide actuating system:
A typical mould design is given in Figure 11-24. The mould is shown closed and open. In this system, the ejector system is utilized to open the splits. Guide dowels are fitted at an angle to the underside of each split. These guide dowels pass through holes machined at an angle , in the enclosed chase bolster. These holes are often bushed. When the ejector system is actuated, the relative movement between the ejector plate and the enclosed chase bolster causes the guide dowels to be moved forward, and, as these fitted at an angle, the splits are caused to open.
The opening movement of the splits in this design shown in Figure 11-25 is controlled by a cam track. When the splits are actuated, studs fitted to each end of the split slide along this cam track. Actuation of the splits is by means of a conventional pin ejector system. The number of ejector pins used depends upon the size of the split.
Spring actuating system:
A simple method of actuating angled lift splits is by the use of spring. The .forward movement of the split is limited, and because of this feature, the design is used for components which have comparatively shallow undercuts.
The figure 11-26 shows a part section of the system.
Since we have different method of split actuating method, the decision as to which operating method to adopt depends on the following
1 amount of split movement required
2 whether a delay period is required
3 length of a delay period
4 ease with which the moulding can be removed
5 whether a short or long production run is anticipated
6 whether the available machines programmed for ancillary cylinder control
7 whether moulding inserts are to be incorporated
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