Mouding Internal Under cuts

13.0 INTERNAL UNDERCUTS & THREADS

13.1 Introduction:

An internal undercut is any restriction which prevents a moulding from being extracted from the core in line of draw. Various methods are used for relieving internal undercuts; the specific design adopted depends upon the shape and position of the restriction. Figure below shows examples of components with internal undercuts; where necessary part of the component only is shown to reveal the undercut portion.

(a) (b) (c)

(f) (g) (h)

Internal undercut components.

To illustrate undercut, only half component is shown in some examples

If the undercut is local, the restriction may be incorporated on a form pin which is moved forward during the ejection phase. In the simplest version of this design, the form pin has a straight action (similar to an ejector pin) and the moulding is subsequently removed from the form pin at right angles to the mould's axis. The alternative method is to mount the form pin at an inclined angle so that during ejection the top face of the pin moves inwards, thereby relieving the undercut.

When an undercut extends completely along one internal wall of a component, it may be moulded by incorporating a core which is constructed of two parts. The part which incorporates the restriction is subsequently moved forward during ejection, thereby permitting the moulding to be extracted. The movement of the moving part, termed the split core, may be either straight or inwardly inclined. This latter design may also be used for components which incorporate undercuts on opposing faces as illustrated by the components (g) and (d).

The conventional side-core design (Chapter 12) may be used for certain internal undercut components but this method is restricted to those components which incorporate an open side, as for example (e) and (f). Certain types of undercut components may be stripped from the core and as this method allows for relatively cheap mould construction it should not be overlooked. Two typical components which would be moulded in this manner are shown at (g) and (h).

Each of the above designs for dealing with the internal undercut type component will now be dealt with in more detail.

13.2 Form Pin

A component which has a local undercut portion can be successfully moulded in the conventional mould by incorporating the undercut form on a form pin. Two examples are shown in Figure (a)

( c) (d) (b)

Form pin for undercut components

The top moulding (a) has a small projection on one of the inner walls, while the lower

moulding (b) has a recess in a similar position. Enlarged views of the form pins which form these restrictions are shown at (c) and (d), respectively. Note that the face of the pin forms part of the impression and must, therefore, have the same surface finish as the remainder of the core. The form pin will leave a witness mark at Y on the mouldings and careful mould fitting is essential to prevent flash occurring at this point. With this design there are two basic alternatives; the form pin can either have a straight action, i.e. as for an ejector pin, or it can have an angled action.

This design is normally used for components which incorporate an undercut on one internal wall only. It is impracticable to use this design for a component which has an undercut on two facing walls

13.2.1 Form Pin – straight action:

The mould assembly for this design is identical to the basic pin ejection design except that one (or more) of the ejector pins is replaced by a form pin which contains the undercut form. An enlarged view of the relevant parts is shown in the following figure which also illustrates the principle of operation.

(a) Mould closed (b) Mould opened

The closed mould is shown at (a) and the form pin, in its rear position, forms part of the impression. The moulding shrinks on to the core and is withdrawn from the cavity when the mould is opened. The form pin is moved forward during ejection allowing the moulding to be removed in the direction indicated by an arrow in (b). The form pin is attached to the ejector assembly in a similar manner to an ejector pin, but a locking dowel pin must be incorporated to prevent rotation

13.2.2 Form pin - angled action

The basic feature of this design is that the working face of the form pin is caused to move inwards relative to the core during ejection, thereby relieving the undercut. It can be used for components which incorporate internal undercuts on one or more walls. The basic design and method of operation is illustrated in Figure.

(a) Moulding position (b) Ejected position

The form pin which incorporates the undercut form is fitted at an inclined angle Ii1 in the mould plate and it is maintained in contact with the ejector assembly by means of a key plate suitably attached to the mould plate. When the ejector assembly moves forward (relative to the core) the undercut is relieved by the lateral movement of the form pin and the moulding can be lifted or blown clear

The form pin rests on the ejector assembly in this design and, because of the angle; a relative movement takes place between them during ejection. Because of this movement, either the retaining plate should be hardened or a local insert incorporated below the form pin to prevent undue wear occurring. The actual movement across the retaining plate is equal to the withdrawal movement at the top of the form pin.

The basic spherical headed form pin design shown above figure is not favoured by many designers because there is only point contact with the retaining plate and the following alternative designs are commonly used.

The following figure shows a spherical headed form pin mounted in a nest block

which is free to slide on the retaining plate. All the other features are the same as for

Form pin-angled action – Head design 1. Form pin- angled action – head design 2

the basic design. The advantage of using the nest block is that a greater contact surface is obtained while the self-aligning feature of the spherical end is retained.

Figure (Head design 2) shows another variation on the basic approach but in this design the form pin head is angled to present a large contact surface to the retaining plate. Note that very careful fitting is necessary with this design to ensure that the improved surface contact is in fact achieved. Though it is not shown (for reasons of clarity), the form pin is spring loaded as in the previous designs.

In the design given below, the form pin, which has a standard ejector pin head, is attached to a slide block by a small retaining plate. The slide block is of a general T-form and is mounted in guides which are attached to the ejector plate.

Rotation of the form pin is prevented by the dowel pin. When the ejector plate is actuated, because the form pins is constrained to move at an angle, the slide block will move in direction V across the face of the ejector plate. Similarly, as the ejector plate is returned, the slide block will be progressively returned to its original position. It does not rely on a spring to return the form pin to the moulding position and, because of the guides, permanent flat sliding surfaces in contact are assured.

13.3 Split Cores

The split core design is used for components which have extensive internal undercuts that cannot be incorporated on a form pin. Typical components in this category are shown in Figure to.1b, d, g. To permit this type of component to be moulded

successfully it is necessary to manufacture the core in two (or more) parts and that part in which the undercut is formed is subsequently moved forward during ejection to permit the moulding to be extracted. This split core, as it is termed, may be moved forward either in a straight plane or an angled plane. In both designs the core is constructed as shown in Figure above. The movable part, the split core, nestles in a pocket machined in the moving mould plate. The sides of the split core and of the complementary pocket walls are angled as shown. This minimizes the wearing action which would occur if the sides were straight. When in position, the fit between the split core and the main core must be precise to prevent the very undesirable feature of flash developing between the two parts (i.e. along line Y). The main core may be machined from the mould plate, as with the integer method, or it may be incorporated as an insert. This latter method is normally preferred as it permits simpler machining of the split core pocket.

13.3.1Split cores-straight Action

This design is used for components which incorporate an external undercut on one wall only. It is not a practicable design for components which have undercuts on opposite walls. The design and operating principle are shown in Figure given below. The component chosen to illustrate this design is box-shaped and incorporates a projecting bead on one of the inside walls. This bead is formed by a recess which is machined into the split core as shown in the lower drawing. The split core is directly attached to the ejector assembly by means of a tie rod, so that when the ejector assembly is actuated the split core moves forward in a straight line. The moulding can then be removed at right angles to the mould axis (i.e. in direction V). Face X is used as an ejector face but additional ejection will be required on the other moulding walls as indicated.

13.3.2 Split cores-angled action

In this design the split core is caused to move inwards during the ejection stroke, thereby withdrawing the restriction and allowing the moulding to be extracted in line of draw. The advantage of this is that the withdrawing action is automatic and the moulding does not have to be removed at right angles to the mould's axis as in the previous case. This design can therefore be used for components with undercuts on opposing faces in addition to being used for components with an undercut on one wall only.

There are two principal ways by which the split core can be actuated and there are many variations of these two methods. The first design is. shown in Figure above. The split core is constrained to move at an angle with respect to the line of draw by means of guide pins attached to it as shown. The operating angle (Ø) must be less than that chosen for the fitting angle (θ) to prevent fouling when the core is actuated. The guide pins are not attached to the ejector assembly as, due to the angle, there is relative movement between these two members. When the mould opens and the ejector plate is actuated, the dome-headed guide pins and the split core to which they are attached move forward inwardly inclined, thereby relieving the moulding's undercuts (indicated by the chain-dotted line). When the mould is closed the split core is returned nominally by the spring but it is finally held in position by the fixed mould half acting on face x.

The alternative design is shown in Figure below.

Here again the split core is constrained to move in a path at an angle to the line of draw by means of guide pins. But in this design the pins are used only for guiding and they are independent of the ejector system. The split cores are actuated by ejector pins fitted to the ejector assembly in the normal way. When the mould opens and the ejector plate is actuated, the ejector pins act on the base of the split core, and because of the angle pins the split core moves forward obliquely, thereby relieving the undercuts. There is relative movement between the top face of the ejector pin and the split core, therefore the base of the latter must be sufficiently wide (W) to allow for this. When the mould is closed the ejector assembly is returned to the rear position by pushback pins (not shown).

When undercuts occur on faces of a component directly opposite each other care must be exercised in the design to ensure that the two split cores do not foul each other as they are moved obliquely forward. The limit of this movement is determined by the point

at which the split cores just touch. From the illustration given below, it will be observed that the maximum forward movement of the split cores is dependent on both the operating angle (θ) and the width (G) of the main core.

13.4 Side Cores

Some components with internal undercuts can be moulded using a conventional side core design as discussed in Chapter 12. However, this method is limited to components which have at least one open side. This permits the side core to be withdrawn through this side to relieve the undercut and allow the moulding to be ejected in-line-of-draw. Typical examples of components of this type are shown at (e) and (t), Figure shown in first page

13.5 Stripping (Jumping) Internal Undercuts

A simple and effective way of dealing with a particular type of internal undercut is to strip (jump) the moulding off the core. Whether or not a moulding can be moulded in this way depends on several factors which include: (i) the shape of the undercut, (ii) the elasticity of the material, and (iii) whether the external form permits expansion during ejection. Ideally the undercut should be of the form shown in Figure shown below but relatively sharp-cornered undercuts are stripped in practice, an example being shown at (c). (Note that while the components illustrated

Edge shape limitations for striping

Stripping internal undercuts – valve ejector method

13.6 Moulds for threaded components

Thread is form of undercut. Thus increases the intricacy of the design. The extend of complication varies and it is depending on number of factors as follows

1) Type of thread:Thread may be internal or external; it can be continuous or discontinuous.

2) Method: Thread form is moulded or incorporated by the use of a metal insert.

3) Type of production:Whether it is manual, semi-automatic or fully automatic

4) Other consideration:Does the required thread form allow for stripping etc.

13.6.1 Component design:

The moulded component which incorporates a thread may be primarily classified as external (male) and internal (female). An example is shown in the following figure.

13.7 Moulds for internally threaded components:

The internal thread comes within the broad definition of an internal undercut in that the threaded forms a restriction which prevents the straight draw removal of the component from the core. There are a number of alternative designs of the mould are available.

a) Single interrupted thread design

b) Stripping (jumping off) thread design

c) Split core design

d) Fixed threaded core design

e) Loose threaded core design

f) Unscrewing mould design

13.7.1 Single interrupted thread design

This is the one exception to the statement that a thread forms a restriction which prevents straight draw removal of the moulding from the core. Providing that a single thread is adopted and that this thread is non-continuous (i.e. less than one complete revolution) the thread can be moulded in-line-of-draw. Consider the example illustrated in Figure. This shows a hollow cylindrical moulding with one internal thread. Providing that a sufficient gap is provided between the beginning and end of the thread, the parting surface of the two core faces may be stepped and butted together as shown in Figure. This feature allows for straight draw removal of the moulding. Note that this drawing of the relevant mould parts is purely for explanation purposes only. It is not a cross-section through the centre of the impression. A witness line will be visible on the

Internal surface of the moulding. Box-shaped cylindrical mouldings are more difficult to accommodate under this heading. However, providing the component designer accepts some latitude in his design in that he does not insist upon a completely solid base, then this component too may be moulded in-line of draw.

An illustrative example of an internally threaded box type component is shown in figure below. Note that the single thread is interrupted in two places, and that the thread consists of two short lengths, one on either side of the vertical centre line (as drawn).

A local cross-section through the relevant part of the mould impression is shown. Note that two local core inserts (1) protrude through the base of the impression to form the underside of the thread. These local cores are accommodated in complementary slots in the core (2) as shown. Very careful mould fitting is essential for this design. The shape of the local core inserts is approximately trapezoidal in form and this feature will be apparent upon inspection of the moulding. Once again, a witness mark will be apparent on the moulding's internal surface.

13.7.2 Stripping (jumping) internal threads

The internally threaded component may be stripped (jumped) from the core using the basic stripper plate design providing

a) a roll thread is required

b) The plastic material has sufficient elasticity during the ejection phase..

c) the moulding must be free to expand during ejection to permit the moulded undercut to ride over the restriction on the core, means that the outside form of the component must be such that it can be formed in a cavity which is fully contained in one half of the mould. (If part of the moulding form is incorporated in the stripper plate it would tend to restrict the required expansion.)

A mould of this type is illustrated in Figure below. This diagram shows a single impression mould for an internally threaded cap. The moulding is formed by the cavity and the core. Ejection is normally by means of a stripper plate as shown. Let us briefly consider the mould operation.

13.7.3 Split core design

Providing the threaded form is discontinuous, and then the split-core design discussed in this section for undercut type components may be used. Note that this design is not restricted to a single thread but may be used for an extended threaded section.

13.7.4 Fixed threaded core design

This mould design is the male counterpart of the threaded cavity design. Figure 7 shows a section through a simple mould of this type. The thread form is incorporated on a non-rotating core attached to the moving mould plate. An integer type cavity forms the external shape of the moulding.

In operation, when the mould is opened, the moulding remains on the core and is subsequently unscrewed by the operator. The external shape of the component must be such that it provides facility for this, commensurate with the devices available to the production department concerned.

The advantages of this design, compared with the unscrewing type mould design, are as follows:

(i) Mould cost: considerably cheaper (no unscrewing or ejector mechanism is required).

(ii) Servicing: no moving parts within the mould (servicing costs are kept to a minimum).

The major disadvantage of this design, particularly for multi-impression moulds, is that the individual mouldings must be unscrewed manually, thereby increasing considerably the moulding cycle time.

13.7.5 Loose threaded cores

In cases where a large component incorporates a local internally threaded hole (see, for example, Figure below) or has several internally threaded holes in close proximity to each other, the loose threaded core technique should be considered. This technique obviates automatic unscrewing, thereby considerably reducing the cost of the mould. Where a number of holes are closely spaced, automatic unscrewing often becomes impracticable anyway.

The basic principle of the loose threaded core design is illustrated in Figure above. At (a) the mould is shown closed. The threaded hole in the moulding is being formed by the loose threaded core. This loose core, which has a valve head type seating, is accommodated in a pocket machined into the main core. When the mould is opened (b) the moulding is ejected by an ejector pin system. The loose threaded core is ejected with the moulding, and is subsequently unscrewed.

Two sets of loose cores should be used during production. At the end of the first moulding cycle, the second set of cores can be inserted into the mould and the next cycle commenced. During this cycle the first set of loose cores can be removed from the first moulding and made ready for insertion into the mould immediately the next component is ejected and so on.

13.7.6 Unscrewing moulds

High labour costs and other modern production requirements demand the maximum use of automatic operation. Manual unscrewing of individual components from the mould is thereby precluded from all but experimental runs or small batch production.

The more complex unscrewing mould must therefore be considered where the component design does not permit stripping of the threads.

In an unscrewing type mould, either the cores or the cavities are rotated to automatically unscrew the component from the mould. To provide the required rotary motion, an unscrewing unit is fitted behind the moving mould plate in place of the conventional ejector unit. In certain designs however, where positive ejection is required, an ejector system may also be incorporated. From the impression construction standpoint, there are a number of alternative designs to consider as follows:

a) The axially fixed core design in which the threaded core is merely rotated to remove the moulding.

b) The extractor plate design in which an extractor plate is actuated at the same time as the threaded core is rotated.

c) The withdrawing rotating core design in which the threaded core, in addition to being rotated is simultaneously withdrawn through the core plate.

d) The rotating cavity design.

There are two basic impression layouts applicable to this section; they are (i) the pitch circle diameter (PCD) layout, and (ii) the in-line layout. Various power systems are available to actuate the unscrewing mould and there are also alternative methods of linking the power source to the mould. These may be broadly classified as follows:

Power source

1) Manual, 2) Machine 3) Hydraulic or pneumatic 4) Electric

Transmission system

1) Gear train, 2) Chain and sprocket 3) Rack & pinion 4) Worm and worm wheel

Axially fixed rotating core.

This design is particularly suitable for a component whose external form permits the cavity to be located in the same mould half as the threaded core (Figure 11.13). When the threaded core is rotated, in an axially fixed position, with respect to the cavity, the moulding is progressively ejected provided the cavity is maintained in a stationary position. The external shape of the moulding must be such that it cannot rotate with the core during the ejection phase. (Let us relate this to the function of unscrewing a nut from a bolt. If we rotate the bolt with the left hand, and prevent the nut from rotating with the right hand then the nut will progressively ride up the bolt until the threads disengage.) Smooth cylindrical components would obviously be unsuitable subjects for this technique because the moulding will tend to rotate with the core once the initial cavity-moulding adhesion is broken.

Withdrawing rotating core:

The principle of this design is illustrated in Figure below. The threaded core is unscrewed from the moulding by progressively withdrawing the, core through the mould plate as it is rotated. (If we relate this action to the simple nut and bolt analogy discussed in the previous section, this time we hold the nut firmly in the right hand and progressively unscrew the bolt with the left hand. Note that when we rotate the bolt one complete revolution it moves out of the nut a distance equal to the lead of the thread. Thus in our mould design we must arrange for the threaded core to be withdrawn through the mould plate a distance equal to the lead of the moulded thread for each revolution of the threaded core.)

Principle of withdrawing withdrawing type rotating core

rotating core design is mounted in mould

Rotating cavity:

When gate marks are not permissible on the external surface of the moulding then the rotating cavity design may be considered. In this design, the feed is arranged to pass through the centre of the core thereby permitting the pin gate to be adopted (Figure below). For convenience the core is usually mounted on the injection half. The type of feed system adopted can be based on either the underfeed design (as shown) or on one of the runnerless designs.

(a)

(b)

Let us now consider the sequence of operation for this type of mould. In Figure the impression is filled via the feed system as shown at (a). The rotation of the cavity insert starts immediately the mould opening commences, and, providing the moulding cannot rotate within the cavity, the moulding is progressively unscrewed (b). The final drawing (c) shows the mould fully open. The moulding, having been unscrewed from the core, is ejected from the cavity (in this example air ejection is adopted). The feed system is removed from between the floating cavity plate and the feed plate. The mould is closed and the cycle continues.