9.0 MOULD COOLING
9.1 General
One fundamental principle on injection moulding is that hot material enters the mould, where it cools rapidly to a temperature at which it solidifies sufficiently to retain the shape of the impression. The temperature of the mould is therefore important as it governs a portion of the overall moulding cycle. While the melt flows more freely in a hot mould, a greater cooling period is required before the solidifies in a cold mould it may not reach the extremities of the impression. A compromise between the two extremes must therefore be accepted to obtain the optimum moulding cycle.
The operating temperature for a particular mould will depend on a number of factors which include the following, (a) type and grade of material to be moulded, (b)length of flow within the impression, (c) waII section of the moulding; length of the feed system, etcIt is often found advantageous to use a slightly higher temperature than is required just to fill the impression as this tends to improve the surface finish of the moulding by minimizing weld lines, flow marks and other blemishes.
To maintain the required temperature differential between the mould and plastic material, water (or other fluid) is circulated through holes or channels within the mould. These holes or channels are termed flow-ways or water ways and the complete system of flow ways is termed the circuit
During the impression filling stage the hottest material will be in the vicinity of the entry point, i.e. the gate, the coolest material will be at the point farthest from the entry. The temperature of the coolant fluid, however, increases as it passes through the mould. Therefore to achieve an even cooling rate over the moulding surface it is necessary to locate the incoming coolant fluid adjacent to ‘hot’ moulding surfaces and to locate the channels containing ‘heated’ coolant fluid adjacent to cool’ moulding surfaces. However, as will be seen from the following discussion, it is not always practicable to adopt the idealized approach and the designer must use a fair amount of common sense when laying out coolant circuits if unnecessarily expensive moulds are to be avoided.
Units for the circulation of water (and other fluids) are commercially available. These units are simply connected to the mould via flexible hoses. With these units the mould’s temperature can be maintained within close 1 limits. Close temperature control is not possible using the alternative system in which the mould is connected to a cold water supply
9.2 Cooling Integer - Type mould plates
The temperature of a mould plate of the integer type is controlled by circulating water through holes bored in the plate. The holes are normally interconnected to form a circuit. The circuit may be at one or more levels, the number of which will depend on the depth of the mould plate.
As the circuits for a integer cavity plates and integer core plates are generally dissimilar, they will be treated separately.
a) Cooling integer-type cavity plate
Let us begin by considering the simplest case, that of a mould plate which incorporates a small cavity. The simplest approach to a circuit which we can adopt is to drill two flow ways, one on either side of the cavity, and to connect these at one end by means of a flexible hose, adapters being fitted into the ends of the flows way (Fig. 9-1).
To obviate any form of external connection, the two flow ways can be inter-connected internally by means of an internal drilling (Figure 9-2). This forms a U-circuit and it is useful, in particular, for cooling long, narrow cavities.
Instead of using an internal cross drilling, a milled slot may be used in conjunction with a connecting plate. There are two basic designs. Figure (9-3) shows the two drilled flow-way holes interconnected by a channel milled in the side Wall of the mould plate to provide a continuous flow path for the coolant. A connecting plate is sunk into the side wall and secured by screws, as shown. A gasket is incorporated to prevent leakage of the coolant.
Fig. 9-1 Fig. 9-2
Fig. 9-3 Fig. 9-4
The alternative design is shown in Figure (9-4) and it is similar to previous design. Except that the connecting channel, in this case, is machined into the connecting plate, and the latter is attached directly to the side wall of the mould plate (i.e. the connecting plate is not sunk into the side wall). The latter design is, therefore, the cheaper of the two methods but it suffers from the disadvantage that the connecting plate may be disturbed during the mould setting operation and fluid leakage may occur.
Fig. 9-5 Fig. 9-6
The rectangular circuit (Figure 9-5) which incorporates further internal drilling ensures that the flow ways are close to all four walls of the cavity, allowing a more even temperature control. For a large-area, shallow cavity, the above circuits are not suitable. Now consider a circuit which can be positioned directly below cavity. The mould plate is drilled, plugged and baffled (Figure 9-6). This forms a flow path of a Z-configuration, through which the coolant is circulated. As mentioned previously, the temperature of a coolant progressively increases as it passes through the mould. Thus the cooling effect is going to be more marked on the left side of the mould plate than it will on the right side. This means that a temperature variation across the cavity face will occur with this flow-way design, with possible resulting problems.
A preferred design using basically the same circuit is shown in Figure (9-7). This is called the balanced Z-circuit. Note that the drilling for the left side of the mould are diametrically opposite to the drilling on the right side. The coolant inlets are ‘A’ and ‘B’ respectively and both drilling are adjacent to the vertical center line and thus pass close to the sprue entry.
A modified approach is shown schematically in Figure (9-7 C). In this circuit design all of the inlet and outlet ports are arranged on the same side to facilitate mould setting.
c
Fig. 9-7
In the balanced Z-circuit, baffles are necessary to block certain flow ways to provide a continuous circuit without allowing sections to be bypassed and to become ‘dead waters’. The baffles should be incorporated in such a manner that they are readily accessible should leakage past them occur.
A correctly and incorrectly fitted baffle is illustrated in Figure 9-8. Note that should leakage occur past the baffle fitted as in (b) the mould plate will have to be removed in order to permit a repair to be carried out. This is not necessary with method (a). In general, when a large number of cross drilling and baffle have to be incorporated in a design, it is good drafting practice to include a schematic drawing of the flow path to assist the mould-maker (see, for example, Figure 9-6).
Fig. 9-8
The pressure plug may also be used to seal ends of drilled flow-way holes, thereby replacing the conventional taper pressure plugs. This eliminates the necessity for the normal tapping operation. Now consider the cooling of deep integer-type cavities. For these, a multi-level system is adopted. The circuit on each level is arranged to follow the contour of the cavity as far as possible. For a regular component, such as a box, this usually means that a number of identical circuits of rectangular type are arranged within the mould, one above the other, to provide for the transfer of heat from the walls of the cavity. (See circuits W and X, Figure 9-9.) The final circuit (Y) is normally of a Z configuration to allow for the transfer of heat from the base of the cavity. In the example given, the individual circuits are interconnected by axial drilling so as to form one continuous circuit. The advantage of doing this is that the mould has only one inlet one outlet which simplifies the connection to the supply.
Fig. 9-9 Fig. 9-10
The disadvantage, however, of adopting a continuous circuit for large moulds, is that close temperature control of the mould walls is impossible to achieve. The coolant fluid progressively gains heat during its passage through the mould, therefore opposite mould walls are likely to be at different temperatures. This feature may create moulding problems.
By adopting the individual circuit design, that is, each circuit is kept as a separate entity, the moulder can connect up the various circuits to achieve optimum results. To achieve a balancing effect it is desirable that the coolant flow through successive alternate circuit layers is reversed. This feature is illustrated in Figure (9-10)
Another method is to connect internally only certain of the circuits. This is most beneficially achieved in pairs, as shown in Figure (9-10 b), which, while achieving balanced counter flow, reduces the number of external connections required.
For irregularly shaped components, particularly those which require a differently shaped circuit on each level, internal connection of the various circuits is often impracticable. In these cases the circuits can be connected externally using pipe fittings of the conventional type.
If control of the temperature of the individual walls of cavity is required then the coolant plate method can be considered. In this design coolant plates are attached to the side faces of the mould plate and the coolant flows through channels machined into these plates. Drilled holes are therefore dispensed with, apart from the inlet and outlet apertures.
The individual coolant plates for the four sides are self contained and each has an inlet and an outlet. Note that while the channel cross-section may be kept relatively small to ensure turbulent flow, the length of the flow path can be extensive as required for the specific application. Turbulent flow is necessary to ensure good transmission of heat from the mould to the coolant fluid.
The coolant plate design suffers from the same problems that arise with drilled holes, namely corrosion and built-up of lime deposits. Thus, in operation, both designs become progressively less effective from the heat transfer viewpoint as time passes. It is common practice to clean the channels or holes at reasonable time intervals, and it is with respects to the removal of corrosion and lime deposit that the coolant plate method shows considerable savings in maintenance time. The coolant plates are relatively easy to remove; they are light and can be worked upon independently of the main mould structure. Problems associated with the removal of corroded plugs from drilled holes, and in the cleaning of corroded and blocked holes are thereby avoided.
Phosphating or similar protection of the channels or drilling is a sound investment to reduce the maintenance time spent on coolant system Leakage of fluid is prevented either by a gasket or by an ‘0’ ring
b) Cooling Integer - type core plate :
Providing the depth of the core is fairly shallow (under 25mm (1 in) the Z type single-level system can be adopted, the water ways being situated beneath the core in a manner similar to that for integer cavities discussed in the preceding section. For deeper cores, however, the single -level circuit is not sufficient to permit the coolant to transfer heat away from the core surface fast enough. Some arrangement must, therefore, be made to permit the circulation of coolant inside the core. There are several alternative ways of doing this and the method adopted will be determined to some extent by the actual shape of the core.’
Angle hole system (Figure 9-11):. Flow ways are drilled at an angle from the underside of the core plate so that they interconnect at a point (X) relatively close to the surface. Each of these drilling is plugged. The inlet (Y) and outlet (Z) holes are drilled from either side of the mould and break into the angled waterways, as shown
Fig. 9-11
Baffled - straight hole system : In this system (Figure 9-12) holes are bored at right angles to the rear face of the core plate The lower end of each boring is plugged The borings are interconnected by a hole (X) drilled from a side face To ensure the coolant passes down each hole, baffles are fitted as shown
Fig. 9-12
Stepped circuit :
To obtain cooling channels which are positioned fairly close to the top surface of the core, the stepped circuit can be considered. In this system (shown in Figure 9-13) holes (X) are drilled through the side wall of the core, parallel to the core face. These holes must be very carefully plugged and finished as they form part of the impression. Badly fitted plugs on a moulding surface cause considerable moulding difficulties, and for this reason this particular design is not favoured by many designers. The stepped configuration of drilling, as shown, is necessary to provide a suitable inlet and outlet connection position.
Fig. 9-13
The above three circuits can be used singly, as shown, for fairly narrow cores. For wider cores a number of identical circuits can be incorporated, positioned at suitable intervals along the core. The individual circuits can be internally or externally connected, whichever is the more convenient.
c) Multiple circuits :
Example of typical multiple circuits are shown schematically in Figure9-14). The first figure (9-14a) illustrates the angled hole system. Note that the six individual circuits have been connected as two sets of three. This feature permits a balanced temperature gradient across the core to be achieved. Figure ( 9-14 b) shows a pair of baffled hole circuits coupled together, while Figure (9-14 C) illustrates a multiple stepped circuit.
Fig. 9-14
9.3 Cooling Insert – bolster assembly
We will discuss cooling of the insert-bolster assembly under two headings, (i) cooling the bolster, and (ii) cooling the insert. The latter is further classified depending upon which type of insert is to be cooled (cavity or core).
9.4 Cooling bolster
In moulds constructed on the insert-bolster principle, where the depth of the impression is relatively small, the circulation of the coolant is often confined to the bolster. The designer relies on the reasonably good thermal conductivity of steel to allow the heat to be rapidly transferred from the impression as required. Even better results can be achieved using a material with a higher thermal conductivity, such as beryllium-copper, for the insert.
The method adopted for cooling the bolster is identical to that described for cooling the integer cavity block. That is, holes are drilled through the bolster and are interconnected, either externally or internally, to permit the circulation of a coolant.
It is desirable that these flow ways are positioned as close to the insert as practicable. For a shallow depth of insert, the holes may be situated directly below the insert (Figure 9-15). A Z-type layout is normally adopted. The alternative method is to arrange holes close to the sides of the insert (Figure9-16).
Fig. 9-15 Fig. 9-16
In this case the rectangular type of circuit is used. For deeper inserts, a multi-level system is desirable. This is simply a combination of both the above layouts (Figure 9-1Ob)
9.5 Cooling Cavity Inserts
The method adopted for cooling cavity inserts depends, to some extent, upon the shape of the insert. This can broadly be classified as either rectangular or circular. The circulation of fluid within the insert is easily achieved, but a complication exists in that the flow-way cannot be drilled into the insert from the bolster without incorporating some form of seal to prevent leakage.
9.6 Cooling rectangular Inserts :
A typical rectangular insert is shown (Figure 9-17). Now, while the simplest of circuits only is illustrated, more complicated single- and multi-level circuits can be adopted. This will be determined by the shape and the depth of the cavity. All drilling within the cavity insert should be interconnected, plugged, and baffled, so as to necessitate the minimum of external couplings. The designer should aim to have only one outlet per insert.
The mould setter can more quickly set up a mould for production if the supply and returns lines can be attached directly to adapters which project from the side walls of the mould. This means that the designer should provide some positive connection between the insert and the outside of the mould. Three alternative methods are illustrated in Figure (9-17)
(i) An extension piece is screwed directly into the insert through a suitable clearance hole in the bolster (Figure 9-17). This method has one disadvantage in that the connection between the extension piece and the insert must be broken each time the insert is removed from the bolster.
Extension pieces incorporating an integral nipple at one end are available as standard parts. This component allows ‘quick connection adapters to be used, and thereby usefully permits a reduction in the mould setting
Fig. 9-17
(ii) An extension piece is screwed directly into the insert through a suitable slot machined in the bolster (Figure 9-17c). This design overcomes the disadvantage stated in (I) but the slot does constitute an open crevice which may become a material trap if the mould flashes. Build-up of plastic material in this trap is difficult to remove and can damage the opposite face of the mould, if allowed to become excessive. However, providing the slot is well clear of the impression, this feature does not constitute an undue hazard.
(iii) Providing the depth of the bolster is sufficient, drilling can be made through the bolster below the insert and the two systems interconnected by cross-drilling (Figure 9-17 d). Leakage between the insert and the bolster is prevented by an 0-ring round each cross-drilling, suitably accommodated in a recess in the bolster as shown. This design does not suffer the disadvantages associated with .the previous methods and is therefore the preferred design.
(iv) Narrow inserts are difficult to cool in the manner described above as there is usually insufficient space to incorporate a conventional circuit. To overcome this problem, a water cascade junction may be considered. This unit is available from DME* as a standard part.
The mould insert is bored and tapped to accommodate the outlet pipe of the water cascade junction assembly. The outer end of this outlet pipe is coupled to a junction block which is adjacent to the lower mould face. An inlet pipe, similarly connected to the junction block, extends through the center of the outlet pipe and into the insert coolant hole, as shown.
Suitable connection of coolant lines to the inlet and outlet ports of the water cascade junction permits a flow of coolant to be provided through the insert. The water cascade junction may also be considered for cooling other small mould parts such as inserts, side cores and splits.
Another method of cooling inserts is by the use of copper pipes. This design is suitable when mould space is restricted. Pipes can be laid in channels which may be immediately adjacent to other holes in the mould plate without the possibility of leakage occurring.
Fig. 9-18
A number of channels (X) are machined through the bolster (Figure 9-18). A copper pipe is bent to a suitable shape so that it can be placed into the channels (see plan view). The insert when fitted to the bolster is in direct contact with the pipe. To increase the contact area and to improve heat transfer, the space around the pipe can be filled with a low-melting-point alloy (see inset), or a square-section pipe can be used.
9.7 Cooling Circular Inserts
The drilling methods discussed for rectangular inserts cannot normally be adopted for cooling circular inserts due to space limitations. However, because the insert has a circular form, an annular groove can be incorporated quite simply. Most design for direct cooling of circular cavity inserts are based upon this principle.
Fig. 9-19 Fig. 9-20
Consider Figure (9-19). The circular cavity insert is shown fitted in a standard type of frame bolster. A coolant annulus (X) is machined on the periphery of the insert, and additional grooves (Y) provided above and below the coolant annulus to accommodate 0-rings. When fitted to the bolster, these 0-rings prevent leakage of fluid between the insert and the bolster. Some care must be exercise to prevent the 0-ring being damaged when the insert is fitted. A lead-in on the bolster hole at Z facilitates this operation.
The annulus is connected to the supply and return line via drilling through the bolster. For multi-impression moulds, the inserts can be positioned in lines so that a vertical drilling interconnects each annulus to form a continuous circuit (Figure 9-20).
Fig. 9-21 Fig. 9-22
Instead of machining the coolant annulus into the periphery of the cavity insert ,or coolant sleeve , the annulus can be incorporated as a groove machined into the mould plate as shown in Figure (9-21). The main object of this approach is to lay out the impressions so that the individual grooves interconnect and thereby avoid the necessity of additional drilled holes. While drilled holes are simple to incorporate for inserts arranged in line (see Figure 9-20), the holes are more difficult to incorporate to interconnect inserts which are arranged on a pitch circle diameter. It is for this latter layout that the interconnecting groove design is often adopted.
A plan view of a mould plate in which inserts are arranged on a pitch circle diameter, and located so that the individual grooves interconnect, is shown in Figure (9-22). To provide for a definite flow path for the coolant, a gap must be left between two of these grooves as shown as ‘X’. These latter grooves are connected to the inlet and outlet apertures respectively.
To achieve this design of interconnecting grooves, a large amount of steel must be removed from the central region of the mould plate and this will have a considerable weakening effect. Therefore if this design is adopted the designer must ensure that the mould plate is adequately supported by the backing plate.
An alternative design is shown in Figure (9-23). A plain cavity insert is fitted into a coolant sleeve which incorporates both the coolant annulus (X) and the 0-ring grooves (Y). This design has the advantage over the standard coolant annulus method (Figure 9-19) in that once the sleeves are fitted the 0-rings need not be disturbed. The plain inserts can be removed and replaced without affecting the coolant system.
Fig. 9-23
9.8 Cooling Core Inserts
System evolved for the efficient cooling of mouldings produced from the insert-bolster design; normally involve passing a coolant fluid directly through channels or holes incorporated within the body of the insert. The design adopted depends to a large extent upon the size and shape of the insert. Cooling the core for a large shallow box, for example, will require an approach quite different from that for cooling a long, small-diameter core for a pen barrel.
a) Cooling Shallow Core Inserts:
Once the designer has decided not to rely on conducting the melt heat away from the core to rather remote holes drilled in the bolster, he must then alternatively consider incorporating holes or channels directly into the core insert. One method for doing this drilling holes using a basic ‘U’ circuit configuration. Useful variations on this approach use either the ‘Z’ or ‘balanced Z’ designs.
An alternative design which can be adopted for cooling shallow core inserts is the ‘spiral circuit’. This design basically consists of a channel machined into the rear face of the core insert in the form of a spiral. Unfortunately in practice the spiral form is both difficult and expensive to produce, therefore Compromise ‘spirals’ are normally adopted. Example of these are shown in Figure (9-24), for cooling a large round insert, and in Figure (9-25), for cooling a large rectangular insert respectively.
In the first example a series of concentric grooves are machined, and these are interconnected via channels, as shown; the flow path is established by suitably positioned baffles. The inlet and outlet holes are machined into the bolster as indicated by the chain
Fig. 9-24 Fig. 9-25
dotted lines. Note that the 0-ring encompasses all grooves and thereby prevents leakage of the coolant fluid. The second example, using a basic rectangular layout, is shown. The drawing shows the underside of the core insert only. In this design one continuous flow path is adopted without baffles. Alternatively a similar arrangement to that adopted for the round insert (Figure 9-24) may be used.
b) Cooling Deep Core Insert :
Only by circulating the coolant fluid deeply inside the core insert can efficient transfer of heat from the core surface be achieved. There are many alternative arrangements for cooling deep core inserts, and a number of these designs will be covered in the following discussion.
i) Deep chamber design : In the first example shown in Figure (9-26) the rear face of the core insert is recessed to form a deep chamber (U). This chamber is normally circular for ease of machining. The insert is firmly held down on to a flat face at the base of a pocket machined in the bolster by screws (not shown). Leakage between the two surfaces is prevented by a 0-ring fitted into a groove. In operation the chamber is completely full of water. The incoming coolant passes from the inlet (X) through the internal drilling and pipe to impinge on the center of the chamber (at Y). On single- impression moulds this is likely to be the hottest part of the core insert as it is directly opposite the sprue. While being the cheapest of the deep cooling methods to incorporate, the deep chamber design suffers from two major disadvantages-
(correct) (incorrect)
Fig. 9-26
(a) The flow rate of the coolant drops markedly as it enters the chamber. This means that the required turbulent flow is not achieved in this region and that the transfer of heat to the coolant is less effective.
(b) It is possible by incorrect design or incorrect tool making for an air pocket to be formed at the top of the chamber as shown in Figure (9-26b).
An uneven temperature profile, with associated moulding problem, will result. The air pocket is created by the incorrect positioning of the outlet port (Z) in relation to the chamber. It is essential that this port is always situated at the highest point of the chamber when the mould is mounted on the injection machine (compare Figures 6-26a and 9-26b.) It is for this reason that moulds incorporating the deep chamber design should be mounted on the machine.
The design can be usefully used for moulds in which the production rates are not important and where mould costs must be kept to a minimum. The deep chamber design forms the basis of more efficient designs, examples of which are discussed in Section (ii) and (iii), which follow.
(ii) The Deep Chamber Design with Central Support: This system is illustrated in Figure 9-27. The support feature is provided by a central column which can be integral with the bolster, as shown, or be a separate member. Obviously if the depth of the chamber necessitates a column which is relatively long, it is preferable to use the latter design.
The design has two primary objectives:
(a) To support the central region of the core against possible deflection.
(b) To have the central region solid to permit a valve type ejector element to be incorporated. Note that if the later design is used in conjunction with a separate central column, then as additional 0-ring must be incorporated to avoid fluid leakage past the stem of the valve.
Fig. 9-27
The disadvantage which applied to the deep chamber design, apply to this design as well. That is, the flow rate drops as it enters .the annulus, and air pockets may be formed. If a large diameter ejector valve is incorporated, with its own coolant, then the results of the above disadvantages are lessened. This is because as efficient coolant circulation system is incorporated at the point where it is required, that is, at the hottest part of the namely the front surface.
(iii) The Helical Channel Design: The more efficient design than either of the two previous systems is the helical channel design, which is illustrated in Figure (9-28) It is more costly design, however, from the mould manufacturing viewpoint.
In this design the deep chamber is plugged with a close fitting block of steel or brass (the latter being preferable from the corrosion resistance and block removal viewpoints). A helical channel (U) is machined into the outside of this plug. The internal drilling through the bolster and plug are arranged so that the coolant is directed from the inlet (V), through a central drilling (W), across the face of the plug (see cross-sectional drawing A-A) and so to the outlet port (X) via the helical channel. The core insert is fitted into a bolster of solid type and 0-rings are fitted to prevent leakage.
Fig. 9-28
This design ensures that the coolant follows a precise path and no ‘dead waters’ are possible. The plug forms a positive support to the insert shell and, therefore, the wall section of this shell (Z) can be less than for either of the previous designs. A more rapid transfer of heat from the moulding is, therefore, achieved as the coolant passes relatively close to the impression.
(iv) Baffled Hole System: A completely different approach is illustrated in Figure (9-29). This design utilizes a system of baffled holes (U).The holes, drilled into the rear face of the insert, may either be at right angles to the base or be parallel to the outside wall of the core. (The latter design is illustrated.) The diameter of the hole is normally in the range 13mm (1/2in) to 25mm (1 in), depending on the size of the insert, To provide a flow path for the coolant the individual holes are interconnected by an annulus (V) which is machined into the base of the insert. A baffle is fitted into an end-milled Slot which is machined at right angles to the annulus (Figure 6-29 C). The inlet (W) and outlet (X) drilling through the bolster are situated on either side Of this baffle (see plan view, Figure 9-29 b).
Fig. 9-29
To ensure that the coolant circulates down each individual hole, baffles must be fitted into each. The baffle, the top end of which is shown in Figure (9-29 e), is usually made of brass. The actual flow path of the coolant is shown in Figure 9-29 d. The Core insert is fitted into a bolster of solid type and an 0-ring incorporated to prevent leakage. Each baffle must be flush with the rear face of the insert, to prevent the hole being bypassed.
The designs discussed so far in this section have been mainly applicable to relatively large core inserts. The following design examples offer alternative methods for cooling the smaller insert, particular on multi-impression moulds.
(v) Baffled Hole System For Small Inserts: In the design illustrated in Figure 9-30, the impression are arranged in line (S), and the inserts, being circular, fitted into a frame type of bolster. Each insert incorporates a chamber (U) which is in alignment with a drilling in the bolster. To prevent leakage of coolant a small 0-ring is fitted in a recess below each insert. The individual drilling are interconnected by a hole (V) drilled completely through the mould. The lower end of this hole is the inlet (W), and the top end becomes the outlet (X). To ensure the coolant passes down each chamber, baffles are necessary. The baffles are mounted in each insert chamber at right angles to the main drilling (see plan view). Note that the lower end of the baffle incorporates a radius to match that of the main drilling.
The flow path of the coolant is shown in Figure (9-30 b). As the coolant progressively gains heat as it passes through the mould, this design is not efficient for cooling more than three or four impressions.
Fig. 9-30
(vi) The Bubblier System: The design is basically the same as the deep chamber design (i), suitably adapted for small inserts. A relatively small diameter hole is machined into the rear face of the insert as shown in Figure (9-31). A ‘bubblier’ pipe is fitted in the backing plate and protrudes into this hole, as shown, thereby forming an annulus. Suitable inlet and outlet holes are drilled in the backing plate.
One type of circuit for which this system can be used is illustrated. The coolant passes from the inlet hole ‘U’ up the inside of the bubblier pipe, and then down the outside, into the outlet hole.. A schematic drawing of the Complete circuit is shown in Figure (9-31). Note that the temperature of the coolant is approximately the same in each insert as they are all connected in the same way.
Fig. 9-31
(vii) The Spiral Plug System: This method of cooling small core inserts is an alternative to the bubblier system discussed in subsection (vi). The only basic difference between the two designs is that a spiral plug replaces the central pipe of the above system. The spiral plug is essentially a hollow cylinder, the external surface of which is machined to form a single-start constant -depth thread. The spiral plug is fitted into a close-filling accommodating hole in both the core insert and in the backing plate as Shown in Figure (9-32). The inlet hole bored in the backing plate is suitably coupled to the spiral plug’s central drilling, while the adjacent outlet port is aligned with the start of the spiral groove. Thus the incoming coolant is directed through the center of the assembly after which it passes down the spiral groove to the outlet port. Note that a dowel pin should be incorporated to ensure that misalignment between the respective holes does not occur due to the possible twisting of the spiral plug during production.
Fig. 9-32 Fig. 9-33
A similar but alternative design is illustrated in Figure (9-33). This time the spiral plug is formed from a solid cylinder, the external surface of which is machined to form a two-start constant-depth thread. Fitting details as above. A straight-through drilling in the backing plate connects with the two starts of the spiral, thus forming a continuous circuit. The incoming coolant passes down one groove of the spiral and then back to the outlet hole via the second groove. A suitable gap must be provided between the top of the spiral plug and the insert drilling to provide a flow path for the coolant between the two spirals.
Both the single- and two-start spiral plugs are available as standard parts from Hasco.
(viii) Heat-rods.: This system is normally adopted for situations in which it is impracticable to incorporate an internal fluid circulating system within a core insert because of size limitations. A heat rod is basically a cylindrical metal rod which is inserted into an accommodating hole machined in the core insert Its purpose is to facilitate the conduction of heat away from the impression.
(ix) Heat-pipes: When viewed side by side, the heat-pipe and the heat-rod appear to be identical in form. Thus the heat-pipe may be fitted in an identical manner to that of the heat-rod. This drawing illustrated a heat-pipe fitted for an identical application to that discussed above in (vii). However, the similarity ends at this point. The heat-pipe is a commercially available heat transfer device which is capable of transmitting heat energy at relatively high velocities. Unlike the heat-rod it does not rely upon the thermal conductivity of the metal used.
The principle of the heat-pipe has been used in many chemical engineering heat transfer application for several decades. They have proven in practice to be both a reliable and effective method of heat transmission.
Heat-pipes are available as standard parts from several manufactures, for a range of sizes. The fitting arrangements vary slightly between these manufactures, and for this reason the respective catalogue should be consulted before incorporating a heat-pipe into a design.
The heat-pipe design is based upon a physical fact that the boiling point of a fluid depends upon the pressure of its environment. While at a pressure of one atmosphere, water boils at 100oC As the pressure is reduced within a closed chamber, (that is a vacuum is applied) the boiling point falls correspondingly.
9.9 Cooling other mould parts
1. Other mould plates
On multi-plate moulds it is necessary to consider the cooling of other mould plates in addition to that of the primary cavity and core plate. In particular, the stripper plate in a stripper plate mould, and the feed plate in a mould of the underfeed type. Separate control of the temperature of these plates is necessary to achieve the optimum production cycle.
In general, the maintenance of temperature of these plates is achieved in a manner identical to that described for cavity plates of integer type. Flow ways are drilled and interconnected so that a coolant can be circulated through the plate.
2 Cooling the Sprue Bush
A relatively large bulk of plastic material is contained in the sprue, which must be cooled during each cycle to a temperature at which it is sufficiently solid to allow for its removal from the mould. It is therefore desirable to incorporate a separate sprue bush cooling circuit so that heat can be transferred from this member as efficiently as possible
The shape of the sprue bush is similar to that of circular inserts, so the methods illustrated for cooling circular inserted (Figure 9-20 and 9-23) are equally applicable for cooling the sprue bush.
9.10 Water connections and seals
1 Adapters
As mentioned, the majority of moulds are drilled to provide a flow-path through which the coolant can be circulated. These drilling are connected to the supply and return lines via adapters. The adapter is a standard mould pipe fitting which can be obtained in a number of alternative designs and sizes.
Fig. 9-34
| (BSP) | Tapping size (in) | Depth of threaded hole | Diameter of flow way | ||||
| Method A | Method B | ||||||
| mm | inch | mm | inch | mm | inch | ||
| 1/8 | 11/32 | 11 | 7/16 | 8 | 11/32 | - | - |
| 1/4 | 29/64 | 13 | 1/2 | 11 | 7/16 | 7 | 1/4 |
| 3/8 | 19/32 | 13 | 1/2 | 15 | 19/32 | 10 | 3/8 |
| 1/2 | 47/64 | 14 | 9/16 | 18 | 23/32 | 13 | 1/2 |
A typical basic design of adapter is illustrated in Figure 9-34. One end of the adapter is threaded to correspond to that of the flow-way tapped hole, the other end incorporates either:
(i) External serration (as illustrated) for use with PVC hose, utilizing hose clips for attachment purposes;
(ii) Threads, for use with a corrugated metal hose system with in-built threaded couplings. This system offers advantages in that it is more leak free in operation, and it will withstand higher pressures.
Naturally the initial overall cost of this water connector system is greater than that of (i) above.
Two alternative methods of fitting the basic adapter are shown in Figure 9-34. In method ‘a’ the tapped hole size governs the diameter of the water way. As the minimum spacing between adjacent water-ways depends on the ‘space required to insert and remove the adapter (using an appropriate spanner) the smaller adapter is useful. However, this method has the disadvantage that the flow of the coolant is restricted by the small-diameter hole in the adapter.
In the second method (b) an adapter is chosen which has an internal bore which closely matches that of the water-way. Thus with this design the flow- way aperture is not restricted by the adapter.
2 Quick connection adapters
The disadvantages of the fixed adapter design is two-fold.
(1) The rubber hose must be connected and disconnected each time the mould is set on the machine.
(ii) The adapter projects a considerable distance from the side of the mould.
The first point results in an extension of the setting time, while the second point makes the mould setting just a little more difficult in that the projecting adapters tend to get in the mould setter’s way.
Fig. 9-35
To overcome these disadvantages various quick connecting adapters have been designed and are commercially available so as to minimize mould setting time. Those shown in Figure 9-35 are from the DME* (USA and Europe ) range. There are two types in this range called ‘Jiffy-tite’ and ‘Jiffy matic’ respectively. Both types consist of two parts: the first part, shown on the left of the drawing, is the ‘connecting plug’ and it is screwed into the mould like a conventional adapter. The second part, termed the ‘socket’, is attached to the rubber hose.
By a suitable push action the two parts can be connected or disconnected very simply. The ‘Jiffy-matic’ has the additional advantage in that as soon as the socket is disconnected, the coolant flow ceases. immediately. With the ‘Jiffy-tite’ system the coolant flow must be shut off at the source. The connecting plug (with either type) may be sunk below the surface of the mould wall, so that it does not protrude above the relevant surface. This design feature prevents the plug from being damaged during the setting operation.
3 Position of water connections
All inlet and outlet holes, should, whenever practicable, be positioned either at the base or at the rear of the mould. Connections at the front tend to get in the operator’s way. With connections at the top of the mould, should leakage occur, water may get on to the polished face of the impression.
It is desirable that the position of all water connections (this includes holes U which are interconnected externally) is such that the associated adapters and hoses do not interfere with the bolting of the mould on to the machine.
4 Plugs
As discussed previously, plugging the ends of certain holes is necessary to form a continuous circuit. These plugs are variously termed ‘taper pressure plugs’ (DME* and Desoutter*) hexagon socket pipe plug (Hasco*) and pressure plugs (DME*). The plugs are available from the above manufacturers as standard parts in the following size range:
Metric:M8;M1O;M12;M14.
Imperial: 1/8; ¼, 3/8; 1/2.
Note that the above size range is not necessarily available from one particular manufacturer. The tapping size and length of thread h is the same as that indicated for adapters (figure 9-34). The plug has a square or hexagonal projection (or depression) so that a suitable key or wrench can be used to ensure a leak free joint when the plug is screwed into the mould. By convection, plugs are shown with a dotted cross-hatch on the plan view of mould drawings (Figure 9-16). This simplifies the tracing of the required coolant flow path.
5. 0-rings
A 0-rings (0-seal) is a synthetic rubber ring which is incorporated in a suitable recess in a mould for the purpose of preventing leakage of the coolant fluid. For this function to e achieved effectively, the O-ring must be suitably compressed by a specific amount in order to achieve the required leak free joint. While two alternative cross-sectional shapes are available, namely circular and rectangular, the circular cross sectional 0-ring is the one that is normally adopted in mould design practice. An 0-ring primarily used is of the following cases:
(I) To prevent fluid leakage from between two adjacent plates. Typical examples of this case is illustrated in Figure 9-26,and 9-27 respectively. This is the simpler of the two cases in that the 0-ring is’ simply laid into a recess in one plate and when the second plate is secured to the first, the 0-seal is compressed the required amount. An enlarged view of the fitting details is shown in Figure 9-36
Fig. 9-36
(ii) To prevent fluid leakage from between adjacent curved surfaces. This is the case which results when a cavity or core insert incorporates an annulus for the circulation of the coolant fluid (refer to Figure9-19). This necessitates a pair of 0-rings being mounted, one on either side of the annulus, as shown. An enlarged view of this second case, with relevant fitting details is shown in Figure 9-37a. Note that the assembly operation involves the 0-ring being expanded over the outside diameter of the cavity or core insert in order to fit it into its accommodating groove.
Fig. 9-37
Useful equations which may be used as a guide to the dimensions of the recess in this case are as follows:
Do=1.8D (6.7)
W=1.3d (6.5)
t = 0.8d (6.6)
Where Do= the outside diameter of the circular insert. All other symbols are designated above.
An alternative and less complex method of fitting the lower 0-ring is shown. Figure 6-40b. In this design the lower 0-ring is accommodated in a recess into the mould plate adjacent to the fitting diameter of the insert. Note that this design necessitates a larger than normal flange. The advantages of this design is that it eliminates the necessity of stretching the O-ring in order to fit it into its accommodating groove as discussed for design ‘a’.
The relevant equations which apply in this case are as follows:
Do= D-0.3d (6.8)
W= 1.3d (6.5)
t = 0.8d (6.6)
Where Do=The outside diameter of the circular insert. All symbols are as designated above.
An 0-ring which fails in operation does so for one of the following reasons:
(i) The 0-ring has been fitted into an accommodating recess, the dimensions of which are not suitable. Whenever possible the designer should refer to the supplier’s catalogue for the specification for the recess given in Figure 9-36 and 9-37, are intended as a guide, and should only be used if information from the 0-ring supplier is not available.
(ii) The 0-ring is damaged during the fitting or refitting operation. This usually occurs if the internal and external corners are left sharp. With Reference to Figure 9-36 and9-37, note where specific radii have been specified. A lead-in angle of 10-15 degree machined at the aperture entry is advisable when fitting circular inserts which incorporate 0-rings (see Figure (9-37 a)
(iii) The incorrect material has been specified for the 0-ring with respect to the circulating medium used, (e.g. certain oils require a different grade of synthetic rubber).
(iv) The incorrect material has been specified for the 0-ring with respect to the temperature of the circulating cooling mediumGo to next chapter
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