GB2164895A

GB2164895A - Injection moulding apparatus

Info

Publication number: GB2164895A
Application number: GB08523759A
Authority: GB
Inventors: Keith John Cockrill; Alan William Jordan
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1984-09-26
Filing date: 1985-09-26
Publication date: 1986-04-03
Also published as: GB8424357D0; GB2164895B; GB8523759D0

Abstract

Injection moulding apparatus comprises a pump arranged to pump moulding material 2 along a conduit and into a mould inlet (not shown). A pressure transducer 4 monitors the pressure of the moulding material 2 in the region of the mould inlet and the drive power of the pump is controlled by control means, in accordance with the output of the pressure transduce 4 so as to maintain said pressure substantially constant at a preselected maximum. The apparatus finds particular application in the installation of sheath materials at cable joints. The pump is a pneumatic piston 1, cylinder 7 arrangement. <IMAGE>

Description

SPECIFICATION Injection moulding apparatus The present invention relates to injection moulding equipment and finds particular application in the installation of sheath materials at cable joints.

Certain types of cable, for instance many communication cables, are provided with an outer, protective sheath. The sheath may be electrically insulating where the cable concerned carries electrical power, as well as impermeable to water. Suitable materials out of which such sheaths have been made include thermoplastic polymers which can be extruded onto a cable during manufacture.

Problems arise when a joint must be made between one section of cable and another.

Joints may be necessary because the distance over which the cable must extend is greater than the length of cable which can conveniently be laid as a continuous stretch. Alternatively, it may be necessary that the cable is cut in order to carry out tests on a particular stretch of cable.

The problems lie in installing an effective protective sheath over the completed cable joint which also makes a successful seal to the sheath which the rest of the cable already carries. To do this, it is known to use injection moulding techniques. A mould is mounted to encase the cable joint, including a short distance of the cable itself which is adjacent either end of the joint. Molten sheath material is then injected into the mould until the mould is full, and allowed to cool. Optionally the mould may be heated before and during the injection stage. The aim is that either the hot molten material, or the preheated mould, will soften the sheath material already in place on the cable enough for the injected sheath material to amalgamate homogeneously with it and form a hermetic seal.

The integrity of the protective sheath produced over the cable joint, and the seal between it and the sheath material already on ;the cable, are influenced by a combination of factors. In particular the temperature and pressure profiles of the injected sheath material during the installation process are very important. The temperature profile is partly determined by the rate at which sheath material is injected into the mould and is closely linked to the pressure profile since temperature and pressure are directly dependent on each other.

If the mould is unheated and the sheath material is injected too slowly, the material already in the mould can cool, tending to solidify. Subsequent injected material may not then amalgamate successfully with already injected material. Voids and discontinuities can thus be created in the finished sheath which reduce or destroy its effectiveness. Even if the mould is heated, uneven injection rates are undesirable not least because the injection stage will be carried out with reduced efficiency as a result. If the sheath material is injected too fast however, whether the mould is heated or not displacement of the cable joint relative to the mould can occur.This is because injection of sheath material into the mould takes place through a lateral aperture in the wall of the mould and, if injected too fast, can exert an unbalanced lateral force on the cable joint which is sufficient to displace it in the mould.

In an attempt to overcome the problems outlined above, it has been done to control the motive force applied to the sheath material during injection. A maximum motive force is selected and the injection apparatus concerned is allowed to run at a pumping rate limited by that maximum motive force. When the mould is full, the injection apparatus ceases to move to and fro since to continue doing so would mean applying a motive force which is greater than the selected maximum. That is, although the motive force is maintained at the maximum, no more sheath material will go into the mould.

Results which have been obtained when using injection apparatus controlled in relation to a maximum motive force have been found variable in quality. During filling, the resistance offered to the motive force can vary. This may be a result of the shape of the mould being filled in combination with the changing proportion of mould remaining empty. Or it may be a result of the quantity of sheath material within the injection apparatus against which the motive force is acting, which quantity can vary significantly during a single pumping cycle. If the resistance offered to the motive force varies, the injection rate may become uneven, leading to the inconsistent quality of sheath installation which has been observed.

An object of the present invention is to provide injection moulding apparatus which is capable of being used to fill moulds of different designs, and of being used with different moulding materials, which is still capable of producing consistently acceptable results.

According to a first aspect of the present invention there is provided a method of injection moulding, for use in installing protective material on a cable, which comprises the steps of: (i) mounting an outlet of a conduit having a heating zone so as to communicate with a mould inlet; (ii) applying drive power to a pump so as to pump moulding material along the conduit, through the heating zone, and into the mould; (iii) monitoring the flow of the moulding material from the conduit into the mould by means of monitoring the pressure of the moulding material in the conduit at a position between the heating zone and the outlet of the conduit which communicates with the mould inlet; and (i) varying the drive power so as to maintain said pressure of the moulding material within a preselected range of values suitable for filling the mould.

According to a second aspect of the present invention there is provided injection moulding apparatus, for use in a method according to the first aspect of the present invention, comprising a conduit having a feed inlet for the reception of moulding material, a heating zone for melting moulding material received at the feed inlet, and an outlet adapted to be mounted so as to communicate with the inlet of a mould, the internal volume of the mould being greater than the internal volume of the conduit, a pump for pumping moulding material along the conduit from the feed inlet through the heating zone to the outlet, and a power source for supplying drive power to the pump, wherein the apparatus further comprises a pressure transducer mounted to respond to the pressure in the moulding material in the conduit at a position between the heating zone and the outlet of the conduit, and control means which controls the drive power supplied to the pump, the output of the pressure transducer being operatively connected to an input of the control means.

An advantage of apparatus according to embodiments of the invention is that a mould may be filled with moulding material at a preselected rate. The rate concerned can be chosen to be suitable to a particular mould and moulding material and is substantially independent of fluctuations which may be introduced by varying conditions during the injection moulding cycle.

Injection moulding apparatus according to an embodiment of the invention will now be described with reference to the accompanying figures in which: Figure 1 shows schematically injection moulding apparatus suitable for use in installing or reinstating a polythene sheath over a joint in a telecommunication cable; Figure 2 to 5 each show a three-quarter view of part of the apparatus shown in Fig. 1; Figure 2 showing a piston; Figure 3 showing a cylinder a cylinder end plate; Figure 4 showing a feed hopper; and Figure 5 showing a framework for mounting and alignment of the apparatus.

Referring to Fig. 1, the injection moulding apparatus comprises a pneumatically powered piston 1 which acts, in use. on polythene granules 2 in a conduit to push them through a heating zone 3 of the conduit and into a mould (not shown). At the exit of the heating zone 3 of the conduit there is mounted a pressure transducer 4 whose output is used in controlling the air supply which powers the piston 1.

In more detail, referring to Figs. 1 and 2, the piston 1 comprises a disc 5 with a central, solid, cylindrical projection 6. The disc 5 is mounted in a right, circular cylinder 7, parallel with the ends thereof, each of which comprises a plate 8. The projection 6 extends, perpendicular to the disc 5, through an apertuere 9 in one end plate 8 of the cylinder 7.

The disc 5 is provided with a seal 10 around its edge which seal 10 allows the disc 5 to slide along the cylinder 7 under pneumatic pressure. The seal 10 is of known type, comprising a PTFE, rectangular section ring, preloaded outwards from the disc by an underlying ''O" ring. The aperture 9 through which the projection 6 extends is also provided with a seal 11 which is substantially airtight but allows sliding movement of the projection 6 through the aperture 9.

For cleaning purposes, the projection 6 is provided with a screw mounted copper tip 35. The tip 35 is tapered slightly, being mounted on the projection 6 at its smaller surface. The larger endface of the tip 35 has a clearance of 0.5 mm from the internal surface of the filling chamber 15 and has a greater diameter than that of the projection 6.

Because no direct contact is made, in use, between the projection 6, or the tip 35, and the surface of the filling chamber 15, contamination by slivers of material produced by wear between those members is avoided. Copper is selected for the material of the tip 35 because polythene grannules do not generally adhere to copper.

Referring to Figs. 1 and 3, each end plate 8 of the cylinder 7 is rectangular. The plates 8 are each provided with a circular seal 21 against which the curved wall of the cylinder 7 can seat. One corner of each plate 8 is extended to provide mounting for a solenoid actuated gas valve 12 which controls a gas supply through a bore (not shown) in the plate 8 which opens through an aperture 22 into the assembled cylinder 7.

Three position sensing transducers 13 are mounted on the outside surface of the curved wall of the cylinder 7 in order to give an indication of the position of the piston 1 relative to the cylinder 7. One transducer 13 lies adjacent each end plate 8 of the cylinder 7 while the remaining transducer 13 lies approximately halfway along the cylinder 7. The piston disc 5 is provided around its circumference with a magnetised, ferrite loaded "0" ring 14, and the position transducers 13 are magnetic sensing. Each position transducer 13 is adapted to give an indication when the piston disc 5 lies in its close vicinity inside the cylinder 7.

Referring to Figs. 1 and 4, the aperture 9 in the end of the cylinder 7 communicates with a filling chamber 15. The filling chamber 15 forms a first portion of the conduit referred to above. A hopper 16 is mounted above the filling chamber 15, in use, opening into its uppermost wall. The body of the hopper 16 has a rectangular cross section and pairs of parallel walls. At its lower end, the hopper 16 is provided with a tapered spout 17 suitable for feeding polythene granules 2 into the filling chamber 15. The hopper 16 is tilted at 450 relative to the filling chamber 15, its upper end being tilted towards the piston disc 5.

The most exposed major wall is provided with a viewing window 27.

During use of the apparatus the projection 6 of the piston 1 will travel to and fro along the filling chamber 15. The hopper 16 is positioned relative to the filling chamber 15 such that each full rearward stroke of the projection 6, out of the filling chamber 15, allows a charge of granules 2 to be fed into the filling chamber 15 from the hopper 16 while each full forward stroke will drive that charge along the filling chamber 15 into a heating zone 3 which is clear of the hopper spout 17. This type of feed arrangement is known and further details are not therefore given.

The heating zone 3 forms a second portion of the conduit and communicates directly with the filling chamber 15. It is provided with electrical heating elements 23 mounted on the outer surface of its walls. Suitable forms and arrangements of heating elements are known and they are not therefore described.

Between the heating zone 3 and the filling chamber 15 are provided, circumferential grooves 18 on the external surface of the conduit. These grooves 18 act to reduce undesirable heat conduction from the heating zone 3 to the filling chamber 15.

Beyond the heating zone 3 with respect to the filling chamber 15 there is an outlet nozzle 19 which is suitable for attachment to a mould inlet. Set into the wall of the outlet nozzle 19 is a pressure transducer 4 respon sive to the pressure prevailing at the internal surface of the nozzle 19.

Referring to Fig. 5, the apparatus is held in alignment by means of a framework compris ing the cylinder 7 and its end plates 8, four headless bolts 25, four tapped pillars 20 and an additional support plate 28. The support plate 28 has substantially the same dimen sions as the two end plates 8, excluding the extended corner each end plate 8 is provided with for mounting the gas valves 12. The support plate 28 is arranged parallel to the two end plates 8, the three plates 8, 28 be ing mounted at spaced intervals by means of the bolts 25 and the pillars 20. The bolts 25 extend between the corner regions of the two end plates 8, co-operating at a first end plate 8' with retaining nuts 26 and at the second end plate 8" with the tapped pillars 20 to hold the plates 8 against the ends of the curved wall of the cylinder 7.The curved wall of the cylinder 7 seats against the circular seals 21 as mentioned above. The tapped pillars 20 extend between the second end plate 8" and the support plate 28, the support plate 28 being retained against the pillars 20 by short retaining bolts 30.

Referring to Fig. 1, when the injection moulding apparatus is assembled the framework holds the filling chamber 15 in alignment with the projection 6 of the piston 1 by means of an aperture 31 in the centre of the support plate 28 and a cylindrical sleeve 36 mounted at the aperture 9 in the second end plate 8". The filling chamber 15 extends into the sleeve 36 and through the aperture 31, which makes a precision fit with the external surface of the filling chamber 15. The latter is prevented from movement away from and towards the cylinder 7 by means of a machined circumferential flange 32 and a circlip 37 respectively which abut either face of the margin of the aperture 31.The reson for the discrepancy in retaining means for the two directions is that the flange 32 must stand up to what is transmitted to it of the driving thrust of the piston 1 whereas the circlip 37 only has to stand up to any transmitted effects of piston withdrawal.

For ease of handling of the apparatus the support plate 28 is provided with a handle 33 and the framework is mounted on a sled 34 constructed out of a single bent metal tube.

In use, to install a protective sheath over a cable joint in a polythene insulated cable, a suitable mould is mounted to encase the cable joint and a short length of cable to either side of the joint. A set of injection moulding apparatus is mounted at an end of the mould such that the nozzle 19 is coupled to a mould injection point, communicating with the mould interior.

Before the first, mould filling stage of the installation process, the mould is heated. The hopper 16 is charged with polythene granules 2 and the piston 1 then reciprocated to pump granules 2 through the heating zone 3 and into the mould, the piston 1 being pneumati catty driven by compressed air fed in alternation through the gas valves 12. During this filling stage the heating element 23 are activated to heat the polythene granules 2 in the heating zone 3 to a suitable temperature for injection as a melt and the mould continues to be heated.

The pressure transducer 4 monitors the melt pressure at the nozzle 19 and its output is used in controlling the piston motion by modifying the action of the gas valves 12. The maximum melt pressure is maintained at a suitable pressure for injection, such as 300 psi, by this means.

The position sensing transducers 13 can play an important part during this stage. Since the melt pressure at the nozzle 19 will vary during each reciprocation of the piston 1, it can be important to know at which point in its reciprocation cycle the piston 1 lies in or der to avoid compensating for a drop in melt pressure caused only by a return stroke of the piston 1.

By measuring melt pressure at the nozzle 19 it is also possible to avoid inefficient strokes of the piston 1. The polythene granules 2 in the filling chamber 15 will be compacted to varying degress dependent at least in part on the weight of granules 2 present in the hopper 16. If the granules 2 are particularly loosely packed, a forward stroke of the piston 1 may have the effect of compacting the granules 2 rather than of propelling them through the filling chamber 15. By correlating the outputs of the pressure transducer 4 and the position sensing transducers 13, it is possible to detect such a compaction effect when the piston 1 stimulates the middle of the three position sensing transducers 13 on a forward stroke. The piston 1 can then be returned, rather than being allowed to perform the full forward stroke, so that more granules 2 can drop into the filling chamber 15.

There are two alternative methods of detecting that the mould is full. Firstly, the piston 1 will cease to make forward strokes since an attempt to do so would encourage the melt pressure to rise above the selected maximum. The position sensing tranducers 13 will detect the stopping of the piston 1 and this will indicate that the mould is full. Secondly, as a result of the melt not being able to continue flowing into the mould, the temperature gradient from the filling chamber 15 to the nozzle 19 will start to level out, the temperature at the nozzle 19 therefore rising.

Since the volume of melt at the nozzle 19 must remain constant, the melt pressure will start to rise, producing a characteristic surge in the output of the pressure transducer 4. Of the two methods, the first has the advantage of speed.

When the mould is full, the second, cooling stage of the installation process can be entered. This stage involves high pressure combined with slow polythene injection in comparison with the first, filling stage which involves relatively low pressure and fast polythene injection. During the cooling stage, the pressure transducer 4 is set to detect an increased maximum pressure, for instance 1200 psi, and the supply of heat to the mould is stopped.

As the melt in the mould cools it contracts and the high pressure acts to force in more polythene to prevent voids forming in the mould.

Lastly, when the melt has cooled to a temperature at which it begins to solidify, in the case of polythene granules this may be at about 110 C, the maximum pressure the pressure transducer 4 is set to detect is changed to a low setting, such as 150 psi. This avoids any skin formed by the melt, which tends to cool fastest at the mould surface, being pushed inwards to form dimples.

The purpose of the pressure transducer 4 in monitoring the melt pressure at the nozzle 19 during the mould filling stage is to assess the rate of flow of the melted polythene granules 2 from the heating zone 3 into the mould.

Although the pressure transducer 4 does not necessarily have to be positioned as described above, at the nozzle 19, there are therefore constraints on its position. It must lie between the heating zone 3 and the point at which the pressure of the melted granules 2 drops to substantially atmospheric pressure as they enter the mould during the mould filling stage.

Unless this is so, the pressure being monitored will not be indicative of the flow of the melted granules 2 as required. In the present embodiment, the pressure monitored by the pressure transducer 4 is greater than that at which the granules 2 actually enter the mould because of the presence of the viscous melted granules 2 in the nozzle 19 between the pressure transducer 4 and the mould.

However, in other embodiments of the present invention, in order that the pressure monitored during the mould filling stage lie in a useful range of values for indicating flow of whatever moulding material is used into the mould, the pressure transducer 4 may either be placed closer to the heating zone 3 or a restriction may be provided in the nozzle 19 between the pressure transducer 4 and the mould.

It is particularly convenient, though not essential, that the complete sheath installation process described above is controlled by a microprocessor. The outputs of the various transducers 4, 13 can be used to supply the information to a microprocessor which is all it requires to complete each stage of the process and trigger the next. Microprocessor control can also be extended to cover any heat sources involved.

Although the embodiment of the invention described above includes position sensing transducers 13 to monitor the position of the piston 1, it will be clear that there are alternative ways of monitoring the position of the piston 1 which position is closely related to the status of the gas valves 12. Said position can therefore be derived for instance from control signals fed to the gas valves 12. The arrangement may indeed be found to be simplified by the absence of the position sensing transducers 13. In such an arrangement, of the two methods mentioned above for detecting that the mould is full, it would be convenient to use detection of the characteristic pressure surge.

Although an embodiment of the invention is described above with reference to cable joint sheath installation, other arrangements may be found suitable for different purposes. It may then of course be necessary to select different criteria regarding for instance the maximum melt pressures or the position of the piston.

This may also apply were a material other than polythene to be moulded. Microprocessor control is well-suited to such modification.

It is not necessary that a pneumatically driven piston and cylinder arrangement provides the motive power for melt injection but the arrangement as described above lends itself both to cable joint sheath installation, avoiding risk of contamination of the melt, and to automatic control.

It may be found advantageous to line the filling chamber and the heating zone with a non-stick material to improve the reliability of the injection moulding apparatus in action. Further the walls of the filling chamber may be constructed out of a badly heat-conducting material such as steel while the walls of the heating zone may be constructed out of a better heat-conducting material such as brass.

This helps to confine heat to the heating zone.

It should be noted that because the hopper 16 is tilted at 450 to the filling chamber 15, the apparatus can be used with the filling chamber 15 extending at an angle which is anything from horizontal to vertical without necessarily rendering the hopper 16 signficantly inefficient. This arrangement of hopper 16 is not essential but allows the apparatus to be used for a variety of cable installations.

Other angles may be used instead; for instance, any angle lying in the range from 350 to 55Q may be found suitable.

The arrangement described above lends itself particularly well to disassembly for cleaning purposes. For example, the heating zone 3 and filling chamber 15 can be removed without significant disturbance of the cylinder 7.

Because the melt injection to the mould takes place in rechargeable batches, it is not important what volume the mould has. Hence the apparatus is versatile in being usable to fill a variety of moulds.

Analysis of sample runs using apparatus according to an embodiment of the invention has demonstrated that results can be achieved which are so consistently good that such apparatus can be used to provide reference conditions for the analysis of changes to mould designs.

Claims

1. A method of injection moulding, for use in installing protective material on a cable, which comprises the steps of: (i) mounting an outlet of a conduit, the conduit having a heating zone, so as to communicate with a mould inlet; (ii) applying drive power to a pump so as to pump moulding material along the conduit through the heating zone, and into the mould; (iii) monitoring the flow of the moulding material from the conduit into the mould by means of monitoring the pressure of the moulding material inside the conduit at a position between the heating zone and the outlet of the conduit which communicates with the mould inlet; and (iv) varying the drive power so as to maintain said pressure of the moulding material within a preselected range of values suitable for filling the mould.

2. A method of injection moulding according to claim 1, the pump pumping material into the mould during step (ii) by means of reciprocation, which further comprises the step of: (v) monitoring the progress of the pump through its reciprocation cycle; wherein the drive power is varied in a preselected manner in accordance with the relationship between said pressure of the moulding material and the progress of the pump through its reciprocation cycle.

3. A method according to claims 1 or 2 which further comprises the steps of: (vi) detecting a rise in said pressure of the moulding material to a threshold value above the preselected range of values; and subsequently (vii) varying the drive power so as to maintain said pressure of the moulding material within a new range of values, which range comprises values greater than those in the range suitable for filling the moulding.

4. A method according to claims 1 or 2 which further comprises the steps of: (viii) detecting a reduction in the rate of progress of the pump through its reciprocation cycle to a rate which is below a selected minimum rate; and subsequently (iv) varying the drive power so as to maintain said pressure of the moulding material within a new range of values, which range comprises values greater than those in the range suitable for filling the mould.

5. A method according to any one of the preceding claims, for use in installing protective material on submarine telecommunication cables.

6. A method of injection moulding substantially as described hereinbefore with reference to the accompanying Figs.

7. A method of injection moulding sheath material onto a cable, by means of an injection moulding cycle which comprises mould filling and cooling stages, comprising the following steps: (a) mounting an outlet of a conduit, the conduit having a heating zone, in communication with the inlet of a mould; (b) applying drive power to a pump so as inject moulding material along the conduit, through the heating zone, into the mould; (c) monitoring the pressure of the moulding material inside the conduit at a position between the heating zone and said outlet of the conduit; and (d) varying the drive power to the pump such that said pressure of the moulding material follows a predetermined pattern through out the injection moulding cycle.

8. Injection moulding apparatus, for use in a method according to any one of the preceding Claims, comprising a conduit having a feed inlet for the reception of moulding material, a heating zone for melting moulding material received at the feed inlet, and an outlet adapted to be mounted so as to communicate with the inlet of a mould, the internal volume of the mould being greater than the internal volume of the conduit, a pump for pumping moulding material along the conduit from the feed inlet through the heating zone to the outlet, and a power source for supplying drive power to the pump, wherein the apparatus further comprises a pressure transducer mounted to respond to the pressure in the moulding material inside the conduit at a position between the heating zone and the outlet of the conduit, and control means which controls the drive power supplied to the pump, the output of the pressure transducer being operatively connected to an input of the control means.

9. Injection moulding apparatus according to claim 8 wherein the pump functions in a reciprocating manner,#he apparatus further comprising pump monitoring means adapted to monitor the progress of the pump through its reciprocation cycle, the output of the pump monitoring means being operatively connected to an input of the control means.

10. Injection moulding apparatus according to claim 9 wherein the pump comprises a piston which can be driven to reciprocate along the conduit, the pump monitoring means being adapted to monitor the position of the piston along the conduit.

11. Injection moulding apparatus according to claim 10 wherein the pump further comprises a plate and a chamber, the plate being adapted to be driven pneumatically to reciprocate in the chamber, the piston being mounted on a major face of the plate so that such reciprocation of the plate drives the piston to reciprocate in the conduit.

12. Injection moulding apparatus according to claim 11 wherein the pump monitoring means comprises a marker mounted on the plate and detecting means mounted on the chamber, the detecting means being adapted to generate an output indicative of the position of the plate relative to the chamber.

13. Injection moulding apparatus according to claim 12 wherein the output of the detecting means is capable in particular of indicating when the plate is substantially at either end, or in the middle, of its reciprocation path.

14. Injection moulding apparatus according to claims 12 or 13 wherein the marker comprises a magnetised, ferrite loaded ring of elastic material mounted in a groove in the periphery of the plate, and the detecting means comprises a plurality of magnetic sensors mounted so as to be spaced apart on the outer surface of the chamber.

15. Injection moulding apparatus according to any one of claims 8 to 14 which further comprises a feed hopper adapted to feed moulding materials by gravity into the conduit via the feed inlet, which hopper is mounted in use at an angle in the range 1250 to 1450 with respect to the axis of the outlet, in the vertical plane which contains that axis.

16. Injection moulding apparatus substantially as described hereinbefore with reference to the accompanying Figures.