WO1999012720A1

WO1999012720A1 - Method and arrangement for supporting the ends of a resistance wire in an injection mould during manufacturing of electrically heat-weldable plastic tube fittings

Info

Publication number: WO1999012720A1
Application number: PCT/FI1998/000691
Authority: WO
Inventors: Jouni Aleksi Alkio
Original assignee: Tooler Oy
Priority date: 1997-09-05
Filing date: 1998-09-04
Publication date: 1999-03-18
Also published as: EP1011949A1; NO20001086L; HUP0004237A3; FI103023B; EA001743B1; CA2302208A1; TR200001578T2; EA200000199A1; FI973607A; HUP0004237A2; FI103023B1; AU9074198A; KR20010023300A; JP2001515804A; BR9812180A; FI973607A0; NO20001086D0; IL134849A0; CN1291131A

Abstract

The invention relates to a method and an arrangement for supporting the ends of a resistance wire in the manufacture of electrically heat-weldable plastic tube fittings in an injection mould. In the method, an electrically heated resistance wire (4) is wound about the core (1), the ends of the resistance wire (4) are connected to pegs (5, 6), pins (9, 10) are fitted into recesses (holes) (7, 8) provided in the core (1), and the pegs (5, 6) are supported on the core (1) by means of pins (9, 10) connected to them. In accordance with the invention, a guide bushing (11, 12) is disposed in contact with each pin (9, 10) and the connected peg (5, 6), each guide bushing (11, 12) is supported in a recess (13, 14) provided in the injection mould (2), an elastic actuator such as a spring (15, 16) is disposed to act on each guide bushing (11, 12) so that its holding power presses the guide bushing, the peg and the pin into position against the core (1), the injection moulding operation is started by introducing injection moulding compound around the core (1) in the moulding space (3) in a manner known per se, whereby, as the injection moulding compound reaches each pin (9, 10), the plastic pressure lifts the pin (9, 10), the peg (5, 6) and the guide bushing (11, 12) from the core (1) against the holding power of the actuator, such as a spring (15, 16), and after the injection moulding the core (1) is withdrawn from the mantle (2), the mantle (2) including the guide bushings (11, 12) is opened and the finished fitting is removed from the injection mould.

Description

METHOD AND ARRANGEMENT FOR SUPPORTING THE ENDS OF A RESISTANCE WIRE IN AN INJECTION MOULD DURING MANUFACTURING OF ELECTRICALLY HEAT-WELDABLE PLASTIC TUBE FITTINGS

The invention relates to a method as defined in the preamble of claim 1 for supporting the ends of a resistance wire during the manufacture of electrically heat- weldable plastic tube fittings in an injection mould.

The invention also relates to an arrangement as defined in claim 6 for applying the method.

Heat-weldable, thermoplastic fittings, i.e. muffs are intended to tightly fit together plastic tubes made of similar material. Such pipings made of plastic tubes are used e.g. as gas pipes in the processing industry.

The fitting comprises a body with one or more adapter parts for the plastic tubes to be joined. The adapter parts in turn comprise fitting cavities into which the plastic tubes are fitted. The fitting cavities are joined by tube sections. The fitting, in 0 particular the adapter parts, are equipped with a resistance wire. The resistance wire is disposed in connection with the fitting cavity of each adapter part so that it passes by the fitting cavity in the vicinity of its inner surface and extends at least over the length of the joint between one fitting cavity and the plastic tube. The tube sections to be joined are disposed in the fitting cavities of the fitting. An electric current 5 suitable for heat welding is disposed to pass through the resistance wire over a certain period of time, and then both the fitting, the adapter part and the plastic tube mounted in the fitting cavity will be plasticised and melt in the joint area, subsequently solidifying to form a uniform tight unit.

0 Plastic tube fittings are manaufactured with injection moulding techniques. Before the moulding step, the resistance wire is disposed inside the injection mould. The resistance wire is supported on the moulding core either by means of a premoulded fitting body or support pins.

5 In a first method, a first thin precasting, i.e. fitting body, is first manufactured around the moulding core in an injection mould, and after it has cooled, a resistance wire is wound around the body. The fitting body is preferably provided with recesses for connecting pegs, to which the ends of the resistance wire are attached. After the resistance wire including the connecting pegs have been fitted in position in the fitting body, this is returned into the injection mould and a second moulding is performed, whereby the fitting assumes its final shape. The connecting pegs extend beyond the moulded fitting and through these, electric current is supplied to the resistance wire in heat welding. This method involves the problem of the moulding being carried out in two steps. This slows down the production process and entails an increase in production costs.

In a second method, the resistance wire is wound directly on a moulding core, as described for instance in EP patent specification 260 014. In this embodiment, the moulding core has been provided with recesses, into which the connecting peg is attached by means of the support pins for the duration of the moulding. The support pins are removed either entirely or partly from the recesses after the moulding. The portion of the pin located within the recess in the core is removed by cutting. The pin is provided with a contraction such as a groove at the cut. The cutting is performed by means of the moulding core: the core is withdrawn from the finished, cooled fitting, the pins being cut off at the contraction. The top portion of the pin remains within the connecting peg, whereas the remainder remains within the recess in the moulding core, from where it is removed before the following moulding operation. In this conjunction, the removal of the remainder of the pin from the moulding core constitutes a problem: in order not to deteriorate the following fitting moulding, these remaining portions must be carefully recovered. The additional operation step consisting of the removal of the pin remainder constitutes another problem. Optionally, the pin is entirely removed from the core recess by means of an appropriate tool; the pin and the recess are for instance threaded, the pin being then screwed out from the recess after the moulding. This embodiment involves the problem of an additional operation step: removal of the pin from the recess with a tool after the moulding step.

The purpose of the invention is to provide a new method and arrangement for supporting the ends of a resistance wire in the manufacture of electrically heat- welded plastic tube fittings in an injection mould, which resolves the problems described above. To achieve this, the method in accordance with the invention is characterised by the features defined in the characterising clause of claim 1. The arrangement in accordance with the invention, in turn, is characterised by the features defined in the characterising clause of claim 6. Preferred embodiments of the invention are described in the remaining claims. The invention is explained in greater detail below with reference to the accompanying drawings, of which

figure 1 is a vertical cross-sectional view of the support arrangement of a resistance wire of the invention in the manufacture of electrically heat-weldable plastic tube fittings in an injection mould before the injection moulding operation, figure 2 is a vertical cross-sectional view of the support arrangement of the ends of a resistance wire of the invention in the manufacture of electrically heat- weldable plastic tube fittings in an injection mould after the injection moulding operation, and figure 3 is a vertical cross-sectional view of the finished thermoplastic fitting, and figure 4 A is a top view of the attachment of the end of the resistance wire to the peg and the screwing of the peg in the injection moulding step and figure 4B is a top view of the position of the resistance wire and the pin at the end of the injection moulding step, figure 5 shows a second support arrangement for the end of the resistance wire, however, illustrating one single peg and pin.

An arrangement for supporting the ends of a resistance wire in the manufacture of electrically heat-weldable plastic tube fittings in an injection mould is illustrated in figures 1 and 2. The injection mould comprises a core 1 and a mantle 2, and a moulding space 3 provided between these. The electrically heated resistance wire 4 is wound around core 1. Resistance wire 4 is attached at either end 4a, 4b to peg 5, 6. Core 1 has been provided with two recesses 7, 8, preferably holes or bores. Both pegs 5, 6 are connected to a pin 9, 10, which is fitted in recess 7, 8. With the aid of pins 9, 10 and pegs 5, 6, resistance wire 4 is retained in position around core 1 during the moulding. Resistance wire 4 is additionally fitted in the area of core 1, where the adapter parts of the fitting are formed in the moulding step for the tubes to be joined. The ends of both pegs 5, 6 will project from the finished fitting (cf. figure 3), to allow the connection of a power source during the joining of the tubes, as described below.

The arrangement for supporting the ends 4a, 4b of resistance wire 4 further includes two guide bushings 11, 12, disposed to support two pins 9, 10 respectively, and recesses 13, 14, such as holes or bores for each guide bushing 11, 12, in the mantle 2 facing the moulding space 3. With these recesses 13, 14, guide bushings 11, 12 are supported essentially vertically against the surface of core 1 and movable towards recess 7, 8 into the mantle 2 of the injection mould. The arrangement also comprises a flexible actuator disposed in connection with each recess 13, 14, in this case a spring, particularly a coil spring 15, 16. It is disposed behind guide bushing 11, 12 in a recess, such as a hole, thus being supported on the one hand by the recess bottom, i.e. in this case the hole stopper means 17, 18 acting as a recess, and on the other hand by the rear surface of guide bushing 11, 12. Under the springback action, spring 15, 16 thus presses guide bushing 11, 12, and through this also pin 9, 10 and peg 5, 6, retaining them in position in recess 7, 8. Pins 9, 10 and also guide bushings

11, 12 are disposed to rise against the springback action of spring 15, 16 in recesses 7, 8 under the pressure of the moulding compound during the injection moulding step. In the beginning of the injection moulding step, spring 15, 16 ensures that no moulding compound enters the space between pin 9, 10 and guide bushing 11,12.

An elastic actuator can be provided also in some other way than by means of coil spring 15, 16 as described above. The purpose of the elastic actuator is on the one hand to retain pin 9, 10 in position in recess 7, 8 before the injection moulding step starts and on the other hand to yield during the injection moulding step, as molten plastic is being introduced into the moulding space 3 of the mould, its pressure acting against the holding power of the actuator. It should be further noted that this holding power, by means of which the elastic actuator has been disposed to act on guide bushing 11, 12 and through this on pin 9, 10 and peg 5, 6, should be of and order of magnitude such that it does not press these means 11, 12; 9, 10; 5, 6 back into the molten moulding compound, as moulding space 3 is being filled with molten plastic and the plastic feed pressure subsides at the end of the injection moulding step.

The elastic actuator used is for instance a hydraulic or pneumatic cylinder, by means of which guide bushing 11, 12 and through this pin 9, 10 and peg 5, 6 are maintained inserted into recess 7, 8, the pressure of the molten plastic composition acting against the cylinder pressure in the injection moulding step. The cylinder is preferably mechanically driven so that it is pulled up along with guide bushing 11,

12, pin 9,10 and peg 5, 6 as the injection moulding step proceeds, and remains in the upper position at the end of the injection moulding step, these means being thus prevented from being pressed back into the molten plastic and solidifying in precisely the desired position in the moulding compound.

In a preferred embodiment of the invention, recesses 7, 8 in pins 9, 10 are disposed to extend through the core 1 of the injection mould. Thus, during the injection moulding step, the plastic pressure will act on pin 9, 10 directly through recess 7, 8 on the opposite side of core 1.

In a preferred embodiment of the invention, pins 9, 10 and guide bushings 11, 12 are disposed to raise to the desired level above the surface of core 1 under the action of the plastic pressure. For this purpose, a stopper 19, 20, such as a pin or annular constriction of recess 13, 14 is disposed in recess 13, 14 of guide bushing 11, 12. Stopper 19, 20 is disposed at a distance from the surface of core 1 and at the same time from the rear surface of guide bushing 11, 12 supported on pin 9, 10 or similar under the action of spring 15, 16 such that guide bushing 11, 12 may rise to the desired height, i.e. at least to a level high enough to allow pin 9, 10 to be entirely withdrawn from recess 9, 10 in the injection moulding step. Thus, after the injection moulding step, core 1 may be withdrawn directly from mantle 2 and the finished fitting.

In a preferred embodiment of the invention, each pin 9, 10 includes a flange 21, 22 or a similar annular projection. It is disposed in the upper portion of pin 9, 10 so as to remain outside recess 7, 8 and hence on the surface of core 1 or above this. Peg 5, 6 is connected with pin 9, 10 on the other side of flange 21, 22. Peg 5, 6 and pin 9, 10 constitute a coaxial uniform piece. Guide bushing 11, 12 is disposed to rest on flange 21, 22. If flange 21, 22 is disposed so as to form a gap 23, 24 between the flange and core 1, as illustrated in figure 1, the injection moulding compound will act on flange 21, 22 through gap 23, 24 in the moulding step and lift pin 9, 10, even if recess 7, 8 in pin 9, 10 would not extend through core 1. It should be noted that the cross-sectional area of the end of guide bushing 11, 12 essentially equals the area of flange 21, 22, and hence the pressure of the plastic to be injected will not act directly on the end of guide bushing 11, 12, the pressure being applied through the lower surface of flange 21, 22 to flange 21, 22 and further to guide bushing 11, 12 and to spring 19, 20 connected to this, and acting against the springback factor.

A preferred attachment of the end 4a of resistance wire 4 to peg 5 and especially to associated pin 9 is illustrated in figures 4 A and 4B. These figures show detail A- A as a top view of figures 1 and 2. End 4a of the resistance wire is attached to peg 5 and pin 9 so that resistance wire 4 is wound about peg 5 and pin 9. As peg 5 and pin 9 rise in the injection moulding step under the pressure of the molten plastic against the holding power of guide bushing 11 (cf. figure 1 and 2), such as the springback factor of spring 15, resistance wire 4 is unwound from peg 5 and pin 9, and thus the resistance wire acquires additional length to the extent required by the rising of peg 5 and pin 9. In figure 4 A end 4a of resistance wire 4 has been spot- welded at point a on pin 9, particularly at the edge of flange 21 pertaining to pin 9, and resistance wire

4 has been wound e.g. less than one turn, such as half a turn about flange 21. As peg

5 and pin 9 rise from recess 7 in the moulding step (cf. figures 1 and 2), they are also wound relative to recess 7 (direction B) and resistance wire 4 is released, i.e. is unwound from flange 21, and owing to this the attachment of end 4a of resistance wire 4 remains intact, while there is still sufficiently of free resistance wire 4 to allow pin 9 to rise from recess 7. The length of pin 9 and at the same time the minimum depth of recess 7 is e.g. of the order of 5 mm, the diameter of flange 21 being e.g. 10 mm. When resistance wire 4 is wound 1/4 turn about flange 21, the 90° rotation of pin 9 and flange 21 in direction B produces more than 7 mm of released wire 4.

In a second preferred embodiment of the invention, illustrated in figure 4, each pin 25 includes a flange 26 or a similar annular projection. Pin 25 is fitted into recess 27 in core 1 so that flange 26 is on the surface of core 1 (or optionally above this, as in the preceding embodiment example). Guide bushing 29 rests on flange 26. In this embodiment, one or more grooves or cavities 30 are provided in the lower surface of flange 26. In the injection moulding step, the plastic pressure will thus be applied through cavity 30 to the lower surface of flange 26 and further to guide bushing 31 and to the associated spring (not illustrated in this figure) lifting pin 26 including peg 28 from recess 27. Except for the design of flange 26, this arrangement corresponds to that of figures 1 and 2 on principle; however, recess 27 in pin 25 does not extend through core 1 (even if this is feasible).

With reference to figures 1 and 2, the following process steps are performed in the method of the invention for supporting the ends of the resistance wire in the manufacture of electrically heat-weldable plastic tube fittings: a) The electrically heated resistance wire 4 is wound about core 1 in the area of core 1 in which adapter parts 33, 34 of the fitting will be formed in the moulding step (cf. figure 3) for the tubes to be joined. b) The ends 4a, 4b of resistance wire 4 are connectd to pegs 5, 6 e.g. by spot welding. Usually one end of resistance wire 4 is first attached to the first peg 5 and the wire is preferably wound about this, and then the resistance wire is wound on the surface of core 1, and only then the second end of the resistance wire is attached to the second peg 6 and wound about this as well (cf. figures 4A and 4B). c) Pins 9, 10 are fitted in the recesses, such as holes 7, 8 provided in core 1. d) At the same time, pins 5, 6 are supported on core 1 with the aid of pin 9, 10 included in each peg. e) Guide bushing 11, 12 is disposed in connection with each pin 9, 10 and peg 9, 10. Most preferably, guide bushing 11, 12 is disposed to rest on flange 21, 22 belonging to pin 9, 10. f) Each guide bushing 11, 12 is supported in a recess 13, 14 provided in mantle 2. g) Spring 15, 16 or a similar actuator is disposed to act on each guide bushing 11, 12 so that its holding power presses guide bushing 11, 12, peg 5, 6 and pin 9, 10 into position against core 1. In particular spring 15, 16 presses guide bushing 11, 12 against the flange 21, 22 of pin 9, 10. h) Injection moulding is started by introducing molten moulding compound through opening 40 into moulding space 3 in a manner known er se, the moulding compound spreading about core 1. i) During the injection moulding process, as the molten compound reaches each pin 9, 10 (either through a recess 7, 8 extending through core 1 and/or through a gap 23, 24 between flange 21, 22 and core 1, acting on the lower surface of flange 21, 22 and/or through cavities 30 in flange 26 acting on the lower surface of flange 26), the plastic pressure will lift pin 9, 10, peg 5, 6 and guide bushing 11, 12 from core 1 against the holding power of an elastic member, such as spring 15, 16, as illustrated in figure 2. Pin 9, 10, peg 5, 6 and guide bushing 11, 12 will preferably remain at least nearly in upper position, pegs 5, 6 then distinctly projecting from the finished fitting. j) After the injection moulding step, core 1 is withdrawn from mantle 2, mantle 2 and guide bushing 11, 12 are opened and the finished fitting is removed from the injection mould.

In this arrangement, the injection mould is intended for the manufacture of a thermoplastic fitting comprising a straight cylindrical body 32 and two adapter parts 33, 34 disposed at the ends of body 32, as shown in figure 3. Adapter parts 33, 34, in turn, include fitting cavities 35, 36, into which the plastic tubes are fitted. Fitting cavities 35, 36 are interconnected either directly or with a short tube section 37. The fitting, in particular adapter parts 33, 34, are equipped with a resistance wire 4. Resistance wire 4 has been disposed in the join area between the tube to be connected, the fitting cavity 35, 36 of each adapter part 33, 34 and the fitting. Resistance wire 4 passes by fitting cavity 35, 36 in the vicinity of its inner surface and extends at least over the length of the joint between one fitting cavity and the plastic tube. The tube sections to be joined are fitted in the fitting cavities 35, 36 of the fitting. An electric current suitable for heat welding is connected to pegs 5, 6 and is thus disposed to pass through resistance wire 4 for a determined period, and then both the fitting, adapter part 33, 34 and the plastic tube fitted in fitting recess 35, 36 will be plasticised and melt in the join area and subsequently solidify to form a uniform, tight unit.

The invention has been described above by means of a number of preferred embodiments, however, the invention can of course be applied in many ways within the scope of the inventive idea defined in the accompanying claims.

Claims

1. A method for supporting the ends of a resistance wire in the manufacture of electrically heat-weldable plastic tube fittings in an injection mould, whereby - an electrically heated resistance wire (4) is wound about the core (1),

- the ends of the resistance wire (4) are connected to pegs (5, 6),

- pins (9, 10) are fitted into recesses (holes) (7, 8) provided in the core (1), and

- the pegs (5, 6) are supported on the core (1) by means of pins (9, 10) connected to them, characterised in that

- a guide bushing (11, 12) is disposed in connection with each pin (9, 10) and the connected peg (5, 6),

- each guide bushing (11, 12) is supported in a recess (13, 14) provided in the injection mould (2), - an elastic actuator, such as a spring (15, 16), is disposed to act on each guide bushing (11, 12) so that its holding power presses the guide bushing, the peg and the pin into position against the core (1),

- the injection moulding process is started by introducing moulding compound around the core (1) in the moulding space (3) in a manner known er se, whereby - as the injection moulding compound reaches each pin (9, 10), the plastic pressure lifts the pin (9, 10), the peg (5, 6) and the guide bushing (11, 12) from the core (1) against the holding power of the actuator, such as a spring (15, 16), and that

- after the injection moulding, the core (1) is withdrawn from the mantle (2), the mantle (2) including the guide bushing (11, 12) is opened, and the finished fitting is removed from the injection mould.

2. A method as claimed in claim 1, characterised in that the recess (7, 8) in each pin (9, 10) is disposed to extend through the core (1).

3. A method as claimed in claim 1 or 2, characterised in that the pin (9, 10) and the guide bushing (11, 12) are disposed to rise under the plastic pressure to the desired level above the core (1).

4. A method as claimed in claim 1, 2 or 3, characterised in that the resistance wire (4) is wound into contact with the peg (5, 6).

5. An arrangement for supporting the ends of a resistance wire in the manufacture of electrically heat-weldable plastic tube fittings in an injection mould, comprising the core (1) and the mantle (2) of the injection mould and a moulding space (3) provided between these, the electrically heated resistance wire (4) being wound about the core (1), the arrangement comprising

- pegs (5, 6), the ends of the resistance wire (4) being attached in connection with these,

- recesses (7, 8) provided in the core (1),

- pins (9, 10) for supporting the pegs (5, 6), the pins being fitted in the recesses (7,

8), characterised in that the arrangement further comprises - guide bushings (11, 12), which are disposed to support the pins (9, 10),

- a recess (13, 14) in the mantle (2) for each guide bushing (11, 12), the guide bushings (11, 12) being supported through the recesses (13, 14) on the mantle (2) of the injection mould, and - an elastic actuator, such as a spring (15, 16) in each recess (13, 14) behind the guide bushing (11, 12), the actuators (15, 16) serving to retain the guide bushings (11, 12) and with these the pins (9, 10) in position, the pins (9, 10) and further the guide bushings (11, 12) being disposed to rise from the recesses (7, 8) under the action of the plastic pressure against the holding power of the actuator (15, 16) during the injection moulding step.

6. An arrangement as claimed in claim 5, characterised in that the recess (7, 8) in each pin (9, 10) is disposed to extend through the core (1).

7. An arrangement as claimed in claim 5 or 6, characterised in that each pin (9,10) and guide bushing (11, 12) is disposed to rise under the plastic pressure to the desired level above the core (1), a stopper (17, 18) having been positioned at this point.

8. An arrangement as claimed in any of claims 5 to 7, characterised in that each pin (9, 10) comprises a flange (21, 22) or a similar projection.

9. An arrangement as claimed in claim 8, characterised in that the flange of the pin (25) is provided with a groove or cavity (30) on its lower surface.

10. A method as claimed in any of claims 5 to 9, characterised in that the resistance wire (4) is wound into contact with the peg (5, 6), especially the pin (9, 10).