CA1261582A

CA1261582A - Method of and apparatus for spin-welding

Info

Publication number: CA1261582A
Application number: CA000509871A
Authority: CA
Inventors: Malcolm G. Collins; Kenneth R. Clark; Keith E. Nuttall; Suresh G. Panvalkar
Original assignee: MB Group PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1985-05-24
Filing date: 1986-05-23
Publication date: 1989-09-26
Also published as: IN167280B; ZW10486A1; GR861340B; ZA863590B; ES555264A0; AU581760B2; NZ216197A; AU582852B2; GB2175536B; FI83941B; NO862067L; FI862175A0; NZ216199A; FI862177A0; DE3669128D1; US4743331A; GB8513240D0; ATE50531T1; IN167320B; MY101219A

Abstract

METHOD OF AND APPARATUS FOR SPIN-WELDING

ABSTRACT
During spin-welding of an end ring to a container body, the weld area is subjected to a radial pressure by means of a wire cable tourniquet under the action of a pneumatic cylinder. The interference fit between the ring and body is measured by reference to the torque required for their slow speed relative rotation or to the axial force required for their assembly and, according to the measured interference fit, the radial pressure applied by the tourniquet during welding is controlled.

Description

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METHOD OF ~ND APPARATUS FOR SPIN WELDING
BACKGROUND OF THE INVENTION
The inven-tion relates to spin-welding, which is a known techni~ue for welding together plastics components which are assembled with opposed annular surEaces, in which one of the components is spun at high speed relative to the other to cause melting and subseguent fusion of the plastics material at the interface of the opposed surfaces.
DESCRIPTION OF THE PRIOR ART
Our co-pending Canadian patent application No. 507,787 filed April 28, 1986 describes a method of spin welding a plug fit end component within a side wall for a container. During spinning, the end component and container body are urged together by both axial and radial forces. The radial forces acting on the surfaces to be welded are the result of an interference fit between the components and an external radial compressive force which is applied during welding. Due to the variations in size of each component, manifest in plastic moulding tolerances, -the interference fit between any two components may vary considerably. Such differences in component size can occur, for example, through inconsistencies in any one polymer grade used and also, more importantly, as a result of the general industrial practice of basing plastics moulding production on at least two alternative grades of polymer from different suppliers, such grades often exhibiting signi~icantly different mould shrinkage characteristics when converted into moulded articles.

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Significant dimensional variation in mouldings may also arise from alteration of mould cycle time, injection pressure, melt temperature and temperature of any cooling water used as is well understood in the art.

Since the rate of frictional heat generation during the spinning o~
the components ls directly proportional to the contact pressure between the welding surfaces, for any one chosen set of welding conditions defining spin duration and speed, the weld quality can range from a tacky weld, as a result of insufficient melt being formed at the interface between excessively loose fit components, to a fully-fused ~leld with massively excessive melt formation between excessively tight fit components. In the latter case, it is poss;ble to over-weld to such an extent that any anti-flash features provided adjacent the main weld area become overwhelmed by the melt produced to such an extent that aesthetically unacceptable debris or flash becomes visible on the external surface of the finished article.

The ideal axial pressure needed to maintain the components in their assembled condition and, where necessary, to cause a small inward progression of one component rèlakive to the other for reasons associated with anti-flash measures, will also vary according to the contact pressure between the opposlng surFaces during weld~ng.

~Z6:~5~32 SUi~MARY OF THE INYENTION
1, The object of the present invention is to provide a method and apparatus for spin-welding together two opposed annular surfaces of plastic components wherein ~he contact pressure between the two opposed surfaces during spin-welding is maintained approximately equal to a predetermined.value, despite the variations in size of each component which result from the normal plast-cs moulding tolerances, through the controlled application of a radial pressure.

According to a f;rst aspect of the -invention there ls provided a method of spin-welding together two opposed surfaces of thermoplastics components which when assembled together prior to welding have an interference fit, the method comprising the steps of:
a~ engaging the components with one another and moving them into the assembled position;
b) applying a rad;al pressure to the assembled components in the region of the opposed surfaces; and c) whilst the radial pressure is applied, spinning the components relative to one another at a speed and for a time sufficient to cause welding of the opposed surfaces;
d) measuring a parameter which is a function of the force required to cause relative movement of the engaged components before applying the radial pressure; and e) selecting the value of the radial pressure applied according to the value of the measured parameter such that the ~l2~3L~ Z

contact pressure between the opposed surfaces is controlled during welding.

According to a second aspect of the invention, there is provided apparatus for spin-welding together two opposed surfaces of thermoplastics components which when assembled together prior to welding have an lnterference fit, comprising means for applying a radial pressure to the assembled components in the region of the opposed surfaces, means for spinn1ng the components relative to one another while subjected to the radial pressure, means for measur;ng a lo parameter which is a function of the force required to cause relative movement of the engaged components before application of the radial pressure, and means for selecting the radial pressure according to the value of the measured parameter.

One advantage of the invention is that satisfactory welds can be achieved for components over a large range of interference fits There is a further advantage in the use of an automatically compensating radial pressure, in that combinations of components having different diametric interferences no longer require different axially applied pressures to prevent axial outward d~splacement of one component relative to the other during the welding cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE.l shows a longitudinal section through a cylindrical container body and an end component therefor;
fIGURE 2 is a side view of a spin-welding machine; fIGURE 3 ;s a dlagrammatic sectional view of the mach~ne oF Figure 2 taken along line A-A;
fIGURE 4 is a longitudinal sectional view through the spin-welding head of the mach~ne;
FIGURE 5 is an enlarged view of part of the spin-welding head showing a container body and end component engaged therewith;
FIGURE 6 is a side elevational view of a device for exerting radial compressive pressure which is mounted on the spin-welding head;
FIGURE 7 is a diagrammatic sketch showing the overlap of a wire 1~ cable employed in the spin-welding head of the machine;
as shown with EIGURE 5;
FIGURE 8 is a graphic representation of the machine cycle;
FIGURE 9 is a diagrammatic time/velocity graph for the spin motor of the machine; 0 FIGURE 10 is a block diagram showing the control system for the spin welding machlne; and FIGURE 11 is a block diagram showing the control system For the device shown in Figure 6.

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DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Figure l, there is shown a container comprising a moulded plast~cs cylindr1cal body l provided wlth an lntegral bottom panel 2 and a plug f1t end component in the form of a moulded plastics ring 3 adapted to be assembled into the open end of the body~ The bottom panel of the body has a plurality of external webs 4 wh1ch engage fixed pins located on the ram of the mach1ne such that they prevent the body rotating during welding. The ring 3 also has a number of external webs 5 wh1ch engage one or more driv~ing pins in the spinning head of the machine and thereby provide drive for the spin-welding process. Components such as shown in Figure l are described in greater detail in our co-pending Canadian patent application ~ao.
507,7~7.

lS As shown ~in Figure 2, the spin-weldlng machine is supported on a frame 6 and has a main AC drive motor 7 driv1ng a primary drive shaft 8 through a geared speed reductlon unit 9, dr-ive belt 90, and a pneumatic clutch 10. The clutch is remotely operated by a programmable control system (F-igures lO,ll). A hand wheel ll may be used for manual rotation of the drive shaft during sett-ing up. A
brake (not shown) may also be prov-ided. The feed mechanism 12, by which pre-assembled conta1ners are fed to the work station, is dr-iven from a secondary dr-ive shaft 13 wh-ich is itself driven in an index-ing motion from the shaft 8 via a Geneva mechan-ism 14. A ram assembly lS, !32 also driven from shaft ~, is operative to push the contalner at the work place into and out of engagement with a spin-welding head 16.
The spin-weld1ng head is driven by a servo motor 17 controlled by a switch unit 18 driven off the shaft 8.

The body l and ring 3 are pre-assembled before welding and are fed to the machine by a feed mechanism shown in Figure 3. The pre-assembled containers have already been turned on their sides before be~ng fed into the machine so they can roll down the infeed chute 19. A gate 20, which is shown only diagrammatically in Figures 2 and 3, stops them before they can reach a transfer turret 21 mounted for intermittent rotation on the secondary drive shaft 13. The gate is t;med in sequence with the machine from the switch unit 18 and is operated when the transfer turret has stopped rotating. The gate 20 moves sideways until a container therein is lined up with turret guides. The container is then free to drop under gravity into the transfer turret. The sideways movement of the gate 20 causes it to interfere with the next following container in feed chute 19, preventing it from dropping. After a predetermined period, the gate is returned to its original position, allowing the next conka~ner to drop into the gate. A pneumatic cyllnder 200 with a solenoid valve (not shown) is used to operate the gate.

Rotation of the transfer turret 21 carries the containers from the ~nfeed to the work station W and then to the discharge chute 25. In this example the interrupted motion of the transfer turret is provided

2~ 3 by the Geneva mechanism 14. The transfer turret comprises a pair of plates 23 mounted on shaft 13 and having peripheral part-circular cut-outs therein to support the containers during their travel thereon. Outside guides and a rail prevent the containers being dlsturbed while the turret rotates.

The sp~n-weldlng process ~s carried out at the work stat~on which is shown at ~ in Figure 3. At the work station, the container is held between the spin welding head shown in Figures 4 and 5, and the ram lo assembly 15.

The spin-welding head comprises a low inertia mechanism driven by a DC
low inertia rare earth brushless servo motor 17 as shown in Figure 4.
Drive is taken to the spin welding head shaft 26 via a toothed belt 27. The driven pulley 37 is mounted on one end of the shaft 26 which is horizontal. A disc 28 of li~htweight alloy is bolted directly to the opposite end of the shaft 26. Machined in its exposed ~ace, the disc 28 has driving pins 29 which engage in the ring 3 and cooperate with the external webs 5 thereof to cause the ring to be driven in rotation. In order to keep friction as low as poss~ble the shaft 26 ~s mounted in two ball races 30.

The ram movement, to push a container ~nto the sp~n welding head, ls actuated by a cam 38 (F~gure 2) dr~ven at the mach~ne cycle speed on ~2~S~Z

the shaft 8. This cam action is transferred to the ram slider by a lever arm 51 pivoted at 52 and a connecting link 50. ~ogs 151 located on the front face of the ram engage with the webs 4 to prevent the body l from rotating during welding.

~Ihen the container formed by the assembled container body l and end component 3 is pushed by the ram assembly 15 into engagement with the spin-welding head 16, the end face of the end component 3 co~es into contact with an ejector ring 46 (Figure 5) which yields axially under the action of a plurality of coil springs 47 spaced circumferentially lo around the ring 46. The ring 46 is held in the position shown in Figure 5 during welding and the correct end pressure for the welding process is provided by the coil springs 47 via the ejector ring.

Radial pressure is applied during the welding process by means of a tourniquet comprising a loop of steel wire cable 39 which is retained ~n an annular groove 42 i~ a cable retaining housing 45 mounted on the spin-welding head. When the tourniquet is in a relaxed condition, ~t forms a loop having a diameter sl1ghtly greater than that of the container to be welded. When no container is held in position for welding, the ejector ring moves axially under the influence of the springs 47 to close off the annular groove 42 and to retain the wire cable 39 therein, as shown in Figure 4.

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As shown in Figure 6, one end oF the cable 39 is rigidly anchored at 40 whilst the other end ;s attached to a pneumatic cylinder 41. A
small release area 43 is cut out of the cable retaining houslng 45 to allow the cable to cross over at 44 at the cable's entry and exit po1nts. During the operation of the machine, the assembled conkainer body and end component are fed into the cable retainlng housing and through the loop of the cable 39. The driving pins 29 are located as described below and the pneumatic cylinder 41 is operated to apply a tension to the cable such that the cable loop diameter is decreased, thereby producing the desired external pressure on the body l necessary for the spin-welding process. After welding, the pneumatic cylinder 41 is returned to its original position, releasing the external pressure and allowing the container to be ejected. As the ram assembly 15 moves back, the ejector ring 46 is free to move forward, thereby pushing the now welded container out of the cable retaining housing and closing the annular groove 42. The manner of overlap of the wire cable is shown more clearly in Figure 7.

For any given components to be spin-welded, there will be a preferred or medium interference fit which occurs when both components conform ~0 exactly to their design dimensions. Due ko the variations in component size, within normal plastics moulding tolerances, the interference fit between any two components may differ significankly from the preferred value. A range of interference fits which can lead ~z~

to successful welds under commercial conditions may be defined as extending from a loose fit having a diametric interference substantially less than the med~um fit to a tight fit having a diametrlc 1nterference substantially greater than the medium fit. The specific values of diametric interference for "loose", "med~um" and "tight" fits will, of course, vary according to the nature of the components being welded.

In the case of the components constructed and dimensloned as described 1n our co-pending Canadian patent application No. 507,787, the preferred or medium interference fit between the components is about 0.75mm (that is, the internal diameter of the body 1 when relaxed is 0.75mm less than the external diameter of the ring 3 when relaxed~.
In this case, a loose fit may have a diametric interference of about 0.25mm and a tight fit may have a diametric interference of about 1.25mm. As a consequence of these variable degrees of fit between end component and side wall, the contact pressure between the surfaces to be welded, without application of addit1Onal radial pressure, can be calculated to range between 0.02 Newtons/mm2 and 0.17 Newtons/mm2.

For one set of welding conditions, for example, derived from exper~ments us~ng polypropylene rlngs and bodies, the ideal contact pressure between the surfaces dur~ng weld~ng has been found to be about 0.23 Newtonstmm2. It has been shown that for the example / r~\

~2~ 32 quoted, the radial pressure required to maintain the contact pressurebetween the welding surfaces at about 0.23 Newtons/mm2 needs to be about 0.25 Newtons/mm2 for the loose fit situation of 0.25mm diametric interference, and about 0.07 Newtons/mm2 for the tight fit situation of 1.25mm dlametric interference. Such pressures enable both extremes of fit to be satlsfactorlly spin-welded to give a fully-fused integral jo;nt in a total t1me of about 0.21 seconds at lOOOrpm, wherein 0.05 seconds is requlred to accelerate the end component to the required speed, and 0.08 seconds is required to electromagnetically brake the lo system to the stationary position. This total time can be reduced by choosing a greater contact presssure or by increasing the motor speed, or both.

In order for the correct radial pressure to be applied by the tourniquet during welding, it is necessary for the interference fit between the container body and the end component to be measured. Two methods have been shown to be useful and are explained below as examples.

The first is based on the force required to fully assemble the end component 1nto the side wall of the conta1ner prior to spin-weldlng.
~ It has been found that a substantially linear relationship exists between th~ interference fit and maximum force of assembly, durin~
which the side wall 1s caused to deform in an elastic fashion because of the diametric interference between the end component and the side wall, although the general shape of the insertion force profile can be influenced by the design of the container and the end component. The measurement of the insertion force may be made on a prelim~nary assembly machine whlch fits rings 3 to bodies l or at an assembly station that forms an integral part of the spln welding apparatus.

The second method is based on the measurement, on the spin-welding machine itsel~, of the current required by the spin-welding servo motor to produce a torque sufficient to spin, at a low speed such as lo 70rpm, the end component within the side wall when fully assembled.
This is a particularly appropriate method since it is convenient to programme the servo motor to spin at such low speed for say 0.10 seconds at the very beginning of the welding cycle to facilitate engagement of the drive webs 5 on the plug by the correspond~ng drive pins 29 on the spin-welding head before accelerating to the much higher welding speed. The engagement time can be extended ~or a short period such as 0.1 second during which time the torque required for slowly sp~nning the end component within the side wall can be measured and related to a diametric interference between end component and side wall by means of a previously establlshed correlation relationship.

For the components shown in Figure 1, which relate close1y to those described ~n Canadian patent application No. 507,787, the current ,, \~, ...
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required to slow~spin the end components within the side wall has been shown to vary in an essentially l;near fashion with diametric interference between end component and side wall. Thls relationship can be made even more pronounced if a radial pressure ls applied to the side wall during measurement. Thus the degree of fit ex1sting between a container body and an end component therefor to be we1ded can be measured since d1fferent fits will requlre diFferent currents to drive the servo motor at the chosen fixed low speed.

A voltage is generated by a servoamplifier which is an analogue of the lo motor current. This measured voltage is received in a control system where it is compared with the voltages predetermined to represent various fit types e.g. loose, medium and tight. ~ach of the various fit types will in practice represent a band of the full range of possible fits extending from the very loose fit to the very tight fit. The sensitivity of the system will depend on the number of such bands which are distinguished.

The control system will also identify and lead to the ejection of assembled components wherein the interference fit is either so loose or so tight that a satisfactory weld will not be possible under the prevailing operat1ng conditions.

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When the type of fit has been identified by the control system, a signal is sent to the machine's pneumatic system, choosing one of a series of solenoid valves that each have had their pressure pre-set to suit one of the fit types. Therefore, in this way, the correct external pressure can be applied by the pneumatic cylinder 41 to the cable 39 for any range of flts between the end components and the container bodies. There is, of course, no limit to the fit types whlch may be identified in this way and the system can provide a direct correlation between the interference fit of the components and the appropriate corresponding radial pressure to be applied.

Although a measurement of voltage has been used in this example as a means for comparing the torque applied by the motor to spin the end component relative to the container body against the frictional force of the interference fit, other parameters relating to this torque could be measured as an alternative according, for example, to the type of motor employed. Thus a predetermined constant torque may be applied by the motor and the resulting speed of rotation measured.

The drive of the servo mokor is governed by an ampli~ier which can be controlled through a programmable logic controller to prov~de the requ~red time velocity profile for the motor during the welding process. Operatlon o~ the motor 17 is t1med to the machine cycle ~rom the switch unit 18. On initiation o~ the weld process the amplifier i82 is energised and the motor is run at a slow speed for a short time to enable the driving pins 29 of disc 28 to engage the external webs ~ of ring 3 and fixed dogs 151 of the ram to engage the webs 4 on the base of the container. After the webs have been engaged, the current required to run the servo-motor at the predetermined slow speed is measured, to classify the intenference fit, and the appropriate radial pressure to be applied by the tourniquet is selected. After this the output of the amplif1er ls ramped such that the motor is rapidly accelerated to its welding process speed and retained at this speed for a period decided by the nature of the particular container type being welded. At the end of the weld time the amplifier is de-energised and the motor is stopped by the friction generated a~ the weld. Stopping of the motor can also be assisted by braking or by powered ramping down of the speed.

Figure 8 is a graphic representation of the machine cycle oYer one rotation of the primary shaft 8. Lines AB and HJ represent the movement of the turret through one indexed motion (i.e. 90 of the secondary shaft 13). The curves CD and FG represent the forward and return movements of the ram. As can be seen, there is a slight overlap of the ram motion with the movement of the turret. In the period defined between po~nts D and F the ram is stationary in its forward position and during this period the welding takes place.
During the period D to E the motor 17 is driven at slow speed to ~z~

enable the fixed pins 29 on the spin welding disc 28 to locate on the webs 5 of the ring and the dogs 1~1 on the ram to locate on the webs 4 on the base of the container and to allow the appropriate radial pressure PW to be selected. Between the points E and F the motor 17 is accelerated up to weld speed, retained at weld speed for the required per~od for welding to occur, and stopped either through friction at the weld or through braking means referred to earller.
Figure 9 is a diagrammatic timett)/velocity(v) curve for the motor 17 during the period between the points D and F. From Figure 9 it will be seen that the motor 17 is stopped before the point F. The period XY during which welding takes place can be altered according to the nature of containers being formed.

The current taken by the motor 17 over the welding period XY may be monitored and compared with a previously established datum. If the load on the motor applied by the weld is below a predeterm~ned level, a satisfactory weld will not be formed. By monitoring the current taken by the motor during the weld period, unsatisfactory welds can be identified and the container rejected.

Suitably, the period defined between the points D and F may be of the order of 0.4 seconds.

Wh~lst a pneumatlc cylinder 41 has been described for applying tension 2 r to the cable 39 in order to apply an inwardly directed radial pressure to the wall 1 the cylinder 4 may if desired be replaced by a solenoid or a servo motor system connected directly to the cable.

Whilst the cable 39 is able to apply a narrow hoop of radial pressure to ~he end wall 1 it may be replaced, lf desired by a ring of segments each urged against the wall 1 by, for example, pneumatic or hydraulic means.

Figure 10 shows diagrammatically the functional interrelationship between the different parts of the apparatus, and in particular, the control connec~ions between the spin-welding machine and the programmable logic controller.

Figure 11 is a diagrammatic representation of the system for controlling the application of radial pressure, and shows in particular the control connections between the electronic system for measuring the parameter which represents the interference fit, the programmable logic controller and the tourniquet adjusting system which adjusts the radial pressure applied during weldlng.

The method and apparatus descrlbed are particularly suitable for welding container components made from thermoplastics materials such as polyekhylene, polypropylene, copolymers thereof, or polyamides when ~L2~;~S~3~
,9 in a form flexible enough to yield under the imposed inwardly directed radial force to achieve the interference fit necessary for spln weldlng. Whilst the inventlon has been described in terms of radially contracting a cyllndrical wall against a plug thereln, the same controls of lnter-component lnkerference may be achieved ln principle by spreading the plug agalnst the interior of the cyllndrical wall.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of spin-welding together two opposed surfaces of thermo plastics components which when assembled together prior to welding have an interference fit, the method comprising the steps of:
a) engaging the components with one another and moving them into an assembled position;
b) applying a radial pressure to the assembled components in the region of the opposed surfaces;
c) whilst the radial pressure is applied, spinning the components relative to one another at a speed and for a time sufficient to cause welding of the opposed surfaces;
d) measuring a parameter which is a function of the force required to cause relative movement of the engaged components before applying the radial pressure; and e) selecting the value of the radial pressure according to the value of the measured parameter such that contact pressure between the opposed surfaces is controlled during welding.

2. A method according to Claim 1, wherein the measured parameter is a function of the force required to cause relative axial movement of the engaged components towards the fully assembled position.

3. A method according to Claim 1, wherein the measured parameter is a function of the force required to cause relative rotary movement of the engaged components when in the fully assembled position at a predetermined speed which is low compared with that required to cause welding.

4. A method according to claim 1 wherein the value of the measured parameter is compared with predetermined values of that parameter for assembled components exhibiting different degrees of interference fit and wherein the value of the radial pressure is selected according to that comparison.

5. A method according to claim 4 wherein the predetermined values of the measured parameter includes values which relate to assembled components having a diametric interference fit corresponding to one of 0.25 mm, 0.75 mm and 1.25 mm: denoted loose, medium or tight, respectively.

6. A method according to claim 1 wherein the thermoplastics material of the components is chosen from the group consisting of polyethylene, polypropylene, copolymers of polyethylene, copolymers or polypropylene, and polyamides.

7. Apparatus for spin-welding together two opposed surfaces of thermoplastics components which when assembled together prior to welding have an interference fit, comprising means for applying a radial pressure to the assembled components in the region of the opposed surfaces, means for spinning the components relative to one another while subjected to the radial pressure, means for measuring a parameter which is a function of the force required to cause relative movement of the engaged components before application of the radial pressure, and means for selecting the radial pressure according to the value of the measured parameter.

8. Spin-welding apparatus according to claim 7 wherein the means for applying a radial pressure to the assembled components comprises an elongate flexible element formed into a loop surrounding the assembled components in the region of the opposed surfaces wherein one end of the element is fixed and the other end is connected to means for applying tension to the element thereby applying radial pressure to the assembled components.

9. Spin-welding apparatus according to claim 8 wherein the means for applying tension to the element is a pneumatic cylinder operated by one of a series of solenoid valves according to the radial pressure selected.

10. Spin-welding apparatus according to claim 8 wherein the elongate flexible element is a wire cable guided in an annular groove into which it retracts when not in use.