WO1979000756A1

WO1979000756A1 - Plastics coating apparatus

Info

Publication number: WO1979000756A1
Application number: PCT/GB1979/000042
Authority: WO
Inventors: P White; D Tilley
Original assignee: Mallatite Plastics Ltd; P White; D Tilley
Priority date: 1978-03-14
Filing date: 1979-03-08
Publication date: 1979-10-04
Also published as: DE2940979A1; FR2419807A1; JPS55500147A; GB1571763A

Abstract

Apparatus for coating articles with plastics material comprises a number of workholders (45) each consisting of a conduit and at least one jig (57) attached to the conduit. Each jig is adapted to support one or more articles to be coated. The apparatus further includes means for passing a continuous flow of cooling fluid through the conduits when the apparatus is in operation, an oven (58) for heating the articles on the workholders, a fluidizable bed (62) of plastics coating material and means for lowering the heated articles into the bed (62) for a predetermined period to allow coating of the articles to a predetermined thickness to take place and for subsequently lifting the articles from the bed. The lowering and lifting means comprise a parallelogram linkage consisting of a straight arm (43) and a cranked arm (44) for each worlholder which is pivotally connected to its respective arms. The cranked arm (44) is arranged to be acted on by a pneumatic cylinder (7O) whereby movement of the associated piston in one direction is effective to lower the and the associated worlholder and movement of the piston in the other direction is effective to lift the arm and workholder. In operation of the apparatus, the workholders are moved through a circular path whereby at a first station the jigs are loaded with one or more articles, the workholders then pass through the oven (58) to heat the articles, each workholder is then brought in turn to a station at which the associated arm (44) is acted on by the cylinder (70) and the articles suspended from the workholder are lowered into the fluidized bed (62) and then lifted out of the bed, the articles are then allowed to cool and are finally removed from the jigs at an unloading station. The jigs and workholders are maintained at a temperature below the fusion point of the plastics material by the cooling means so that they are not coated while the articles are being coated with said plastics material.

Description

Title; Plastics Coating Apparatus This invention relates to the coating of articles with plastics materials. The invention is particularly concerned with holding devices for handling articles to be coated.

Articles can be coated with plastics material by heat¬ ing said articles to a temperature above the fusion temperature of the coating material and then immersing said heated articles in a fluidized bed of the coating material. A serious problem associated with this method is that devices for holding the heated articles also tend to be heated and coated with the articles and this is undesirable. Various methods have been tried in the past to prevent the holding devices from being coated with plastics material when the articles which they- are holding are being coated. One such method is described in U.S. Patent Specification No-. 3,440,078 in which an article holding device comprises a pair of tongs by means of which an article to be coated may be held. The tongs have jaws which form an air box or casing and which are porous for the discharge of air there¬ through against the body of an article held therein. Means are provided for introducing air under pressure into the box and the air is introduced at a chilled or low temper¬ ature to minimize the absorption by the air box or casing of heat from the heated article to be coated and accordingly ^* to prevent or reduce to a minimum the coating of the air box or casing. This arrangement requires the use of an expen¬ sive compressor in order to generate the compressed air and means must also be provided for cooling the compressed air flow. In addition to the expense, a further disadvantage is that the flow of compressed air must be very accurately controlled when the article held by the air box or casing is immersed in a fluidized bed of coating material if the article is to be coated properly. Another method is disclosed in U.S. Patent Specifi¬ cation No. 3,226,245 in which an article to be coated is held in a holder which is hollow and through which water is passed in order to cool the holder. While this arrange- ent is cheaper, it is not possible to coat the whole of article to be coated in one operation because part of the article is ma__ked by the cooled holder.

An arran ement for coating the whole of an article t be coated in one operation is disclosed in German Offen- legungsschrift No. 2648093 in which the article is susp from a hollow jig which is connected to a holder in the f of a reservoir for a cooling liquid. In this case, there are no means for removing the cooling liquid from the jig and the liquid can only escape if it evaporates. In the 'case in which water is used, the boiling point of which i 100°C, the jigs can attain a temperature which is close t the fusion temperature of some plastics materialscommonly used for coating (approximately 120°C). Very accurate co trol is therefore necessary if the jigs themselves are no to be coated entailing expensive control systems.

Yet another arrangement for the plastics coating of articles is disclosed in German Of enlegungsschrift No. 2625 123 in which articles to be coated are suspended fr hollow.hooks connected by a feed pipe to a cooling unit i the form of a tank. The articles are heated in an oven a then the heated articles suspended from the hooks are coated with plastics material by raising a tank containin pulverized plastics into the path of movement of the articles suspended from the hooks. This arrangement can

♦only be used for coating small light articles having a sm configuration. Uniform coating of heavy articles or articles having a complex configuration cannot be achieve by this method. In order to achieve even and uniform coating, we have found that the articlesmust be moved wit a tank containing pulverulent plastics material in a substantially horizontal direction and that the level of powder in the tank must remain substantially constant. I the tank is raised up to the articles, the powder starts to sway within the tank and it is not possible to achieve even and uniform coating. On the other hand, if the arti are lowered into a stationary tank the only movement of the powder iε caused by disturbance of the articles movin y^

( ^" through the powder and even and uniform coating of the article can be achieved.

The present invention therefore aims to provide apparatus for coating articles with plastics material which overcomes the above mentioned disadvantages and which does not require the use of accurate and expensive control systems.

According to the invention, there is provided apparatus for coating articles with plastics material, said apparatus comprising at least one holder consisting of a conduit and at least one jig attached to the conduit, the or each jig being adapted to support one or more articles to be coated; means for passing a continuous flow of cooling fluid through the or each conduit when the apparatus is in operation; means for heating the or each article on the or each holder; a bed of plastics coating material; and means for lowering the heated article(s) into said bed for a predetermined period to allow coating of the article(s) to a predetermined thickness to take place and for subsequently lifting the article(s) froπi the bed, the lowering and lifting means being operatively connected to the holder(s) and including at least one pivotal arm to which the or a respective holder is pivotally connected. The bed of plastics coating material is preferably a fluidizable bed.

The cooling fluid is preferably water and a plurality of jigs are desirably detachably secured to the or each holder.

The invention will now be further described, by way of example, with reference to the drawings, in which:-

Figure 1 is a plan view which shows, diagrammatically, one embodiment of plastics coating apparatus according to the invention;

Figure 2 is a diagrammatic section taken on the line II-II in Figure 1 ; and

Figure 3 __hows a detail of the apparatus shown in Figure 2 to an enlarged scale.

It will be noted that Figure* 3 of the drawings shows the apparatus in more detail than the other two figures which show only those parts of the apparatus which are necessary for an understanding of the manner in which th apparatus operates. For details of the cooling means fo the work holders reference should be made to Figure 3 of the drawings. Referring to the drawings, the apparatus comprises central framework 20 above which is rotatably mounted an indexing table 22. The table 22 includes a central boss 23 and extending radially outwardly from said boss are f arms 24. A ring 25 of L-shaped cross-section is secured to the outer ends of the arms 24 and a circular tray 26 having a sloping base is secured to the ring 25. An ape ture is provided in the tray 26 which is aligned with th boss 23 and through which extends a tube 27. The tube i secured, e.g. by welding, at one end to the boss 23 and also secured to the tray in the region of the aperture therein. A number of holes 28 which serve as drain hole in a manner hereinafter described are provided in the wa of the tube 27 adjacent the bottom region of the tray 26 which said aperture is located. The tube 27 i^ mounted in bearings 29 which are ^• mounted on webs 21 forming part of the framework 20 and gear or like wheel 31 is secured on said tube between the two webs 21. The wheel 31 is arranged to be driven by s able drive means (not shown) via a belt, chain or the li (not shown). The drive means are arranged to drive the ^* wheel 3 , and hence the tube 27 and indexing table 22 in stepping fashion and may comprise, for example, a Geneva wheel movement or an electric stepping motor.

Extending axially through the tube 27 is the upper e of a feed pipe 33 which is connected at its lower end to the output from a pump 34. The inlet to said pump is in communication with a water tank 35. The pipe 33 is held against rotation and the boss 23 is rotatably mounted on the upper end of this pipe by means of a plain sleeve be 36. A rotary pipe union 37 is mounted on the said upper of the pipe 33, the pipe union consisting of a fixed par which is mounted on the pipe 33 and a rotatable part whi is mounted on, but is rotatable with respect to, the fix ^y^S

( _ part. A feed pipe 38 leads from the rotatable part of the pipe union 37 to a ring feed pipe 39 which is mounted on the indexing table by means of stanchions 41 secured to the arms 24. Secured to the ring 25 at equally spaced intervals are a series of twelve blocks 42 on each of which is pivotally mounted a respective parallelogram linkage consisting in each case, of a straight arm 43 and a.pantograph type cranked arm 44. A respective workholder 45 is pivotally connected to the other ends of the arms 43 and 44.

As shown in Figure 3 of the drawings, each workholder

45 consists of a conduit having a vertical feed section 45a, an upper horizontal feed section 45b, a lower horizontal section 45c_, an upper horizontal return section 45d and a vertical return section 45e_. A feed pipe 46 is connected between the feed section 45a and a valve 47. Each valve 47 . is connected to the ring feed pipe 39 by a first feed pipe 48 in which a regulating gate valve 49 is provided and by a second feed pipe 51 in which no valve or control is provided. Each valve 47 is normally in a position in which only a regulated flow from the associated feed pipe 49 is allowed to pa__s through it but is provided with a control device 52 which, when operated, opens a port in the valve to allow full flow from the pipe 51 to pass through said valve 47. The operation of the control device 52 is described ♦hereinafter. A return pipe -56 leads from the return section 45e_ of the workholder into the tray 26 of the indexing table. Each return pipe preferably consists of a rigid portion 56a and a flexible portion 56b. Similarly, each feed pipe 46 preferably consists of a rigid portion 46a. and a flexible portion 46b. In each case, the rigid portions of the pipes

46 and 56 are connected to the work holder. A plurality of jigs 57 (in this ca.'.e four) are secured to each workholder 45 (Fig. 2) and preferably have a large area of contact with the associated workholder in order to facilitate cooling of the jigs in a manner hereinafter described.

Turning now to Figure 1 of the drawings, it will be seen that each of che sets of arms 3, 44 occupies one of twelve

OA.PI ^{~ "} ~^" stations 1 to 12 of the apparatus. Station 1 is a loading station at which articles to be coated with plastics mater are suspended from the jigs 57 on the workholder at that station. Stations 2 to 7 are located in an arcuate oven 5 which is designed to heat articles on the jigs to a temperature of from 350°C to 450OC. The heating means for the oven may comprise a number of gas burners 59 located i a duct 60 extending from one side of the oven to the other An air circulating fan or blower 61 is located between the outlet from the oven and the inlet to the duct 60 to force air through the burners 59 and then into the oven 58. Preferably, direct heating takes place only at stations 2 to 5 while "soaking" takes place at stations 6 and 7. A fluidizable bed 62 of plastics coating material is located at station 8 and a cooling tank 63 is located at station 9 Air coaling means (not shown) are provided at station 10 a station 11 is an unloading station at which coated article can be removed from the jigs. Station 12 is an- idle stati at which unloading can^' be completed if necessary and, if desired, loading commenced.

When the apparatus is required for use, the burners 5 and fan or blower 61 for heating the oven 58 are turned on the bed 62 is fluidized, the cooling means at station 10 are rendered operative and the pump 4 is switched on. Th pump delivers water from the tank 35 up the feed pipe 33 through the pipe union 37 and feed pipe 38 to the ring fee pipe 39. From the ring pipe 39, the water is pumped throu the regulator valves 49 and feed pipes 48 (in some* cases also through the second feed pipes 51 as described herein- after), through the valves 47 and feed pipes 46 to the wor holders 45. The water passes through each of the sections 45a, 45b, 45£, 45d and 45e_ of the workholders and is then pumped along the return pipes 56 to discharge into the tra 26 of the indexing table 22. The water leaves the tray through the drain holes 28 and passes down the tube 27 int a collection tank 32 and thence into a cooling system com¬ prising a cooling radiator 64 which is arranged to be cool by a fan 65. After being cooled by the cooling system, th y^ water is returned to the tank 35 for re-circulation via a pipe 66, the flow of water through said pipe, and hence through the radiator 64, being controlled by a valve 64a. Once the jigs 57 on the workholder 45 at station 1 have been loaded with articles to be coated and the oven has attained the desired working temperature (350-450°C), the drive means are started and the loaded workholder is moved from station 1 to station 2 owing to the rotation of the indexing table under the action of the drive means. Rotation of the indexing table is then stopped for a predetermined period.

As the workholder is moved from station 1 to station 2, a roller 67 on the arm 44 is in engagement with a cam track (not shown) which ensures that the arms 43 and 44 are main- tained in the raised position indicated at the right hand side of Figures 2 and > .

Further indexing steps of the table 22 bring the work- holder in turn to stations 3_. 4 and 5 at each of which the burners 59 in the oven serve to heat the articles to be coated to the desired temperature. At the same time, the jigs 57 are kept at a lower temperature by virtue of the flow of cooled water through the workholder. At the next step, the workholder is brought to station 6 at which no direct ^'heating is applied, but the articles are allowed to "soak" in the heat in the oven.

The workholder is brought at the next step to station 7 and at this point a roller 53 on the control device 52 engages a cam track 54 mounted on supports 55 which are secured to the framework 20. The engagement of the roller 53 with the cam track 54 causes the control device 52 to open the valve 47 to permit unrestricted flow of water through the feed pipe 51 to pass through the valve 47 and feed pipe 46 to the workholder 45. Thus, while the workholder remains in the oven at station 7 its cooling is rendered more effec- tive by the increased flow of cooling water through it. As a result, the cooling of the jigs is increased while the articles on the jigs continue to "soak" in the heat in the oven. The cooling of the workholder does, however, result in some cooling of the articles, the temperature of whic reduced to about 250°C, but this is still well above the fusion temperature of plastics coating materials.

At the next step, the workholder is moved out of th oven to station 8 at which the fluidized bed 62 of plast coating material is located. The cam track which is eng by the roller 67 on arm 44 ends at this station and the roller moves onto a pad 68 which is provided on the end a piston rod 69 of a pneumatic ram or lifting cylinder 7 The cylinder is actuated to retract the piston whereby t piston rod 69.and pad 68 are lowered which allows the ar 43, 44 and the workholder 45 to be lowered so that the w holder is lowered into the bed 62 as shown in Figure 2. The workholder 45 is allowed to remain in the lowered position, in which the jigs and articles are immersed in the fluidized bed of coating material, for a predetermin period to allow for coating of the heated articles to a determined desired thickness after which the cylinder 70 actuated to force the piston rod 69 upwards. This upwar movement of the piston rod 69 causes in turn the arm 44 hence the workholder 45 to be moved upwards out of the b 62. The workholder is then preferably subjected to agit means to shake off loose particles from the coating whic loose particles are returned to the bed. Since the work holder is cooled by an unrestricted rapid flow of coolin "water through it, both said-workholder and the jigs held thereon are maintained below the fusion temperature (120 of the coating material and thus are not coated with the plastics material. Any loose powder collected on the wo holder and/or jigs is shaken off by the agitation which be accomplished by rapid actuation of the lifting cylind 70 in both directions although other agitation means may employed if desired.

The workholder is next indexed to station 9 at whic it is lowered into the cooling tank 63 of water or other cooling fluid in a similar manner to lowering the workho into the fluidized bed 62. The cam track 54 ends at thi point and the roller 53 is therefore allowed to drop dow so that the control device 52 is effective to close the inlet from the feed pipe 51 to the valve 47. The flow of cooling water through the workholder 45 is thereafter reduced to the controlled flow permitted by the regulator valve 49. The time during which the workholder remains in the cooling tank is so regulated that the residue of heat in the components will tend to dry off the coolant once the workholder is lifted out of said tank. At the end of said time, the arm 44 is raised by a lifting cylinder similar to the cylinder 70 to raise the workholder clear of the tank before the next indexing step when the workholder is moved to station 10 at which said workholder and the associated jigs and articles are cooled and additionally dried by one or more air jets. At the same time, the roller 67 is again engaged with the first-mentioned cam track (not shown) whereby the arms 43 and 44 are again maintained in the raised position. This cam track extends from between stations 9 and 10 right round through stations 11 to 6 to between stations 7 ar_d 8. During raising and lowering of the arms 43 and 44, the workholder is displaced vertically under the action of the parallelogram linkage.

The workholder is next moved to station 11 at which the coated articles are unloaded from the jigs. If not completed at this station, the unloading can be completed at station 12 at which loading can also be commenced.

The jigs 57 are preferably detachedly mounted on the workholder whereby they may be replaced by others more suitable for use with any particular article to be coated and can also be removed for cleaning and/or replacement. As shown in Figure 2, the arcuate oven 58 is prefer¬ ably provided with an arcuate slot 74 in its roof which permits the passage of the workholder 45. The oven is designed to heat articles to be coated to a temperature of 350-450°C in order to burn off any traces of oil and grease on the articles to be coated which can interfere with adhesion of the coating material to the articles.

The return pipes 56 may be replaced or supplemented by steam escape pipes if desired but this is not preferred. Preferably, means (not shown) are provided for moving the articles horizontally while they are immersed in the plas coating material to provide a more even coating.

The apparatus may further be provided with means for varying the depth to which articles suspended from the ji are lowered into the* fluidized bed 62. This is particula useful when large articles are to be coated since they ha to be lowered further into the bed than small articles in order to be coated. Further, if only part of an article to be coated with plastics material, this can be achieved arranging the article on a jig so that the part to be coa is lowermost and by lowering the article to such an exten that only the part to be coated is located in the fluidiz bed 62. The means for varying the lowering depth of the arti

.may comprise a valve 72 which is actuable by an extension member 71 secured to the pad 68. The valve is so arrange that, when it is engaged by the extension member 71 it is effective to stop the movement of the cylinder 70 in the direction in which said cylinder is operative to lower articles suspended from the jigs into the fluidized bed 6 The valve can be mounted on a vertical rod 73 and can be adjustable on said rod to vary the position at which it i contacted by the extension member 71. Thus, the lower th valve 72 is located on the rod 73, the deeper will the

^* articles be lowered into the bed 62 and raising the posit of the valve 72 on the rod 73 will reduce the lowering de of the articles into the fluidized bed 62. Just as the cooling, tank 63 may be provided with a lifting cylinder similar to the cylinder 70, so this cylinder may also be provided with a valve arranged to operate in a similar ma to the valve 72 in order to adjust the depth to which articles are lowered into the cooling tank 63. *

If it i__ not desired to cool the articles too quickl the step of lowering them into the cooling tank may be omitted. This may be achievedeither by removing the cool tank or leaving it empty or by preventing the cylinder associated with the cooling tank from operating so that t

__0Λ coated articles are not lowered into the cooling tank.

Other embodiments and modifications are possible without departing from the scope of the invention as defined by the appended claims. For example, if thermosetting plastics are being used as the coating materials, a curing stage is required between stations 8 and 9 but this is not required for ther o-plastics materials. Moreover, although the above description relates to intermittentoperation, it is also possible for the apparatus to operate continuously and the invention is not restricted to intermittent operation.

Further, the pump 34 may be arranged to supply a continuous flow of cooling liquid to the cooling tank 63 in order to prevent the cooling liquid from being over-heated by the hot coated articles which are lowered into it. The cooling tank may be included in the cooling system whereby it receives a continuous supply of cooled water from the pump 34. How¬ ever, it is also possible to connect the feed pipe 33 direct to the water main so that the cooling water is supplied direct from the mains and the pump and cooling system can be dispensed with. In this case, the cooling tank can also be supplied v/ith a steady flow from the mains. According to a further alternative, a circulating system for the cooling water may be provided as shown in the drawings but without a cooling system and with provision for adding some water from the mains and allowing some of the heated water ^• to overflow to waste whereby a continuous supply of cooling water is furnished. In yet another alternative, the cooling tank may be dispensed with and the cooling achieved by spraying.

Claims

1. Apparatus for coating articles with plastics material, said apparatus comprising at least one holder consisting of a conduit and at least one jig attached to the conduit, the or each jig being adapted to support one or more articles to be coated; means for passing a con¬ tinuous flow of cooling fluid through the or each conduit when the apparatus is in operation; means for heating th or each article on the or each holder; a bed of plastics coating material; and means for lowering the heated article(s) into said bed for a predetermined period to allow coating of the article(s) to a predetermined thick¬ ness to take place and for subsequently lifting the article(s) from the bed, wherein the lowering and lifting means are operatively connected to the holder(s) and include at least one pivotal arm to which the or a respec tive holder is pivotally connected.

2. Apparatus according to claim 1, wherein the or eac arm is pivotally mounted on a movable conveyor means.

3. Apparatus according to claim 2, wherein the movabl conveyor means comprise a rotatably mounted table on whic a plurality of said arms, each carrying an associated hol are pivotally mounted.

4. Apparatus according to claim 3, wherein the table •arranged to be rotated intermittently in steps correspond ing in number to the number of holders pivotally connecte to said table.

5. Apparatus according to claim 3 or claim 4, wherein the heating means comprise an oven arcuately arranged around part of the periphery of said table.

6. Apparatus according to claim 5, wherein the oven takes the form of an arcuate tube having a slot to accomm date the passage of the arms when the associated holders advanced through the oven.

7. Apparatus according to any one of claims 3 to 6, wherein the means for passing cooling fluid through the conduit comprise a pump and a feed pipe connected to the put from the pump and leading to the conduits. _^___ y^

8. Apparatus according to claim 7, wherein the feed pipe is connected to a rotary pipe union which in turn is connected to a ring pipe feed mounted on the table for rotation there¬ with, the conduits being connected to said ring pipe feed.

9. Apparatus according to claim 8, wherein each conduit is connected to the ring pipe via a valve allowing restricted flow in some radial positions of the associated conduit and unrestricted flow in other radial positions of the said associated conduit.

10. Apparatus according to any one of claims 7 to 9, wherein a return pipe leads from each of the conduits to the table which is constructed in the form of a tray.

11. Apparatus according to claim 10, wherein the tray is provided with drainage means leading to a cooling system for cooling the fluid returned from the conduits.

12. Apparatus according to claim 11, wherein the fluid cooled by the cooling system is fed to a tank from which the pump receives its supply.

13- Apparatus according to any one of claims 3 to 12, wherein the lifting means include a pneumatic ram or lifting cylinder engageable with each arm in turn as said table is rotated.

14. Apparatus according to any preceding claim, wherein the or each holder is provided with a plurality of jigs detachably secured thereto.

'15« Plastics coating apparatus substantially as described herein with reference to the drawings.