Patent Publication Number: US-2021170442-A1

Title: Method and device for producing a powder coating melt

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of European Patent Application, Serial No. 19 214 205.7, filed Dec. 6, 2019, the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
     FIELD OF THE INVENTION 
     The invention concerns a method and a device for producing a powder coating melt. 
     To produce powder coating powder, a powder coating premix is prepared by means of a screw machine and the prepared powder coating melt is extruded. The extruded powder coating melt is then cooled and crushed into powder coating powder. The powder coating powder is then separated into a useful part and a fines part. The useful part, which comprises powder coating particles with the desired particle dimension or grain size, is used for powder coating while the fines part, which comprises smaller powder coating particles, is not suitable for powder coating. In addition, during powder coating, powder coating particles which are unsuitable for powder coating are deposited for example in particle filters of spray booths. Such powder coating particles or fine powder coating particles are known as fines or super-fines. 
     SUMMARY OF THE INVENTION 
     The invention is based on an object of creating a method for producing a powder coating melt which, in a simple, flexible and efficient manner, allows a recycling of powder coating particles. In particular so-called off-spec powder coating particles shall be recycled. Off-spec powder coating particles are powder coating particles, which are outside of a desired specification with respect to at least one characteristic, such as colour and/or particle size for example. 
     This object is achieved by a method for producing a powder coating melt, comprising the following steps:
         provision of a screw machine,   production of a first powder coating melt from a powder coating premix by means of the screw machine,   supply of powder coating particles into the first powder coating melt present in the screw machine, and   production of a second powder coating melt by mixing the first powder coating melt and the powder coating particles by means of the screw machine.       

     According to the invention, it has been found that the powder coating particles to be recycled are produced from a homogenised powder coating melt or powder coating powder, and thus for recycling no longer need be subjected to intensive homogenisation. Accordingly, in the method according to the invention, firstly a first powder coating melt is produced from a powder coating premix by means of the screw machine. The powder coating particles to be recycled are then supplied to this first powder coating melt and then merely mixed with the first powder coating melt by means of the screw machine, so that a second powder coating melt is produced which contains the powder coating particles to be recycled. The second powder coating melt is then discharged or extruded from the screw machine and processed further into powder coating powder in the usual fashion. The powder coating particles to be recycled are in particular fine powder coating particles. 
     The first powder coating melt is preferably fully prepared and suitable for further processing into powder coating powder. The first powder coating melt is in particular plasticised and homogenised. The powder coating particles are in particular mixed homogenously in and/or melted on, preferably completely melted, after being supplied to the first powder coating melt and before its discharge. Due to the complete melting of the powder coating particles, in particular so-called off-spec powder coating particles can be recycled. 
     Because the powder coating particles are merely mixed into the first powder coating melt, production or preparation of the first powder coating melt from the powder coating premix is not adversely affected. The first powder coating melt may be produced in the usual quality and/or with the usual rate. Then the powder coating particles can be mixed into the first powder coating melt in a simple and flexible fashion. The supplied quantity of powder coating particles may vary within wide ranges, in particular between zero and an intake limit Intensive homogenisation of the powder coating particles which have been mixed in, in particular melted on or melted, is not necessary by means of the screw machine. 
     The screw machine is in particular configured as a multi-shaft screw machine, preferably as a two-shaft screw machine. The multi-shaft screw machine in particular comprises at least two treatment element shafts which can be driven in rotation in the same direction. The at least two treatment element shafts are preferably configured to intermesh tightly. 
     A method, wherein the supplied powder coating particles each have a maximum particle dimension A max , wherein for at least 50%, in particular for at least 70%, and in particular for at least 90% of the supplied powder coating particles: A max ≤100 μm, in particular A max ≤90 μm, in particular A max ≤80 μm, and in particular A max ≤60 μm, guarantees a simple, flexible and efficient recycling of the powder coating particles. Because of the maximum particle dimension A max , the powder coating particles are not suitable for use as powder coating powder but must be recycled. The supplied powder coating particles can easily be mixed into the first powder coating melt. The percentage figure designates the number of particles or the mass proportion. The maximum particle dimension A max  designates in particular a maximum grain diameter. 
     A method, wherein the supplied powder coating particles each have a maximum particle dimension A max , wherein for at least 50%, in particular for at least 70%, and in particular for at least 90% of the supplied powder coating particles: A max ≥1 μm, in particular A max ≥5 μm, and in particular A max ≥10 μm, guarantees a simple, flexible and efficient recycling of the powder coating particles. Because of the maximum particle dimension A max , the powder coating particles can easily be supplied and mixed into the first powder coating melt. The percentage figure designates the number of particles or the mass proportion. The maximum particle dimension A max  designates in particular a maximum grain diameter. 
     A method, wherein the powder coating particles are melted before the second powder coating melt is discharged from the screw machine, guarantees a simple, flexible and efficient recycling of the powder coating particles. The powder coating particles are in particular completely melted after being supplied into the first powder coating melt and before being discharged through the discharge opening. Because the powder coating particles are melted before discharge through the discharge opening of the screw machine, the powder coating particles are optimally bonded into the powder coating melt so that the second powder coating melt is homogenous. The supply of powder coating particles lowers the outlet temperature of the second powder coating melt. This is advantageous for cooling and hardening of the second powder coating melt after discharge. 
     A method, wherein the powder coating particles are supplied downstream of a plasticisation zone, in particular downstream of a homogenisation zone, of the screw machine, guarantees a simple, flexible and efficient recycling of the powder coating particles. The screw machine comprises, successively in a conveying direction, an intake zone, a plasticisation zone and a homogenisation zone. In the intake zone, the powder coating premix is supplied to the screw machine and conveyed to the plasticisation zone. In the plasticisation zone, the powder coating premix is melted and then homogenised in the homogenisation zone. The powder coating particles are supplied downstream of the plasticisation zone so that plasticisation of the powder coating premix is only affected, or adversely affected, as little as possible. Preferably, the powder coating particles are supplied downstream of the homogenisation zone, so that the powder coating premix is plasticised and the first powder coating melt homogenised optimally before the powder coating particles are supplied. Thus there is substantially no adverse effect on the preparation of the powder coating premix into the first powder coating melt, so that the first powder coating melt can be prepared with a high quality and at a high throughput rate. In the plasticisation zone, the screw machine preferably comprises kneading elements, in particular kneading discs. This guarantees rapid and efficient plasticisation of the powder coating premix. In the homogenisation zone, the screw machine preferably comprises kneading elements, in particular kneading discs. This guarantees an efficient homogenisation of the polymer melt. 
     A method, wherein the powder coating particles are supplied in a supply zone of the screw machine which is arranged downstream of a plasticisation zone and upstream of a mixing zone, guarantees a simple, flexible and efficient recycling of the powder coating particles. In the supply zone, the powder coating particles are supplied to the screw machine. Here, preferably, a supply device is used which in particular is connected to the screw machine. The supply zone is arranged downstream of the plasticisation zone in a conveying direction, so that the powder coating particles are supplied to the plasticised powder coating premix or powder coating melt. Preferably, the powder coating particles are supplied downstream of a homogenisation zone, so that the powder coating particles are supplied to the fully prepared polymer melt, and there is substantially no adverse effect on the plasticisation and homogenisation of the powder coating premix. Downstream of the supply zone, a mixing zone is provided in which the supplied powder coating particles are mixed into the first powder coating melt, so that before the discharge there are now no free powder coating particles, i.e. no powder coating particles which are not bonded into the powder coating melt. The mixing zone is preferably configured and/or arranged such that firstly, mixing of the supplied powder coating particles before discharge is guaranteed, and secondly the shear forces acting on the powder coating particles before discharge are as low as possible. The mixing zone comprises at least one conveying element, in particular at least one screw element and/or at least one kneading element, in particular at least one kneading disc. The mixing zone comprises for example several conveying elements arranged in succession, so that the first powder coating melt and the supplied powder coating particles are conveyed together long enough for the powder coating particles to be mixed and/or melted, preferably completely melted. The mixing zone comprises for example several kneading elements arranged in succession, in particular several kneading discs. This improves mixing. Preferably, at least one conveying element and at least one kneading element are arranged in succession in the mixing zone. The at least one conveying element initially conveys the first powder coating melt with the supplied powder coating particles away from the supply zone so that no build-up occurs in the supply zone. By means of the at least one kneading element, the supplied powder coating particles are mixed into the first powder coating melt over a short length. 
     A method, wherein the powder coating particles are supplied to the screw machine by means of a supply device which in particular comprises a dosing unit and/or a supply screw machine, guarantees a simple, flexible and efficient recycling of the powder coating particles. The supply device allows an automated supply of powder coating particles. The supply device in particular comprises a gravimetric dosing unit and/or volumetric dosing unit and/or a supply screw machine. The supply screw machine is in particular configured as a side feed device. The supply screw machine in particular comprises two screw shafts which in particular can be driven in rotation in the same direction. The supply screw machine is connected to the screw machine. For this, the screw machine comprises a housing with a supply opening through which the supply screw machine supplies the powder coating particles to the screw machine. The supply device in particular is coolable. Preferably, a dosing unit and/or a supply screw machine is coolable. The supply device preferably comprises a cooling device for cooling. 
     A method, wherein the powder coating particles are provided by a separating device, guarantees a simple, flexible and efficient recycling of the powder coating particles. During production of powder coating powder, the hardened powder coating melt is ground into a powder coating powder which is then separated into at least two particle size fractions. Separation takes place by means of a separating device which is formed for example as a screen or cyclone. The powder coating particles which are separated by means of the separating device and are unsuitable for powder coating are returned to the screw machine where they are supplied to the first powder coating melt, for example by means of a supply device. The powder coating particles which are not usable for powder coating are thus supplied directly for recycling. 
     A method, wherein the powder coating particles are supplied by means of a supply screw machine, which is in particular cooled, guarantees a simple, flexible and efficient recycling of the powder coating particles. The supply screw machine guarantees a simple and even supply of the powder coating particles into the powder coating melt present in the screw machine. The supply screw machine comprises at least one screw shaft, preferably at least two screw shafts. The supply screw machine is preferably connected to the side of the screw machine, so that the powder coating particles are supplied to the screw machine through a side supply opening 
     A method, wherein the supply screw machine is connected to the screw machine by means of a supply insert, wherein the supply insert is in particular cooled, guarantees a simple, flexible and efficient recycling of the powder coating particles. The supply insert is connected to a housing of the supply screw machine and opens into a housing opening of a housing of the screw machine, so as to form a supply opening for supply of the powder coating particles. The supply insert is detachably connected to the housing of the supply screw machine and/or to the housing of the screw machine. The supply insert can be exchanged or removed for cleaning. Preferably, the supply insert comprises a flange and a supply channel component attached thereto. The supply insert preferably comprises at least one cooling element. The at least one cooling element is preferably configured as a cooling channel. Preferably, the flange and/or the supply channel component can be cooled by means of a cooling fluid. For this, the flange and/or the supply channel component comprises at least one cooling element, in particular at least one cooling channel. The at least one cooling channel is formed in the flange and/or the supply channel component. Cooling reduces, in particular largely prevents, adhesion by melting of the powder coating particles in the supply insert and/or in the supply screw machine. For example, a respective cooling channel is formed in the flange and in the supply channel component so that the flange and the supply channel component may be cooled separately and/or jointly. By means of a control device, the corresponding cooling circuits can be controlled and operated separately from each other and/or jointly. For example, the cooling circuit for the flange is constantly operated, and the cooling circuit for the supply channel component is operated depending on certain operating states, such as for example during a stoppage of the screw machine or when the temperature exceeds a predefined level in the region of the supply channel component. 
     A method, wherein a housing and/or at least one screw shaft and/or an inlet hopper of the supply screw machine is cooled, guarantees a simple, flexible and efficient recycling of the powder coating particles. By cooling the inlet hopper and/or the housing and/or the at least one screw shaft, an adhesion by melting of the powder coating particles on the inlet hopper and/or on an inner wall of the housing and/or on the at least one screw shaft is reduced or substantially prevented. At least one cooling device, which is configured for example as a compressed air cooling device and/or a water cooling device, serves for cooling. Preferably, at least one cooling channel is formed in an inlet hopper and/or a housing of the supply screw machine and/or in the at least one screw shaft. A cooling fluid flows through the cooling channel for cooling. 
     A method, wherein the powder coating particles are supplied by means of a dosing unit, wherein the dosing unit is in particular cooled, guarantees a simple, flexible and efficient recycling of the powder coating particles. The dosing unit allows an exact dosage of the powder coating particles so that the dosed powder coating particles can be melted and homogeneously mixed in the powder coating melt. The dosing unit is designed as a gravimetric dosing unit or a volumetric dosing unit. Due to the cooling of the dosing unit an adhesion by melting of the powder coating particles is reduced or essentially prevented. In particular, a container and/or a discharge pipe of the dosing unit is cooled. At least one cooling device serves for cooling, which is for example designed as a compressed air cooling device and/or a water cooling device. Preferably, the dosing unit and the supply screw machine have a common cooling or rather cooling device. 
     The invention is furthermore based on an object of providing a device for producing a powder coating melt which, in a simple, flexible and efficient manner, allows a recycling of powder coating particles. 
     This object is achieved by a device for producing a powder coating melt, comprising
         a screw machine with
           a first supply opening for supplying a powder coating premix, and   a second supply opening arranged downstream for supplying powder coating particles,   
           a supply device for supplying the powder coating particles to the screw machine through the second supply opening.       

     The advantages of the device according to the invention correspond to the advantages of the method according to the invention. The device may in particular also be refined with the features of the method for producing a powder coating melt according to the invention. 
     The screw machine is in particular configured as a multi-shaft screw machine, preferably as a two-shaft screw machine. The screw machine comprises a housing, at least one housing bore formed in the housing, and at least one treatment element shaft mounted rotatably in the at least one housing bore. When configured as a multi-shaft screw machine, at least two mutually penetrating housing bores are formed in the housing, in which at least two treatment element shafts are arranged. The at least two treatment element shafts can preferably be driven in rotation in the same direction. The at least two treatment element shafts are in particular configured to intermesh tightly. 
     The device preferably comprises a control device for controlling the screw machine and/or the supply device. The supply device comprises in particular a cooling device. The cooling device serves in particular for cooling a dosing unit and/or a supply screw machine. In particular, a cooling device for cooling the supply device may be controlled by means of the control device. 
     A device configured such that the second supply opening is arranged between a plasticisation zone, in particular a homogenisation zone, and a discharge opening, in particular a mixing zone, of the screw machine, guarantees a simple, flexible and efficient recycling of the powder coating particles. The screw machine comprises, successively in a conveying direction, an intake zone, a plasticisation zone and a homogenisation zone. Because the second supply opening is arranged between the plasticisation zone and the discharge opening, the powder coating particles can easily be mixed into the plasticised powder coating premix or powder coating melt. Preferably, the second supply opening is arranged between the homogenisation zone and the discharge opening. Through this, the powder coating particles are supplied to the fully prepared powder coating melt, so that the preparation of the powder coating melt from the powder coating premix is not substantially influenced or adversely affected. Preferably, downstream of the second supply opening, the screw machine comprises a mixing zone arranged upstream of the discharge opening. The screw machine comprises at least one conveying element and/or at least one kneading element in the mixing zone. Preferably, the screw machine comprises at least one conveying element and at least one kneading element in succession in the mixing zone. 
     A device configured such that the supply device comprises a dosing unit and/or a supply screw machine, wherein the dosing unit and/or the supply screw machine are in particular coolable by means of a cooling device, guarantees a simple, flexible and efficient recycling of the powder coating particles. The supply screw machine comprises a housing in which at least one housing bore is formed. The housing has a supply opening into which in particular an inlet hopper opens. At least one screw shaft is arranged rotatably in the at least one housing bore. Preferably, at least two mutually penetrating housing bores are formed in the housing. At least two screw shafts are arranged rotatably in the two housing bores. The supply screw machine is in particular configured as a side feed machine. The supply screw machine is in particular connected to the side of a housing of the screw machine. The supply screw machine and/or the dosing unit can preferably be cooled by means of at least one cooling device, for example a compressed air cooling device and/or a water cooling device. The at least one cooling device serves for cooling the inlet hopper and/or the housing and/or the at least one screw shaft of the supply screw machine and/or a container and/or a discharge pipe of the dosing unit. Preferably, the supply device comprises a gravimetric or volumetric dosing unit which supplies the powder coating particles to the supply screw machine. The powder coating particles are preferably provided by a separating device. The inlet hopper is preferably configured with double walls. Because of the double-walled design, the inlet hopper comprises in particular a cooling channel. The cooling channel is in particular part of at least one cooling device. A cooling fluid, for example compressed air or water, can flow through the cooling channel. The cooled inlet hopper prevents adhesion of the powder coating particles. 
     A device configured such that the supply device comprises a supply insert for connection to the screw machine, wherein the supply insert is in particular coolable, guarantees a simple, flexible and efficient recycling of the powder coating particles. This supply device preferably comprises a supply screw machine which is connected to the screw machine by means of the supply insert. The supply insert comprises in particular a flange and a supply channel component attached thereto. The supply insert comprises at least one cooling element, in particular at least one cooling channel. The at least one cooling channel serves for cooling the flange and/or the supply channel component. The flange is in particular attached to a housing of the supply screw machine so that the supply channel component extends into a housing opening of the housing of the screw machine. 
     Further advantages, features and details of the invention arise from the following description of several exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows in partly sectional view, a device for producing a powder coating melt according to a first exemplary embodiment with a multi-shaft screw machine for preparation of a powder coating premix, and a supply device connected thereto with a supply screw machine for the supply of powder coating particles, 
         FIG. 2  shows a partly sectional, top view of the device in  FIG. 1 , 
         FIG. 3  shows a perspective view of a coolable supply insert for connection of this supply screw machine to the multi-shaft screw machine, 
         FIG. 4  shows in partly sectional view, a device for producing a powder coating melt according to a second exemplary embodiment, and 
         FIG. 5  shows a partly sectional, top view of the device in  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first exemplary embodiment of the invention is described below with reference to  FIGS. 1 to 3 . The device  1  shown in the figures serves for production of a powder coating melt  2 ,  3 . 
     The device  1  comprises a multi-shaft screw machine  4  which has a housing  5  made from several successively arranged housing portions  6  to  14 . The housing portions  6  to  14  are connected together to form the housing  5 . Two mutually parallel and mutually penetrating housing bores  15 ,  16 , which in cross-section take the form of a horizontal figure of eight, are formed in the housing  5 . Two treatment element shafts  17 ,  18  are arranged concentrically in the housing bores  15 ,  16  and can be driven in rotation about respective rotational axes  20 ,  21  by a drive motor  19 . A branching gear mechanism  22  is arranged between the treatment element shafts  17 ,  18  and the drive motor  19 . A coupling  23  is in turn arranged between the drive motor  19  and the branching gear mechanism  22 . The treatment element shafts  17 ,  18  can be driven in rotation in the same direction, i.e. in the same rotational directions, about the rotational axes  20 ,  21  by the drive motor  19 . 
     The multi-shaft screw machine  4  comprises, successively in a conveying direction  24 , an intake zone  25 , a plasticisation zone  26 , a homogenisation zone  27 , a supply zone  28 , a mixing zone  29  and a discharge zone  30 . 
     A powder coating premix  31  is supplied to the multi-shaft screw machine  4  in the intake zone  25 . For this, in the first housing portion  6 , a first supply opening  32  is formed through which the powder coating premix  31  can be introduced into the housing bores  15 ,  16 . To provide the powder coating premix  31 , the device  1  comprises a first supply device  33  which is connected to the first supply opening  32 . The first supply device  33  comprises for example a gravimetric dosing unit. 
     In the intake zone  25 , the supplied powder coating premix  31  is conveyed to the plasticisation zone  26 . In the intake zone  25 , the treatment element shafts  17 ,  18  comprise screw elements  34 ,  34 ′ which are arranged rotationally fixedly on assigned shafts  35 ,  36  and serve to convey the powder coating premix  31 . 
     The supplied powder coating premix  31  is melted in the plasticisation zone  26 . In the plasticisation zone  26 , kneading elements  37 ,  37 ′ are arranged rotationally fixedly on the shafts  35 ,  36  for melting and dispersal. The kneading elements  37 ,  37 ′ are configured as kneading discs. Preferably, in the plasticisation zone  26 , kneading blocks comprising several integrally formed kneading discs are arranged on the shafts  35 ,  36 . 
     The powder coating premix  31 , which has been plasticised or melted in the plasticisation zone  26 , is conveyed to the homogenisation zone  27 . In the homogenisation zone  27 , the plasticised powder coating premix  31  is homogenised into a first powder coating melt  2 . In the homogenisation zone  27 , the plasticised powder coating premix  31  is intensively mixed so that the first powder coating melt  2  is fully prepared at the downstream end of the homogenisation zone  27 . In the homogenisation zone  27 , kneading elements  38 ,  38 ′ are arranged rotationally fixedly on the shafts  35 ,  36 . The kneading elements  38 ,  38 ′ are in particular configured as kneading discs. Preferably, kneading blocks, which are formed from several integrally formed kneading discs, are arranged in the homogenisation zone  27 . 
     In the supply zone  28 , powder coating particles  39  are supplied to the fully prepared powder coating melt  2 . The powder coating particles  39  are not suitable for powder coating and must be recycled. The supply of the powder coating particles  39  to the first powder coating melt  2  reduces its temperature. This is advantageous for further processing of the powder coating melt  2 . 
     To supply the powder coating particles  39 , the device  1  comprises a second supply device  40 . The second supply device  40  comprises a gravimetric dosing unit  41 , an inlet hopper  80 , a supply screw machine  42 , and a supply insert  43 . The supply screw machine  42  is connected to the side of the housing  5  by means of the supply insert  43 , so that a second supply opening  44  is formed in the housing  5  in the supply zone  28 . The second supply opening  44  opens into the housing bores  15 ,  16 , or into the housing bore  16 . The metering unit  41  is connected to the supply screw machine  42  by means of the inlet hopper  80 . 
     In the supply zone  28 , screw elements  45 ,  45 ′ are rotationally fixedly arranged on the shafts  35 ,  36 . The screw elements  45 ,  45 ′ convey the first powder coating melt  2  with the supplied powder coating particles  39  to the mixing zone  29 . 
     In the mixing zone  29 , the supplied powder coating particles  39  are mixed into the first powder coating melt  2  and melted therein. For this, screw elements  46 ,  46 ′ and kneading elements  47 ,  47 ′ are arranged successively and rotationally fixedly on the shafts  35 ,  36  in the mixing zone  29 . The screw elements  46 ,  46 ′ serve to convey the first powder coating melt  2  with the supplied powder coating particles  39  away from the second supply opening  44 , so that no build-up occurs in the supply zone  28 . By means of the kneading elements  47 ,  47 ′, the powder coating particles  39  are mixed into the first powder coating melt  2  as gently as possible and without intensive shearing, so that a second powder coating melt  3  is produced. The kneading elements  47 ,  47 ′ are preferably configured as kneading discs. Kneading blocks for example, which are formed from integrally interconnected kneading discs, are arranged in the mixing zone  29 . 
     After the powder coating particles  39  have been mixed in the mixing zone  29 , the second powder coating melt  3  is discharged in the discharge zone  30 . In the discharge zone  30 , screw elements  48 ,  48 ′ are rotationally fixedly arranged on the shaft  35 ,  36  for discharging the second powder coating melt  3 . On the last housing portion  14 , a nozzle plate  49  is arranged which terminates the housing  5  and forms a discharge opening  50 . Depending on the design of the nozzle plate  49 , the discharge opening  50  may be arranged vertically so that discharge takes place horizontally, or arranged horizontally so that discharge takes place vertically. Vertical discharge is in particular pressureless.  FIGS. 1 and 2  show a horizontal discharge. The second powder coating melt  3  is discharged through the discharge opening  50 . The second powder coating melt  3  is fully prepared so it now contains no free powder coating particles  39 . The powder coating particles  39  are mixed in and melted by the time of discharge. 
     After discharge of the second powder coating melt  3 , this is cooled in conventional fashion. The cooled and hardened powder coating melt  3  is then ground into powder coating powder  51 . 
     For separating the powder coating powder  51 , the device  1  comprises a separating device  52 . The separating device  52  separates the supplied powder coating powder  51  into a useful fraction F 1  and a recycling fraction F 2 . The useful fraction F 1  is suitable for powder coating. In contrast, the recycling fraction F 2  contains the powder coating particles  39  which are not suitable for powder coating. The powder coating particles  39  are provided to the second supply device  40  from the separating device  51 . 
     The powder coating particles  39  each have a maximum particle dimension A max , wherein for at least 50%, in particular for at least 70%, and in particular for at least 90%: A max ≤100 μm, in particular A max ≤80 μm, and in particular A max ≤60 μm. Furthermore, for at least 50%, in particular for at least 70%, and in particular for at least 90% of the powder coating particles  39 : A max ≥1 μm, A max ≥5 μm, and in particular A max ≥10 μm. 
     Firstly, the powder coating particles  39  reach the gravimetric metering unit  41 . The gravimetric metering unit  41  is connected to the supply screw machine  42  and supplies the powder coating particles  39  thereto. 
     The supply screw machine  42  is configured as a two-shaft side feed machine. The supply screw machine  42  comprises a housing  53  with mutually penetrating housing bores  54 ,  55  formed therein which in cross-section take the form of a horizontal figure of eight. Two screw shafts  56 ,  57  are arranged in the housing bores  54 ,  55 , and can be driven in rotation in the same direction about associated rotational axes  60 ,  61  by a drive motor  59  via a branching gear mechanism  58 . A supply opening  62 , into which the metering unit  41  opens above the inlet hopper  80 , is formed in the housing  53 . 
     The supply screw machine  42  is connected to the multi-shaft screw machine  4  by means of the supply insert  43 . The supply insert  43  comprises a supply channel component  63  and a flange  64  attached thereto. The supply insert  43  is attached to the end of the housing  53  by means of the flange  64  such that bores  65 ,  66  formed in the supply channel component  63  align with the housing bores  54 ,  55 . The bores  65 ,  66  formed in the supply channel component  63  are mutually penetrating and in cross-section take the form of a horizontal figure of eight. The supply channel component  63  is arranged in an associated housing opening  67  of the housing  5 , so that the bores  65 ,  66  open into the housing bores  15 ,  16  or the housing bore  16  and form the second supply opening  44 . 
     The supply insert  43  comprises a cooling channel  75 . The cooling channel  75  has an inlet opening  76  and an outlet opening  77  for a cooling fluid  78 . The cooling channel  75  extends through the flange  64  and through the supply channel component  63 . 
     To cool the second supply device  40 , the device  1  comprises a cooling device  68 . The cooling device  68  comprises a compressed air supply unit  69  with a valve  70 . A compressed air line  71  leads from the compressed air supply unit  69  via the valve  70  into the supply opening  62 . By means of compressed air which is supplied to the supply opening  62  via the compressed air line  71 , a compressed air stream can be created from the supply opening  62  through the housing bores  54 ,  55  up to the second supply opening  44 , so that firstly a slight positive pressure is generated which prevents an outflow of waste heat from the multi-shaft screw machine  4 , for example when the multi-shaft screw machine  4  has stopped, and secondly achieves a cooling of the housing  53  and the screw shafts  56 ,  57 . 
     The cooling device  68  furthermore comprises a cooling line  72 , a heat exchanger  73  and a pump  74 . The cooling line  72  is connected to the heat exchanger  73  and pump  74 , and attached to the inlet opening  76  and outlet opening  77  of the cooling channel  75 . The pump  74  pumps the cooling fluid  78 , for example water, through the cooling line  72  to the cooling channel  75  and from there to the heat exchanger  73 , so that heat is extracted from the supply insert  43  and the supply insert  43  is cooled. The heated cooling fluid  78  is cooled again in the heat exchanger  73 . 
     The inlet hopper  80  is formed with double walls so that the inlet hopper  18  forms a cooling channel. The cooling channel is part of the cooling device  68  and is cooled for example by means of compressed air or water. 
     The cooling device  68  reduces, and preferably prevents, an adhesion by melting of the powder coating particles  39  on the inlet hopper  80 , the inner wall of the housing  53 , the screw shafts  56 ,  57  and/or the supply insert  43 . 
     The device  1  comprises a control device  79 . The control device  79  controls the drive motor  19 , the supply devices  33 ,  40  and the cooling device  68 . Because the powder coating particles  39  are supplied to the fully prepared first powder coating melt  2 , the preparation of the first powder coating melt  2  is not influenced or adversely affected. Since the powder coating particles  39  have already been prepared, these need merely be mixed into the first powder coating melt  2 . Mixing can take place gently. Since preparation of the first powder coating melt  2  is not substantially adversely affected, the quantity of supplied powder coating particles  39  may be varied within broad limits Thus the powder coating particles  39  may be recycled flexibly depending on the quantity occurring. The powder coating particles  39  are recycled gently since there is no second intensive mixing and homogenising. This is advantageous since the powder coating particles  39  are more reactive than the powder coating premix  31 , and thus a deterioration in the quality of the second powder coating melt  3  is avoided. The powder coating particles  39  are thus recycled in a simple, flexible and efficient fashion. 
     A second exemplary embodiment of the invention is described below with reference to  FIGS. 4 and 5 . In contrast to the above-mentioned exemplary embodiment, the device  1  has a first supply device  33 , which comprises a gravimetric dosing unit  81  and a supply screw machine  82 . The dosing unit  81  opens into the supply screw machine  82 . The supply screw machine  82  is connected directly, i.e. without a supply insert, into the intake zone  25  on the housing  5  so that a first supply opening  32  opens into the housing bores  15 ,  16  or the housing bore  16  on the side of the intake zone  25 . The dosing unit  81  and the supply screw machine  82  are formed similarly to the dosing unit  41  and supply screw machine  42 . 
     The supply device  33  serves to supply powder coating particles  84  which are unsuitable for powder coating. The powder coating particles  84  are supplied to the intake zone  25  together with the powder coating premix  31 , melted together with the powder coating premix  31  in the plasticisation zone  26 , and then homogenised in the homogenisation zone  27  into the first powder coating melt  2 . The quantity of powder coating particles  84  which can be supplied with the powder coating premix  31  is determined by the subsequent plasticisation and homogenisation. Thus powder coating particles  84  may be supplied in a quantity which does not adversely affect the preparation of the powder coating premix  31 , for example does not cause a reduction in quality and/or throughput rate. The supply and recycling of the powder coating particles  39  take place similarly to the first exemplary embodiment. With respect to the further construction and further function of the device  1 , reference is made to the previous exemplary embodiment. 
     In general, the following applies: 
     In the mixing zone  29 , exclusively screw elements  46 ,  46 ′ may be arranged. In this case, the mixing zone  29  must be sufficiently long to mix in and in particular melt the supplied powder coating particles  39 . The length of the mixing zone  29  may be shortened by the arrangement of kneading elements  47 ,  47 ′ in the mixing zone  29 . 
     Powder coating particles  39  with any grain size or any grain diameter may be supplied downstream of the plasticisation zone  26 , in particular the homogenisation zone  27 . Preferably, fine powder coating particles are supplied. The necessary length of the mixing zone  29  depends on the viscosity of the powder coating melt  2 , the melt temperature, the rotation speed of the treatment element shafts  17 ,  18 , and the supplied quantity of powder coating particles  39 . The powder coating particles  39  may also be supplied to the middle of the homogenisation zone  27 , as long as this does not adversely affect the preparation of the powder coating melt  2 . 
     A comparatively small quantity of powder coating particles  84  may also be supplied to the intake zone  25  as long as this does not adversely affect the preparation of the powder coating premix  31 . 
     Since the quantity of supplied powder coating particles  39  may vary within broad limits, it is also possible to supply the powder coating particles  39 , which have just been produced in the separating device  52 , directly into the supply screw machine  42 , i.e. without the interposition of a dosing unit, for example by means of a pneumatic conveyor. 
     The powder coating particles  39  may also be supplied to the second supply device  40  via so-called big bags. 
     The cooling by means of compressed air may take place in particular during a stoppage of the multi-shaft screw machine  4 . The stoppage is detected for example from the rotation speed of the drive motor  19  or from control signals present in the control device  79 .