Patent Grant
7732020
The present invention relates to a method and apparatus for the electrostatic application of powder material to solid dosage forms.
A solid dosage form can be formed from any solid material that can be apportioned into individual units and is, therefore, a unit dose form. A solid dosage form may be, but is not necessarily, an oral dosage form. Examples of pharmaceutical solid dosage forms include pharmaceutical tablets and other pharmaceutical products that are to be taken orally, including pellets, capsules and spherules, and pharmaceutical pessaries, pharmaceutical bougies and pharmaceutical suppositories. Pharmaceutical solid dosage forms can be formed from pharmaceutical substrates that are divided into unit dose forms. Examples of non-pharmaceutical solid dosage forms include items of confectionery, washing detergent tablets, repellents, herbicides, pesticides and fertilisers.
The electrostatic application of powder material to solid dosage forms is known. Examples of patent specifications describing such applications are WO 03/061841 and WO 02/49771.
When coating solid dosage forms electrostatically with powder, it is desirable to accurately control the coating process so that the powder coating on each solid dosage form is as even as possible and of the appropriate thickness. This is done by positioning each solid dosage form appropriately in relation to the coating powder supply and by controlling the properties of the powder supply.
In the applicant's co-pending application no PCT/GB2004/005458, the solid dosage forms are conveyed on platens which move along a drive path. The accurate positioning of the solid dosage forms relative to the coating powder supply is achieved via a guide on the drive path, which fixes each platen at a selected vertical position for the duration of the coating process. Thus, the distance between the powder supply and the surface of the solid dosage form to be coated is accurately controlled. Whilst this method has proved to be very successful, further improvements can be made by controlling the arrangement for supplying the coating powder and the way in which it is applied to the solid dosage forms.
When coating solid dosage forms electrostatically with powder, the coating powder must be charged so that it can be transferred from the coating powder supply to the solid dosage form. This charging may be achieved by mixing the coating powder and shearing the coating powder sufficiently to impart an electric charge. The charging occurs to a large extent by triboelectric charging, for example by the contact between the coating powder and carrier particles mixed with the coating powder. If it is desired to apply powder to solid dosage forms at a reasonably high rate, as required for industrial production, this mixing process must be very efficient in order to supply sufficient quantities of charged coating powder.
It is an object of the invention to provide an improved method and apparatus for the application of powder material to solid dosage forms.
According to a first aspect of the invention, there is provided apparatus for electrostatically charging powder material and supplying it to an applicator for electrostatically applying the powder material to solid dosage forms, the apparatus comprising:
The solid dosage forms may be oral dosage forms, for example, pharmaceutical tablets.
The use of two elongate mixing shafts promotes fast charging of the powder material by a shearing action. One or both of the mixing shafts may include slots for increasing the rate of charging of the powder material.
In an embodiment of the invention, the feeder comprises a rotatable paddle wheel. The paddle wheel may be magnetic.
The apparatus may further comprise a replenisher for replenishing the powder material in the sump. Preferably, the replenisher is connected to a sensor for monitoring the amount of powder material in the sump.
Advantageously, the mixer further comprises a third elongate mixing shaft substantially parallel to the first and second elongate mixing shafts, the third mixing shaft being positioned between the first and second mixing shafts, having mixing paddles thereon and being arranged to rotate in either direction, the paddles on the three mixing shafts being arranged to mesh as the mixing shafts rotate.
The use of three elongate mixing shafts promotes even faster charging of the powder material by a shearing action.
One or all of the mixing shafts may include slots for increasing the rate of charging of the powder material. The slots create more shearing sites for the powder material which increases the rate of electrostatic charging.
In an embodiment of the invention, the apparatus further comprises a sump of powder material. Preferably, the sump of powder material further comprises a magnetized carrier material mixed with the powder material. This is particularly useful where a magnetic feeder and/or applicator are used.
According to the first aspect of the invention, there is also provided a method for electrostatically charging powder material and supplying it to an applicator for electrostatically applying the powder material to solid dosage forms, the method comprising the steps of:
One or both of the mixing shafts may include slots for increasing the rate of charging of the powder material.
Preferably, the step of removing the electrostatically charged powder from the sump comprises rotating a paddle wheel, the paddle wheel removing powder material from the sump. The paddle wheel may be magnetic.
Preferably, the method further comprises the step of monitoring the amount of powder material in the sump.
Preferably, the method further comprises the step of replenishing the powder material in the sump.
In an advantageous embodiment of the invention, the step of mixing comprises rotating three substantially parallel elongate mixers, the third mixing shaft being positioned between the first and second mixing shafts and having mixing paddles thereon, the paddles on the three mixing shafts meshing as the mixing shafts rotate.
One or all of the mixing shafts may include slots for increasing the rate of charging of the powder material.
According to the first aspect of the invention, there is also provided apparatus for electrostatically charging powder material, the apparatus comprising a mixer for mixing a sump of the powder material to electrostatically charge the powder material, the mixer comprising three substantially parallel elongate mixing shafts, the first mixing shaft and the second mixing shaft having oppositely angled mixing paddles thereon and being arranged to rotate in opposite directions, the third mixing shaft being positioned between the first and second mixing shafts, having mixing paddles thereon and being arranged to rotate in either direction, the paddles on the three mixing shafts being arranged to mesh as the mixing shafts rotate.
According to the first aspect of the invention, there is also provided a method for electrostatically charging powder material, the method comprising mixing a sump of the powder material to electrostatically charge the powder material, the mixing comprising rotating three substantially parallel elongate mixing shafts, the first mixing shaft and the second mixing shaft having oppositely angled mixing paddles, the third mixing shaft being positioned between the first and second mixing shafts and having mixing paddles thereon, the paddles on the three mixing shafts meshing as the mixing shafts rotate.
According to a second aspect of the invention, there is provided an applicator for electrostatically applying powder material to solid dosage forms, the applicator comprising:
The solid dosage forms may be oral dosage forms, for example, pharmaceutical tablets.
In an embodiment of the invention, the applicator comprises at least one magnet inside the sleeve for applying the rotating magnetic field to the sleeve. In one embodiment, the applicator comprises a plurality of magnets positioned in a cylinder inside the sleeve, the cylinder being arranged to rotate. Preferably, the cylinder is eccentrically mounted within the sleeve, so that the magnetic field provided by the magnets is higher in one portion of the sleeve than in another portion of the sleeve.
In an embodiment of the invention, the applicator comprises a second sleeve for receiving a mixture of electrostatically charged powder material combined with a magnetized carrier material from the sump, the second sleeve being arranged to have a rotating magnetic field applied thereto for rotating the mixture around the second sleeve and the second sleeve being arranged to have an electric potential applied thereto to drive the electrostatically charged powder material onto the solid dosage forms passing alongside the second sleeve.
In an embodiment of the invention, the applicator comprises at least one magnet inside the second sleeve for applying the rotating magnetic field to the sleeve. In one embodiment, the applicator comprises a plurality of magnets positioned in a cylinder inside the second sleeve, the cylinder being arranged to rotate. Preferably, the cylinder is eccentrically mounted within the second sleeve, so that the magnetic field provided by the magnets is higher in one portion of the second sleeve than in another portion of the second sleeve.
The first sleeve and the second sleeve are preferably arranged to have oppositely rotating magnetic fields applied thereto.
Providing two sleeves instead of one enables the rate at which substrates can be coated with powder to be increased. Further, rotating the magnetic fields of the sleeves in opposite directions tends to improve the uniformity of the coating.
It is advantageous if the applicator further comprises a blade alongside the sleeve or sleeves for controlling the height of the mixture on the sleeve or sleeves. The amount of powder material applied to the solid dosage forms can thereby be controlled. This is particularly advantageous if the distance between the applicator and solid dosage forms to which coating powder is applied is very small.
Advantageously, the solid dosage forms may be earthed before passing them alongside the sleeve or sleeves.
In an embodiment of the invention, the sleeve or sleeves are substantially cylindrical. In an alternative embodiment of the invention, the sleeve or sleeves are substantially in the shape of a cylinder but having a flattened portion running substantially the length of the sleeve located on the sleeve where the solid dosage forms are arranged to pass alongside the sleeve or sleeves. The provision of a flattened portion of the sleeve where the solid dosage forms pass alongside the sleeve assists in providing an even coating of the solid dosage forms. In another form of the invention, the flat top described above is replaced with a top that slopes down towards the offload side of the sleeve. The provision of a sloping top tends to reduce the edge effect that can occur in applicators of the form described herein.
In an embodiment of the invention, the sleeve or sleeves include a magnetic shield arranged to provide a localised reduction in the magnetic field strength at the surface of the sleeve at an offload position of said sleeve. In this embodiment of the invention, the offload position, that is, the position at which the magnetised carrier leaves the sleeve, can be controlled by controlling the location and thickness of the shield. The shield is preferably a mu-metal shield.
The reduction of the magnetic field strength at the offload position of the surface of the sleeve results in a significant reduction in the build up of magnetised carrier particles on the sleeve.
Preferably, the sleeve or sleeves are made from stainless steel. In one form of the invention, the sleeve is formed of a plastic inner sleeve with a thin metal shell over the top.
According to the second aspect of the invention, there is also provided a method for electrostatically applying powder material to solid dosage forms, the method comprising the steps of:
Preferably, the method further comprises the steps of:
Preferably, the rotating magnetic field applied to the first sleeve rotates in the opposite direction to the rotating magnetic field applied to the second sleeve.
In an embodiment of the invention, the method further comprises the step of returning the magnetized carrier material to the sump.
Preferably, the method further comprises the step of controlling the height of the mixture on the sleeve or sleeves. The step of controlling the height of the mixture on the sleeve or sleeves may be achieved by a blade alongside the sleeve or sleeves.
Advantageously, the method further comprises the step of earthing the solid dosage forms before passing them alongside the sleeve or sleeves.
The rotating magnetic field may be applied to the sleeve or sleeves by at least one magnet inside the sleeve or sleeves.
In an embodiment of the invention, the sleeve or sleeves are substantially cylindrical. In an alternative embodiment of the invention, the sleeve or sleeves are substantially in the shape of a cylinder but having a flattened portion running substantially the length of the sleeve located on the sleeve where the solid dosage forms are arranged to pass alongside the sleeve or sleeves. In another form of the invention, the flat top described above is replaced with a top that slopes down towards the offload side of the sleeve.
In an embodiment of the invention, the sleeve or sleeves include a magnetic shield arranged to provide a localised reduction in the magnetic field strength at the surface of the sleeve at an offload position of said sleeve. In this embodiment of the invention, the offload position, that is, the position at which the magnetised carrier leaves the sleeve, can be controlled by controlling the location and thickness of the shield. The shield is preferably a mu-metal shield.
The sleeve or sleeves may be made from stainless steel. In an alternative form of the invention, the sleeve is formed of a plastic inner sleeve with a thin metal shell over the top.
According to the second aspect of the invention, there is also provided an applicator for electrostatically applying powder material to substrates, the applicator comprising two sleeves for receiving a mixture of electrostatically charged powder material combined with a magnetic carrier material from one sump, the sleeves being arranged to have electric potentials applied thereto to drive the electrostatically charged powder material onto substrates passing alongside the sleeves, the sleeves being arranged to have rotating magnetic fields applied thereto for rotating the mixture around the sleeves, the magnetic fields applied to the two sleeves being arranged to rotate in opposite directions.
Providing two sleeves instead of one enables the rate at which substrates can be coated with powder to be increased. Further, rotating the magnetic fields of the sleeves in opposite directions tends to improve the uniformity of the coating.
According to the second aspect of the invention, there is also provided a method for electrostatically applying powder material to substrates, the method comprising the steps of:
According to the second aspect of the invention, there is also provided an applicator for electrostatically applying powder material to substrates, the applicator comprising:
The provision of a flattened portion of the sleeve where the substrates pass alongside the sleeve assists in providing an even coating of the substrates.
According to the second aspect of the invention, there is also provided a method for electrostatically applying powder material to solid dosage forms, the method comprising the steps of:
According to the second aspect of the invention, there is further provided an applicator for electrostatically applying powder material to substrates, the applicator comprising:
The reduction of the magnetic field strength at the offload position of the surface of the sleeve results in a significant reduction in the build up of magnetised carrier particles on the sleeve.
According to the second aspect of the invention, there is also provided a method for electrostatically applying powder material to solid dosage forms, the method comprising the steps of:
According to a third aspect of the invention, there is provided apparatus for electrostatically applying powder material to solid dosage forms, the apparatus comprising apparatus as hereinbefore described according to the first aspect of the invention and an applicator as herein before described according to the second aspect of the invention.
According to the third aspect of the invention, there is also provided apparatus for electrostatically applying powder material to solid dosage forms, the apparatus comprising:
The solid dosage forms may be oral dosage forms, for example, pharmaceutical tablets.
According to the third aspect of the invention, there is also provided a method for electrostatically applying powder material to solid dosage forms, the method comprising a method as hereinbefore described according to the first aspect of the invention and a method as hereinbefore described according to the second aspect of the invention.
According to the third aspect of the invention, there is also provided a method for electrostatically applying powder material to solid dosage forms, the apparatus comprising the steps of:
According to the third aspect of the invention, there is also provided apparatus for electrostatically applying powder material to substrates, the apparatus comprising:
According to the third aspect of the invention, there is also provided a method for electrostatically applying powder material to substrates, the method comprising the steps of:
According to the third aspect of the invention, there is also provided apparatus for electrostatically applying powder material to substrates, the apparatus comprising:
According to the third aspect of the invention, there is also provided a method for electrostatically applying powder material to substrates, the method comprising the steps of:
According to the third aspect of the invention, there is also provided apparatus for electrostatically applying powder material to substrates, the apparatus comprising:
According to the third aspect of the invention, there is also provided a method for electrostatically applying powder material to substrates, the method comprising the steps of:
According to the invention, there is also provided apparatus according to the third aspect of the invention further comprising a sump of powder material. Preferably, the apparatus is suitable for pharmaceutical applications and the powder material in the sump is pharmaceutically acceptable.
Preferably, the sump of powder material is contained in a replaceable cartridge. Preferably, the cartridge is replaceable by the user. Preferably, the cartridge is suitable for pharmaceutical applications.
According to the invention, there is also provided a sump of powder material for use with any aspect of the invention. Preferably, the powder material in the sump is pharmaceutically acceptable. According to the invention, there is also provided a cartridge comprising such a sump of powder material. Preferably, the cartridge is suitable for pharmaceutical applications.
The invention may also be applicable to the electrostatic application of powder material to other products, in particular medical products, for example stents, and the reader will understand that, where the term solid dosage form is used, the term stent may equally be used.
It should be understood that any features of the invention which are described with reference to one aspect of the invention may be equally applicable to another aspect of the invention.
Embodiments of the invention will now be described with reference to the accompanying schematic drawings of which:
As already mentioned, sump 101 comprises powder material mixed with a carrier. The powder material will be used for coating the solid dosage forms and is a toner-like material which is capable of being electrically charged. For pharmaceutical applications, the powder material must, of course, be pharmaceutically acceptable. The carrier is any suitable material capable of being magnetised. In this embodiment, the carrier is a quantity of permanently magnetised strontium ferrite beads. The powder material and carrier are mixed in a prescribed ratio which will be described in more detail below.
The active mixing and shearing system causes the powder material to electrically charge and attach to the carrier particles. The charging occurs to a large extent by triboelectric charging for example due to the frictional contact between the powder material and the carrier particles. The number of shearing sites (and hence the speed of charging) are increased by having a number of slots or holes in the paddles 203a, 203b (not shown), which results in greater agitation of the powder material/carrier blend. Of course, with slots or holes in the paddles, the amount of material which can be turned over by the paddles decreases. Thus this serves to decrease the amount of shearing whereas the holes themselves increase the amount of shearing. Thus, the optimum arrangement is one in which the overall shearing by these two routes is maximised.
It can be seen in
In use, the shaft 301 and magnets 303 remain stationary while the outer sleeve 305 rotates in the direction shown by the arrow 309. The bucket loader 105 is positioned above the mixer shafts so that the powder material and carrier are pulled up into the buckets 307 by the 6 o'clock magnet 303. (It will be remembered that the carrier is magnetised so is attracted by the magnets 303. The powder material is electrically charged due to the shearing provided by the mixers and is therefore attracted to the carrier as it moves up into the buckets.) As the outer sleeve 305 rotates, the powder material and carrier remain in the bucket by virtue of the magnets 303. There is sufficient magnetic strength to maintain material in the buckets until it reaches approximately 9 o'clock at which point the material remains in the bucket by virtue of gravity. As the bucket rotates further, the magnets on the rotor/sleeve arrangement attract the powder material and carrier onto the sleeve of the rotor/sleeve arrangement 107.
Of course, the bucket loader may be arranged to rotate in the opposite direction, in which case the magnets will instead be positioned from 6 o'clock round to 2 o'clock (in the anti-clockwise direction).
The effect of the magnetic fields is to create a series of opposite poles around the sleeve, shown schematically by dotted lines 407. The poles run in lines parallel to the axis of the sleeve. Because the rotor is not concentric with the sleeve, but is mounted more closely to the sleeve at the top and left, the magnetic field on the sleeve is stronger at the top of the sleeve than at the bottom of the sleeve and is stronger at the left hand side of the sleeve than at the right hand side of the sleeve.
In the arrangement of
Of course, the rotor may be arranged to rotate in the opposite direction i.e. clockwise, in which case the carrier and powder material will progress around the sleeve in the anti-clockwise direction.
A metering blade (not shown) forms a slot between the blade and the sleeve 401 so as to form the brush into a constant height. The speeds of the bucket loader 105 and the rotor 403 are chosen to supply an abundance of material to the sleeve/rotor arrangement so that, after the metering blade, the brush is of a controlled predetermined height.
A high voltage supply (not shown) is applied to the sleeve 401, the polarity chosen to create a potential difference that will drive the charged powder material particles towards any lower voltage parts. As the solid dosage forms 505 pass across the top of the sleeve 401, the solid dosage forms 505 are very close to the brush. The solid dosage forms 505 are arranged to be at, or close to, earth potential such that the electric potential on the sleeve is sufficient to drive the powder material 503 onto the exposed surfaces of the solid dosage forms 505. As the powder material deposits on the exposed surfaces of the solid dosage forms, a voltage builds up. This eventually balances the electric potential on the sleeve, so that no more powder material is driven onto the solid dosage forms. Thus, the electric potential applied to the sleeve can be used to control the amount of powder material deposited on the solid dosage forms. The distance d (see
The carrier material 501, however, remains magnetically attracted to the rotor magnets so remains on the sleeve. The carrier 501 continues to progress around the sleeve 401 in the clockwise direction, as shown by the arrow 509, as the rotor 403 rotates in the direction shown by the arrow 409 and eventually the carrier material 501 falls off the sleeve 401 and returns to the sump. The lower magnetic field at the offload portion of the sleeve (because of the eccentrically mounted rotor) facilitates this.
It will be appreciated that, because the powder material is being used up to coat the solid dosage forms whereas the carrier material is not being used up, if the sump were not monitored, the ratio of powder material to carrier would change. A concentration sensor is used for this purpose.
In this embodiment, the concentration sensor uses a ferrite core differential transformer to sense the permeability of the carrier/powder material mixture. In order for the concentration sensor to operate successfully, there must be a reasonable quantity of mixture in the sump so that there is sufficient mixture in front of the sensor to achieve a reasonable sensitivity. In practice, this may be a depth of about 5 mm of mixture. As the relative proportions of the carrier and the powder material change, the permeability of the mixture changes and the coupling between the transformer elements in the concentration sensor changes. A replenishment system, connected to the concentration sensor, adds new powder material to the sump so that the carrier to powder material ratio is maintained.
It should be noted that the bucket loaders 605a and 605b could rotate in the opposite directions to the directions shown in
The advantages of the arrangement of
The two counter-rotating brushes also gives a more even coat on the tablet by minimising what is known as the “edge effect”. The edge effect can be described as follows. As the carrier progresses around the sleeve, it eventually falls back into the sump. However, because of the magnets on the rotor there is a tendency for some carrier particles to remain on the sleeve even though the magnetic field strength at the bottom portion of the sleeve is lower. Thus, there can be a build up of carrier particles causing an “edge” of surplus carrier material which, as it extends around the sleeve, can inhibit the powder material from being driven onto the solid dosage forms. The two counter-rotating brushes in
The third mixer 603c is positioned between mixers 603a and 603b. Mixer 603c comprises a shaft 701c with a number of crescent shaped paddles 703c. The paddles 703c on mixer 603c are not angled in either direction, but are perpendicular to the shaft 701c axis. Thus, when mixer 603c rotates it does not tend to drive the powder material and carrier to either end of the mixer, but simply mixes the powder material and carrier in situ. The mixer 603c can be arranged to rotate in either direction.
Just as with the two mixer arrangement of
A second alternative form of sleeve/rotor arrangement (not illustrated), may be used in the arrangement of
As shown in
The offload position is dependent on the position and thickness of the mu-metal shield. Accordingly, the offload position can be controlled. This may be advantageous, for example, in order to return the magnetised carrier material 501 to the sump in the optimum position for combining with new material. By way of example, the offload position may be selected so as to maximise the time that the magnetised carrier material 501 is mixed with the material in the sump.
It should be noted that the mu-metal shield 501 can be located inside the sleeve 401 (as shown in
The arrangement of
Of course, the solid dosage forms described herein are just two of many possible solid dosage forms that could be used with the present invention. The solid dosage form could be any shape that is suitable for its particular application.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0407312.8 | Mar 2004 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2005/001247 | 3/31/2005 | WO | 00 | 3/8/2007 |
| Publishing Document | Publishing Date | Country | Kind |
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| WO2005/095002 | 10/13/2005 | WO | A |
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