Electrostatic application of powder material to solid dosage forms utilizing an electrically conductive shield
An apparatus for electrostatically applying a powder material to a solid dosage from includes a source ( 1 ) of charged powder material, a support assembly ( 2 ) for supporting the solid dosage form ( 3 ) with a front face in the vicinity of the source of powder material and facing the source of powder material, the support assembly ( 2 ) including an electrically conducting member ( 5 ) in the vicinity of the rear face of the solid dosage form and an electrically conducting shield ( 8 ) disposed closely around the solid dosage from ( 3 ) between the front face and the rear face of the solid dosage form, and means ( 4 ) for creating a potential difference between the source of powder material and the electrically conducting member and for maintaining the electrically conducting shield at a potential more similar to that of the source of powder material than to that of the electrically conducting member.
1. An apparatus for electrostatically applying a powder material to a solid dosage form having a front face and a rear face, the apparatus comprising:
a source of charged powder material;
a support assembly for supporting the solid dosage form with the front face in the vicinity of the source of charged powder material and facing the source of charged powder material, the support assembly comprising an electrically conducting member in the vicinity of the rear face of the solid dosage form and an electrically conducting shield disposed around the solid dosage form between the front face and the rear face of the solid dosage form; and
means for creating a potential difference between the source of charged powder material and the electrically conducting member and for maintaining the electrically conducting shield at a potential more similar to that of the source of powder material than to that of the electrically conducting member.
2. An apparatus according to claim 1 , wherein the electrically conducting shield extends continuously around all of the solid dosage form.
3. An apparatus according to claim 1 , wherein the electrically conducting shield has a cylindrical part defining a cylindrical opening for accommodating the solid dosage form.
4. An apparatus according to claim 3 , wherein the length of the cylindrical part of the electrically conducting shield is less than the depth of the solid dosage form, measured as the maximum separation between the front and rear faces of the solid dosage form.
5. An apparatus according to claim 1 , wherein during use, the solid dosage form is supported on the support assembly and there is a gap of not more than about 1 mm between the solid dosage form and the shield.
6. An apparatus according to claim 1 , wherein the electrically conducting shield comprises an electrically conducting element covered by a layer of insulating material.
7. An apparatus according to claim 1 , wherein the electrically conducting member is adjacent to the rear face of the solid dosage form.
8. An apparatus according to claim 7 , wherein the electrically conducting member includes a shaped receiving part for receiving the rear face of the solid dosage form, with the rear face conforming closely to the receiving part over a major part of the area of the rear face.
9. An apparatus according to claim 1 , wherein the potentials at which the electrically conducting shield and the source of powder material are maintained are of the same sign.
10. An apparatus according to claim 1 , wherein the electrically conducting member is arranged to be maintained at earth potential.
11. An apparatus according to claim 1 , wherein the support assembly is suitable for supporting a plurality of solid dosage forms and comprises a plurality of electrically conducting members, each in the vicinity of a rear face of a respective one of the solid dosage forms, and a plurality of electrically conducting shields, each disposed around a respective one of the solid dosage forms between the front face and the rear face of the respective solid dosage form.
12. An apparatus according to claim 1 , wherein the means for creating a potential difference between the source of powder material and the electrically conducting member comprises a voltage source for applying a bias voltage between the source of powder material and the electrically conducting member.
13. An apparatus according to claim 1 , wherein the electrically conducting shield is disposed closely around the solid dosage form between the front face and the rear face of the solid dosage form.
14. An apparatus for electrostatically applying a powder material to solid dosage forms each having a front face and a rear face, the apparatus comprising:
a source of charged powder material;
a support assembly for supporting the solid dosage forms with front faces of the solid dosage forms in the vicinity of the source of charged powder material and facing the source of charged powder material, the support assembly comprising an electrically conducting member in the vicinity of the rear faces of the solid dosage forms and an electrically conducting shield disposed around the solid dosage forms between the front faces and the rear faces of the solid dosage forms, the shield including a plurality of openings for receiving respective solid dosage forms; and
means for creating a potential difference between the source of charged powder material and the electrically conducting member and for maintaining the electrically conducting shield at a potential more similar to that of the source of powder material than to that of the electrically conducting member.
15. A method of electrostatically applying a powder material to a solid dosage form having a front face and a rear face, the method comprising the steps of:
providing a source of charged powder material:
supporting a solid dosage form on a support assembly with the front face in the vicinity of the source of charged powder material and facing the source of charged powder material, the support assembly comprising an electrically conducting member in the vicinity of the rear face of the solid dosage form and an electrically conducting shield disposed around the solid dosage form between the front face and the rear face of the solid dosage form;
creating a potential difference between the source of charged powder material and the electrically conducting member; and
maintaining the shield at a potential more similar to that of the source of powder material than to that of the electrically conducting member, whereby powder material is applied to the solid dosage form forward of the shield but substantially not rearward of the shield.
16. A method according to claim 15 , wherein the solid dosage form is a domed tablet having a pair of opposite domed end faces joined by a cylindrical side wall.
17. A method according to claim 16 , wherein the electrically conducting shield is disposed closely around the cylindrical side wall, and powder material is applied to the part of the side wall forward of the shield but not to the part of the side wall rearward of the shield.
18. A method according to claim 15 , wherein the potential difference created between the source of powder material and the electrically conducting member includes a bias voltage that is a steady DC voltage.
19. A method according to claim 15 , wherein an alternating voltage is superimposed on the DC voltage.
20. A method according to claim 15 , further comprising the step of treating the powder material to fix it on the solid dosage form.