Method and apparatus for electrostatic powder coating

An electrostatic coating method for coating a workpiece with chargeable powder particles, comprising the steps of:

• supporting a workpiece to be coated with powder particles and a silent discharge plate electrode in a spaced opposed relationship;
• applying a d.c. voltage across the workpiece and said silent discharge plate electrode to establish an electric field in a space therebetween;
• applying an a.c. voltage to said silent discharge plate electrode across two spaced points thereon;
• introducing into said space between the silent discharge plate electrode and the workpiece for coating the workpiece charged powder particles having the same polarity as the silent discharge plate electrode,
• prior to introducing the charged powder particles into said space fully charging the particles to be coated on the workpiece externally of said space and with a separate d.c. voltage independent of the first-mentioned d.c. voltage and in the same polarity as that of the silent discharge electrode with a sufficient charge of said polarity on the maximum number of particles charged to effectively have the charged powder particles driven toward the workpiece when introduced into said electric field;
• and
• said a.c. voltage being applied to effectively repel the charged powder particles from an electrode surface confronting said space and to maintain silent discharge at the electrode surface very small to effectively prevent neutral uncharged particles and particles charged oppositely to said polarity from depositing on said surface of the silent discharge electrode, whereby the powder particles in said space are driven toward the workpiece to coat the workpiece and prevented from accumulating on said silent discharge electrode surface.