1. Field of the Invention
The present invention relates to electrographic apparatus and more particularly to such apparatus with an improved construction for controlling the application of particulate developer to an electrographic image member.
2. Description of the Prior Art
In electrographic imaging an image member, bearing an electrical image pattern, is developed by application of marking particles which adhere to the member in accordance with the electrical pattern. Many acceptable techniques exist for applying developer; however, one general approach, which is often used commercially, is to attract particulate developer to an applicator surface and move the applicator into a transfer relation with the image member so that marking particles can adhere to the member in accordance with the image pattern.
Most commonly the applicator is a roller which rotates so that its peripheral surface moves between a developer supply location and a zone in transfer relation with the image member. Adherence of the developer to such applicator rollers can be accomplished in various ways including, e.g., adhesive or electrical attraction, but the most prevalent commercial technique utilizes magnetic attraction and applicators using this technique are often called magnetic brushes. Developers used with such magnetic brushes can be single component (in which case toner is magnetically attractable) or comprise two components (in which case the toner particles are electrostatically attracted to magnetically attractable carrier particles in the developer mixture). The magnetic brush applicators can take various forms; however, a typical configuration comprises a non-magnetic outer cylinder which surrounds an array of magnets located within its inner periphery. Developer transport is effected by rotation of the outer cylinder and/or the interior magnet array.
In using such magnetic brushes (and in other applicators such as mentioned above), the uniformity of image-development often depends significantly upon control of the quantity and density (i.e., compactedness) of developer adhering to the applicator surface. Developer shortage can cause incomplete development and developer excess can cause scratching and other non-uniformities in the developed image. Some development apparatus use conductive or partially-conductive developers to provide an electrically biased development field; and, in such devices, the thickness and degree of compaction of the developer "nap" should be uniform so that predetermined field conditions will exist at the development zone.
The classic technique for controlling nap thickness of such brushes has been a doctor blade that extends across the applicator surface and is spaced a uniform distance from the surface. Such doctor blade is located between the developer supply and the image member so as to skive off excess developer adhering to the roller. Thus, the goal is to position the skiver precisely parallel to the transporting cylinder to form a predetermined gap which controls the amount or thickness of toner passing to the development zone. Although proper skiver positioning is critical, it is extremely difficult to achieve and maintain. An expensive mechanism is necessary if easy adjustment is needed. More often, compromises are made which make such skivers difficult to adjust or not adjustable at all. In addition to the foregoing problems such skivers, or doctor blades, necessarily cause unwanted compaction of the adhered developer, prior to its entering the development zone.