Contact copier

Johnson contact copier early (1930's)

A contact copier (also known as contact printer), is a device used to copy an image (negative or positive depending on the process), by illuminating the photolith that contains it, placing it in direct contact with a photosensitive surface (film, paper, plate or sheet) on which a (negative) image is formed from the photolith Original. The light source can be an actínic bulb internal or external to the device[1]

It consists essentially in a flat glass on which the original photolith is placed in direct contact with the surface (paper, film, plate) to be exposed, with a pump for creating a vacuum around both (optional), and a light source (actinic UV tubes in the case of offset and PCB manufacturing) making use of an automatic timer (optional).[1]

Uses

This contact copying process that was used in the early photography days, it is also used, among others: in silkscreen printing, in blueprint copying, in the CTF offset printing processes and in the PCB manufacture.

The contact copier comprises a source of intense light projected through a photolith film that only lets light pass through in clear areas (those not printed). Silk frames, offset plates or PCB, are usually impregnated with a photosensitive material (varnish type) which reacts with light that modifies its surface characteristics. Subsequently, some portions of the photo-sensitized varnish may be removed, leaving the final product ready for use .

Photography

contact copier used in photography (with an external light source)

The contact copier is used for duplication of negative prints , that is, to reproduce on paper, a photographic negative in exactly the same format. It was the common mode to make prints until it began the use the alternative photographic enlarger. There are some models with internal light source constructed as a closed box, in which one or more lamps illuminate the negative through an opal or frosted glass.

The reproduction is done by placing the negative on top of the glass and then the photo paper with the emulsion in contact with the negative. The negative role fits the body, the lid is closed applying some pressure against the glass to prevent blur; then proceed to impress photographic paper by turning on the interior light (contact copier) or an external focus, in the last case, usually from a photographic enlarger. Exposure time can be controlled "manually" or using a timer that controls the light source for a preset time and with greater precision.

Whiteprint

Diazo copier by Ozalid

In the process of whiteprint copying ,[2] (essentially a diazotype process), a dynamic contact copier is used (similar to the manual action of exposing both sheets strongly bonded directly to the sunlight.). The exposition is made progressively as the twin rollers pull the two papers together (original and copy) against a source of ultraviolet light, typically a powerful black light lamp . The original plan (on a transparent support) and the diazo paper are introduced, in perfect contact, within the pinch rollers of the contact copier. The sensitized paper, has a photosensitive coating -an impregnation of diazo - covering the surface of the paper. Once exposed, the copied paper is immersed in a developer solution made from ammonia (or ammonia vapor) converting the parts of the paper not exposed to the light source to a characteristic dark violet colour (blue-line).

Silkscreen

Contact copier for Silkscreen

In screenprinting it is normally use a contact copier box with several fluorescent tubes close to the silk frame. The emulsion reacts depending on the amount of light received, for that reason it is important to make some tests for determining the exposure time. Usually for a "box type" contact copier, exposure is usually not more than one minute. In the industrial type contact copiers with actinic lamp placed at a distance, exposure time can be about 20 minutes.

The photolith (also called "Art") is placed under the frame against the emulsion. The black portions of photolith film does not let light into the relevant parts of the coated silk, therefore, in these parts Emulsion will not heal and may be subsequently washed, keep in mind that there must be no gap between the photolith and the silk, to achieve that purpose any element with enough weight may be used to press the silk against the photolith (or a vacuum pump in professional machines).

Offset

contact copier used in CTF

In CTF offset technology, a photolith is placed in direct contact with the printing plate and pressed with an opaque lid that sometimes has a vacuum pump that helps making a good contact. The plate comes coated with a layer of photoresist in which a (negative) inverted image is formed with respect to the original photolith and after the transfer of information (burning) it's accomplished, then, after undergoing a developing process, the plate will be ready to be used in an offset printer.[3]

The process shares some fundamental principles with the photographic processes, since the pattern engraved on the plate is generated by exposure to light with an image created in a contact copier using an optical mask. This procedure is comparable to the high-precision method of the version used to make printed circuit boards.

Printed circuits

Contact copier for PCBs

The contact copier is used today, particularly in the areas hobbyist, for the photoengraving of prototype printed circuit boards (PCBs) before being sent to production (artisanal creation ). Substantially similar to the contact printer used in photography, this variant usually uses ultraviolet lamps to impress a copper base specifically pre-sensitized.

Burned by exposure to light parts reproduce patterns drawn on a transparent photolith film on a pre-sensitized plate (epoxy or Bakelite). This pre-sensitized plate comprises an insulating plate (epoxy resin or bakelite), adhered with a layer of copper, and coated with a varnish layer sensitized. The varnish is sensitive to UV rays, which weaken or strengthen its structure depending on whether a "positive" or "negative" process. Photolith film printed parts (usually black), inserted between the light source and the pre-sensitized plate, protect the varnish from the UV. The base copper impressed must then be engraved in a specific bath (usually iron chloride -FeCl3-), that removes the excess copper.

See also

References

  1. 1 2 Hearst Magazines (November 1948). ? id = 89gDAAAAMBAJ & pg = PA216 Popular Mechanics Check |url= value (help). Hearst Magazines. pp. 216–. ISSN 0032-4558.
  2. ↑ Blueprints replaced by whiteprints
  3. ↑ NIIR board (1 October 2002). id = aMvZQJwVfk0C & pg = PA222 Hand Book on Printing Technology (offset, Gravure, Flexo, Screen) Check |url= value (help). National Institute Of Industrial Re. pp. 222–. ISBN 978-81-7833-087-7.

External links

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