Control Strips

In offset printing it is essential not to rely fully on visual, subjective quality checks, but rather to carry out objective measurements as well.  Only measurements will provide data which may be used for process control. It is a prerequisite that suitable control strips are employed during plate making, proof and production printing.  A number of control strips and other quality control devices will be described which are compatible to the BVD/FOGRA standardization system. The FOGRA Precision Measuring Strips PMS I , PMS I /N and the PMS Print Control Strip, as well as the UGRA Plate Control Wedge 1982, are film strips which provide various line screen, halftone, solid tone and continuous tone panels.  These provide visual and instrumental control of image transfer steps in plate making and printing.  They were designed primarily as measurement aids for high quality 60 I./cm ( 150 I./in) four color jobs. The FOGRA PMS is a universally applicable, compact control strip for plate making and printing. Apart from a solid tone panel it comprises elements in the highlight (7 % area), mudstone (39%) and shadow (80%) ranges.  The halftones contain circular dots or holes of 59 I./cm screen ruling.  Additionally, there are three doubling and slur elements (D) and a microcline wedge (K) for the control of plate making. The UGRA plate Control Wedge 1982 is used for the control of plate making and the detailed evaluation of print reproduction curves.  The wedge comprises a continuous tone wedge, a micro line panel, a halftone wedge, doubling and slur elements and highlight/shadow panels. The FOGRA PMS Control Strip is used specifically for the control of four-color proof and production printing. At the sacrifice of about 10 mm (1/2") space in the printing direction, it provides the full set of test patches for the control of plate making and printing which are required for carrying out the BVD/FOGRA Standardization. 

Standardized Plate making

Dot gain is the dominant property determining the visual agreement between proof and print. Therefore, it is important to watch dot gain closely and to standardize the dot area change during plate making. For the purposes of standardization, plate types are classified, according to resolution, into groups with similar reproduction properties.  The tonal value transfer of each group is given by a Copy Table, see Fig.4, which also shows the recommended standard range for the micro line element. The resolution is determined as follows: A series of exposures of an UGRA Plate Control Wedge 1982 or a FOGRA PMS I is carried out on the same plate and at a fixed position on the vacuum frame.  Every successive exposure should be a fixed multiple of the previous exposure.  We then select the exposure where reproduction of both positive and negative microclines starts in the same segment.  The number of this segment indicates the resolution in terms of micrometres(um). Plates for proof and production printing should be exposed and processed so that the micro line reading lies within the standard range of the corresponding Copy Table. This assures a standardized dot area loss of about 3.5% in the mid-tone for positive plates and a dot area increase of about 4% for negative plates. 

Apparent Dot Gain

The apparent dot gain of a print is defined here as the difference between the tonal values on the print and on the corresponding film area.  The tonal value of the print is determined densitometrically on the image of a control strip by means of the Murray-Davies formula. The principal aim of standardization is the matching of dot gain in press proof and production printing for all colors.  In practice, scattered forms, consisting of color separations from various trade houses, are quite common.  Therefore, a general agreement on dot gain is necessary so that compatibility problems can be reduced.  The standardization recommends four distinct sets of dot gain values, see Table 2.  All gloss-coated and most matt-coated and all non-coated papers are ranked as B.  The criteria for matt-coated papers is the apparent dot gain shown in a print test. If the dot gain is not within tolerances, it can be influenced by changing material or production parameters bearing in mind the following order of effectiveness: (a)  Ink flow properties, (b) The criterion for matt-coated  papers the apparent dot gain shown in a print test. If the dot gain is not within tolerances, it can be influences by changing material or production parameters bearing in mind the following order of effectiveness: (a)   Ink flow properties, (b) Blanket type, (c) Squeeze pressure and machine type, (d) Stock type, (e) plate exposure.

Solid Tone Density

In proof and production printing, the inking level of the primary colors Cyan, magenta and Yellow is determined by Derived Color Standards.  These are solid tone color specimens which are printed on production stock, with production ink, by one of the three methods.  Only inks conforming to ISO 2846 are recommended for proof and production printing The standardization does not advocate the indiscriminate use of standard color densities as this practice does not assure consistent visual appearance. The uniformity of solid tone density across the printing direction should be within +_4% for proofs and +- 6% for other prints. The variation of densities during a production should be within +-8% for every color. 

Standard Color Sequences

4-color-printing : B+C+M+Y
2-color-printing: C+M, B+Y.
1-color-printing : C, M, Y, B or, preferably, B.C.M.Y.

Table 1 : Example of a Copy Table which applies to positive working plates of 5 pm to 8 pm resolution.  In this case, 12 pm to 15 pm is the recommended exposure range.

Table 2: Standard apparent dot gain values for 60 I./cm (150 I./in) ruling.