Format size:

Short and simple CMYK 300 dpi. In detail…

Pictures used in design must have a minimum resolution whatever the format of those pictures are (TIFF, PSD, JPEG or some other type). This minimum resolution is calculated by multiplying resolution of printing plate (for example 46 lines per centimetre) with 2,54 (46 lpc × 2,54 =117 lpi) and 2 (117 lpi x 2=234 dpi). The two at the end represent main rule in flexography, that the picture or image must be at least two times larger than resolution of printing plates. That means that it must be at least 117 lpi × 2 =234 dpi on the monitor for raster. Recommended size is a little bigger varies between 254 and 300 dpi. Resolution larger than that, for example 400 dpi, is useless since the details in the image cannot be printed and the oversized images take much space. It must be noted that during printing you may lose parts of colour space shown on the monitor if the image is in RGB and not in CMYK format and this part can be important for some end costumer.

lpi=line per inch
lpc=line per cm
dpi=dot per inch

Vectors and text format:

Do not use raster process on vectors and text unless it is for some special intentional effect. If the format is only CMYK (text/logo) do not rasterize it, because it is possible that the text will be made as a special spot colour (different than the raster process colour). In that case, during prepress the text will be extracted by hand, converted into vector, and convert into another colour, followed by trapping process. Used colour on colour trapping is minimum 0,1 mm (depend on the machine). Conclusion is that flexography sometimes does not allow printing a regular image with the text in CMYK format like in offset technique.

Do not forget to add the font you used or alternatively convert the text into vector (function expand in Illustrator). Printed minimum text size is small (6, 5 or even 4 pt), while for invert text minimum of 8 pt bold is recommended for gold, metallic colours, and strong colours. 6 or 5 pt is for white on black background film. If you are working with vectors, you should use original program like Illustrator, since it is not recommended to leave the vectors in Photoshop. It is a good thing to add a JPEG or PDF preview file to see the original idea.

Shadow, minimum, maximum:

In flexography process there can be a dot gain which can result with something like a step (tonal jump effect). One percent on the monitor can go as far as 20% in print especially if solid colour (100 % full tone) is used with gradient in raster on the same printing plate. That is also the reason the gradient is sometimes separated from the same spot colour. Tonal jumps occur with CMYK images in sharp contrast usually on black and cyan colour, and with certain techniques the tonal jump can be minimized (e.g., avoiding 0% CMYK colours, softening the contrasts, etc.). In addition to problems with minimum, problems regarding filling up may also occur. For example, 95% of defined colour can occur better than 100%. Plate producers use this effect for the best results in flexographic. That also can be avoided by using softer contrast or by simply not using the colour definition above 95%.

Number of colours:

The maximum number of colours must be respected. It is impossible to print design with defined fourteen spot Pantone colours with an 8-colour printing machine (real, not so uncommon example). Do not forget white ink. If we print on transparent film one colour is usually used as a white background colour, so just seven colours are left. Same goes for mat varnish or glossy/protection varnish. Solid and raster area in the same colour will be also separated in most case and they will be used as two separate colours with different ink thickness. Logo and vector text must be also defined as Pantone, HKS and not as a CMYK colour. Anilox roller for CMYK raster has 3-4 times less ink transfer as a spot colour anilox roller. So, if you define a text in 100 % black in CMYK and use as a CMYK colour the colour will be dark grey and if we use same ink with 3-4 time stronger anilox roller the test will be very dark black.

Several types of pigments can give the same colour shade but with different characteristic. For example, red Pantone 485 C can be made with mixture of yellow and magenta, or orange and magenta, warm red and yellow…etc. The colour shade will be same but effects as bronzing with orange, low light fastness with warm red can occur. Metallic inks (gold, silver…) have an aluminium or other pigments with metallic glow which can’t be measured. Such colours can’t be used like normal colours because they need bigger viscosity and low-resolution big ink transfer anilox.

Conclusion

Flexography is determined by: preparation, block (hardness and resolution), glue for block (hardness), anilox rollers (volume, resolution), ink (viscosity, density) and printing machine itself (the central cylinder, pressure, automatic viscosity…) with other variables.