Materials and consumables
Materials questions test whether you can specify a substrate, ink and consumable that survive both the printing process and the finishing, and store them so they behave. Expect gsm and calliper arithmetic, grain direction, ink-paper interaction and shelf life.
Paper and board
- Grammage in g/m2 multiplied by area gives weight; calliper in micrometres and bulk in cm3/g describe thickness and stiffness. Coated grades hold finer screens; uncoated needs lower total area coverage.
- Grain runs parallel to the longer sheet dimension unless the ream is marked short grain. Fold parallel to the grain, and perfect-bind with the grain parallel to the spine, or the book cockles and the text block will not lie flat.
- Condition paper at 20 plus or minus 2 degrees Celsius and 50 plus or minus 5% relative humidity for 24 hours before printing. Paper expands across the grain as it takes up moisture, which is where register and folding problems start.
Inks, toners and plates
- Offset inks are paste inks balanced between tack and viscosity: too much tack picks the paper, too little emulsifies with fountain solution.
- UV inks cure by photoinitiator activation rather than evaporation; never mix UV and conventional ink and never leave UV ink sitting on a blanket.
- CTP plates are grained and anodised aluminium, thermal at 830 nm or violet at 405 nm; handle violet plates under yellow safelight and keep plates dry, flat and out of sunlight.
- Compressible blankets are specified by thickness, commonly 1.70 mm or 1.95 mm, and must match the packing calculation, or squeeze and excess dot gain follow.
Adhesives
- EVA hot melt for general perfect binding, PUR for lay-flat and coated stock, water-based adhesives for case making, starch for corrugated.
Worked example
Calculate paper for 5,000 sheets of A3 at 120 g/m2. A3 is 0.297 x 0.420 = 0.1247 m2, so one sheet weighs 0.1247 x 120 = 15.0 g and the run weighs 5,000 x 15.0 g = 74.8 kg. Add 3% spoilage and you order about 77 kg, that is 10 reams of 500 sheets plus a little spare.
Traps that cost marks
- Ordering short-grain stock for a folding job and then blaming the folder for the cracking.
- Storing plates or paper next to a drying press, a radiator or an open window.
- Assuming every ink sold as eco-friendly is solvent-free; check the safety data sheet for VOC content.