Correcting a carton net for board thickness, bleed and A3 fit
A bakery wants a shelf-ready carton for six shortbread fingers. The internal dimensions must be 160 mm × 60 mm × 25 mm. The carton is to be made from 350 gsm folding boxboard, printed in two spot colours, with a 3 mm bleed, a 4 mm glue tab and a crash-lock base. The student’s net uses the external dimensions as 160 × 60 × 25 mm, draws the creases as single lines, and places the front-panel logo 1.5 mm from a crease. The preflight report shows the two spot colours are 0.3 mm out of register at the crease, and the net measures 424 mm × 300 mm, so it will not fit the A3 sheet (420 mm × 297 mm) with bleed. The bakery also requires the carton to run on its existing A3 press. The student argues that “the printer can scale it down”. The packaging technician has asked for a corrected net and a production-ready artwork plan.
1. Determine whether the carton can be produced on A3, and propose the most suitable fix.
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The carton cannot be made on A3 in one piece at the required internal size. For a simple four-panel tube, the blank width is 2(L + W) plus the glue tab. Using corrected external dimensions, L = 160 + 2(0.35) = 160.7 mm and W = 60 + 0.7 = 60.7 mm. Blank width = 2(160.7 + 60.7) + 4 = 446.8 mm. Adding 3 mm bleed on each side gives 452.8 mm, which exceeds the 420 mm width of A3; the 297 mm height is also tight once the base and lid panels are included. Rotating the net does not help because 452.8 > 420 and the other direction is also over 297. Scaling to fit is not acceptable: it would reduce the internal length by about 6% and the biscuits would not fit. The practical options are to use a larger sheet such as SRA3+ (460 × 320 mm) or to redesign as a two-piece sleeve and tray, with the sleeve printed and the tray die-cut separately. The student’s 424 mm net was already over A3, so the “printer can scale it” argument fails.
2. Correct the dimensional basis of the net, including board thickness, crease allowance and bleed.
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Start from internal dimensions because they control the product fit: 160 × 60 × 25 mm. For 350 gsm board, t = 0.35 mm. External dimensions are L = 160 + 2t = 160.7 mm, W = 60.7 mm, H = 25.7 mm. Add a crease allowance of about 0.4 mm per fold, tested on the actual board, so the panels do not bind when folded; this is added to the flat pattern, not to the internal size. Use a 4 mm glue tab, 3 mm bleed beyond trim, and keep all live artwork at least 3 mm inside creases and trim. Draw creases as dashed or chain lines, not as single solid cut lines, so the die-maker can distinguish cut from fold. The crash-lock base needs its own locking tabs and should be proofed with a white dummy before print. Dimension the net from the internal cavity and note the board grade and grain direction; grain should run along the main folds to reduce cracking.
3. Advise on artwork placement and colour registration for the crease problem.
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The 0.3 mm misregistration at the crease is a warning that the artwork is too close to a fold and that the two spot colours may not trap correctly. Move the logo so its nearest edge is at least 3 mm from any crease and 3 mm inside trim. Where the two spot colours meet, add a 0.3 mm trap (overlap) on the lighter colour, and avoid fine reverse type across a fold. Extend the background bleed 3 mm past the trim on all sides, but do not carry it into the glue tab unless the tab is hidden. Add registration marks, a colour bar and a legend outside the trim. Proof the revised net on the actual 350 gsm board, fold it, and check the register at the crease under a loupe. If the press still cannot hold register, simplify the design to one spot colour plus the board colour rather than accept a blurred fold line.