4 essential printing production processes you must know for imposition, otherwise it will be very troublesome
Book imposition is a very important prepress task and serves as the core guide for the entire production process. Since book production involves many processes, different imposition methods have varying impacts on printing quality, efficiency, and cost. A reasonable imposition method can ensure smooth printing production. Improper imposition can cause unpredictable issues in production, making completion impossible or even requiring rework. This article will explore the impact of book imposition on different printing processes, hoping to provide guidance for peers.
Impact of Imposition on Printing Processes
Printing is one of the most crucial steps in book production, and the quality of imposition directly affects printing costs and efficiency. Generally, if the order quantity is large, priority is given to using sheet-fed presses, eight-color presses, or even web presses, each of which has different imposition requirements, as discussed below.
When using a sheet-fed press, the first step is to check whether the ordered paper size meets the requirements. Once this basic condition is satisfied, the folding method should be determined according to the number of folds possible in later processes. Most conventional folding machines cannot handle folding full sheets on sheet-fed presses and require splitting, such as combining two 16-page sections. Sheet-fed presses are slower than dual-press machines, but the overall production efficiency is twice that of a dual-press, although setup and alignment take longer, making them suitable for large orders. When the order quantity exceeds 10,000 copies and the page count exceeds 256 pages, using a sheet-fed press can be considered. The drawback of sheet-fed presses is that color control is not as good as dual-press machines, and without specialized reversing equipment, manually flipping sheets can be labor-intensive.
For large print runs with many pages, eight-color presses are also suitable. Eight-color presses come in various types; from an imposition perspective, they can be mainly divided into single gripper and double gripper, with different imposition placements. For single gripper presses, the front and back are printed simultaneously, making front-to-back registration easier to control, only leaving a normal color strip at the plate end. However, printing the bottom side together carries risks such as smudging in areas of heavy ink coverage, and some models require intermediate placement (suction positions). If not placed properly, later folding may result in uneven page sizes, making online stitching difficult, and may even require additional waste sheets to achieve proper binding. For double gripper presses, imposition must reserve the gripper edges because changes in the gripper often cause misregistration, reducing print quality. Eight-color presses are suitable for print runs over 8,000 copies, with page paper weights of 100–140g/m² and orders that do not require high color accuracy.
Web presses are characterized by high speed, inline folding, and quick turnaround, making them less suitable for small orders. When imposing for web presses, attention should first be paid to grippers and color bars. On sheet-fed presses, the gripper and color bars are usually at each end of the sheet, with the color bar at the end of the plate. On web presses, the gripper and color bars are in the same position, saving some paper edges. Another consideration is that the circumference of a web press is fixed, meaning the sheet size (including gripper edge) cannot be adjusted and always has a fixed dimension. Therefore, when considering using a web press, the first consideration should be whether the finished size matches the press circumference. Otherwise, even if efficiency is improved, paper edge waste may increase costs, especially when multiple layouts are imposed on the last plate, adding manual pagination costs. Generally, web presses are suitable for print runs over 30,000 copies, with paper weights of 64–105g/m² for the inner pages, provided the finished size matches the foldable imposition method and the equipment's maximum usage of the circumference is no less than 75%.
When imposing, in addition to considering the characteristics of the above presses, attention should also be paid to not imposition images with large hue differences in the same column (top and bottom positions); elements requiring precise registration, such as small reversed text, should be placed near the gripper; for very thin paper, such as Bible paper with a weight below 48g/m², do not impose too many pages together, as this may frequently cause double sheets, reducing press efficiency, and may also require extra alignment before folding due to uneven stacking.
Impact of Imposition on Surface Finishing Processes
Although books and periodicals do not require flashy surface treatments like packaging, some craftsmanship is usually added to the cover design, which leads to differences in imposition compared to the inner pages. For inner page imposition, it is sufficient to maximize the use of the printing press size, whereas covers need to be considered more comprehensively. For cover printing, it is best to use a four-open press, without arranging too many copies per sheet; even for large print runs, at most 4 copies can be arranged. For products that require very precise registration for certain surface treatments, it is best to arrange only one cover per sheet, regardless of the print run, otherwise it will increase the difficulty of subsequent processes or even make them impossible, such as hot stamping, spot UV, or embossing, which all require precise alignment with the printed content.
Most imposition personnel often impose self-reverse plates to save materials, which is usually suitable for products without surface treatment. It is not recommended to do this for covers or areas with surface treatments, as it is prone to errors and makes color consistency on the front and back more difficult.
Inner pages may also undergo surface treatment, so areas that require treatments should be placed on the same side as much as possible, otherwise problems may occur. For example, if spot UV is needed on both sides, the back side will be affected because the paper has already experienced drying from ultraviolet lamps or heating modules on the front side, causing expansion and deformation, which increases registration difficulty, and in some cases, the paper may curl and cannot go through the press.
Moreover, for products that require post-press die-cutting, the location for printing allowance differs from ordinary products, and it is necessary to comprehensively refer to the parameters of the die-cutting and printing machines to determine the correct positioning.
**Impact of Imposition on Folding and Gathering**
Imposition mainly serves post-press processes. In book and periodical printing, the main post-press processes, excluding surface treatments, are folding and gathering, as well as binding the finished products. Some also refer to these as semi-finished products and finished products.
Common folding methods for book inner pages include 4PP, 8PP, 12PP, 16PP, 24PP, 32PP, etc. These folding methods are primarily based on the paper thickness and whether the pre-fold and post-fold dimensions fall within the equipment parameters of the folding machine.
For commonly used inner page papers such as book paper and coated paper, if the book paper exceeds 120g/m² or coated paper exceeds 180g/m², using the 32PP imposition method can easily cause paper wrinkles.
Book imposition is usually carried out on bookmaking machines or the bookmaking units of perfect binding lines. Some are done on integrated imposition machines, where the machine parameters must be considered to determine how many sections can be imposed at a time.
For example, if a bookmaking machine can impose 20 sections at a time, and 16PP imposition requires a total of 26 sections, two sets must be imposed. After checking the paper thickness and dimensions, if using the 32PP method, only 13 sections need to be imposed at once, significantly improving efficiency.
In processes such as gathering, thread stitching, and saddle stitching, it is necessary to leave allowances for large and small margins to improve production efficiency. During imposition, extra spacing for top and bottom margins needs to be added, usually with at least 5mm difference, and it is necessary to standardize whether the bottom or top margin is larger. Paper thickness and the total thickness of the book must also be considered. If the paper is too thin, pages with a large count should be gathered together. For example, if using 70g/m² coated paper with a total of 40 sections, printing can only be done 16PP at a time. Inner, middle, and outer sections can be gathered together, then processed through the folder (same saddle stitch folding group), with thread stitching done per 48PP section, reducing the number of end pieces and making the spine more square and aesthetically pleasing. If there are leftover 4PP sections, they should also be gathered with other main sections.
Many books design fold-out pages within the inner pages, which can be single fold-outs or double fold-outs. Special attention is required in layout setup regarding size and folding method. The dimensions of the extended pages should be slightly smaller than the finished product width, mainly to ensure smooth trimming and extension of the fold-out pages.
Gate-folded double fold-out pages that require sewing are generally more complicated to handle, and there are two main methods:
The first method is an 8PP gate fold (if the equipment cannot perform gate folding, a reverse accordion fold in the middle is needed), pasted at a designated position. After the finished product is created, this position will be overcome, which may slightly affect it. If there is a cross-page image nearby, the pasted position may impact the unfolding of the cross-page.
The second method involves sewing along the fold. Since gate-fold fold-outs have no large and small edges, conventional methods cannot insert the stitching needle. It usually requires several sets of 4PP scrap papers to be included, sewing through them, and then removing the scrap. This method makes the final product sturdier and more aesthetically pleasing but increases the cost of several processes.
Experienced layout designers may use a special 12PP version for folding. This 12PP version is different from the conventional 12PP folding method and is more similar to the folding method of a standard 16PP version. This method is mature and avoids unnecessary processes such as including scrap pages.
Impact of Layout on Binding Processes
Reasonable folding and imposition layouts can basically solve most problems in the binding process. When saddle-stitching books with a large number of pages, or as mentioned above, multiple pages grouped together for stitching, it is necessary to consider whether the content at the trimmed edge may be cut off. If there is a risk, prepress settings should include creep adjustments, with the specific distance set according to the thickness of the signature or the entire book.
For perfect bound and hardcover books, the spine adhesive between signatures should ideally be recessed to leave space. The purpose of leaving the gap is to allow better adhesion to the glue. For cross-page images between the cover or endpapers and the inner pages, overlapping must be created so that the cross-page can span correctly after binding.
The cover of a perfect-bound book needs a reserved glue area. The standard is that the cover should wrap around the half-finished book block; otherwise, during perfect binding, the adhesive can easily transfer along the spine to the cover. Small-size perfect-bound books often use a double-book imposition method, and the spacing between the two books also requires careful adjustment according to the parameters of downstream equipment to prevent cutting issues.
Proper imposition is crucial for printing production, as it can improve efficiency while reducing production costs. Layout personnel need to have a thorough understanding of the entire printing process and equipment parameters, and ideally, a dedicated person should oversee quality control and implement a review mechanism.