Care and Handling of Coverboard in Book Binding
Dave Bird, Technical Services Manager at Rock-Tenn, shares tips on storage and handling of binders board for consistently flat book covers.
Traditionally, bookbinding has been a relatively simple operation. Regardless of whether a book is being bound for the first time, or otherwise rebound, the process consists basically of sewing or gluing pages together to maintain an order, preparing a cover and gluing it over cover boards and stamping, or otherwise, decorating the completed cover, then attaching the cover to the text.
In recent years, competition in the book industry has become fiercer than ever. One inevitable outcome of such competition has been the race by many manufacturers to apply quality manufacturing methods such as Lean or Six Sigma to the bookbinding operation in order to reduce working capital, increase inventory turns, or improve productivity. Ironically, while great progress has been made in decreasing turnaround time from publishing to book delivery, several of these same companies report increasing numbers of rejections due to poor quality, despite the adoption of these advanced manufacturing techniques. And as every book manufacturer knows, a “good” book is one that doesn’t come back!
One aspect that many quality gurus miss is a proper understanding of what happens to cover board during a bookbinding operation. Coverboard, after all, is a natural fiber and as such, responds to natural elements - particularly moisture – in different ways. Further, cover curl - an occurrence in which the covered panel of a case-bound book does not remain flat – is one of the leading factors in “cover board quality” as experienced by book manufacturers, publishers and component vendors.
Understanding the time required for moisture to diffuse and equilibrate with the environment is essential to minimizing cover curl. Cover curl typically increases during the dry, winter months and modifying your standard approaches to manufacturing may help you control cover curl and increase overall throughput. Here are some things to keep in mind to keep your bookbinding operations running as smoothly as possible.
Maintaining a Consistent Environment
During dry winter months, manufacturers often do not compensate for the lower humidity. As the air becomes drier, cover board will typically lose some of its internal moisture as it equilibrates with a dry environment. This can sometimes be seen as the top board units of an opened pallet start to curl upwards in the cross-grain direction. This movement is natural with cellulose fiber but can sometimes lead to feeding difficulties on the case-maker. When board is delivered from a cold warehouse or off the back of the shipping truck, it needs to acclimate with the manufacturing environment.
Tips to manufacturing success
- Store received board in an environment similar to the manufacturing area.
- Before running board on the case-maker, allow time so that the board’s stabilized temperature is within 15 deg. F of the case-making area.
- Open board loads, removing stretch film but allowing the board to be top-weighted during acclimation to minimize moisture dissipation. If some of the top boards start to curl in the cross-grain direction, just set them aside in a stack with the curl facing down and they will shortly flatten out.
- Eliminate any fans or blowing air onto the board load during case making. In many situations, an overhead blower or fan is creating excessive air movement and accelerating moisture migration.
Maximizing Moisture Acceptance
The case-making process consists of two primary steps. First, adhesives are applied to the cover material and depending upon the absorption ability of the cover material and the amount of any filler or coating, the adhesive may sit on the surface for a period of time before starting to penetrate the cover material surface. The second step involves the placement of cover board onto the cover material where, upon contact, the adhesive will start to penetrate the board surface.
This interaction of bonding between the two substrates, using a water-based adhesive is referred to as a moisture event. Minimizing the effects of a moisture event lead to the best results for flatness and occur when there are strong similarities of moisture acceptance and moisture dissipation between the two bonding substrates.
Moisture acceptance can vary greatly with the wide variety of cover materials being used today. Production managers should make sure they are familiar with the dissipation characteristics of each cover material type used. Film coated and liquid laminated covers will not have the same absorption rate or dissipation rate as natural cloth or uncoated paper-base covers because the outer side of the material has been sealed. Synthetic cover material will have a limited amount of moisture acceptance and rely on a suction bond.
In each event, the additional moisture from the adhesive must go somewhere. That somewhere is into the board, resulting in additional fiber swell and longer time required for moisture dissipation. Depending upon the cover material, its level of moisture compatibility, and the characteristics of the board, the time to reach equilibrium with the environment may be anywhere from 12 to 24 hours. During this time, the made-cover is open to manipulation.
Tips to delivering a flat cover
- Maintain an adhesive solids content above 50%.
- Use a minimal amount of adhesive to create the bonding of substrates. If 1 mil (.001) thickness is adequate to create the bond and 2 mil (.002) is applied, the moisture addition to substrates has doubled.
- Whenever possible, adjust the adhesive viscosity by adjusting the temperature rather than adding additional water. There is a fairly wide range in temperature application and flow characteristics can be improved without inducing add’l moisture. Your adhesive supplier can assist in creating a temperature/viscosity range chart.
- Learn which substrate combinations generate problems. Pull 2 made covers off the case maker while running. Lay one cover face-up and the other facedown without weighting them. Watch for curl reaction within a few minutes. If curl is excessive, the made covers should be brick-stacked and top-weighted to maintain flatness during curing.
Stacking and Storing Finished Books
Proper stacking to maintain flatness requires positioning stacks of proportional size (of made covers) onto a supported flat surface and the addition of weight upon the stack to insure flatness as moisture dissipates. There are a variety of methods used to stabilize made covers during equilibration but the most common method is brick stacking onto a pallet.
Brick stacking uses an overlap of each cover layer to maintain flatness and weight. A wooden top can be used to weight the top stack layer. The key elements to success in brick-stacking are: a) maintaining a consistent stack height that provides minimal weight build on the cover turn-in area, b) placing cover stacks so that their weight is distributed to the outer edges of the stack below and, c) overlapping layers to stabilize the pallet for movement to the next operation.
If made covers are not controlled during equilibration and there are dissimilarities between the bonded substrates, cover curl can result. As mentioned previously, the rate of moisture intake and the rate of moisture dissipation between cover material and cover board represent two factors of consideration. Another factor is the adhesive. If excessive adhesive is applied, the excessive moisture can affect substrate interaction. If the adhesive solids content is low, additional moisture is added and that moisture must eventually dissipate. With proper and timely stabilization of the made covers, the interaction of these variables can be reduced.
In-line case-making and case-in operations present a slightly different set of equilibration issues. As long as there is minimal time between case making and case-in, the made cover will not have time to react to the moisture intake. This reduces the possibility of cover curl happening before the made cover reaches the case-in cover hopper. Problems generate when the case-in portion of the in-line operation is down and case making continues to run product. Made covers should be brick-stacked and stabilized so that they can be fed back in-line without presenting problems to the case-in cover hopper.
Tip 3: Flat covers for the case-in operation
- Brick stacking off the case-maker should always result in the cover turn-in area being supported by the cover turn-in area of the stack below it.
- Cover stacks should be made with a consistent amount of covers in every stack.
- Stack height of covers should allow for no more than 3/8” differential between the measurement at the turned-edge and measurement at the board joint area.
- Pallets of made covers remain flat when a weighted top is placed upon the load during curing.
Improving the Case-in Operation: Another Moisture Event
During case-in, application rollers meter adhesive onto the outer end leaf. As the end leaf adhesive makes contact with the exposed surface of the cover board, there exists another opportunity for a moisture event.
In this case, the moisture from the water-based adhesive will begin penetrating the end leaf fibers and the board fibers. A time period ranging from 12 to 24 hours is necessary for the moisture to dissipate and the substrates to reach equilibrium with the environment. During this time, the bound book and the made cover can be manipulated or allowed to find equilibrium by stabilizing the bound book. This is frequently done by stacking the books onto pallets and weighting them with a pallet top or light banding. If books are packed directly after case-in, a properly fitted bulk carton will stabilize the bound books.
Tip 4: Flat books out the door
- Case-in adhesive should have a high solids ratio..…60 to 65% and the application rate should be minimal to make the bond.
- If curl reactivity was found to be an issue in case making, monitor the results during case-in by placing a bound book on a table and watching it’s reaction over a 3-5 minute timeframe. If there is an exaggerated tendency for cover uplift, be sure that palletized book stacks are weighted during curing. If books are going directly into cartons, be sure that the carton is a good fit and doesn’t contain excess “head” space.
- Track flatness issues and the type of end leaf used to see if there is correlation over time.
Understanding the substrates, the interactions and the process
Proper handling within the process and understanding of the interactions of the components is vital to manufacturing high quality books. As noted above, when changes to the environment, the types of cover board or cover material, glue, or method of stacking occur, it is best to consider how this will effect the moisture dissipation. In this way, returns and allowances can be kept to a minimum and overall book manufacturing productivity improved.
- Cover material – A variety of different materials are available for the outer covering of the book cover including paper-based printing stock, natural cloth, and synthetic woven & non-woven materials. These may be used in their natural state or coated with a variety of fillers to enhance performance characteristics, such as latex saturated, acrylic filled or paper-backed.
- Cover board – Refers to recycled cellulose fiber as the dominant base substrate used to support the cover material. Cellulose fiber provides an inexpensive “body” to the case-bound cover and allows water-based adhesives to be used in the bonding process.
- Adhesive – Water-based adhesives are the most functional, allowing considerable “open-time” to position the cover board onto the cover material and create the turned-in edges. Dextrin and polyvinyl-acetate adhesives are used but protein-based heated glue (normally referred to as “animal glue”) is the dominant adhesive used to bond cover materials to cover board.
- Moisture Event – The joining of substrates with a water-base adhesive and the corresponding interaction between the substrates as they absorb and dissipate moisture.
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