Research & Innovation
Back to the Future in Sheeting and Converting
By Pulp & Paper Canada
There are many reasons why paper and paperboard sheets, such as folio-sized paper for offset printing short runs, are needed. It is more economical to use sheet-fed printing presses and they offer mor...
By Pulp & Paper Canada
There are many reasons why paper and paperboard sheets, such as folio-sized paper for offset printing short runs, are needed. It is more economical to use sheet-fed printing presses and they offer more flexibility in the size of the final product. Flexibility can be increased further through several printing presses, as the investment cost is relatively low (when compared to web presses). Sheet-fed offset printing presses ensure an excellent register and print quality and eliminate both curling and cracking of the bulky folding boxboard.
Packaging and paperboard sheeting must meet the increasing requirements of customers and modern society, as more attractive surface on packaging materials is an excellent media for information.
An increasing number of ink jet and laser printers both in home and office have boosted the consumption of cut size sheets.
Paper and paperboard converting and sheeting is not a goal in itself. Neither is it always profitable unless done with superior expertise. This is a typical characteristic of paper and paperboard converting, as the field is multidisciplinary. Four principal product groups exist in paper and paperboard converting:
1. Packaging media
2. Industrial papers and boards
3. Household and similar papers
4. Paper and plastic bags and sacs
These items are used by some of the following sectors: the food, printing, paper and steel industries, household goods and schools, communications, offices, techno chemical industries, agriculture, cable, textile and garment industries, sanitary products, hospital products, sack industry, folding and corrugated board box manufacturers, automotive industry, etc.
It is necessary to establish quality standards to the requirements of the printer, mill or packaging standards in order to protect the finished product as well its appearance in the market place.
Quality standards are usually specific to a grade or category of paper determined by its end use. An example of this would be cut size paper for office use on small printers or photocopiers. These will not exceed 11″x17″ size sheets and most common 8.5″x11″ or legal documents 8.5″x14″. These specifications will show a sheet 8.5″x11″ to be plus or minus .031″, square to five degrees, and curl not to exceed a given dimension. Curl charts are used to do this.
Folio sheet sizes will be greater than 11″x17″. For example, a typical folio sheet will read 23″x35″, and the specification will read 1/16 oversize, plus or minus 1/32.
Modern day printing has brought an additional category of users called the digital printing grades. These also have specific standards for sheet sizes, which vary from one mill to another. Common sizes are 11″ x 17″ or 12″ x 18″ grain long or 18″ x 12″ grain short. The size specs vary on width based on grain direction as well as length.
Sheeting folio or cut size requires adhering to the standards for the sheet size and the quality of the knife cut and the slitter cut. The sheet must be square to meet the standards. Excessive roll curl must be decurled in the backstand, in order to give a flat sheet. If there is static in the paper, does it run well on the sheeter or does the static cause jam-ups on the sheeter? If the static stays in the paper it will become a problem for the pressman on the printing press.
Quality sheeting aspect
It is time to define what “quality” means in terms of sheet finishing and how the quality of a sheeting operation is measured. This does not include paper defects such as holes, lumps, poor formation, coating defects, etc.
Quality of cut edge, accuracy of width, accuracy of length, accuracy of square or angle and accuracy of the pile or jog must meet the quality standards established by the customer, whether loose in cartons or loose on skids. The sheets must be clean, square and without scuff marks if coated. The paper and board product quality is still a paper machine or off machine coater operator’s responsibility.
If the sheeting firm or sheeter operator is conscientious, all paper or board defects should be identified to the customer. A typical operation would advise the mill or customer quality control department of such issues and discontinue sheeting the defective product until further instructions are received.
From roll to sheet
Roll build and quality of the roll to be sheeted is very important for further processing on converting equipment. The roll is sized by diameter and width to accommodate the finished sheet size. The edges must be clean and dust-free, especially if sheeted in multiple webs at a time. If there is edge dust, it will be pulled onto the surface of the sheet by the vacuum created by the webs. The core of the roll must be tight in order to control unwinding. For web tension to be maintained and to prevent the roll from unwinding on an emergency stop or web brake, the roll must be round and the hardness must be even across the width for optimum control.
The web is guided through a web tension control system that controls the braking system at the backstand. It is then put through a decurling station and a web guiding system and then on through a splice detector and into the slitter section. The web tension control will help maintain sheet length accuracy as well as added tension to help decurl if required. The objective is to have a flat sheet within the specifications of the customer. The splice detector rejects splices by activating the reject gate (splices left for the printer will damage the blankets on the press). The slitter section will cut the sheet or sheets in the width required and trim the edge off the roll. The edge trim is removed to eliminate damaged edges on the sheet and guarantees the squareness of a finished sheet. At this point the edge of the sheet is usually the feed side into an offset printing press and therefore must be clean cut.
Quality cut edge is measured by microfiche and the length tolerance is measured on a scanning table in some locations. The cut should be as smooth as possible to avoid paper dust and loose fibres from accumulating on the printing press. Many converting or sheeting centres use a premeasured template and a microfiche as a means of measuring the quality of cut for both slitter and knife cut. In many cases the knife cut on a sheeter is the side of the sheet on a printing press, and the cut edge with dust will eventually cause problems for the pressman.
This is an extremely important requirement for precision sheeting. The sheets that are not squared to close tolerances cannot be printed in perfect register in multicolor jobs. A conscientious sheeting operation will have properly calibrated dimension gauges and the sheeter operator will control the quality of the product being sheeted. Dimension gauges should be calibrated regularly, approximately every two months and every time it is relocated or moved.
Operating a folio sheeter can get quite complex given the following variables. Short sheets grain long, or short sheets grain short, or coated papers and coated board, cutting any of the above on a sheeter requires knowledge and experience. The speed at which a sheeter must run in order to be financially successful is as critical as what product it is cutting. For example, cutting short sheets does not always allow the nip or roller with belts to control the sheet at high speeds. When cutting coated paper or board, the pressure of the belts will mark or scuff the coated sheet. A critical area is in the overlap sectors and the accuracy of piling is also very important. The jog must be plus or minus 1/32″. These standards may vary from one mill to another. However, when “press ready” skids are required by a printer, the pile will not be redone by the press feeder and thus the jog must be acceptable to the press sheet pickup mechanism, or the pressman may reject the skid. The right sheet count must also be met, whether i
n cartons or loose on a skid.
Basis weight variation refers to the difference in basis weight from one roll to another off the same order. The average mill has plus or minus 5% variation in its standards. A sheeting operation processing orders by M weight (the weight of one thousand sheets of paper of a specific size) and sheet count will run into problems. If the basis weight or GSM is off, the sheet count on a skid of paper will be short if the weight is on the high side. However, if the basis weight is on the low side, there will be more sheets on a skid. If the proper sheet count is on the skid, the weight of the skid will be lower than expected based on the calculated weight per thousand sheets. This is critical to a converter and customer or end user. The same will apply to measuring shrinkage caused by the process of converting rolls into sheets. If the weight of the paper or board is off, the shrinkage will not be accurate. These are only a few of the critical items of concern when sheeting paper or board.
The environment in which a sheet of paper is produced should be as close as possible to the environment in which it will be converted or printed. Relative humidity and temperature are critical aspects to be considered. Good sheet-fed printing requires a pressroom temperature of 70-85 F(21-29C) and room humidity between 35% to 50%. A room with humidity in the 40-45% range is considered optimum for many sheetfed offset plants.
Static control is also important and easier to control if the room humidity is high. Traceability of the roll from the sheet is very important and all skids must be identified by means of tracing the roll from which they were generated. Rejected sheets and product shrinkage will play an important part in the cost of sheeting, as down time to load rolls or remove skids from the layboy adds to the economics of sheeting paper.
The economics of sheeting paper and paperboard has changed over the years. No longer are the long runs on sheeters and converting equipment for standard stock sizes or large inventories. While paper mills are competing in a global market, paper merchants are competing domestically for any business the printing industry might have.
Sheeting is no longer a function reserved solely for paper mills. Sheet sizes are very different and ordered in different quantities. Orders are typically much smaller and more frequent. The same is often required within 24 hours from the time of order, which makes it very difficult for a mill to compete on the same playing field. The product that fills the order is what the merchant has in stock. Mills will turn an order around in 24 hours with a converting centre or contract sheeting operation, holding roll stock inventory.
More mills are outsourcing the sheeting of their products because more and more sheeting operations control the quality of converting paper products better or equal to themselves.
Future means of getting sheets of paper and paperboard to customers could be done via a customer service centre, where parent rolls are inventoried. This service would be outsourcing the operation after the paper mill roll production to finished sheets going to the customer. The customer service centre would be an independent company filling orders for a paper mill from customer service representatives through sheeting the paper and shipping the product, to the paper merchant/distributor or as by the instructions from the merchant, ship direct to the end user.
This would allow the concentration of funds and skilled people to focus on helping paper mills compete in the global market. The reduction in handling rolls for internal processing, as well as all required supplies, couples with the high investment required in the finishing rooms would allow for considerable cost savings that would directly affect the bottom line.
Mark-free sheeting of coated paper and board
Damage to the surfaces of sensitive paper and board grades in the form of bruising, marks and scratches is a problem paper mills and converters are often facing today. In conventional sheeting machines these marks are caused by physical interaction in the sheet transport and in the overlapping section through contact with the material. The consequence: every year, tonnes of coated paper and board have to be taken back by converters and paper mills as a result of customer claims.
One of the companies of the Krber PaperLink Group, E.C.H. Will has addressed those elements of folio sheeters which caused the problems. The solution is a new kind of sheet transport and overlapping system that works with a vacuum belt system and carefully controlled positive electrostatic charges instead of conventional overlapping fingers and break rollers to slow down and overlap sheets. Consequently, with this system there is no danger of the machine marking materials like conventional machines do. Instead, it promotes continuous production and highest quality, even at higher speeds.
Convertingblog.com provides online community for converters
Converters, industry journalists and interested parties are invited to join a web log, presenting a forum for converters to share ideas and gain knowledge about the converting industry, business and applications in this online discussion forum, with topics ranging from application specific to general business operations.
Maxcess International announced the launch of Convertingblog.com, facilitated by Tim Walker of TJ Walker and Associates and other industry experts. Through dialogue with their peers, participants are able to share experiences and collaborate on issues that they face everyday in an environment that provides incubation of good ideas to the point that they may become the new industry practice.
Jean-Pierre Dupr is the president and CEO of Trillium Converting Corporation, ON, and La Corporation de Transformation Iris, QC.
1 The author admits that, still hanging on his office wall, is the handmade sheet distributed during PaperWeek International 2005 in Montreal in recognition of the 200th anniversary of papermaking in Canada.