Crack Prevention in Corrugated Paper Boards

IntroductionPackaging boxes are made by creasing and folding corru-gated board to impart stiffness. For proper folding of corru-gated board, neat fold lines (creases) are required to be cre-ated. These creases weaken the section along the fold line to facilitate a neat fold.A schematic of the creasing process is shown in the figure below.ProblemsThe top liner cracks during creasing if the crease is too deep.The bottom liner cracks during folding if the crease is too shallow.Paper is highly sensitive to temperature and humidity.Fiber orientations of paper vary even in its width.Project GoalPrevent cracking of corrugated board during creasing and folding operations. This shall be accomplished by:Thoroughly understanding the mechanical behavior of corrugated boards.Modifying the current creasing and folding process to prevent cracking of corrugated paper boards.Experiments and Finite Element SimulationThe experimental creasing test setup consists of a corrugated board held firmly against an anvil, on which a creaser (knife) indents a crease. The creasing test setup is shown below:The finite element model for the creasing simulation is shown in figure below:ResultsThe experimental load-displacement curve for the creasing process along with the simulation is shown below:The finite element simulation for creasing process is in good qualitative agreement with experimental observations. How-ever, quantitatively, the simulation deviates from experi-ments, specially in the post-buckling regime.ConclusionsPaper, the constituent material of corrugated boards is me-chanically characterized. The creasing process can be sim-ulated well in a finite element analysis framework. How-ever, more accurate characterization of paper is required and micro-mechanical model for cracking in paper is required to be developed.

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9 Corrugated Box Testing Methods to Ensure Packaging Quality

Corrugated Box Testing Methods:1. Bursting Strength Test2. Edge Crush Test 2 Single3. Water Resistance of the Gluing4. Cobb Sizing Tester5. Paper Grammage and Thickness6. Puncture Resistance7. Scuff Resistance Test8. Box Compression Test9. Chemical Analysis in Corrugated Box TestingAs per FICCI, cardboard products make up more than 30% of the packaging industry. That’s because corrugated boxes are pretty popular. Regular slotted containers, for example, are one of the most widely used types of boxes for shipping and storage. Corrugated cartons are low cost, durable, and eco-friendly. However, not all boxes are made equal. That is to say, they vary a lot in terms of compressive strength, thickness, chemical resistance, and other factors.You’ll need to know this when you have to choose a suitable corrugated box for your packaging needs. The cardboard strength and construction directly impact the amount of weight a box can carry. A box too weak might puncture or collapse. On the other hand, an excessive box means you pay extra for material and add to environmental wastage.To determine how corrugated boxes hold up against external influences — so as to help you make an informed decision when picking boxes — manufacturers use multiple testing procedures. Here’s a round-up of common corrugated box testing methods used to check the strength of corrugated boxes. #1– Bursting Strength TestWHAT: Test box strength when subjected to pressureWHY: To determine exact weight a box can carry, rough handlingThe Mullen Test or Bursting Strength Test checks the toughness of the walls of the corrugated board when subjected to force or pressure. In this testing procedure, a rubber diaphragm is used to apply pressure to the walls of the corrugated box until it bursts. The diaphragm is expanded using hydraulics and as the diaphragm expands, the corrugated board bursts under pressure.We measure the bursting strength in Kilograms per square centimetre. The Bursting factor is given as a thousand times the bursting strength, divided by the grammage of the board.Bursting factor = Bursting strength (kg/cm2) x 1000/Grammage (gm/m2)#2 – Edge Crush TestWHAT: Cross-direction crushing of a corrugated boardWHY: To test material quality, stacking strength, pallet shipping enduranceOne of the most popular methods to test the stacking strength of corrugated boxes is the Edge Crush Test or ECT. Every side of a corrugated box is made up of sheets that have three or more layers. Generally, a thin sheet with grooves and ridges is sandwiched between two flat sheets. These grooves and ridges, also known as flutes, provide anisotropic strength to a box. This means the boxes can bear a greater amount of force from one direction. Edge crush test, right test, or flat crush test helps determine this. In the Edge Crush test, force is applied to one side of the box, perpendicular to the ridges, until the box gets crushed. In the Ring Crush test, a ring cut out from the box is used to test the same. While in the flat crush test, excessive force is applied on one side. The results of the Edge Crush test are measured in pounds per lineal inch of load-bearing edge but represented as ECT.Reference: Standford UniversityECT gives you an accurate idea of a box’s strength when they are stacked and shipped by pallets. Also, an ECT rated corrugated box uses less material than its equivalent Burst Test rated box, to provide an equivalent level of strength. This means it’s cheaper and less wasteful to purchase ECT rated boxes in bulk.However, it is important to know that the above results (overall strength of a box) may vary based on the makeup of each corrugated sheet. Hence, knowing the type of corrugation is important before you make a decision.#3 – Water Resistance of the GluingWHAT: Test water-resistance of the glue lines of a corrugated boardWHY: To test the impact of climatic conditions, moisture permeability, water absorptionEven though the fiberboard itself can absorb and retain water, testing the water-resistance of gluing or sealing is also important. For certain applications, the FEFCO 9standard is used to test the water-resistance of the gluing of the corrugated cardboard boxes.In this type of test, the corrugated board is immersed in water while exposing the glue lines to check for bond strength and water absorption.#4 – Cobb Sizing TesterWHAT: Check water resistance and measure weight increase when exposed to water   WHY: To test the quality of the box, porosityThe raw material that is used in the manufacture of corrugated fiberboards has a tendency to absorb water and retain it. The Cobb Sizing tester is used to the degree to which water is absorbed. In this test, corrugated fiberboard is first subjected to water. Then, the water is squeezed out of the sample using pressure. Usually, depending on the quality, all of the water is not removed in spite of the board being pressed by a heavy steel roller. The difference in weight because of the retained water is known as the Cobb value.The lower the Cobb value the better the water resistance. The Cobb test is required for certain certifications, especially those involving hazardous material packaging.#5 – Paper Grammage and ThicknessWHAT: Test the areal density and thicknessWHY: To test box quality and rigidityGrammage and thickness are the two most fundamental properties of the corrugated fiberboards that determine the quality of the box. There is no “best” grammage or thickness mentioned in corrugated boxes specifications and it is completely based on your requirements. When more padding is required, a box with a higher thickness is used. The flutes of the corrugated board are larger and pack more air in them. Thin boards with dense flutes have a high grammage. Such boxes are needed when the packaging should be more compact and rigid.Image Courtesy: Shanghai GLThickness is usually measured in (millimetres) mm and grammage is expressed in terms of grams per square meter (g/m2).Thinner boards are easier to fold, lighter, and more suitable when it comes to printing or detailing. Thicker ones are more sturdy and appropriate for heavy-duty shipping. Usually, the type of flute (A, B, C, E, or F) determines the cardboard strength with C being the most common (4.0mm).#6 – Puncture ResistanceWHAT: Test resistance to penetration by sharp, solid objectsWHY: To test cardboard strength and sturdiness during transportationPuncture resistance tests how well the box can handle the impact made with a pyramid or triangularly shaped weight. The corrugated box testing standards for puncture resistance include FEFCO 5 or ISO 3036.#7 – Scuff Resistance TestWHAT: Check durability of printed or painted boxes to resist abrasionWHY: To ensure printed text on cardboard box can sustain rubbing, wearingPrinting is an integral part of packaging. With different types of printing methods being used, it’s important to check how well labels or prints can handle scuffing or abrasion. For this purpose, scuff resistance or rub proof tests are employed. There is the Sutherland Rub Test, which is an industry-standard testing procedure. Coated surfaces such as paper, films, paperboards and all other printed materials are tested using this procedure.Alternative corrugated box testing methods are also available to test resistance to abrasion. These include rotary abrasion testers and even manual wiping. Scuff Resistance test is especially important for pharmaceutical or medical industries where label legibility is of prime importance. #8 – Box Compression TestWHAT: Test compressive strength of a corrugated boxWHY: To measure the stacking strength of a pallet loadBox compression test, also known as container compression test, is a way to test how much load the box can take on top of it before it gets deformed and the extent to which it gets deformed. It gives us a good idea of how many boxes can be stacked together without damaging the contents of the lowermost box. This test is crucial to check the strength of boxes and is required by most industries transporting boxes in bulk.Image Courtesy: Testing InstrumentsThe boxes are tested in different orientations. If the box design makes use of inner supports, such as wood support or corrugated board cushioning, then such factors are also taken into account.The compressive strength of a corrugated box can also be calculated using the McKee formula where the Edge Crush Test (ECT), Caliper or corrugated board thickness (CBT) and box perimeter (P) values are taken into account.As per the McKee formula: Compression Strength = 5.874 * ECT * CBT0.508 * P0.492#9 – Chemical Analysis in Corrugated Box TestingWHAT: Test resistance of box to chemicalsWHY: To check product formulation and regulatory complianceChemical analysis is required for certain applications where the nature of fiberboard, as well as its resistance to certain chemicals, are checked. The analysis of the fiberboard involves microscopic examination of the board to see what types of paper are used to make the fiberboard. Moreover, the moisture content, as well as the pH of the board, is also determined.Picking the Right Box for Your NeedsCorrugated box testing procedures help you determine the right kind of box for your business requirements. Most boxes carry a Box Maker’s Certificate stamp that lists specifications and box strength.For example, the ECT value lets you calculate the amount of weight a package can carry. Or, the Box Compression Test helps determine stacking strength that’s especially important when shipping and transportation. Take time to find the perfect box. It can save you a lot of costs and lower material wastage.

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Make glue line analysis part of your quality control routine

This quick and easy test procedure can help you keep tabs on adhesive application and common machine issues Soaking board apart and then staining it with iodine remains one of the most useful diagnostic tools at the disposal of the corrugator crew. It is easy to do, doesn’t require a lot of training to interpret the results and can help pinpoint many common machine or operational problems quickly. The test results can even be recorded photographically for later interpretation or comparison.This test works because iodine turns starch purple. Since corrugating adhesive is starch based, the purple stains reveal where adhesive has been applied.Be aware that paper with a lot of sizing will also turn purple and will make reading the glue lines more challenging.However, at least this is valuable information also, as highly sized paper may have penetration or bonding issues and this will give you a clue to take corrective actions at the machine.The quality of the glue line reflects the quality of bond.Glue lines are influenced by several process variables, from the application of the adhesive to the tips of the flutes at both glue mechanisms, to the final bonding in the double backer itself. The conditions under which this occurs are altered by changing machine speeds.This image shows the liner (left) and the medium (right) from thesame board. The glue roll speed issue can be seen on the liner while the medium shows how high viscosity is making it worse by causing dribbling down the flanks.Examining the linerGlue lines on the liner are examined for acceptable quality; they should be solid and have consistent width all the way across the web. It is useful to mark the direction the board ran on the machine. What the medium can tell youIn the corrugating process we apply adhesive to the medium and then transfer it to the liner when the two are joined. So it is valuable to look at the medium side of the soak samples, as well, since that is where the adhesive was applied (the liner shows only the adhesive that was transferred). Sometimes the glue lines will appear acceptable on the liner but the medium will reveal issues not apparent by analyzing only the liner. Troubleshooting the double face glue linesThe double face bond is formed under low-pressure conditions (unlike the single face side which was formed under much higher pressure) and is therefore more affected by process variables. As speeds are increased, these variables can affect the transfer of adhesive to the flute tips and/or transfer from flute tip to liner. Application of the glue to the flute tip can be affected by the gap setting of the rider roll or contact bar, by the speed ratio of the glue roll to web, and by malformed or low flutes. If the applicator or metering rolls are worn or dirty, the application will not be even across the web. Flute tips should have a consistent film of starch covering the radius of the flute tip.( Starch on the flanks of the flutes often indicates a glue roll speed issue.) Here are some issues that can be diagnosed with this method1. Application rate:Glue lines should be measured with a gauge to determine their width, which will indicate whether the application rate is adequate. This is a valuable tool for determining the correct glue roll-to- metering roll gap settings. Determining the correct application rate is important not only to control adhesive consumption, it also affects run speeds, bond strength, and board quality. Proper adhesive application will help avoid problems such as warp and washboarding. 2. Glue roll speed: Looking at where the adhesive was deposited on the flute tips will help determine the correct glue roll-to-paper speed ratio. Adhesive on the trailing flanks of the flute indicates a glue roll that is turning too fast in relation to the paper. Conversely, adhesive on the leading edges of the flute tip point to a glue roll with too much lag.3. Glue rolls out of parallel: One of the most useful tests for determining uneven application is to comparethe glue lines from the drive side with those on the operator’s side. A difference in the width of the glue lines almost always indicates an out-of-parallel condition.4. Worn or dirty glue rolls:This test is similar to the above test, except here we know the rolls are in parallel. Again, we compare the widths of the glue lines across the entire web. Since a worn or dirty area of the glue roll will transfer less adhesive, glue lines that are consistently narrower in any region of the board will point to the problem area of the glue roll.5. Hold-down pressure: Assuming adequate flute tip coverage, poor adhesive transfer to the double face liner will usually indicate inadequate pressure in the hot plate section. Flute tips must keep contact with the liner. If the gelatinization process of the starch starts before good liner contact is established, the transfer will be reduced. Comparing the flute tip application to the liner application will help identify the true cause. Double face liner glue lines that are varying in width or broken (often called Morse Code) across the web can point to this, as well.Glue line gages and pictures are available from your Harper/Love representative; these are great troubleshooting aids and valuable training tools.6. Slinging or dribbling: This problem usually can be revealed only by examining the medium. Adhesive can sometimes sling or dribble and end up in the valleys of the flute (typically, it is not visible on the liner). Slinging or dribbling can be caused by improper viscosity, inadequate formulation (too much borax) or some mechanical issue. Whatever the cause, it puts adhesive where it does not belong, and contributes nothing to the bond. Adhesive is wasted.Studying glue lines reveals a great deal of useful information, particularly about how the adhesive was applied to the medium and then transferred to the liner. A soak tank and a spray bottle of iodine should be part of every corrugator’s  arsenal.

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Why is Eliminating Warp Still Such a Mystery?

If you manufacture corrugated paperboard, eliminating warp is a familiar challenge. There are a number of factors that contribute to warp and an equal number of techniques for reducing it, but the ability to completely eradicate warp still remains a mystery. In this post, we provide an overview of paperboard warp and provide several options for decreasing its occurrence.What is Warp?A simple definition for warp is “. . . to become bent or twisted out of shape, typically as a result of the effects of heat or dampness”.  In the corrugated paperboard arena, this definition of warp applies to one of the most common problems that these manufacturers face – producing bent or twisted corrugated board that results in excessive scrap, waste, and reduced production rates.What Causes Warp?There are several main causes of warp:· Moisture imbalance between the top and bottom liners· Tension imbalance between the top and bottom liners· Corrugator component misalignmentMoisture ImbalanceIn the process of applying adhesive to the liners, there should be a delicate balance of heat and bonding between the layers with the adhesive consistently applied to the flute tips. Exposure to high humidity, applying excessive adhesive, inconsistent adhesive application and improper glue roll to paper speeds will all cause moisture imbalance which in turn leads to warp in the cross-machine direction. Moisture related warp defects will display as:Eliminating warp continues to be a challenge faced by many corrugated board manufacturersUp Warp – board bows up (convex)Down Warp – board bows down (concave)S Warp – board bows up on one corner and bows down on the opposite cornerDiagonal or Twist Warp – board curves along the diagonal of the sheetTechniques for reducing moisture related warp include:· Increase corrugator speed (if possible)· Adjust web and liner wrap – either reduce double back wrap or increase single face wrap· Increase adhesive settings· Equalize the temperature across the width of the sheet by inspecting for parallel misalignment between the preheater drums and wrap arms, and make proper alignment adjustments so that they are parallel one anotherTension IssuesIn any process line that is web fed, tension problems can occur and corrugating lines are no exception.  A difference in tension between the top and bottom of the sheet can result in machine direction warp – also known as end-to-end up warp or end-to-end down warp. Conditions that can attribute to this tension related warp are:· Preheaters turning improperly (or not at all)· Rolls not turning freely· Inadequately functioning roll stand brakes· Poorly maintained web guides· Excessive drag in the hot plate section· Cross tension due to uneven draw in belts· Significant parallel misalignment between units within a specific corrugator sectionThere are several methods for improving tension related warp issues such as:· Adjust wrap when liners do not create enough drag on the preheaters· Perform quarterly inspections and maintenance of splicers and other units to ensure bearings and rolls are moving freely· Perform regular inspections and maintenance on braking systems to check for brake and caliper wear· Perform weekly inspections and cleansing of web guides – vacuum guides/holes can become clogged creating less vacuum and less tension· Perform wear inspections of tension roll covers – worn or smooth covers will not add enough drag to the web· Perform regular inspections and maintenance of hot plates· Inspect for misalignment within all components and rolls and make necessary adjustments so that they are parallel to one anotherCorrugator Component MisalignmentWhen corrugator components are not aligned parallel to one another,  product variations and inconsistencies can be introduced, which can lead to warp issues. Precision alignment of corrugator components helps reduce issues associated with the following:· Paper stresses· Tension issues· Heat variations across the product width· WrinklesConclusionWarp has always been an issue in the manufacturing of corrugated box board – and as outlined in this post, there may be a number of factors that contribute to this problem. However, being aware of these factors and eliminating them one at a time, gives you a fighting chance against this common nuisance.

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