Polyester Cord Strappings And Lashings

June 30, 2009

1)Background
The first US commercial polyester fiber production was by Dupont Company in 1953. At that time, the polyester fibers were primarily used for fabric production.There was a growing movement to develop plastic strappings to replace the steel strappings that were widely used then.
Some time later in the 1950’s , the glue bonded polyester cord strapping was developed .At the early stage , these cord strappings were used for simple bundling of goods .The woven strapping was also developed at about the same time frame.In the 1960’s , the composite strapping was developed .By the 1990’s ,  cord strappings have made major technical advances to largely replace steel strappings in most applications.

2) Types of Cord Strappings
There are 3 main types of polyester cord strappings depending on their manufacturing processes:

2.1 Glue-Bonded Cord Strappings

The linear polyester yarns are held in band by hot-melt glue .This strapping has a soft and tacky feel . Comes only in natural color.

Glue-Bonded cord strapping

 

2.2 Woven Cord Strappings
These are woven from high tenacity polyester yarns and can come in any colour. The woven strapping is soft ,lightweight and flexible .
Woven polyester cord strapping

Woven polyester cord strapping

  

  
2.3 Composite Cord Strappings
The linear polyester yarns are co-extruded with polyethylene to produce a stiff strapping band .Only comes in natural colour.
Composite cord strapping

Composite cord strapping

    
The 3 types of cord strappings are interchangeable  in usage. It is just a marketing strategy by manufacturers to diffentiate themselves in order to compete better. Individual customers may have preferences for a particular type based on their individual  opinion. 
3) Properties
All polyester cord strappings and lashings share the same properties :
3.1 Safe
       It has no sharp edges and does not spring back when cut under tension .
3.2 Strength
       It is really strong as steel !
3.3 No damage to products
       As it is soft and flexible , it does not damage the product being bundled.
3.4 Rot-proof
        It does not rot or stain .
3.5 Lightweight
        It is at least 5 times lighter than the corresponding steel strapping.
3.6 Absorbs Shock Better
       With about 10% elastic elongation, it can absorb shocks very well .
3.7 High system Strength
       When used with a steel wire buckle it provide high system strength and can be retensioned at any time.
3.8 Creep Resistance
       Once tensioned it retains its tension.
4) Performance
     4.1 Linear Break Strength (Tensile Strength)
     The most common measure used is the linear break strength where a length of the strapping is pulled till failure .
     This is following the ASTM standard. 
     4.2 System Strength
     The Europeans decided that the system strength is a better measure as it tests the whole loop including the buckle
     which is how the strapping is used.The system strength shall be double the linear break strength if the buckle is
     100% efficient.
    4.3 Energy To Break
    The latest trend is that the energy to break is the most complete measure of strength . This measures the energy
     required to break the strapping . It is the area under the curve in the force-elongation graph.
   4.4 Joint Strength
   In all applications , strappings have to be joined together into a loop. The joint is also the weakest link in the loop.
   The common jointing methods are heat seal, crimps , seals and buckles. For the polyester cord strappings , buckles
   are the only means of forming strong joints.There are two main types of buckles being used for cord strappings and
   lashings :
   i)Wire buckles are used for strappings up to 32mm width .
  ii) Welded rectangular buckles are used for lashings of width 32mm and above. Buckles with hooks or cam buckles
  may also be usedwhen lashings are used to tie down cargo loads for transportation.
 
Wire Buckles And Welded Buckles

Wire Buckles And Welded Buckles

   
    
     
  
 
   
   The joint strength is normally called the buckle efficiency which is the measured system strength divided by twice
    the linear break strength and given in percentage.
    Buckle Efficiency = (Measured system strength / 2 x Linear break strength)      x 100%
    The buckle efficiency normally range from 75%- 95% , depending on how well the buckle is designed and produced.
   4.5 Creep
   Strappings are expected to maintain its applied tension around the goods over time. Unfortunately, it is a natural
   phenemenon for tensioned strapping systems to lose strength over time and this is called creep . It has been shown
   that the corded polyester strapping with wire buckle system has less than 10% creep and is far superior to other
   strapping and jointing systems.
  4.6 Shock Absorbency
  The polyester strappings have an elastic elongation of about 8-12 %. During the elastic elongation phase ,
  the tensioned strapping will return to its original state once the  tension is released. In application , this acts as a
  shock absorber to absorb sudden load and return to the normal state once the shock  is over.
  5) Accessories
  5.1 Tensioners
  A tensioner (either manual, pneumatic or automatic) is used to tension the strapping looped around the
  goods .Tensioners come in various sizes to suit the strapping width.
 
Manual tensioner

Manual tensioner

   
 
   5.2 Wire Buckles
   The buckles also come in various sizes to suit the width and breaking strength of the strapping .These buckles  have
   either galvanized or phosphate treament finishing. The phosphated buckles (  dull blackish color)are slightly more
   expensive but have better slip resistance as the phosphate treatment gives a rough finish to the surface of the buckle.                                            

Dunnage Bag Terms

April 24, 2009

Common Terms Used In Dunnage Bags
AAR                                                           American Association of Railroads
Air Bladder                                            The inner plastic bag holding the air . Also called liner.

Burst pressure                                      The pressure beyond which the bag may burst

Crosswise bracing                               The dunnage bag is installed with the length parallel to the floor

Deflate                                                      Air is released from the inflated bag

Effective contact area                       The area of the bag directly in contact with the cargo loads or walls. The inflated bag

                                                                    is rounded at the top and bottom so only the central portion is in contact with the

                                                                    cargo load 

Force                                                        The force generated by the inflated dunnage bag. It is proportional to the effective
                                                                     contact area and the inflation pressure.

Gap                                                            See void

GSM                                                          Refers to the weight in gram per square metre of the kraft paper or polywoven fabric

Inflate                                                      Fill the bag with compressed air

Install                                                      Place and inflate a bag in a void

Kraft paper                                            Special paper used to make the outer bag

Laminated paper                                Refers to kraft paper laminated with layer of polywoven fabric

Layer                                                       The number of separate layers of material making up the bag

Leak                                                         The inflated bag losing air over time

Lengthwise bracing                           The dunnage bag is placed with the length perpendicular to the floor

Level 1                                                    AAR classification of dunnage bag with working pressure of 2.5psi and burst pressure

                                                                   of 8psi 

Level 2                                                   AAR classification for dunnage bag with working pressure of 5psi and bust pressure of 

                                                                  17psi

Level 3                                                   AAR classification of dunnage bag with working pressure of 8psi and burst pressure of

                                                                 25psi

Level 4                                                 AAR classification of dunnage bag with working pressure of 10psi and burst pressure of

                                                               30psi

Liner                                                     See air bladder

One-way valve                                 The valve only allows compressed air to enter the bag but not out of it.As such, the bag

                                                                can only be inflated once.

Paper dunnage bag                        A dunnage bag with a paper outer bag

Ply                                                        Refers to the number of separate layers of paper in the outer bag

Polywoven dunnage bag             A dunnage bag made with a polywoven outer bag

Recyclable                                        The material making up the bag can be recyled into raw material for reuse.

Reusable                                           The bag can be used 2 or more times

RoHS                                                  This refers to the European Union directive on Restriction on use of Heavy Metal

                                                              substances

Size of bag                                         Refers to the length and width of the uninflated bag

Two-way valve                               The valve allows compressed air to be pumped into the bag and also allows the air

                                                               inside the bag to be released . This means the bag is reusable.

Void                                                     The empty space between cargo loads or between the cargo load and the wall where the

                                                               dunnage bag is installed.

Working pressure                          The  pressure that the bag can be safely inflated to.

WPP fabric                                         Woven propylene fabric is woven from polypropylene yarn

RoHS in Dunnage Bags

March 30, 2009

heavy-metals-in-plastic-dunnage-bags-60309

How Many Dunnage Bags To Use?

March 18, 2009

no-of-dunnage-bags-needed-for-standard-container1

Dunnage Bag Forces

March 13, 2009

Dunnage Bag Forces

The forces that can be generated by a dunnage bag is determined by the bag size, working pressure and the void size . Charts of the forces for some common sizes and working pressures are listed below.

dunnage-bag-forces2

Dunnage Air Bags

March 13, 2009

Read the rest of this entry »

Protect Your Cargo From Damage By Using Dunnage Bags

February 25, 2009

Introduction

Damage to cargo  is due mainly to cargo loads knocking into each other or against the walls of the container  .Whenever there is a void amongst the cargo loads and walls , the cargo loads will shift during transportation. At the time of loading, it may seem that the voids are very small . However, during transportation the compacting motion will tend to accentuate the voids . Consequently, the cargo loads will shift and collide with each other or against the walls. The impact is determined by the weight of the cargo load and the acceleration or deceleration of the container or truck. So the impact can be quite tremendous !

The Dunnage Bag Solution

The simple solution to the problem is to fill up the voids amongst the cargo loads . Seems like the dunnage bag is perfect for this job.

What Is A  Dunnage Bag?

Essentially it consist of a plastic air bladder fitted with a one-way or two-way valve and placed inside a paper bag ( single or multiple ply ) or a polywoven bag .The outer bag serves to protect the inner bladder and to give it strength when inflated.

Other names used for dunnage bags are air bags, disposable inflatable dunnage and pneumatic dunnage.

Dunnage bags are normally specified by size ( the uninflated width and length) and working ( or operating) pressure  as these attributes determine the force the bag can generate when inflated.

Working Pressure

The common working pressures are 10kPa(1.4psi) , 20kPa(2.9psi),40kPa(5.8psi) and 60kPa(8.7psi). For paper dunnage bags, this roughly correspond to 1-ply for the 10kPa, 2-ply for the 20kPa , 4-ply for 40kPa and 6-ply for the 60kPa bag.In the case  of polywoven dunnage bags, there is only one layer for the outer bag and the density (gsm)  of the woven fabric is the main determinant of the working pressure. For a given working pressure,the  force generated is directly proportional to the size of the bag . The force needed to restraint the cargo loads inside the container is a largely dependent on the gross weight of the cargo.

Sizes

It is important to select the correct size to fit the gross  height  of the stacked cargo loads inside the container. The force transmitted is directly proportional to the contact surface area with the cargo load .It is good to have as as much of the cargo load surface as possible covered by the dunnage bag.

Void 

The inflated bag is rounded at the top  and bottom. This gets progressively worse  (for a bag of given size ) as the bag becomes thicker ( or as the void becomes bigger) . The maximum thickness ( the  maximum void size it can fill)  is largely determined by the width of the bag. Only the flat part of the bag surface remains in contact with the cargo load .Therefore, the void determines the effective contact area for a given bag width.It is recommended that at least 50 percent of the surface area of the bag should be in contact with the cargo load .

            Width of bag                                                                                 Recommended Max.  Void 

              60cm(24″)                                                                                             25cm(10″)

              90cm(36″)                                                                                             35cm(14′)

              120cm(48″)                                                                                          50cm(20″) 

How To Install The Dunnage Bag?

It is recommended that the dunnage bag be placed and inflated between the cargo loads right in the middle of the container or truck. In this configuration the inflated bag will push and hold the cargo loads on both sides of it tightly against the walls.You are effectively using both sides of the bag.

The dunnage bag can also be placed on top of the cargo loads if the void there is within the maximum size recommended.In this set-up , only one side of the dunnage bag is pushing and holding down the cargo loads.

Another workable configuration is to install the bag between the cargo load and the container wall.

The bag can be placed vertically or horizontally , depending on the height of the stacked cargo loads .Heavy cargo normally have lower stacked heights than lighter cargo . The bag should not protrude out of the cargo load by more than 5-10cm  especially at the top . This will prevent any “ballooning” effect . At the same time ,the bag should also be 5-10cm away from the floor.

The sharp edges of the pallets or cargo loads may puncture the inflated bag . It is therefore recommended to sandwich the bag between two pieces of cardboard , chip board or equivalent before installing it.

When the bag is being inflated you can hear the cargo loads shifting. Stop the inflation when the container starts to creak and the cargo loads are pushed and held tightly against each other and against the walls.

How Many Bags Are Needed ?

The rule-of-thumb is that the total restraining forces should be at least one-half the weight of the cargo. For example , a 20-footer container loaded with cargo of weight 20 tonnes (44000 lbs) should have total restraing forces of at least 10 tonnes(22000 lbs) . A typical force chart  for a dunnage bag of working pressure 20kPa  is as follows:

    Bag Size                                              25cm(10″) Void                                       35cm(14″) Void  

    90x 120cm(36″x48″)                 1.3 tonnes (2860lbf)                                1 tonne(2200 lbf)

    90x180cm (36″x72″)                2.1 tonnes(4620 lbf)                                 1.7 tonnes( 3740 lbf)

    90x 220cm(36″x88″)                2.7 tonne(5940 lbf)                                  2.2 tonnes(4840 lbf) 

Note: This is the force generated from any one side of the inflated bag. 

Common Problems With Dunnage Bags 

The No.1 and most serious problem is leakage where the inflated bag becomes deflated over time.  It is important to make sure the valve cap is tightly screwed down and  plugged  after inflation.Leakage is mainly due to defects in the plastic air bladder or heat- welded joints .This can be verified by inflating the bag and leaving it aside for 6-12 hours.

Burst bag can result from using too small a bag or a bag of too low working pressure for the weight of the cargo. It can also be due to “ballooning” where too much of the bag is free to expand .

If the void size is too big for the bag of a given size, then,  it is prudent to use a bag with a larger width or use void fillers to reduce the void size first .

At no time should two dunnage bags be placed together in order to fill a big void .During transportation, the compacting forces will result in one of the bags slipping out of position.

It is not a safe practice to place the dunnage bag between the cargo loads and the container doors. First, you will have a problem to close the second container door. Second, the container doors may swing open suddenly on being unlatched at the receiving dock thus knocking any person standing in the way.

Some Photographs

Inflated Dunnage Bags

Inflated Dunnage Bags

Using the dunnage bag to secure carton boxes

Using the dunnage bag to secure carton boxes

Using the dunnage bag to secure palletised cargo loads

Using the dunnage bag to secure palletised cargo loads

 

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