Archive for February, 2009

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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