Pkg Solutions
Quality Glossary
Unitization and Stacking
The unitization and stacking of packages typically occurs
in three basic forms:
The single load pallet.
The mixed load pallet.
The mixed load, bulk container.
Many products may be, during their distribution chain,
shipped in a mixed load. Your choice of materials should take this into
consideration. It may occur that a given product will travel first on
a single product unitload, next in a mixed load and finally as an individual
parcel. Compression strength is usually the important factor for the first
two legs of the journey and high bursting strength is often more important
for the last leg.
Overhang occurs at three possible points in a unitized load:
Cases overhanging the pallet - the worst type of overhang. Always try to avoid this type
of overhang for three reasons:
Reduction of stacking strength.
Elimination of layout options in aircraft and trucks.
Increased hazard to damage due to pallet marshalling.
Cases offset - upon
each other due to either poor stacking or loose stretch film or bands
will loose much of their stacking strength as well as exposing corners
and edges to additional hazards. Corner boards and stretch wrap film greatly
reduce the tendency for this to happen.
Cases overhanging a deck
board - This also diminishes the stacking strength of the case. This
is a concern with both top and bottom deck boards. Adequate coverage of
deck boards helps this problem.
Stacking Hazards
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Load distribution can be deceptive.
The illustration
left highlights the fact that while it may seem
that the greatest
weight falls on the bottom cases of the
bottom pallet, in fact due to
the small area of the top pallet's
runners, the greatest load
distribution falls on the top layer
of the bottom pallet.
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Interlocking pallet stacking
patterns reduces the compression strength of the unit load significantly.
In situations where product support is negligible it should be avoided
if possible. However, interlocking stacking patterns make the load more
stable in transit. Ways to improve stack stability include: Stretch wrapping
either mechanically or via narrow width hand-wrappers, stretch banding,
heat shrink wrapping or bagging and break-away palletization adhesives.
A Pallet with a single
case size, protected and secured. This is the most common form of
load preparation. Often the load will include the additional protection
of top boards and corner protectors, as illustrated below. The load is
usually two or four way banded, but may also be shrink or stretch wrapped.
Unitization options can also include anti slip agents between cases.
Unitload bulk container
attached to pallet - This method of unitization is appropriate for
mixed products that must travel as a unitload in challenging shipping
environments. Voids could be filled by Kraft paper, braced by empty boxes
marked as "empty box" labels and/or corrugated modular braces.
The Case minimum material requirement should be established, and an example
is show below and should sustain stacking 3 loads high for 3 months at
80% humidity. Metal and plastic cages are often also used for this task,
they have the advantage of enabling a higher stack height in non-racking
warehouse facilities, but can prove expensive if there is a low return
rate in the distribution chain.
Bulk Container on a Pallet Example
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Case
|
Pallet
|
Unitload
|
Dimensions MM
|
800 x 600 x 600 - O.D. |
800 x 600 x 144
|
800 x 600 x 744
|
Dimensions Inch
|
31.5 x 23.62 x 23.62
|
31.5 x 23.62 x 5.67
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31.5 x 23.62 x 29.3
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Recommended Max. Wt. Kg.
|
85 Kg.
|
15 Kg.
|
100
|
Recommended Max. Wt. Lbs.
|
187 Lbs.
|
33 Lbs.
|
220
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Recommended Material
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48 ECT DW 42/26/33/26/42
|
Euro Pallet |
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Bulk Container on a Pallet
Mixed Loads
Unitizing Methods - Whenever possible mixed loads should
be built as shown below in column or layer patterns. This may not be possible
most of the time, but should be followed as closely as possible to maximize
strength and weight distribution.
Layer Stacked - note the even distribution of packs with the same height
in the different layers of the load.
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Column Stacked - Note that similar cases are stacked in there own columns. |
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Pallets
There are numerous sizes and types of pallets, below
are a list of some of the most common kinds that you may encounter.
Euro Pallet - Block Style |
GMA - notched |
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Some Typical Sizes
- Standard (Europe) 1200mm x 1000mm x 144mm
- Euro 1200mm x 800mm x 144mm
- Half Euro 800mm x 600mm x 144mm
- GMA Style 48" x 40" x 5"
For guidance performance parameters as defined by the
Chemical Industry Pallet Specification as shown below:
Chemical Industry Pallet Spec
1. Recommended nominal base dimensions +3mm(1/8") tolerance:
Metric Sizes
|
US equivalent nominal sizes
|
1200 x 1000mm
|
48" x 40" |
1200 x 800mm*
|
48" x 31.5"
|
1300 x 1100mm |
52" x 44"
|
1140 x 1140mm
|
45" x 45"
|
2000 x 1000mm*
|
80" x 40"
|
* Also commonly used sizes.
2. Maximum recommended platform height 155 mm(6").
3. Square in each direction. NOTE: The difference in
length of the two diagonals should not exceed 13 mm(1/2").
4. Four-way, partial four-way, or two way entry.
5. Should be capable of accommodating pallet jacks.
Note: For acceptable performance it is recommended that pallet jack openings
have a minimum width of 305mm(12") and a minimum height of 95.mm(3
3/4) when under maximum load. The bottom surface should have 305 mm(12")
square openings properly placed for pallet jack wheels. The center support
should be a maximum of 155 mm(6") wide
6. Top and bottom bearing surfaces should be designed
to support the intended weight and package type; be flat, non-skid and
have no indentations or protrusions that could cause product damage. NOTE:
For acceptable performance it is recommended that the top bearing surface
have a minimum coverage of 62 percent and the bottom bearing surface have
a minimum coverage of 45 percent.
7. Maximum recommended deflection of loaded pallet if
racked: 12 mm(1/2").
8. No protruding fasteners.
9. Should be made of recyclable materials.
10. Compatible in at least one direction with pallet
conveyors, dispensers and automatic storage/retrieval systems.
11. Minimum recommended load capacity: One metric ton.
12. Should be capable of multiple stackings.
13. Should be capable of multiple trips.
14. Should be capable of safely moving product through
it entire distribution system.
Additional Protection for Unitized Loads
Slipsheets
Slipsheets are placed on the top of the pallet between
the product and the pallet. They protect the product in poor warehouse
and transportation conditions as well as helping to spread the load on
pallets that are not full decked. Typically they are made of B or C flute,
the corrugation direction is optional. The performance of the slipsheet
is judged by its bursting strength, which preferably should be no less
than 200T Mullen (1379 kPa). The use of water resistant liners, while
often more expensive can be advantageous if moisture is a problem in the
shipping environment.
Cap / Top Board
A protective cap and/or top board is recommended for
added protection against stacking and moisture damage. If it is to provide
protection against these threats the material should be specified, please
see the specification section of this KnowledgeBase. Often large sections
of corrugated board are used, however for more challenging distribution
chains the use of chipwood might be considered.
Stretchwrap
As mentioned earlier, Shrinkwrap, or more usually stretchwrap,
can be used to help stabilize a load and give additional environmental
protection. An example of the specification for a typical, good stretchwrap
film is shown below:
Polyethylene
80 gauge (.8 mil) Minimum 250% pre-stretch
Properties (Units) |
ASTM Test |
Values |
Gauge (MIL) |
|
0.8 |
Haze (%) |
D1003 |
< 3.0 |
2% Secant Modulus (KPSI) |
D882 |
|
MD |
|
18.9 |
CD |
|
3.1 |
Tensil Ultimate (KPSI)
|
D882 |
|
MD |
|
4.8 |
CD |
|
575 |
Tensile Elongation (%) |
D882 |
|
MD
|
|
440 |
CD |
|
575 |
Elmendorf Tear (GM) |
D1922
|
|
MD
|
|
250
|
CD
|
|
500 |
Corner protection
Corner protection, please see the illustration for "A
Pallet with a single case size, protected and secured" above,
is recommended for all non-bulk packed unit loads. The corner protector
should be as close to the full height of the load as possible, and preferably
not less than 6" (150mm) from the top of the unit load.
Banding
Strapping or banding is another excellent way of securing
loads. We would recommend that all internationally shipped unitloads or
unitloads bound for air cargo should be banded with 4 bands (2 x 2). Please
see the illustration below.