© 2011 SATRA Technology Centre. Reproduction of SATRA Spotlight in part or in its entirety is not permitted in any shape or form, whether in print or digital media, without prior written permission from SATRA.
Cardboard box and
carton testing
by Steve Rose
at failure is recorded as the burst strength.
It is unclear why burst dominates box
testing procedures. The most likely explanation
is that this test is also often used for leather,
textiles and other sheet materials.
However, what does this test tell us about
the strength or durability of the card packaging
material? The test determines the pressure
required to rupture or burst the side of a
cardboard box. This then gives an indication of
the box’s ability to withstand external or
internal forces. It is a ‘containment’ value – a
measure of how well the box will keep its
contents intact and undamaged during rough
handling. The looser the contents within the
box, the more potential there will be for them
to cause damage to the box during violent
movement. The better the packing, the less
potential for damage.
© Kadmy I Dreamstime.com
How the quality of cardboard used to package products
should be correctly assessed.
With the bulk of footwear, leathergoods and
clothing and other consumer goods being
manufactured in China and other Asian
countries, much of the early life of products
intended for Western markets will be spent in
shipping and storage. Quite correctly,
considerable thought is given to the quality
control of the products themselves. However, is
the same level of consideration – if any –
given to the packaging to which the products
are entrusted for the several months it may
take for them to reach their destined market?
The cardboard used to make the
individual boxes and the shipping cartons
selected to transport goods from
manufacturing base to their intended market
must be of sufficient strength and durability to
protect their contents from damage. How,
though, is this best assessed?
Bu r s t t es t w i d el y u s ed
Many Western brands and retailers sourcing
from Asian manufacturers have a requirement
for burst testing in their specifications. This test,
commonly known as the ‘Mullen burst’, is a
simple diaphragm distension method where
the sample of cardboard is domed until it
bursts. The hydraulic (or pneumatic) pressure
H a n d l i n g v er su s st a c k i n g
Clearly, ‘rough handling’ has many
interpretations. If too rough, it could be
considered as abuse. While boxes need to
withstand robust handling, a test which
simulates only one type of performance
(abnormal performance) should not be relied
upon to be the sole performance indicator.
The principle performance requirement of
boxes is to be able to protect the contents
when stored and transported under normal,
foreseeable conditions. The most stressful
‘normal’ condition will be the applied loads
during stacking. This will take the form of loads
applied to the vertical sheets of cardboard in
the sides of a box. The rigidity of the card is,
therefore, a key factor, and the burst test
cannot determine this.
The higher the boxes are stacked, the
greater the loads that are applied to the lower
boxes. Providing the boxes are correctly
(collinearly) stacked, the loads should be
applied normal to the plane of the box sides.
Perhaps the most obvious method of
stacking behaviour assessment would be to
directly simulate the loads applied to a
representative sample of the boxes in question.
This means applying a compressive force on a
test machine to a complete box. Where the
‘boxes’ in question are shipping cartons, this
means that a very large, heavy-duty
compression machine is required (figure 1).
© 2011 SATRA Technology Centre. Reproduction of SATRA Spotlight in part or in its entirety is not permitted in any shape or form, whether in print or digital media, without prior written permission from SATRA.
This test, commonly known as the ‘Box
Crush Test’ (BCT), and recently published as
SATRA TM439 (‘Shipping Carton
Compression Strength Test’), will determine
the force (loading) under which the carton will
buckle and collapse. The maximum stacking
height (number of cartons) can then be
calculated from this value. However, this
calculation cannot be performed if the carton
is tested empty – it must be tested fully loaded
with identical contents. This is because the
weight of the carton, the type of contents (for
example, boxes or loose bags) and also how
tightly they are packed will all affect the crush
load. It is clear that if a carton is packed
tightly with well-fitting contents, these items will
‘reinforce’ the carton walls and significantly
increase the crushing load.
If, however, products are loosely packed
in the carton (for instance, in a plastic bag),
little support will be given to the external
carton walls. Loosely packed cartons will
also allow movement and settling of the
contents. This could permit (or even cause)
distortion and bulging of the cartons,
increasing the risk of collapse under load.
Indeed, some cartons will have internal
edge and corner supports to reinforce the
structure where the contents are loose.
While individual boxes can be tested
against the BCT on a standard laboratory
tension/compression machine, large shipping
cartons will require specialised test equipment.
C a r d b o a r d E d g e C r us h Te s t ( E C T )
A more convenient test can be substituted
for the BCT, allowing assessment of cartons
without the trouble and expense of finding a
BCT testing service. As stated previously, a
key factor in determining stacking stability is
the load which the card can support when
vertically aligned. This can be achieved by
simply applying a load to a small sample of
card (figure 2).
Such a test, commonly known as the
‘Edge Crush Test’ (ECT), recently published
as SATRA TM440 as the ‘Edgewise
Compression Strength of Corrugated
Fibreboard Test’, has been used in the
paper and packaging industries for many
years and is widely accepted as a definitive
standard for box strength. A small square of
the cardboard box material (usually
corrugated in several layers for a higher
strength-to-weight ratio) is cut and a load
applied to its edge when it is stood on end.
It is critical here that the sample is cut
accurately with clean, undistorted edges. If
the corrugations are damaged in any way,
the sample will collapse prematurely and the
load will not be representative of the
material in service.
From the load required to collapse the
test sample, there are equations and
formulae which can be used to calculate
the equivalent crush value for a complete
carton of known dimensions. Hence, a
BCT value can be estimated from the ECT.
The maximum stack height can then
Figure 1: The SATRA TM439 ‘Box Crush Test’ (BCT)
be estimated from this value and the
weight of each box. This result can be
further refined by adding in other factors,
including the packing effectiveness (for
example, tightly-packed boxes as
compared to loose contents), internal
supports, length of storage, humidity
during storage and stacking method (such
as interlocking or columnar).
E C T v er s u s b u rs t t es t
The ECT is gradually gaining acceptance
within the footwear industry, which perhaps
shows that the previously accepted burst test
is not providing sufficient information to fully
predict box performance. The ECT gives a
more direct method of understanding the
stacking stability of cartons and, as more
users understand the benefits, it will grow in
importance even more.
As the burst test does not determine
rigidity for cardboard packaging materials,
SATRA would be pleased to see the
consumer product industries adopt the ECT
method. This would bring them in line with
the packaging industry and other industries
which use packaging.
S AT R A c a n h e l p
SATRA can carry out the BCT (SATRA
TM439) on individual boxes should
members so require. However, SATRA does
not propose to recommend levels for box
crush strength – each company will have
their own box designs which will depend
upon aesthetics, brand image and,
ultimately, cost, and it is these factors which
will determine ultimate strength.
However, such testing can be of value in
two ways – firstly, to ‘benchmark’ the
company’s particular style of box, and to
check consistency between batches; and
secondly, to help predict the crush strength of
a shipping carton in which they are packed.
The more useful ECT can also be
carried out at SATRA for members. Again,
SATRA does not propose to recommend
levels of performance. Once the crush value
is known, the size of the carton, the type of
contents, additional reinforcements, and so
on can be chosen to enable its use in a
given situation, unless, of course, there
already exists an ECT specification for a
sample, when the test result can easily be
compared to the required level.
Figure 2: The ‘Edge Crush Test’ (ECT) was
recently published as SATRA TM440
How can SATRA help?
Please email footwear@satra.co.uk for
assistance with the testing of cardboard boxes
and cartons.
SATRA Spotlight