JEC COMPOSITES MAGAZINE - Issue #80 - April 2013 - 48

sandwich
A lightweight and durable cargo
hold floor
One difficulty in designing the RAP e2
container was the floor. A low-density core
material was preferred to achieve the best
insulation performance. The density of a
closed-cell foam material is somewhat linear
to its insulation performance, but the compressive strength of a low-density material
is, to say the least, limited. When in use, the
container floor is subjected to pallets loaded
with up to 5 tonnes of goods. Misaligned or
unevenly loaded cargo may result in large
point loads on the floor. Throughout the
whole container, a 60-80 mm thick, very low
density foam core was chosen to achieve the
best insulation performance. To increase the
compressive strength and durability of the
floor while still maintaining a low weight, a
multi-layered sandwich was designed, with
a higher density core material added on top
of the first one. This created a low weight,
highly insulating and compressively strong
double sandwich material.
To protect the glass fibre laminate top layer
from scratches and tears from truck forks
and pallets, many different solutions went
through extensive abrasion testing. Finally, a
layer of HPL (High Pressure Laminate) proved to be the most durable floor material. In
production, this layer is bonded to the multilayered floor during the infusion process.

Designed to confine damage size
The number one cause of damage on the
containers is forklifts. These are heavy machinery with sharp blades sticking out, and
they do occasionally miss assigned forklift
pockets. Protecting a lightweight air cargo
container entirely from holes in the event of
a collision is virtually impossible.

Fig. 2: Insulated light-weight and highly durable
container floor

To ensure the
structural strength
of the container,
multi-axial woven
glass fibre fabrics
are used in the outer
shell of the sandwich
structure. This helps
to evenly distribute
the loads applied
to the structure. In
general, you strive to
Fig. 3: Helping to transport temperature-sensitive products to more than
have a balanced and
symmetric sandwich 100 countries around the world
structure with the
excellent insulation capabilities of the lowsame material on both sides of the core but
density Divinycell foam core. The improved
in this case, it would be a poor solution.
insulation performance has resulted in a
In the event of a collision, of the more
significantly reduced size and increased
critical kind with a forklift, it is likely that
lifetime of the batteries powering the tempethe forks will slit right through the entire
rature control units.
wall. For the outer sandwich sheet, which is
a multi-axial fabric, this is no big problem.
Increased robustness halves
The forks will cut-off the outer fibres, since
damage frequency
the core acts as an anvil or counterhold
Air cargo containers live a rough life, and
tool, causing a confined damage not much
wear and tear is high when they are handled
larger than the cross-section of the fork. This
on airport tarmacs around the world.
damage is fairly easy to repair.
In addition to withstanding all possible
On the backside of the sandwich wall, the
weather conditions, collisions with forklifts
fork will rip the continuous fibres along the
and trucks cause a large amount of damage.
entire inside of the wall when exiting it, just
With the new composite container, statistics
like ripping the grains on the backside of
based on field use show that the damage
a plank when striking a nail through it. To
frequency has been cut down 2 to 3 times,
avoid this, an innovative new way of combiconsequently also repair costs. Although
ning fibres was developed to obtain the same
the sheet thickness of the sandwich is only
confined damage size on both sides of the
1 mm, it has proven to be extremely strong
container shell. The construction was also
and durable to impacts.
cleverly designed to prevent any curvature
or embedded inner tensions due to the
Easy repair allowing worldwide
asymmetric sandwich fibre lay-up.
ground service
In the event of accidental damage, the EnviDoubled insulation performance for
rotainer team has developed a repair scheme
that only requires minimal composite/sandlower energy requirements
As Envirotainer’s composite container fleet
wich knowledge. Containers can be repaired
was growing, the value of the new product
at virtually any airport in the world with
has become validated over and over again.
personnel educated in the repair scheme. No
Compared to the traditional aluminium/
attendance from Envirotainer’s own staff is
polyurethane foam construction, the
needed to restore full air-worthiness of the
insulation performance has doubled. All
containers after this training course has been
thermal bridges through the outer shell of
performed. This enables widespread covethe container have been eliminated, partly
rage of repair stations around the world.
because of replacing the highly thermally
The repair principle is fairly simple: you
conductive aluminium with glass-fibrecut out the damaged laminate and core
reinforced polyester, and also because of the
material, grind and clean the surface around

48 jec composites magazine / No80 April 2013
JEC COMPOSITES MAGAZINE - Issue #80 - April 2013

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