Packaging Unlimited
 |
COMPARATIVE PERFORMANCE OF 48x40-INCH CORRUGATED PALLETS FROM PACKAGING UNLIMITED by John W. Clarke Marshall S. White Ralph L. Rupert Center for Unit Load Design Virginia Tech Blacksburg, Virginia July 5, 2001 1. Introduction Packaging
Unlimited of Louisville, Kentucky has patented a corrugated/hardboard stringer
for use in corrugated pallets. The Virginia Tech Center for Unit Load Design
was contracted to determine the comparative racking strength of 48x40-inch
pallets containing 3 or 4 stringers with 1, 2, or 3 strands of hardboard. 2. Objectives á
To compare the strength of the various pallet designs during racked
across stringer storage across the stringers according to the procedures in
ASTM D1185. 3. Materials Packaging Unlimited of Louisville, Kentucky provided
2 pallets and 5 skids (no bottom deck sheet) for each of 6 general
designs. Some stringers
were painted red to indicate a more recyclable hardboard strand material. All samples were 48x40-inch, 3 or 4 stringer,
partial 4-way, flush, non-reversible designs.
An overview of the pallet designs is given in Table 1. Photographs of the 3 and
4 stringer pallet designs are given in Figures 1-2. Table 1: Description of the 48x40-inch Corrugated Pallets and Skids from
Packaging Unlimited
Stringers were glued layers of double wall CB corrugated (flutes vertical) and hardboard strands (pressed fiberboard). A photograph of the stringer lay-ups is given in Figure 4. Stringer notches were 1.75 inches deep, 8 inches from the ends, 9 inches in length, with a 3/4-inch corner radius. Interior stringers on 4 stringer samples were spaced 11.5 inches outside to outside. All pallets were a flush design. Top decks were AAC triple wall corrugated with flutes oriented perpendicular to the stringer direction. Bottom decks, on pallets only, were CB double wall with flutes oriented perpendicular to the stringer direction. There were no openings in the bottom deck for pallet jack wheels. Adhesives joined the decks to the stringers. All assembly and gluing was performed by Packaging Unlimited.
Figure 1. Overview of the 48x40-inch 3 stringer Pallet
from Packaging Unlimited
Figure 2. Overview
of the 48x40-inch 4 stringer Pallet from Packaging Unlimited
Figure 3. Stringers contained either 1, 2, or 3 layers of hardboard laminated between layers of double wall corrugated. 4. Test
Methods 4.1 Racking Strength
and Stiffness of 48x40-inch Pallets These
tests were conducted according to ASTM D1185-98[1],
Section 8.5. Prior to testing, all pallets were conditioned at 70oF
(20oC) and 90% RH, or approximately 18-20% EMC according to the
procedures outlined in ASTM D1185 for paper-based pallets. The pallets were
supported across the 48-inch length at a free span of 44 inches. Pallets were
not racked across the 40-inch width. Flat metal bars were placed on rack
supports to prevent crushing at the stringer ends. The test load was 1 layer of
boxed sheet paper, 50 lbs per box, 8 boxes per layer (400 pounds). Above this
layer, an inflatable air bag was used to apply a uniform, full coverage load,
similar to an unrestrained boxed or bagged product. Deflection was measured at
three locations under the pallet stringers. The
comparative strength was evaluated in short-term tests to failure. Samples
failed when the pallets exhibited a structural fracture. Knowledge of the
comparative performance can be used to select the design and hardboard lay-ups
for future testing or field trials. In addition, a few samples were tested to
determine the ASTM rated load using longer term (2 hour) creep tests under. The
performance criteria for the long-term tests is 0.50 inches pallet
deflection. Many loads will experience damage when
pallets deflect more than 0.50 inches. In addition, deflections in excess of
½Ó can interfere with automated material handling equipment. 5. Test Results 5.1. Effect of Number of Stringers and
Hardboard Strands on Racking Strength The summary results of the racking tests are summarized in Table 2. There were not enough replicates for statistical analysis, but this study did allow the analysis of trends. The relative performance of each design
was compared with the 3 stringer, 1 strand design. As expected, the strongest skid was the 4-stringer, 3-strand design, at
59% stronger than the 3 stringer 1 strand design. The weakest design, however,
was the 3-stringer skid with 2
hardboard strands. Although the number of replicates was small, this trend was
true for all test replicates, indicating a potential weakness in the stringers
that contained 2 strands. The corrugated paper on the 2 strand stringers was
lighter in color than the paper on the 1 and 3 strand skids, and may indicate a
different paper or storage environment. The 4 stringer 2 strand skids were also
weaker than the 4 stringer 1 strand skids, but only by 2%. On average, the 3-stringer, 3-strand skids were slightly stronger than the 4-stringer skids with 1 or 2 strands. Therefore, for some applications, it may be more cost effective to use 3 stringers and more hardboard strands rather than 4 stringers and fewer strands of hardboard. Following are some recommendations for greater racking strength: á
A 3-stringer design with a wider center stringer may also be stronger
and more functional with handling equipment than a 4-stringer design. á
Taller stringers (3.75 or 4-inches) would increase racking strength. á
Moving the notch closer to the stringer ends (6 inches from the
stringer ends is typical notch location) would increase racking strength. For rigid loads, there may be cost savings in using a lighter
construction top deck and allowing the stringers to bear the weight. Pallets
used in less humid environments may result in greater acceptable loads. Loads
that are more flexible or concentrated than boxed paper, however, may result in
lower rated loads. Users should verify the performance of this design in field
trials with the actual loads before implementation.
5.2. Effect
of Hardboard Grade on Racking Strength The
racking strength of two grades of hardboard strands (red and regular) was
evaluated. The red hardboard was designed to be more recyclable with corrugated
paper, and would be preferred for many applications. No 3-strand samples with
red hardboard were submitted for
this test. The
red hardboard stringer skids exhibited a 5 to 43% greater maximum load than
regular hardboard skids in 3 of the 4 versions of skids tested. The only red
hardboard set that was weaker was the 4 stringer, 1 strand skid, and only one
red replicate was available for testing. These results indicate that the more recyclable
red hardboard is an acceptable substitute for the regular hardboard, and may
even be stronger than the regular hardboard material.
5.3.
Effect of Long term Creep on Racking Strength Several designs were tested according to the ASTM D1185
to see if the design safely supported a test load of 1000 pounds. The results
are given in Table 4. In general, the 4 stringer, 3-strand skid and the 3
stringer 1 strand pallet were the only samples that exhibited less than
½Ó deflection after 2 hours under a rated load of 800 pounds (Test load
was 1000 pounds, or rated load * 1.25 safety factor). The other designs
exhibited slightly more than ½-inch deflection. These preliminary creep
tests suggest that, for boxed loads, the 4 stringer-3 strand skids may be
acceptable for a rated load of 800 pounds, and that the other skids are likely
in the 600-700 pound rated load range. The weakest stringer configuration (3 stringer,
1 strand) in a pallet was as stiff in the long-term creep testing as the
strongest stringer configuration (4 stringer 3 strand) in a skid. The bottom
deck sheets in pallets may be a more economical method of adding stiffness than
bulkier stringer lay-ups.
* The test load was 1000 pounds. The
rated load is the test load divided by a 1.25 safety factor. 5.4.
Effect of Load Type on Racking Strength Most skids were
tested supporting one layer of boxed sheet paper underneath an inflatable air
bag. This study also evaluated the racking performance under the inflatable air
bag alone. The air bag is a worst-case load and represents loads such as bulk
bags of liquid or loose granular products, some bagged products, or small
boxes. More rigid loads (such as larger boxes) can interlock and increase
pallet strength. Less rigid loads (such as bulk bags, liquids, smaller boxes)
will offer less support to the pallet. The results of these tests are given in Table 5. The 1 and 2 strand versions were not tested in this comparison. In general, skids supporting the boxed paper load held twice as much weight as the equivalent design supporting the inflatable air bag. This confirms that the type of load will have a significant effect on the racking performance of these pallets and skids. It is recommended that, when available, actual loads be used to evaluate these pallets and skids in lab tests or carefully controlled field trials before implementation. Note
that the type of load governs racking strength more than the design. That is,
the 4 stringer 3-strand design is 20 to 30% stronger than the 3-stringer
3-strand design when supporting the same load. The strength difference between
loads on the same design is 85-100%.
6.
Conclusions Packaging Unlimited of Louisville,
Kentucky has patented a corrugated/hardboard stringer for use in corrugated
pallets. The Virginia Tech Center for Unit Load Design was contracted to
determine the comparative racking strength of 48x40-inch pallets containing 3
or 4 stringers with 1, 2, or 3 strands of hardboard. The primary load tested was boxed sheet paper. All pallets
were tested at 18% EMC. Following are the observations: á
Skid
designs with stringers that contained 2 hardboard strands were not stronger
than samples with 1 hardboard strand, but designs with 3 hardboard strands were
30 to 33% stronger than the equivalent designs with 1 strand. á
The 3
stringer 3 strand skids were stronger than the 4-stringer skids with 1 or 2
strands. á
Skids
manufactured of stringers with a more recyclable red hardboard grade were
stronger than the regular hardboard grade for 3 of the 4 designs tested. á
When
supporting a boxed paper load, the 3-stringer 1-strand pallet (with bottom deck
sheet) was as strong and stiff as the 4 stringer 3 strand skid (no bottom
sheet). Bottom deck sheets may be an economical method to increase racking
performance. á
The type of
load may govern rackability more than the design. There was a 20-30% difference
in strength between the 4 stringer 3 strand skids and the 3 stringer 3 strand
skids when supporting the same load. There was an 85 to 100% difference between
these same designs when supporting different loads. á The performance and functionality of these designs will vary depending on environmental conditions, load rigidity, and equipment interfaces. Users should field test samples of this pallet design before full-scale implementation. [1] ASTM D1185. 1998. Standard Test Methods for Pallets and Related Structures Employed in Materials Handling and Shipping. American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103. |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
