From the website of:
Corrugated Pallets Inc. - www.corrugatedpallets.com
Phone: 519-576-8181
E-Mail: info@corrugatedpallets.com
Corrugated Pallets Inc. - www.corrugatedpallets.com
Phone: 519-576-8181
E-Mail: info@corrugatedpallets.com
John W. Clarke, Marshall S. White, Ralph L. Rupert
Center for Unit Load Design
Virginia Tech
Blaksburg, Virginia
July 5, 2001
Also Available:
Additional full-pallet test results from November 30, 2001
Wet Process Hardboard Product Description
Wet Process Hardboard Statement of Recyclability
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 reclyclable 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
| Description | Stringers | Top Deck | Bottom Deck (pallets only) | Replicates of each |
| 3 stringers 1 hardboard strand | 3.00"W x 3.50"H | 40x48 inches AAC triple wall | 40x48 inches CB double wall | 4 skids 2 pallets 1 red skid |
| 3 stringers 2 hardboard strands | 3.00"W x 3.50"H | 40x48 inches AAC triple wall | 40x48 inches CB double wall | 4 skids 2 pallets 2 red skids 1 red pallet |
| 3 stringers 3 hardboard strands | 3.00"W x 3.50"H | 40x48 inches AAC triple wall | 40x48 inches CB double wall | 5 skids 2 pallets |
| 4 stringers 1 hardboard strands | 3.00"W x 3.50"H | 40x48 inches AAC triple wall | 40x48 inches CB double wall | 5 skids 1 pallet 1 red pallet |
| 4 stringers 2 hardboard strands | 3.00"W x 3.50"H | 40x48 inches AAC triple wall | 40x48 inches CB double wall | 4 skids 1 pallet 1 red pallet 1 red skid |
| 4 stringers 3 hardboard strands | 3.00"W x 3.50"H | 40x48 inches AAC triple wall | 40x48 inches CB double wall | 5 skids 2 pallets |
Stringers were glued layers of double wall CB corrugated (flutes vertical) and hardboard strands (pressed fiberboard). Stringer notches were 1.75 inches deep, 8 inches from the ends, 9 inches in length, with a ¾ 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.
These tests were conducted according to ASTM D1185-98*, Section 8.5. This test simulates bending stresses on the pallet deck during unit-load block stack storage. Prior to testing, all pallets were conditioned at 70°F (20°C) 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 pounds 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 baged 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 ½" can interfere with automated material handling equipment.
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:
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.
Table 2: Results of the ASTM D1185 static stacking strength and stiffness tests of the Packaging Unlimited 48x40 inch Notched Stringer Skids (18% EMC)
| Design | Number of Replicates | Maximum Load at Failure (lbs) | Comparison with 3-stringer, 1-strand design |
| 3-stringer, 1-strand | 3 | 1743 | - |
| 3-stringer, 2-strand | 3 | 1438 | -17% |
| 3-stringer, 3-strand | 3 | 2313 | +33% |
| 4-stringer, 1-strand | 4 | 2124 | +22% |
| 4-stringer, 2-strand | 3 | 2075 | +19% |
| 4-stringer, 3-strand | 3 | 2766 | +59% |
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.
Table 3: Comparative static stacking strength and stiffness performance of Red Hardboard Stringers versus Regular hardboard stringers for the Packaging Unlimited 48x40 inch Notched Stringer Skids (18% EMC)
| Design | Number of Replicates | Maximum Load at Failure (lbs) |
| 3-stringer 1-strand regular hardboard red hardboard Difference | 3 1 | 1743 1840 +6% |
| 3-stringer 2-strand regular hardboard red hardboard Difference | 3 2 | 1438 2060 +43% |
| 4-stringer 1-strand regular hardboard red hardboard Difference | 4 1 | 2124 1674 -21% |
| 4-stringer 2-strand regular hardboard red hardboard Difference | 3 2 | 2075 2175 +5% |
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 ½" 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" 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.
Table 4: ASTM D1185 Long Term Creep Evaluation of Packaging Unlimited 48x40 inch Notched Stringer Skids and Pallets (18% EMC). Load was boxed sheet paper.
| Design | Replicates | Rated Load | Average deflection after 2 hours under test load (inches) | Average recovery after 1 hour under no load (inches) |
| 3-stringer, 1-strand skid | 1 | 800 | 0.721 | 0.480 |
| 3-stringer, 3-strand skid | 1 | 800 | 0.565 | 0.326 |
| 4-stringer, 1-strand skid | 1 | 800 | 0.609 | 0.417 |
| 4-stringer, 3-strand skid | 1 | 800 | 0.442 | 0.231 |
| 3-stringer, 1-strand pallet | 1 | 800 | 0.425 | 0.240 |
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 along. 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 or 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 the loads on the same design is 85-100%.
Table 5: Effect of Load Applicator on the racking strength of Packaging Unlimited 48x40-inch Notched Stringer Skids (18% EMC)
| Design | Replicates | Boxed Paper: Maximum Load at Failure (pounds) | Replicates | Air Bag: Maximum Load at Failure (pounds) |
| 3-stringer, 3-strand | 3 | 2313 | 2 | 1153 |
| 4-stringer, 3-strand | 3 | 2766 | 1 | 1498 |
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:
» Full Pallet Test Results