Faster cycling, reduced material consumption, and stronger thin walls are among the reasons why three components of a new system for securing shipping pallets are molded using MuCell microfoaming technology.

One company is mounting an injection molded challenge to the by-now nearly universal practice of securing loaded shipping pallets by shrink-wrapping or strapping them. When the Loadhog Lid (made by the eponymous firm Loadhog Ltd. in Sheffield, U.K.) is set atop a loaded pallet, its built-in retractable straps allow a quick and secure connection to the pallet under the load. Whether the load is cartons, bottles, drums, or other containers, if it’s stacked squarely, the Loadhog Lid will hold it all securely in place.

Like shrink-wrap and most strapping, the Loadhog Lid is also plastic—polypropylene to be specific—but with major differences: It’s rigid and reusable. Reusability is a key aspect of Loadhog’s selling proposition, because virtually all the conventional shrink-wrapping and strapping now being used goes straight into the trash after it is stripped from the pallet. The company says the Loadhog also saves much of the time spent applying shrink-wrap or strapping.

Tiny Bubbles, Big Machine

Loadhog studied a number of production methods, including conventional injection, injection-compression, and structural foam, before opting to produce the three main components of the lid using MuCell microcellular foam technology, a choice based on a combination of factors.

For those not familiar with it, MuCell converts an atmospheric gas—usually nitrogen and sometimes carbon dioxide—into a supercritical fluid (SCF) that is injected into the melt in the barrel using a feedback hydraulic accumulator to optimize dosage and mixing. During and after injection, the gas expands to produce a regular foam structure with cells no more than 5 to 50 µm in diameter.

The differences between MuCell and conventionally molded parts reportedly include reduced part weight and material consumption, easier processing, and shorter cycle times—all of which sat well with Loadhog Ltd. The base of the three-part pallet lid measures 1200 by 1000 mm, so even with wall sections of only 2.5 mm, this meant a big machine. Machine supplier Engel says the 1700-ton model of its Duo Series, with a 150-mm screw built for this application, is the largest MuCell system in the world to date. But to Loadhog, it must have seemed small.

Molding the lid conventionally in solid plastic would require a clamping force of about 3000 tons, and not incidentally, would consume more material per part. Loadhog’s machine investment, therefore, was reduced, as well as its material costs. When Loadhog evaluated structural foam using chemical blowing agents, it partially solved warpage and sink mark issues, but the relatively uneven structure of the foam created unacceptable weak spots in this thin-walled part.

Loadhog also benefits from easier processing and shorter cycle times. Trexel Inc. (Woburn, MA), which licenses MuCell technology, says that, compared with conventional melt, the SCF dissolved in the MuCell melt makes it flow more easily. Besides reducing the clamp force required, this also means less injection pressure is needed. The pressure from the gas in the part serves as the holding pressure, which reduces cycle time and avoids the strong orientation of molecular chains that can take place in conventional molding. The resulting reduction of internal stresses in the part minimizes or even eliminates warpage, while supporting high dimensional stability.

MuCell was developed at the Massachusetts Institute of Technology (MIT) and brought to market by Trexel in 1995. Approximately 200 licenses have been issued worldwide, and Engel says its machines account for about a quarter of those.

The 1700-ton Duo at Loadhog brings to 57 the number of Engel MuCell-equipped injection systems currently in operation, and Loadhog also has one of the others. The two upper panels of the Loadhog Lid are made on a 900-ton Engel Duo set up for MuCell, and each machine is equipped with an Engel ERC linear robot. Engel recently dedicated a 1500-ton MuCell Duo system for development and trial work at its facility in St. Valentin, Austria.