Once your pallets have been secured for transit, often using plastic strapping, there are several ways of protecting them from dust or water ingress. Before stretch hooding machines were introduced to the market in the late 1980’s, the two main options were Shrink Hooding and Stretch Wrapping. This article investigates the issues with both packaging methods and explores why stretch hooding has become so popular.
Shrink Hooding involves applying a single layer, heat shrinkable, gusseted polyethylene hood to a pack and then shrinking it to the pack using hot air emitted from gas burners. Most shrink hooders are compatible with LPG or Natural Gas. On modern machines, the flame is only ignited during the shrinking operation. This ensures a considerable energy saving compared to earlier technology, where burners on some models were ignited during the whole production shift. Furthermore, modern machines use enclosed hot air burners to eliminate the risk of the flame coming into contact with the pack. In the past, the burners on some shrink hooders were not enclosed, and the flame came into contact with the hood, causing the pack to catch fire and collapse.
Stretch Wrapping involves applying LLDPE (linear low density polyethylene) stretch film to a pack, either while the pack itself is rotating (Turntable Wrappers) or while the film carriage of the machine rotates around the pack (Rotary Arm or Ring Wrappers). A polyethylene top sheet is often applied to the top of the pack in order to afford some protection from the elements. However, even the most advanced wrappers cannot guarantee that no water can trickle in between the top sheet and the layers of stretch film, and keeping packs dry may be of fundamental importance for certain industries.
Stretch Hooding involves applying a stretchable, gusseted polyethylene hood to a pack, but without using gas burners. Whereas shrink hood film is single layer product, stretch hood film is a 3 layer co-extruded product. Like stretch wrappers, some stretch hooders include a roping function to apply an extra layer of film around the base of the pallet should added stability be required. A unique feature of stretch hooders, compared to shrink hooders, is the ability to re-align the hood and continue hooding if your product is not perfectly aligned on the pallet, rather than stopping the whole line to report a fault.
With both shrink and stretch hooding, gusseted film (plain or printed) is supplied on a reel. The machine draws the film from the reel to the desired length, welds and then cuts it to form the hood. The hood is then lowered over the pack until the machine releases it at conveyor level. Film manufacturers can produce film on three main types of centre core, steel, cardboard and plastic, typically up to 500kg. It is important for any modern stretch hooder to be able to handle various core types and avoid damaging them so that they can be returned to the film producer to be re-used, thereby reducing packaging waste.
Given that stretch wrapping cannot fully protect the pack from water ingress, and shrink hooding involves a costly gas bill (typically €0.05 per pack), Stretch Hooding is rapidly becoming the chosen packaging method for many industries.
Modern stretch hooders can be installed over existing conveyors and can hood the pack right down to bottom of the pallet, without the need for a pack lifting device. Eliminating a pack lifter not only reduces the capital expenditure, but also allows the hooding machine to run at maximum capacity (typically a lifting device can add 5 seconds to the cycle time). A pack lifter is often used on shrink hooders to lock the hood underneath the corners of the pallet thereby preventing the hood from ‘half mooning’ during the shrinking process and leaving the bottom parts of the pack exposed to the elements. Most Stretch Wrappers also require lifting devices to ensure that the film can be applied right to the bottom of the pallet.
Opening the stretch hood is a critical part of the hooding application. Failure to open the hood properly will cause a stoppage that requires the operator to intervene, and will result in the hood being removed and transferred to the packaging waste bin.
Stretch Hood Film and Stretch Hooders have come a long way in the last twenty years. Stretch Hooders used to be significantly more expensive than Shrink Hooders. Furthermore, when stretch hood film was in its infancy the choice of suppliers was limited. Nowadays, both types of equipment are broadly similar in cost, the number of stretch hood film producers has grown, and technology has advanced to a point where stretch hood film is now widely available in 50, 60 and 70 µm, as opposed to 100 µm (or higher) when it first came on the market.
From a packaging perspective, the cost savings are compelling: Based on a 1200L x 1000W x 500Hmm pallet, the cost to stretch wrap a pallet with 23 µm power stretch film + top sheet (in total 600g) is approx. £1.14, whereas the cost to stretch hood the same pallet with 90 µm film is approx. £0.64. Similarly, moving from a 125µm Shrink Hood to a 70µm Stretch Hood could result in a saving of £0.94 per pack.