There are two basic types of slip sheet available, card or plastic. The card slip sheet may either be made from corrugated board only, or corrugated board sandwiched between kraft liner board or made from laminated kraft hardboard. Plastic slip sheets are typically made from layered polythene or polypropylene.
A third option is a mixture of plastic and cardboard consisting of kraft hardboard covered with a polymer.
Slip sheets are primarily used to carry loads in transit. They are typically around 1mm in thickness (although this can vary widely) and will have a footprint made to any custom size. A typical footprint would be 1200mm x 1000mm in order to fit onto a UK/US standard pallet but they can be made to fit a 1000 x 800 pallet, or any other dimensions.
The slip sheet includes one or more lips that protrude about 100mm or more from the edge(s). These lips (or tabs) are used by the push-pull device to grab hold of the load when pulling it onto itself. Different operations require anywhere from one lip to lips on all four sides:
It is common for slip sheet loads to be placed onto a pallet when unloading from a vehicle or container. This allows for manoeuvrability and racking. The load will be removed from the pallet only when further transport is required.
Compared with using pallets, they are considerably less expensive and take up a fraction of the weight and space.
Less weight and less space requirement means lower freight costs. The use of slip sheets for long haul freight (e.g. sea freight) can significantly magnify these savings. To appreciate the space saving, consider that 1,000 slip sheets will typically take up the same space as 10 pallets (depending on the thickness, of course).
Slip sheets are also recyclable and hygienic (when used only once). Slip sheets usually will not require fumigation treatments.
The fork lift needs to have a push-pull attachment in order to safely and efficiently pick up the load. For this operation, extra training is required. The operation requires strict adherence to procedures in order to ensure safety. This requirement applies to both the sender or receiver (unless the load is to be manually un/loaded). For this reason, there may be less flexibility if a load needs to be diverted to a different receiver. Not all loads are suitable for stacking onto slip sheets such as non-uniform shapes, sacks, rolls and very small cartons. Card slip sheets may be vulnerable to water or dampness (see below).
Although some slip sheets are re-usable, most are not.
Card versus Polymer
Corrugated (non kraft) slip sheets will be weakened when in contact with water and dampness and are therefore not suitable for refrigerated supply chains or outside storage and transport. Even in dry conditions, corrugated slip sheets will usually not be reusable. Corrugated is low cost, however, and can be ideal for many non-food items. Kraft board is much stronger and can usually be re-used a few times (perhaps up to 6 times, depending on many variables, including the thickness). Kraft board can be coated with a polymer skin which greatly reduces weakening from water and dampness and increases overall strength and durability. Kraft costs considerably more than corrugated, although it is commonly used, where corrugated slip sheets have limited applications. Slip sheets can also be made of corrugated board sandwiched inside kraft board.
Polythene or polypropylene slip sheets will typically cost even more than kraft (although price comparisons will vary daily because of the fluctuations in raw material prices in both these material groups). There is no problem with water or dampness and they can be re-used several times, depending on many variables, including thickness.
All of these materials are recyclable, although the environmental benefits of recycling material over another depends on market conditions at any given time and is always a matter of debate.
Slip sheets can be made of a composite of card and polymer, although this type can be costly.
Re-usable versus Recyclable
The fact that slip sheets are recyclable is good when compared to one-way pallets or if taken in isolation, but not so good compared to a reusable pallets. When considering a life cycle analysis, it is generally accepted that re-use is preferable to recycling. This generality can be challenged when there is a considerable disparity between the environmental footprint between the two options when they are being used to carry loads. As each slip sheet is sent off for recycling after just one use, the the environmental impact of the recycling effort will favour the re-usable pallet. However, as, over a period of time, the amount of vehicle journeys using slip sheets is lessened as more space and weight is available, thus the slip sheet makes up the ground on the pallet. In short, there is no easy answer. For each intended application there would need to be a life cycle analysis which should also include the environmental footprint in the original manufacture of the slip sheets and pallets, followed by a consideration of the journeys under load that will be taken and the effort required to recycle/re-use the slip sheets. The same arguments can be applied to costs, although prices in raw materials and recyclate tend to be volatile, requiring an overwhelming winner before a solid conclusion can be reached.
Consideration must be given to real-life re-use compared to theoretical re-use. If a company is part of a pallet pool, then there will be procedures designed to circulate pallets around the pool, forcing re-use many times over (if it is a well designed pool). There is a world of difference between this and plastic or laminated kraft slip sheets that may be expected to complete, say, 6 trips. For instance, how do you count the six trips? If you don’t, then how do you ensure that the the slip sheets have not deteriorated to an unacceptable standard in order to avoid dangerous loads. Does there need to be a systematic inspection of each slip sheet before use? One answer is to simply stamp or mark each slip sheet each time is loaded. When unloading slip sheets, if 6 stamps or marks are found to be on the slip sheet then it can be considered to be at the end of its life. This is fine if they are all being used within one organisation. If they are being used within a group of separate companies, there may need to be a system of accounting for end-of-life slip sheets and for their replacements. On the other hand, pool members could form a separate business – perhaps a co-operative – that purchases slip sheets for the whole pool with all members accepting that the distribution of end-of-life slip sheets will be roughly the same across all members of the pool, relative to the volume they handle, and so no administration is required except the marking system described above (or something similar).