| Cushioning materials | [German version] |
Purpose of using cushioning materials
Goods are frequently transported which are particularly sensitive to mechanical stresses and which must consequently be protected from damage due to impact, jolting or vibration in transit. They are thus additionally protected by cushioning materials inside the shipping packaging.
Fragile goods, such as glass, ceramics, porcelain, or sensitive electronic products, such as computers and electronic home entertainment equipment, are particularly susceptible to mechanical stresses and should be protected.
In addition to protecting the package contents, cushioning materials may also be used to adjust the packages to a standard size, in which case they act as adapters between nonstandard package contents and the packaging (modularity of shipping packages).
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Mode of action of cushioning materials
Cushioning materials absorb a proportion of the kinetic energy arising when the package suffers impact or is dropped and increase the braking distance of the package contents. Correct selection and sizing of the cushioning material thus ensure that the package content suffers no damage.
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Required characteristics of cushioning materials
Cushioning materials must in particular fulfill four main requirements:
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Recovery is one of the most important properties of a cushioning material; it ensures that the package contents continue to be protected even when repeatedly subjected to similar stresses. If recovery is too low, the braking distance declines on constant exposure to stress, such that the resultant kinetic energy can no longer adequately be absorbed and the package contents may be damaged. |
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Cushioning materials must be insensitive to climatic conditions, such as moisture due to elevated relative humidity, direct solar radiation and extreme variations in temperature and their action must not be impaired by such exposure. |
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Especially in the case of package contents which are at risk of corrosion, it is important that the cushioning materials are not hygroscopic and consequently do not promote corrosion. They should furthermore not contain any aggressive constituents (neutral pH), which could contribute towards corrosion. The cushioning material and package contents should not interact and possibly impair each other’s properties. |
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Use of the cushioning material should be effective, simple, environmentally compatible and cost-effective. |
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Selection criteria for cushioning materials
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Sensitivity classification of package contents If it is to be possible to dimension the cushioning material properly, it is essential to know what stresses it can withstand without suffering damage. Since industrial equipment in particular today consists of many different components of differing levels of sensitivity, it is very difficult to provide a general classification of goods. The manufacturer will in each instance be able to provide precise details about the sensitivity of their product. The sensitivity classification of a product is determined by the admissible g value. 1g is the acceleration due to gravity (9.81 m/s2), i.e. the force which usually applies to an object on the earth. If an acceleration of 2 g is applied (for example during fast cornering), the weight of the object doubles. This is precisely what happens to an item for transport which is secured on the loading area of a truck or stowed in a sea container. However, in addition to acceleration, the duration of any impact must always also be taken into account. The longer the duration of any impact, the greater is the risk of damage. |
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Stresses during transport The stresses arising during transport are the second important parameter in selecting a cushioning material. These stresses may be highly variable and it is extremely difficult to determine exactly what they will be. The greatest stresses occur if the packaged items are thrown or dropped. This is why the potential drop height of a package as a function of its weight is used as a measure of stress. The regulations of Deutsche Bahn (German railroad operator) and Deutsche Post (German postal authorities) define maximum drop heights for packages as follows:
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Static area load The cushioning material is exposed to both dynamic and static forces during transport and cargo handling, but only static stresses apply during storage. These stresses are known as the static area load acting upon a cushioning material, which is calculated from the weight of the package contents and its bearing area:  The static area load is important for the purpose of selecting a suitable cushioning material, as the material must not lose its recovery when at rest merely under the weight of the package contents. |
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Recovery As mentioned above, recovery is a decisive indicator of the loading capacity of the cushioning material on repeated exposure to stresses. If recovery is too low, the braking distance declines on constant exposure to stress, such that the resultant kinetic energy can no longer adequately be absorbed and the package contents may be damaged. |
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Specific weight Specific weight is stated in kg/m3 and is a measure of the hardness of a cushioning material; the higher is the specific weight, the harder is the cushioning material. |
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Resonance behavior The stresses arising due to the transport of an item on a vehicle are composed of many different and simultaneously acting vibrations and impacts. If theses vibrations are at the natural frequency of the package contents, resonance may occur. The item is consequently exposed to greater acceleration in the vertical direction, the protective action of the cushioning material is canceled out, so exposing the cushioned item to greater risk. Especially when transporting sensitive items, such as instruments or electronic components, the frequency values of the means of transport used and the natural frequencies of the cushioning material and item for transport must be known and adjusted to each other. In this way, by using a truck with air suspension, it is possible to avoid the „excitation“ frequency when transporting electronic components. Under unfavorable transport conditions, this excitation frequency would occur during transport on a leaf-sprung vehicle, so increasing the amplitude of vibration of the package contents and, once the resonant range of the cushioning material had been reached, damaging the package contents. |
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Stress range of the cushioning material Every cushioning material has a stress range within which it exhibits optimum effectiveness. Cushioning curves, which are the plot of maximum impact deceleration against static area load, are used to select suitable cushioning materials. These cushioning curves may be used to determine the cushioning thickness which will provide sufficient shock absorption. Cushioning curves are plotted for a specific drop height. These curves indicate, for example, that a 5 cm thickness of plastic foam cushioning is required to reduce impact forces to the admissible level of at most 30 g. The area required to provide cushioning beneath a packaged item may then easily be calculated. |
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Description of various kinds of cushioning materials
Airbags
Airbags consist of an elastic film which is inflated with air. When at rest, only the static load generated by the weight of the package contents bears upon the cushioning. When dynamic loads occur, these are absorbed by compression of the cushion.
The quantity of inflation air may be varied in accordance with the particular properties and requirements of the package contents. Airbags are commercially available in various sizes and designs, ranging from spheres, standard cushions to corner and edge cushioning and tubular cushioning.
Airbags are mainly used in containers and railroad freight cars and only rarely in trucks.
Advantages of airbags:
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ease of handling |
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nonhygroscopic |
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highly versatile |
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largely insensitive to extreme climatic conditions (heat, cold) |
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elevated recovery and ideal shock absorption characteristics |
Disadvantage of airbags:
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susceptible to pointed and sharp articles, such as nails or the like |
Bubble films
Bubble films function in essentially the same way as airbags. They consist of two plastic films, one of which is completely flat and the other has small, round indentations, which, once the two films have been heat sealed together, contain the necessary air. Bubble films are mainly used inside packaging containers. The advantages and disadvantages are the same as for airbags.
Rubberized fiber cushioning
Rubberized fiber cushioning provides high quality protection for demanding items. This cushioning is made from animal hair or coconut fiber, which is cleaned, converted into nonwoven mats, coated with rubber and vulcanized to form solidly bonded sheets.
Rubberized fiber cushioning is relatively insensitive to the effects of moisture and high or low temperatures and exhibits very good recovery even on long-term exposure to loads.
Plastic foam cushioning materials
Plastic foam cushioning materials are mainly made from polystyrene (PS), polyurethane (PU) and polyethylene (PE). Plastic foams are available in flexible, semirigid and rigid forms. Their cushioning characteristics are determined not only by their specific weight but also by their cell structure. The particular characteristics of the various starting materials are briefly described below.
Polystyrene (PS):
PS is a somewhat soft, elastic foamed plastic with closed cells containing the air required to provide resilience. Recovery is, however, rather limited.
PS is not itself hygroscopic and thus remains fully functional on exposure to moisture. It should be noted, however, that, due to its structure (enclosed capillaries), PS cushioning material nevertheless has a certain tendency to absorb or release water vapor. Appropriate action must accordingly be taken to protect package contents which are at risk of corrosion.
PS cushioning material is produced both as relatively large moldings, such as cushioning frames, edge or corner pads, and as a loose fill cushioning material, known as PS chips. When large moldings are used, the cushioning area often has to be reduced as the static area loads of the package contents are not sufficient to ensure effective cushioning.
Polyurethane (PU):
Polyurethanes are produced in flexible, semirigid and rigid forms with an open cell structure. It is primarily flexible and semirigid grades of polyurethane which are used in packaging applications.
The shock absorbing properties of PU foams increase with foam hardness, while recovery and elasticity decline.
Especially on repeated exposure to identical stresses, this characteristic may cause problems with an excessively rigid grade of foam as there is a continual decline in recovery.
Polyurethane foams are produced as relatively large moldings, generally by direct foaming around the item to be packaged. If this is not feasible, the moldings may also be prefoamed.
One disadvantage of PU foams is their relatively complex production process. Their ideal application is thus not for mass-produced items, but instead for packaging and cushioning constantly differing items.
Polyethylene (PE):
Like polystyrenes, polyethylene foams are closed-cell products. They exhibit excellent cushioning characteristics, which are comparable with those of rubberized fiber cushioning. Even when exposed to major loads, they retain their cushioning capability.
PE foams do, however, have two considerable disadvantages. Firstly, they are costly, which excludes them from many applications, secondly they do not have good weather resistance.
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