Six Steps to Safe Shipment

Step 5. Use Cushioning Material Effectively

Cushioning items with simple shapes and durable surfaces is easy. These objects can be wrapped or covered with a suitable interleaf and then cushioned directly (placed in foam cut-outs). Objects with complex geometry, or those that are highly fragile, are harder to cushion directly. In this case, primary packaging or the use of a double case system can transform difficult shapes into simple ones that are much easier to cushion, and can provide additional protection for fragile projections or surfaces during packing and unpacking. Several examples of this are provided in Table 7.

Table 7. The addition of a mount, intermediate framework, or an inner box can simplify packaging of items with irregular shapes or fragile surfaces. These measures can save time and often result in a better performing package.

Item	How to simplify complex shapes
Unframed painting	Add a travel frame that provides flat durable surfaces for cushion application (see CCI Notes 10/16).
Delicate ornate frame	Fix the object inside a handling, transport, and storage frame (HTS) with flat surfaces (see CCI Notes 10/16).
Fragile pottery item with projections	Use a carved negative mount that has voids carved around the small projections. This mount can than be cushioned or placed in a cushioned inner case.
Several fragile items that will be shipped together	Pack the items into the inner case of a double case system with suitable interleaves or mounts. Use partitions for heavy items.

Cushioning limits shocks and vibration to levels that objects can tolerate without damage. However, the cushioning needs to be properly selected for good performance. Important features of cushioning include:

• material type
• material thickness
• material resilience (the material returns to its original shape after impact)
• correct loading (i.e. the cushion material is not too soft (overloaded) or too hard (insufficiently loaded) for a very fragile object)
• free movement of the cushioned item (i.e. the object is not tightly bound in the cushioning material)

Modern cushioning materials can provide very effective protection for fragile items when they are used correctly. Table 8 lists three material types that can be used in many cushioning applications.

Adequate cushion thickness is necessary for effective cushioning. As a general guide, use at least 50 mm (2 in.) of cushion thickness for fragile items in small- to medium-sized packages. Packages less than 15 kg (33 lb.), or exceptionally fragile objects, may benefit from additional cushion thickness — 75 mm (3 in.) or 100 mm (4 in.).

Resilience is the ability of a material to recover its shape when it is deformed, which makes it effective against repeated impacts. Polyurethane and polyethylene foams are both resilient materials.

Correct cushion loading can be achieved through material choice and pad geometry. Cushion designs may range from complete coverage, as in the case of wrapping and foam cut-outs, to individual pads. For any given cushioning method, the amount of cushion material in contact with the object will establish the load per unit area on the cushioning material. The suitability of a material for any given application can then be verified from cushion performance data (described below).

Note that cushioning is required on all sides of an object in small- to medium-sized packages. However, cushioning requirements decrease for very large packages. Base protection (padding the base of the object only) combined with suitable restraint against rebound can suffice for very large items.

Table 8. The listed cushioning materials can solve many packing problems. Calculating the weight per unit area (W/A) on any given side of an object (see Figure 2) provides a basic assessment of material suitability for the cushion design under consideration.

Material	Information	Applications	Typical W/A range for cushioning
Polyurethane ester	Dark grey coloured (usually) with open-cell structure. Most useful form has a density of 33 kg/m3 (2 pcf*) and a thickness of 50 mm (2 in.). Does not go by a trade name. Avoid direct contact with metals or other artifact surfaces.	Protective cushions. Good choice for cushioning double case packages.	For W in kilograms, A in square centimetres: W/A range is 0.003–0.06 kg/cm2. For W in pounds, A in square inches: W/A range is 0.04–0.8 lb./in2.
Polyethylene	White closed-cell polyethylene foam. Chemically stable. Commonly used density is 33 kg/m3 (2 pcf). Trade names are Ethafoam and Polyplank. Thermoplastic, heat-weldable to itself.	Excellent choice for mount-making due to chemical stability. Easy to work with. Also used for cushioning heavier items.	For W in kilograms, A in square centimetres: W/A range is 0.015–0.1 kg/cm2. For W in pounds, A in square inches: W/A range is 0.2–2 lb./in2.
Bubble pack	Air-encapsulated film. Use with an interleaf material to avoid deposits from release agents. Use a thickness of at least 50 mm (2 in.) on all sides.	Lightweight materials with simple or complex geometry and durable surfaces.	For W in kilograms, A in square centimetres: W/A range is 0.002–0.03 kg/cm2. For W in pounds, A in square inches: W/A range is 0.03–0.4 lb./in2.

* pcf = pounds per cubic foot.

For effective shock and vibration isolation, everything that floats on the cushioning system (i.e. the cushioned item) should be reasonably firm. The cushioned item will then move up and down on the cushioning as a single unit without any secondary movement of its own. To achieve this, gently restrain any loose or vibration-prone items and deal with object vulnerability by treatment, disassembly, or other means as discussed in steps 3 and 4. The cushioning system should be the most flexible part of the package.

Figure 2 illustrates the various forms that cushioning can take for several object geometries. A good material combination for many applications is polyurethane ester [density 33 kg/m3 (2 pcf); thickness 50 mm (2 in.)] for pads and firm polyethylene foam for mounts. The cushion designs in Figure 2 involve straight cuts of foam. Polyurethane ester material can be cut easily on a band saw or with an electric carving knife. Polyethylene foam is chemically stable and can be carved and bonded, making it a popular choice for mount-making applications; it can also be used as a cushioning material for heavy objects.

To verify that a cushioning material is being used correctly, divide the total object weight (W) by the total area (A) of cushioning material on any given side as described in Table 8. If the result is outside of the indicated range, consider another material or change the pad geometry (e.g. increase or decrease the amount of cushioning material in contact with the object).  Corner Pads for Double Case Packages provides instructions for making corner pads for double case packages without the need for calculations. A computer program for cushion design (PadCAD) is also available from CCI, and a trial version of the software can be downloaded from the CCI Web site. This software can be used to design all of the cushioning systems illustrated in Figure 2.

Figure 2. Protective cushioning systems for a variety of object geometries.  Polyurethane ester foam can often be used with very good results. To verify that the material is being used correctly, the load per unit area on the cushions can be calculated by dividing the total object weight (W) by the total area (A) of foam contacting each side.