Drops and Shock Forces in Packaging

Shock is the force applied to a pack by a sudden change in its velocity. This can be envisaged by a pack falling off the back of a truck onto the sidewalk. The change in velocity, in that example controlled by the distance fallen and the nature of the impacting surface, indicates the severity of the shock felt by the pack and its product. Shock is encountered in many areas of the distribution chain, not only in simple drops but also in sudden stops and jolts due to the rigors of transportation.


Drop tests are a common form of proving method for new packs. Over many years a range of likely drop heights have been formulated against particular pack weights. This is illustrated in the spreadsheet below. As one would expect, lighter packs have a greater probability of being dropped from a higher height. This is due to a number of factors, such as their lighter weight making them more easy to be carried more than one at a time. Light weight also makes packs more likely to be thrown. Heavier parcels are more likely to be carried with greater care, to avoid the worker injuring themselves, but rather than being placed on the floor they can have a tendency to be dropped the final few inches to the ground. Below are a few general observations:

The normal result of drops and shocks is damage, to the pack this means that its protection and containment abilities are reduced.

The chart below shows the percentage probability of a particular drop height for a range of pack weights. Naturally this represents only a generalization in relation to all possible distribution cycles. The flat sections of the graphs represent where manual handling is likely to be replaced by mechanical handling.

Shock Conditions

Shock can occur in the mechanical handling systems of the distribution chain, but it is usually less server than the shocks incurred in manual handling. Therefore any pack that has proved to be sufficient in the manual handling section of a distribution chain will usually be sufficient to survive normal mechanical handling. Abrasion can occur in single packs during transportation as they move about. More server damage may happen if the frequency of the transportation system match the natural frequency of the product.

Parcel post and courier services provide a particularly server test for a pack. The products are almost always below 40lbs in weight and experience a great deal of manual handling, resulting in a higher number of drops than average. Postal testing machines are available to simulate these sever conditions and generally take the form of a large wheel with steps inside.

Calculating a Product's Fragility Factor

One of the most frequently used ways to discover the fragility of a product is to repeated drop samples and measure the drop height at which damage starts to occur. However, quantifying fragility as a drop height is useful only if no additional cushioning protection is to be used in the pack. It is helpful for products that may experience drops in their use environment, such as hand-held calculators, telephones, and personal computers. It must be noted that without a knowledge of the products fragility and areas of weakness it is impossible to calculate an efficient cushioning system for a particular product. "G" levels are commonly used to quantify an object's tendency to break when subjected to a shock. The drop height used to determine the G level is normally greater than probable drop height the packed product will experience.

An object will break if subjected to a force greater than its structure can withstand. Force can be calculate from simple school room physics:

Force = Mass X Acceleration

and deceleration are measures of the rate of change of velocity and the forces in action are the same whether the object is accelerating or decelerating; only the direction changes.

Once again school room physics reminds us that "G" is the ratio of acceleration due to gravity to the observed acceleration:

     observed acceleration
G = -----------------------
     acceleration of gravity

If a 200g vase were dropped from one meter, at the moment it reached the floor, its velocity would be 4.43 meters per second. If on hitting the floor it lost this velocity (decelerated) in 0.002 second, the deceleration could be calculated to be 2215 m/s2. Expressing this as a ratio to normal gravity would give a G level of 226. At the moment of impact, the vase would, in effect, weigh 226 times normal 45.2 Kg. Unless it was a very unusual vase, breakage could be guaranteed.

If the vase were dropped onto a sponge rubber pad, the impact velocity would remain the same. However, on impact the rubber pad would deflect, and the time over which the cup lost velocity would be extended. The deceleration would not be as severe and the stop not as abrupt. If the vase now stopped in 0.008 second, the G level would be 56. Another sponge layer might increase the deceleration time to 0.01 second, and the vase would experience 45 G. Adding still more layers would eventually reduce the G level to the point where the vase would not break. This would be one way of determining what cushioning protection the vase needed to protect it from a one meter drop.

However, if the G level that would break the vase was known in advance, that is, if its fragility factor in Gs was known, it would not be necessary to conduct the drop tests; the cushioning needed could be determined by calculation. It can be seen from the vase example that time is needed over which to dissipate the impact velocity and that this time is gained by the deflection of a resilient cushioning material. This is the basic principle of cushioning against shock.

An optimum cushioning material would provide constant deceleration until it is totally compressed by 100% to a thickness of zero. The thickness of cushioning (t) required to adequately retard an object with a fragility factor of G. through a drop height (h), can be expressed as:

t = ---

In reality, cushioning materials do not provide constant deceleration, nor can they be compressed by 100% (60% is usually the maximum, beyond which the material 'bottoms out'). Therefore the thickness of cushioning required will always be greater than equation I shows, by a factor C, which is specific for each type of cushioning material. 

t = --

Where C is the cushion factor, a measure of a cushioning material's efficiency as a shock absorber. Values of C for common cushioning materials are shown in the table below

Material Density kg/m3 Typical cushion factor Maximum static kPa Maximum stress kgf/cm2
Flexible urethane foam + 30 2.2 1.5 0.015
Orientated rubberized hair: style CA 96 2.4 9.8 0.1
Orientated rubberized hair: style CA 64 2.5 6.9 0.07
Orientated rubberized hair: style CA/CA 96 2.6 11.8 0.12
Expanded polyethylene 45 2.6 10.8 0.11
Bonded polyurethane chipfoam 64 3.0 1.8 0.018
Bonded polyurethane chipfoam 96 3.0 2.5 0.025
Expanded polystyrene  16 3.1 13.7 0.14
Rubberized hair 32 3.2 1.0 0.01
Expanded polyethylene 37 3.2 6.9 0.07
Expanded ethylene vinyl acetate 50 3.5 3.9 0.04
Plain rubberized hair 64 3.6 1.5 0.015
Bonded polyurethane chipfoam 144 3.7 5.9 0.06
Bonded polyurethane chipfoam 192 4.1 9.8 0.1
Plain rubberized hair 96 4.3 2.9 0.03

+ This material is likely to be more variable in performance than other materials listed


Estimates of cushioning thickness can be made using dynamic cushioning curves, these are available for most cushioning materials. The information necessary to make these calculations using dynamic cushioning curves is:

Cushion Thickness

To use a dynamic cushioning curve (see the spreadsheet above), locate the curve that crosses the desired critical acceleration line twice. The required foam thickness and the acceptable static load range can then be found. The two places where the critical acceleration line is crossed represent the minimum and the maximum static loads. Usually, a static load near the curve's minimum point would be chosen, but designing with higher static loads would reduce cushion material area.

Understanding shock and fragility factors will help to understand many shipping damages. For example, a refrigerator shipped by road has a compressor motor assembly weighing 15 kilograms. The designer felt safe in securing this assembly to the frame with three fasteners capable of holding 120 kilograms, an ample safety factor. However, during transportation, the refrigerator experiences a 10 G shock and, during that brief moment, the motor behaves as if it had a mass of 150 kilograms. Since the three mounting fasteners can hold only a total of 120 kilograms, they may shear off. The refrigerator sidewall, with a bearing area of 1.5 square meters, and the shipping box are able to distribute the load of a unit suddenly weighing 10 times more. With no external evidence of damage, the refrigerator is accepted at the receiver's warehouse and by the retailer. The problem is discovered only when a final customer plugs it in.

Good manufacturers know the fragility factor for all their products. In many instances, they will redesign products with low G levels, knowing that the saving in protective materials, and the goodwill generated by satisfied customers, will more than repay the cost of added engineering. Fragility may be greatly dependent on how the force is transmitted to the product. An egg on a flat surface has a fragility of 35 to 50 G. depending on the axis of impact. If the egg is supported in a conforming surface, its fragility can exceed 150 G

Below are some examples of fragility factor classes. A manufacturer would be advised to consider redesign of any product with a fragility level of less than about 30 G.

G Factor Classed as Examples
15-25 G Extremely fragile Precision instruments, first-generation computer hard drives
25-40 G Fragile Benchtop and floor-standing instrumentation and electronics
40-60 G Stable Cash registers, office equipment, desktop computers
60 - 85 G Durable Television sets, appliances, printers
85-110 G Rugged Machinery, durable appliances, power supplies, monitors
110 G Portable Laptop computers, optical readers
150 G Hand held Calculators, telephones, microphones, radios

It should be noted that the explanations for shock provided in this web page are simplified. Proper consideration of shock and shock protection takes into account not only peak G but also velocity change. These two factors are usually represented by a "damage boundary curve." The proper method of quantifying shock fragility is through the use of a shock test machine. This device is capable of providing a shock pulse of an accurately defined amplitude, duration, and shape.

Cushioning Against Shock

Any material that will deflect under an applied load can act as a cushioning material. By deforming, the cushioning material reduces the peak G level experienced by the product, compared to the shock pulse felt at the package surface.


A cushioning material reduces the initial shock force at the pack's surface so that the product's response takes place over a longer period of time. The areas under the curve represent energy.

The choice of cushioning materials will be into three main types: Cellulosics, Polymerics, and Long Fibers, such as animal hair bonded with rubber materials and wood wool. Shredded paper, Tissue paper, Corrugated board and molded pulp are examples of cellulose cushioning materials, which can be the most economical. However, such materials may not be as suitable from an abrasive and cleanliness point of view. They can also react corrosively with certain products. Alternatively, molded shapes, from materials such as EPS can prove expensive to produce in anything but high volumes. Loose foam chips are usually a more economical alternative for low quantities.

Polymeric materials can be produced in a wide range of resiliency's and densities. Expanded Polystyrene (EPS), Foam polyurethane and air bubble sheet are examples of polymeric materials. These materials can be more suitable from a cleanliness point of view. Cushioning polymers are not hygroscopic; however, some open-celled foams like polyurethane can absorb liquid when wet.

Loose fill cushioning material are particularly useful for odd shaped products, but can settle in transit. In some cases loose fill cushioning materials can be recovered for reuse. In response to environmental issues, loose fills based on recycled corrugated cases are being used. Foam-in-place polyurethane is labor-intensive in a packing process, but is another versatile cushioning material, enabling custom shapes to be easily produced.

Preshipment Testing Equipment

Vibration Tables

Vibration tables are used to assess product and package responses to the various ranges of vibration that they will experience in the field. They are available in two basic types: Repetitive-shock vibration tables operate at about 1.1 G (acceleration), 1-inch amplitude, and 4.5 hertz. These tables are used in tests specified by the Dangerous Goods Code and in procedures recommended by the ISTA and by ASTM D 4169. They are also useful for determining relative scuff resistance. Variable-frequency vibration tables are programmable to sweep through all com­mon transport frequencies between 3 and 100 hertz. They can be more realistic in rep­resenting the true distribution environment. They are also used to search out resonance weaknesses in the unpackaged product and to locate stack resonance points for stacked packages. The following vibration tests are described by the ASTM:

The principal feature of all drop-test devices is the ability to produce rep drops at selected orientations and from selected heights without imparting roation or other influences. Drop heights can be selected from drop probability tables, from standards set by the ISTA or ASTM, or by the requirement of a danger,: hazardous goods code.

Drop tests are described in the following standards:

Horizontal and Incline (Conbur) Impact Machines

The incline impact machine simulates horizontal shocks such as those expeerienced¬ rail shipment. The shock can be controlled by changing the impact velocir by using impact programmers. By using suitable backloads during the effects of dynamic horizontal compression can also be assessed. With modifications, the incline impact machine is also used to determine the durability of pal­lets to repeated forklift entries.

Incline impact tests are specified by ISTA and ASTM preshipment test meth- ods and are described in the following:

Good packaging laboratories are able to provide a wide range of climatic condi­tions with environmental chambers. They are typically used for preconditioning prior to physical testing. For example, to determine the ability of a plastic pail to survive drops at subzero temperatures or to identify whether a corrugated box loses stack strength at high humidity, both packages would require precondition­ing in the appropriate environment. Such chambers are also used to accelerate aging for such things as long-term storage tests and for environmental stress-crack tests on plastic containers (ASTM D 2561, Environmental Stress-Crack Resistance of Blow-Molded Polyethylene Containers). All standard paper tests should be conducted at 23 ± 2°C and 50% R.H. ± 2%. The highest humidity normally recommended for routine testing is 85%. Beyond this humidity, it becomes very difficult to control the temperature with the accu­racy needed to prevent condensation. To simulate a particular environmental con­dition, the conditions listed in Table 16.4 are the normal choices. Environmental conditioning is described in

Compression Test Systems

Compression strength is directly related to warehouse stacking ability. A com­pression test system is used to determine the load-carrying abilities of a package. Sizes vary from small, for measuring the compression strength of plastic bottles, to units large enough to measure the stack strength of entire pallet loads. Fixed-platen testers tend to cause the specimen to fail at its strongest point. Swivel platens tend to cause the specimen to fail at its weakest point.

Compression tests can be either dynamic, using hydraulically or mechanically driven platforms, or static, wherein a dead load is stacked on a subject containerand the system observed over a period of time. Compression tests are required by most preshipment test procedures and are described in:

Recommended standard atmospheric conditions as provided in ASTM D 4332.

Simulated Environment Temperature Relative Humidity
Cryogenic-55 ± 3°C-
Frozen food storage-18 ± 2°C-
Refrigerated storage5 ± 2°C85 ± 5%
Temperate,humid20 ± 4°C85 ± 5%
Tropical40 ± 2°C85 ± 5%
Dessert60 ± 3°C15 ± 2%

Shock Machines

Shock machines are used to develop fragility boundary curves and to determine G levels used in calculating cushioning requirements or for assessing a product's design fragility. A shock machine consists of a rigid table that can be raised and dropped onto a programming device. By controlling the programming device and the drop height, different G levels, pulse durations, and pulse shapes (sine, square wave, etc.) can be achieved.

Tests using shock machines are described in the following:

ISTA and ASTM Preshipment Testing Procedures

In the late 1940s the Porcelain Enamel Institute's members were experiencing considerable shipping damage. They conducted studies to identify a standard shipment test procedure that would assess the protective characteristics of packaging. A requisite was that damage created in the lab should closely duplicate that observed in the field. The procedure that was developed was found to be usefu other industries and soon was widely adopted.

Modified and updated under the sponsorship of ISTA, these test methods continue to be used today. Briefly, in the Project 1 A procedure, the package is Subjected to 14,200 (11,200 if the package is over 60 pounds) vibratory impacts on a vibration table operating at about 1.1 G and 4.5 hertz. Subsequently, the package is dropped 10 times from a height related to the package weight and in specified orientations. (See Table below).

Packages over 61 pounds may optionally be tested on an incline impact machine, as described in Project 1 for products weighing over 100 pounds.

The ISTA also describes static and dynamic compression tests and recomended tests for export packaging in Projects 2 and 2a. Project 3 is proposed the overnight overnight shipping environment. Incline impact and climatic tests are required by some procedures, particularly for heavy products.

ISTA methods are quick, economical, and simple. However, as knowledge the shipping environment increased, drawbacks became apparent. Damage cannot be duplicated by the ISTA methods is commonly observed. Another short-coming is that shipping vibrations in the real world are not fixed at 4.5 hertz. ISTA data have limited use for package design inputs.

Drop heights and orientations for ISTA preshipment tests.

Package Weight Drop Height Drop Number Orientation
1 through 20.99 pounds 30 inches 1 2-3-5 corner*
21 through 40.99 pounds24 inches2Shortest edge leading out from that corner
41 through 60.99 pounds18 inches3Next shortest edge from that corner
61 to 100.00 pounds12 inches4Longest edge from that corner
5 and 6Fiat on one of the smallest faces and on opposite small face
7 and 8Flat on a medium face and on opposite medium face
9 and 10Flat on one largest face and flat on opposite large face

*The convention for identifying the faces of a package is to place the package in its stable shipping position and to face the end with the manufacturer's joint. Call the top 1, the right side 2, the bottom 3, and the left side 4. The near end is 5 and the far end is 6.

In response to a need for a more flexible preshipment testing methodology, the ASTM published a new standard preshipment testing procedure, ASTM D 4169, Practice for Performance Testing of Shipping Containers and Systems.

The ASTM method recognizes that different distribution elements impose dif­ferent hazards on the product and package. (See Table) It further recognizes that different products might require different levels of assurance against product damage. The ASTM procedure essentially outlines the elements needed to tailor a preshipment test procedure to a specific need.

Summary of ASTM D 4169 distribution testing elements.

Shipping Element Hazard
Element A: Manual handling up to 90.7 kgDrop
Element B: Mechanical handling over 45.4 kgRotational drop
Element C: Warehouse stackingStatic load
Element D: Vehicle stackingStatic load
Element E: Vehicle transport, unitized loadVibration
Element F: Loose load vibrationRepetitive shock
Element G: Vehicle vibrationVibration
Element H: Rail switchingHorizontal impact
Element I: Climate, Atmospheric conditionTemperature and humidity
Element J: Environmental hazardSimilar to military specification MIL-P-116

To use the procedure, you first identify the nature of the distribution environment you wish to simulate in the laboratory and what the shipping unit will be for different stages of the journey. In the ASTM procedure, unlike the ISTA, tests done on the actual unit being shipped, which may vary at different points distribution. For example, a unit load may constitute the test unit for part of the gram, and an individual container may be used for the remainder.

The elements representing the identified shipping environment and the a: priate assurance level are then selected from the test method. A decision as to constitutes an unacceptable level of damage must be made. The test procedure describes different shipping modes, or elements, and provides for introducing atmospheric factors at any point in the test program.

The entire sequential test would be performed when evaluating a new shipping container. Where the package response to a single condition might be needed, only that element needs to be performed. The operator has the option of designing a custom sequence or using one of the 18 predesigned sequences describing most common distribution cycles.

The ASTM procedure is able to simulate more of the hazards encountered in distribution, and in a more realistic way than the ISTA methods. ASTM dures also provide valuable design information. However, establishing a tory capable of performing the ASTM D 4169 tests is more costly orders of magnitude than for ISTA, and skilled operators are required to tests and interpret data. There is also some contention regarding the select:: sequences and their levels. Further, not all shipping problems need the sophistication of the ASTM D 4169 approach.

Preshipment testing is a valuable tool in the development of a suitable bution package or for resolving specific problems. Whatever tests are chosen the damage observed in the laboratory should be similar in appearance observed in the field. One fallacy that must be avoided is that a particular time or exposure in the laboratory is equal to a certain number of kilometers or miles in the field. Finally, it should be understood that the ultimate and true test is a successful shipping history.

Other Test Methods and Standard Practices

The following are selected other standards related to packaging materials.