This introduction to cushion packaging theory is intended to provide a basic understanding of the principles of cushion packaging. The information given should help understand this program and the data provided.
Cushion packages are designed to reduce the severity of shock on fragile articles during handling and storage. Typical cushion packages are designed to reduce the transmitted shock generated by mishandling of the package to a level that will not result in damage to the packaged article by allowing limited but controlled movement within the package.
The extent to which a cushion material (e.g. foam) can absorb shock depends on the physical properties of the cushion material and the dimensions of the cushion.
Dynamic cushioning curves are a characteristic property of each foam grade. They show the level of shock (peak G) transmitted through the foam at a given thickness against a range of loadings. These curves are generated through a series of drop tests. These drop tests are performed by measuring the force transmitted through a foam block of known dimensions. The impacting force is altered by using a set of weights and drop heights.
Cushion curves typically display a minimum in the transmitted force for a specific loading. At lower static loadings the impact energy is insufficient to compress the cushion resulting in a sudden deceleration of the impacting weight and therefore an increased transmitted force. At higher static loadings the impact energy compresses the cushion too rapidly until the cushion “bottoms out” onto the supporting surface.
In practise cushion systems are typically designed to work with a static loading close to the minimum of the transmitted force. The dynamic working range is defined as the range of static loadings of a material that will result in the minimum + 10% force being transmitted through the cushion.
Most designs are based on first drop data these days as the outer packaging and containers used in the overall design give a safety margin to make up for damage of the cushion by an impact. For stiffer foams it may however be advantageous to keep changes of the cushion properties with repeated impacts in mind when designing a cushion. Test of cushion packages have shown that the fatigue effects caused by multiple impacts are significantly reduced after the first three impacts. Therefore this program will provide third drop cushion data as well as the first drop cushion data where such data is available.
When designing a package one should always be aware that cushion performance can be affected by various factors throughout the working life of a package. The most important aspects that need consideration are:
One key parameter for the design is the fragility of the article that needs to be cushioned. This can only be determined accurately by subjecting the article to a series of gradually increased decelerations to determine the deceleration at which damage is caused. Such destructive testing is however not always practical and typical values for certain types of article are given in the table below.
| Peak deceleration | Article type |
| Up to 40 G |
|
| 40 – 60 G |
|
| 60 – 80 G |
|
| 80 – 100 G |
|
| 100 – 120 G |
|
The second parameter of importance for the design is the expected drop height. The program uses the drop heights defined for the design of packaging for military level J (UK ministry of defence specification for packages that are required to last a minimum of 2 years worldwide) as default drop heights on the cushion curve display. These are:
|
Weight of package |
Drop height |
||
|
kg |
lb |
cm |
inches |
|
Up to 30 |
Up to 66 |
100 |
39.4 |
|
31 – 225 |
68 – 496 |
50 |
19.7 |
|
Over 225 |
Above 496 |
30 |
11.8 |
For a more detailed correlation of package mass, handling method and typical drop height refer to the table below:
|
Weight range |
Drop height |
Type of handling |
||
|
kg |
lb |
cm |
in |
|
|
0 – 9 |
0 – 20 |
105 |
42 |
1 person throwing |
|
10 – 22 |
21 – 50 |
90 |
36 |
1 person carrying |
|
23 – 110 |
51 – 250 |
75 |
30 |
2 people carrying |
|
111 – 225 |
251 – 500 |
60 |
24 |
Light handling equipment |
|
226 – 450 |
501 – 1000 |
45 |
18 |
Medium handling equipment |
|
Above 450 |
Above 1000 |
30 |
12 |
Heavy handling equipment |
Depending on the dimensions of the article it may be necessary to subdivide the required area of foam to achieve a stable cushion or use load spreaders to spread the weight of the article over a larger area than its natural side. To maintain a stable cushion when subdividing the foam the thickness of the cushion should not exceed twice its length or width.
The program offers three sections to aid in the design of suitable cushion packages. It uses a set of design parameters which are the user required entries.
This page is the main page of the program. It allows making a choice of design parameters and general settings to design a cushion packaging system based on the specific situation.
The program is available in several languages. The display language can be changed using the drop down menu on the start page or the design page.
For the design the following settings are offered:
Most designs are based on the fragility of the item that needs to be packaged and therefore on the maximum shock level transmitted through the cushion. If a design is needed for a particular shock level but at the same time a limited cushion thickness is available, the design should first be carried out based on the shock level and then modified for the available cushion thickness using the adjuster section.
The metric calculations use centimetre (cm) as the base length measurement, kilogram (kg) for weight and kilopascal (kPa) for loading. The imperial calculations use inch as the base length measurement, pound (lb) for weight and pounds per square inch (psi) for loading.
The performance of a cushion material can be affected by the history of the cushion. Especially when packaging heavier materials that require the use of stiffer (higher density) foams it is helpful to consider the change in performance between first and third drop behaviour.
There are three design parameters that are used as inputs for the program. Two of these are the same independent of the type of design chosen. These are the weight of the article to be cushioned and the drop height.
The weight of the article that is to be cushioned will affect which foams result in a stable cushion under the circumstances used as input for the calculation. If the exact weight of the article is unknown using a weight higher than the actual weight may result in an unnecessarily large cushion thickness and potentially higher shock transmission as the actual weight of the article may be insufficient to compress the cushion in case of an impact. Underestimation of the weight on the other hand may lead to an insufficient cushion thickness and higher than expected shock levels for the packaged article as the cushion may “bottom out” under impact.
The drop height is the height from which the package is expected to drop in an accident. Typical drop heights are defined with regard to the most likely method of transport of a package given its weight, e.g. being carried by one or two people or being lifted using fork lifts. For examples on typical drop heights please check the information in the cushion packaging theory section of this help file.
If a specific level of shock tolerance is chosen as the design parameter, the third required input is the fragility of the article which needs to be cushioned. The fragility of an item, also described as the maximum shock level that an item can withstand without damage, will depend on the material the item is constructed from as well as the complexity of the item. The real fragility of an item can only be determined experimentally, resulting in destruction of the article. If such testing can not be carried out because the item in question is a one off or a very expensive article an estimate should be made based on knowledge of the article and its construction. A table with guidelines of typical fragility factors for several types of items can be found in the cushion packaging theory section of this help file.
If a specific foam thickness is chosen as the design parameter, the third required input is the thickness of foam material that one intends to use in the package design.
Design information is given in the tables at the bottom of the Design screen. These tables contain information on the required amounts of foam and suggested dimensions based on the design parameters entered.
There are three design information options available and the different tables can be accessed via the tabs at the top of the table. The default table shows calculation results for an optimum loading and minimal thickness of the foam. Minimising the cushion thickness will reduce the overall size of the package and therefore help keeping the freight costs low.
The other two tabs give results for either minimum or maximum loading of the foam. Minimum loading will result in a maximum foam volume being used. This configuration is intended for packages that are expected to be used for long term storage of articles. Using a maximum volume of foam will reduce the effects of creep and therefore improve long term stability of the packaging system.
Finally the last set of figures presents information on maximum safe loading of the foam which reduces the foam volume to a minimum. This combination is particularly useful for short term packaging and situations that require the cost of the foam packaging to be kept low.
All three tables give information on the same set of design information values.
Clicking into any cell of the table will highlight the column and select the material as the default material for the cushion curve section and the adjuster section.
Right clicking on any column in the table will allow copying of the presented data in that column, to export it into another program.
The order button will open an email to info@zotefoams.com. The default text is based on the selected foam grade. If you require a quote or more information on a specific grade use this function to send an email with your request to Zotefoams.
The print button will allow printing of the design page as it is on screen. So several print outs may be required to cover all the foam grades on which information is currently available.
This section of the program displays cushion curves for up to three parameter combinations at a time.
The first drop down menu allows selection of the parameters for which drop curves are displayed. These are materials, drop height, drop count and user defined.
The second drop down menu allows changing the selected grade without having to return to the design page. The default selected grade when you first enter this page is the one selected on the design page.
The third drop down menu toggles between first and third drop performance.
For all display options the default thickness and drop height are defined as follows:
The default display is using the materials option. This displays the graph for the selected material plus the materials of immediately higher and lower density (or two higher densities for the lowest density in a range and two lower densities for the highest density in a range).
Selecting the drop height display from the first drop down menu presents the curves for the selected grade at drop heights of 30 cm, 50 cm and 100 cm. These drop heights were chosen based on the design requirements for (UK) military level J packaging.
Selecting drop count in the first drop down menu displays a comparison of first and third drop performance for the default drop height and thickness of the selected material.
The user defined display allows the user to display up to three curves based on their own parameters. These parameters are set using the adjuster section of this program
Cushion curves can be printed using the print button. This will print the graph area of this display.
The adjuster page can be used to optimise the suggested designs. These optimisations can either be driven by a need to utilise available material with least possible waste or analyse the effect of minor modifications, e.g. small changes in cushion thickness to allow use of standardised boxes, on the level of protection provided.
The adjustment assumes that the weight of the article and the drop height remain constant. If only an area or a thickness is entered the other value will be taken from the optimum loading table for the selected grade.
If the suggested adjustments take the parameters outside the dynamic working range of the material “Exceeds cushion factor” will be displayed in the table on the adjuster page.
Adjustments entered here can also be used as the input for the user defined display of cushioning curves.
When the applet has loaded correctly or the application is started the intro page presents the user with an option to select the language for the program, read this help file or start using the program. When the program is started using the “Start” button a warning message will appear regarding the data presented in the program. Please make sure you understand the limitations of this cushion packaging design program before proceeding with your design.
Starting the program will open the design page.
If you are trying to find out which foam grades are suitable for your packaging application this page will allow you to calculate the necessary values. You first need to make selections depending on which parameters you want to focus on and which units you want to use.
Once you have made these selections you can input your design parameters.
Once you have entered these values the table will be re-calculated for your design. To re-calculate the table after changing an input press enter after typing the number or use the “re-calculate” button.
You can now choose the foam best suited for your application based on the presented data. Depending on the estimated length of use of the cushion you may also find the information based on minimum or maximum loading of the cushion useful. These can be accessed by clicking on the tabs above the table.
Please note that the grades with SD and CN suffixes are special grades for packaging of electronic equipment which is sensitive to electrostatic discharges.
If you want to print out a hard copy of the calculated values you can use the print button at the bottom left of the program. This will print the part of the table that is currently displayed.
The following steps are optional depending on whether you want to analyse the cushion curves for certain sets of parameters or adjust your input for a specific grade. The first set of steps will detail working with the cushion curve section while the second set gives information on the adjuster section.
If you need to check the effects of minor alterations on the performance of the cushion or you want to use the user defined setting of the cushion curves display you need to go to the adjuster page. The adjuster page will use the selected grade from the design page as default.