1. Lecture 6: Drop & Cushion Testing presented by David Shires
Packaging Dynamics
Lecture 6: Drop & Cushion Testing presented by David Shires
Editor-in-Chief, Packaging Technology & Science
Chief Consultant, Pira International

2. Does My Product Need Protective Packaging?
To answer this:
Ideally we should know the damage boundaries
We may only know the critical acceleration
We could drop an unprotected product from the expected worst case drop height.

3. Does My Product Need Protective Packaging?
Damage Zone
The critical velocity of the product tells us if we need protective packaging
Drop Height (m)
Impact Velocity (m/s)
0.2
2.0
0.5
3.1
1.0
4.4

4. How Do We Establish Our Design Drop Height?
From test standards
eg ISTA tests
Based on field data
Reasonable basis but generalised
From research reports
Technical journals
Technical associations
ISTA
On-line resources
MADE
SRETS reports
ISTA project 4AB

6. MADE data Europe

7. Establish Design Drop Height
Collect Own Data
Most appropriate data for design
Right pack ?
Right distribution ?
Statistically valid ?
Number and range of trips
Method of analysis

8. Collection & Analysis of Data
Location of data recorder
To measure vibration – on bed of truck
To measure drop heights – inside dummy pack
Size
Weight
Centre of Gravity
Secure location of device

9. Collection & Analysis of Data
Wooden frame
Robust outer
Consistent characteristics
Symmetrical cushion
Response to impact should be same in all directions

10. Beware !!! Miniature data-logger only RMB 2000 Sampling rate is 50s-1
PC interface
Fully configurable
Free software
Sampling rate is 50s-1
1 sample per 20ms
Similar duration to cushioned shock
Longer duration than un-cushioned shock

11. Typical configuration
Sampling Rate 1000s-1
Sample Duration 2s
Trigger level 1G
Pre-Trigger Information 50%
We are interested in time of flight
For a 1m drop time of flight ~ 0.5s

12. Analysis Time of Flight Analysis
During free fall drop effective acceleration = 0
Time of flight = period of zero G

13. Analysis Time of Flight Analysis Works well for free-fall drops
Packages suffer many impacts which are not free-fall drops
All significant impacts are important to record and analyse
We can classify impacts according to their equivalent free fall drop height (EFFDH)

14. EFFDH
Velocity change
= area under acceleration curve =
= vi + vr

15. EFFDH Coefficient of restitution
Can determine C from calibrating drop tests
C depends on impact surface as well as pack

16. Cushion Design Energy taken up by foam: Potential energy +
Heat
Kinetic Energy

17. Cushion Design Cushion too hard Area under curve = energy absorbed
Constant for a given drop
Cushion too soft / too thin

18. Cushion Design σp
Cushion effectively reduces peak stress by increasing strain

19. Cushion Assessment =

20. Cushion Assessment Range of drop heights Range of static stresses
Range of cushion thicknesses

21. Cushion Curves

22. Cushion Curves

23. Cushion Curves

24. Cushion Curves

25. Cushion Curves
Generating cushion curves by falling weight impact tests is large amount of work & needs a lot of samples.
Burgess showed a relationship between impact energy and stress density
From one impact test you can derive the relationship and predict cushion curves
Generation of cushion curves from one shock pulse Packaging Technology and Science Volume 7, Issue 4, Date: July/August 1994, Pages:  Gary Burgess
A Simplified Process for Determining Cushion Curves: The Stress-Energy Method: Matt Daum

26. Main Cushion Materials
Polyurethane Foam
Open Cell
Effective for low static stresses
Polyethylene Foam
Closed cell
Wide range of foam and polymer densities
Polystyrene Foam
Fairly rigid
Suffers more from repeat impacts

27. Alternative Cushion Materials
Starch Foams
Corrugated or Honeycomb Pads
Moulded Pulp
Bio-materials
Suffer on impact
Humidity sensitive
Loose Fill
Extruded foams (best) to shredded paper
Unpredictable performance but convenient for small quantities
Air bags
Low cost, made in-line

28. Creep
Foam cushions are prone to compressive creep – especially at higher static stresses
Product will be loose in the pack
Cushion will be less effective
10% maximum used as a general design guide

29. Creep Rate of creep is a function of static stress and temperature
Generally acceptable at optimum static stress

30. Cushion Design Balance Product fragility Product weight
Contact area with cushion
Cushion thickness
Target drop height

31. Cushion Design
Say max shock is 40G, static stress is 5kPa, worst case drop height is 40cm
Need to check:
Third drop
Creep

32. Cushion Design Moulded Cushion Cushion sheets Corner fitments
Cut & Shaped
Flat
Corner fitments

33. Cushion Design Symmetrical Secure from displacement
Product Centre of Gravity
Buckling or Rotation
Temperature
Creep

34. Cushion Design Product on cushion has natural frequency
How does this compare with natural frequency of product?