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

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.

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

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

http://www.hp.com/packaging/made/

MADE data Europe

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

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

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

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

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

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

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)

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

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

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

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

Cushion Design σp Cushion effectively reduces peak stress by increasing strain

Cushion Assessment =

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

Cushion Curves

Cushion Curves

Cushion Curves

Cushion Curves

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: 169-173 Gary Burgess A Simplified Process for Determining Cushion Curves: The Stress-Energy Method: Matt Daum http://www.hp.com/packaging/misc/Presentation/DaumMatthewDimensions06paper.pdf

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

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

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

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

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

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

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

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

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