Main Contributors to Point Load
- Pallet design/Condition
- Pail placement on pallet
- Placement of upper pallet upon lower pallet
Point Load Manifests itself As
- Collapse/dent/buckling in pail sidewall
- Bending inward of pail bottom (chime)
- Container bulging
How to Tell Where Uneven Force is Being Applied
- Lower sidewall dent/collapse = Pallet related (excessive spacing or pail overhand)
- Upper sidewall near satellite = Placement of upper pallet
Point Load Can Lead To
- Permanent deformation of pail
- Dent/buckling turns into crack—product spillage
- Pallet collapse as load shifts
- External stress point—ESC failure
- Loss of compression strength—reduction in pallet stacking capability = $$
Table of Test Results
|Item#||Test Condition||Average Peak
Load at Failure
|Loss of Compressive
|% Loss of
|1||Top Center Load||3813 lbs.||—||—|
|2A||Bottom 2° Overhang||2152 lbs.||-1661||-43.6%|
|2B||Bottom 1 ■ Overhang||2681 lbs.||-1132||-29.7%|
|3||Top Eccentric Load||1897 lbs.||-1916||-50.2%|
|4A||5″ Deck Board Space||2617 lbs.||-1196||-31.4%|
|4B||3″ Deck Board Space||3115 lbs.||-698||-18.3%|
|4C||2″ Deck Board Space||3336 lbs.||-477||-12.5%|
|5||3/8″ Deck Board Deflection||2278 lbs.||-1535||-40.3%|
Effects of Fill Temperature on Pail Stiffness
Loss of stiffness as a function of temperature
- The stiffness of PE at 140° is 1/2 of that at 73°
- Types of hot fill—pie fillings, fflavorites, sauces, driveway sealers
- Maximum use temperature is 160°. At this temperature some annealing may take place resulting in shrinkage, warpage deformation.
- Pails should not be stacked more than two high
- The contraction of contents upon cooling will create a vacuum.
- Extent of vacuum dependent on headspace and lid seal efficiency. Pail sidewalls give the appearance of being “sucked in”. Stacking should be minimized during the cooling phase.
Environmental Stress Cracking (ESC)
ESC is a type of failure that occurs in molded plastic containers and covers when specific conditions are present.
How Does ESC Occur?
Failure is usually initiated on the surface of a molded part that is under stress and in contact with an aggressive fluid, which promotes initiation and crack propagation. The other factor is time. Failure is primarily a function plastic under stress exposed to certain fluids over a period of time. The addition of heat (e.g. hot warehouse conditions during the summer), will accelerate failures because of the reduction of pail stiffness, thereby increasing stress.
What Does an ESC Failure Look Like?
The failure starts out as a small smooth crack. Under load, the crack will propagate in a tearing manner. Failures are usually found in containers located in the bottom row of a pallet. Once one package fails it’s common for the load to shift in the pallet resulting in a number of containers toppling over.
What are Some Sources of Stress?
There are two type of stresses—External and Internal (molded).
Sources of external stresses include:
- excessive stack load
- point load
- overhand on the pallet
- too much space between pallet boards
- compression through tight stretch wrapping and banding.
- small cover on large pail will introduce compressive forces in the satellite ring area
- mold design/construction—sharp angles, ribs in the parts
- nicks, cuts, pits in the molded parts
Sources of internal stresses (molding related) include:
- short shots in ribs
- gate area (high stress point)
- flawed processing set-up introduces stress—fill velocity, packing pressure, hold time, cooling
- resin (higher melt and density)—certain resins are more susceptible to failure
- colorants—too much color makes parts brittle
What is Environmental Stress Cracking Resistance (ESCR)?
Environmental Stress Crack Resistance (ESCR) is a measure of the resistance of plastics to failure in this mode. To check how well plastic will perform with a specified liquid, two types of lab tests based on ASTM procedures are available. We generally enlist the services of resin suppliers to perform this function. Low surface tension aliphatic-based detergents permeate voids fractions developed under stress which promote separation and fissure between adjacent crystalline regions, resulting in cracking.
- Most detergents are ESC agents
- Dilute solutions are more aggressive
- Other ESC agents are: silicone, acetic acid, alcohols, cutting oils, glycols, de-ionized water, certain spices and picklings
Acids and bases react with PE surface to break polymer bonds. Blistering and cracking occurs as penetration continues through “pitted” or “etched” surface cracks. Embrittlement occurs with resulting loss of elongation properties.
Organic solvents are soluble in polyethylene. After permeating surface, they absorb into the amorphous to cause swelling. This results in a plasticizing effect with a loss in stiffness, buckling, “elephant-footing” and sometimes splitting. Solvent failure is technically not ESC