Heavy Duty Shipping Sacks
26
Failure Analysis: Large Format Bags Toward a material neutral specification for heavy duty shipping sacks
-
Upload
thomas-dunn -
Category
Documents
-
view
726 -
download
1
Transcript of Heavy Duty Shipping Sacks
Failure Analysis: Large Format Bags
Toward a material neutral specification for
heavy duty shipping sacks
2
Objectives
• Multi-wall paper bags Specification and Test Methods
• Other Specifications and Test Methods– Heavy duty plastic bags– Other bags
• Suggest a Sustainable Formula for Success– Engineering properties– Performance-based Specifications
Flexpacknology llc
3
Context• Multi-wall paper bags– Mature Industry– Widespread applications– Renewable/degradable resource
• Heavy duty plastic bags– Polymer improvements frequent – Cost driven down-gauging
• Other bags• Textile bags-new materials• Premium performance
Flexpacknology llc
4
Marketplace• Commodities– Little product differentiation– Much packaging conformity
• FMCGs– Market differentiation– Package formats support segments and trends
• Sustainability Trend– Renewable– Compostable– Source reduced
Flexpacknology llc
5
Marketplace
Commodity
Flexpacknology llc
Segmented Market
Woven
Paper Plastic
6
Segmented Marketplace: Pet Food
Flexpacknology llc
$0.69/lb
$0.64/lb
$1.37/lb
7
Specification: Multi-wall paper bags
• “Kraft Paper”
– Min No. Walls– Min (∑ basis weights)– Min avg (of each wall) dry
tensile strength
– Min avg (of each wall) dry tearing strength
– Min avg outerwall (wet strength Kraft) wet tensile strength
• “Extensible heavy duty Kraft Paper”– Min No. Walls– Min (∑ basis weights)– Min avg (of each wall) dry
tensile energy absorption strength (CD)
– Min avg (of each wall) dry tearing strength (MD+CD)
– Min avg outerwall (wet strength Kraft) wet tensile strength (CD)
Flexpacknology llc
UU-S-48 Version F: 1973
Strength
Tear
WetStrength
8
End Drop Test-CD stress
Flexpacknology llc
Force = mass acceleration
Energy = Force x
9
Flat Drop Test-MD stress
Flexpacknology llc
Force = mass acceleration
Energy = Force x
10
Force: kx
Flexpacknology llc
Incr
easi
ng F
orce
(S
tres
s)
X(strain)
Break
Strength
Elongation
11
Energy: Forcex
Flexpacknology llc
Incr
easi
ng F
orce
(S
tres
s)
X(strain)
BreakSteeper: more energy faster
12
Multiwall bags: springs in series
1 1 1Keq K1 K2
Flexpacknology llc
• #4x “Heavy duty shipping sack extensible kraft paper”– = 3 Walls– ∑ basis weights 180– Avg dry tensile energy
absorption* ( 19 CD; 57 MD+TD)
– Min avg dry tearing strength– Min avg outerwall wet tensile
strength (CD)* Foot pounds
13
Strength so far• Bag’s product accelerates until it stops.• That force elongates bag material until
“limit” is exceeded.Greater mass: more forceHigher drop: more force
• If limit is tensile strength, then bag fails!• If not, product remains contained
Flexpacknology llc
14
Tear•Measures the force –perpendicular to the plane of the paper–required to tear through a specified distance
Flexpacknology llc
15
Notched Tear v. Other•Notched tear–Force applied to thickness of the paper (z)
• Initiating tear–At edge (all 3 dimensions)–Puncture• 2 stages
1. Force to bend/break paper’s x-y plane2. Force to break thickness
• Rate /surface area dependent
Flexpacknology llc
16
Tear so far• More complex than strength• How tear initiated is key• Modeling requires understanding use
context and review of risks.
Flexpacknology llc
17
Wet Strength• Strength of paper does vary with moisture
content.• Suggests other environmental factors may
influence strength of other materials.–Temperature–Chemicals
Flexpacknology llc
18
Stress/Strain of Plastic Films
Flexpacknology llc
Cross Direction
Machine Direction
19
Tensile Energy Absorption of OPP
Flexpacknology llc
Cross Direction177%
Machine Direction 100%
20
Textiles• Variability in all dimensions• Design for worst case results in general over
specification
Flexpacknology llc
21
Textile Tear Testing• Variability in all dimensions–Breaking strands–Breaking bonds between strands–Re-orienting strands
Flexpacknology llc
5 mil hexene lldpe film v. minimum
MD 1% Secant (lb)Tensile @ Yield MD (lb)
Tensile @ Yield TD (lb)
Ultimate Tensile MD (lb)
Puncture Break Energy (in-lb)
Ultimate Tensile TD (lb)Break Elongation MD (%)
Break Elongation TD (%)
Elmendorf Tear MD (g)
Elmendorf Tear TD (g)
Dart Drop Method A (g)
Peak Puncture Force (lb)
-400%
100%
600%
5 mil film
Normalized Minumum C6 LLDPE
Abuse Resistance
StrengthStiffness
After Fiscus, 2005
5 mil hexene lldpe film v. minimum
• Commodity resin optimum– Stiffness: marginally adequate– Strength: slightly over specified– Tear/abuse: mostly over specified
• Converter option for trade off– Blend in another resin
• greater stiffness• Maintain other properties • Down gauge to reduce cost
– Coextrusion for structural effect (“I-beam”)
What Next?
• Strength:– Empirical Values-We (paper/plastic/textile) know
what works – Theoretical Values-Finite element analysis
• Tear & Environment:– What we want to protect against
How?
• Industry committee leadership• Compliance labs-compile empirical data base• MSU: theoretical analysis of strength
dynamics.
