Transcript EPS Expansion & Molding

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Expansion of Expandable
Polystyrene (EPS)
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Expansion Basics
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Heat Is Applied
Beads Expand
Beads Cool
Beads Age
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Expansion - Behind the Scenes
• Heat Is Applied
– Blowing agent(s)
vaporizes
(28oC iso-pentane, 35oC normalpentane, 49oC cyclo-pentane)
– Blowing agent(s)
permeate through the
polymer
(n-pentane<i-pentane<c-pentane)
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Expansion - Behind the Scenes
• Beads Expand
– Polymer/blowing agent matrix reaches it’s glass
transition temperature (Tg)
(about 85oC, varies according to Mw & BA type)
– Polymer chains become fluid
– Internal pressure created by blowing agent
vaporization push apart [unfold] polymer chains,
creating cells
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Expansion - Behind the Scenes
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Expansion - Behind the Scenes
• Beads Expand (cont.)
– Air & steam permeate
into the beads
– As expansion continues,
cell walls become thin
and subjection to high
heat can cause them to
break and rupture
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Expansion - Behind the Scenes
• Beads Expand (cont.)
– Throughout expansion, blowing agent(s)
continue(s) to permeate out of the bead at an
increasing rate [depending on temperature and
thickness of cell walls]
(When permeation rate =/> vaporization rate, expansion ceases)
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Expansion of EPS
(Endotherms, Expansion Rate, & Pentane Loss)
7
6
A
B
5
Endo << D T >> Exo
vs. 40 Mesh Glass
Blowing agents
begin to vaporize
1 ºC
4
Polymer/blowing
agent matrix reaches
its Tg
Cell walls begin to
rupture
3
15X
Expansion
begins after
reaching Tg and
proceeds rapidly
2
Loss of blowing
agent becomes
more rapid as
temperature
increases
10X
5X
25
50
75
100
125
Temperature, ºC
Differential Thermal Analysis
Change in Volume
n-Pentane Loss
1
0
150
% by Weight
C
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Primary Expansion Controls
Density
• Temperature
• Time
Time or Temperature
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Temperature Affects
• Greater the temperature
 Softer the polymer
 Increased expansion rate
• May result in uneven expansion due to inconsistent
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pentane content
bead size
cell structure
raw material “carry over” [extreme cases]
 Increased permeation rate of blowing agent
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Expansion verses Temperature
• <100oC
– Expansion can be sluggish due to stiff polymer
• >120oC
– Polymer is too soft, blowing agent loss is too rapid
• 110-120oC
– Most efficient use of blowing agent, but beads become
sensitive to shrinkage and heat
• 100-110oC
+ Best compromise
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Steam Quality
• Key to Expansion
– Consistent utilities are crucial to achieve consistent
densities with consistent volatile content
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Secondary Expansion Controls
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Volume of Expander
Molecular Weight
Blowing Agent
Bead Size
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Expander Volume
Affects the total heat available to each bead
• Things that can change it
– Drop/charge weight
– Lumps in expander that don’t discharge
– Build up on walls or stir blades
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Molecular Weight
• The lower the molecular weight,
 Increased expandability
 Increased heat sensitivity
 Increased permeation rate of blowing agent
 Increased shrinkage
 Increased collapse
 Structural strength
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Blowing Agent
• Amount
– Higher percentages give greater expandability (to a
point)
– Too high a percentage causes rapid permeation
thus shrinkage, collapse and heat sensitivity
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Blowing Agent
• Type
– The longer the blowing agent stays in the bead,
 Increased expandability
 Reduced shrinkage rate
 Increased prepuff life (from expansion to mold)
– Relative retention
n-pentane < i-pentane < c-pentane
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Blowing Agent
• Degree of Distribution (has a direct affect on
cell size)
 Expandability
 Heat sensitivity
 Structural strength
 R-Factor (insulation properties)
 Permeation rate
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Poor Distribution of Blowing Agent
Blowing Agent
Good Distribution of Blowing Agent
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Bead Size
• The larger the bead, the easier it is to achieve
low densities
– Less surface area for blowing agent to permeate
out of
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Expander Equipment
• Continuous
• Batch
– Wet
– Dry
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Continuous Expanders
• Description
– Material is fed into the bottom of the expansion chamber
where it is subjected to steam under agitation, material
expands and as density decreases, material rises to the top
and out the exit chute.
• Rely on Time & Temperature
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Continuous Expanders
• Main Controls
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Feed rate
Steam pressure (temperature)
Agitation rate
Outlet height
Fresh air introduction (temperature)
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Batch Expanders
• Description
– A pre-weighed quantity of material is dropped (or
charged) into the expansion chamber where either
the expander walls are jacketed with steam (dry)
&/or steam is injected into the chamber (wet). An
agitator keeps material moving. Vacuum, purge or
water inject may be used to stop the expansion.
• Rely on time, temperature &/or volume
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Batch Expanders
• Main Controls
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Steam pressure (temperature)
Volume or height
Time
Charge weight
Vacuum or purge time
Water inject
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Density Check
• Consistency is the Key
• Procedure
– Prepuff is overfilled into a known volume (pretared) container. The container is vibrated or
tapped (vibration is more consistent). A straight
edge is used to strike the top level with the canister.
The canister is reweighed and the density
calculated.
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Expansion - Behind the Scenes
• Beads Age, a.k.a. Maturing or Stabilizing
– Internal moisture [from condensed steam]
permeates out of the bead
– Air permeates into the bead until internal and
external pressures equal
– Blowing agent(s) continue(s) to permeate out of
the bead
(n-pentane>i-pentane>c-pentane)
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Why Age Prepuff?
• Foam becomes more resilient after it’s
stabilized
– Internal vacuum makes beads susceptible to deformation
• Reduces blowing agent levels
– Too high a blowing agent level leads to excessive cool
times and heat sensitivity during molding
• Dry prepuff
– Improves ease of transportation
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Volatile Content on Aging
% Volatiles - X185 RX3859
7.00%
6.00%
% by Volume
5.00%
4.00%
3.00%
R2 = 0.8749
2.00%
1.00%
0.00%
0:00
2:24
4:48
7:12
9:36
12:00
14:24
Time from Expansion
% Volatiles
Poly. (% Volatiles)
16:48
19:12
21:36
0:00
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Aging
• Key
– A consistent environment is important to provide
prepuff to mold with a consistent volatile content
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Aging Time Controls
• Environment
– Air flow Time
– Temperature Time
•  Density Time
•  Bead Size Time
•  Blowing Agent Type(boiling point &
molecular complexity) Time
•  Polymer Mw Time
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Expansion - Troubleshooting
• High Density
– Insufficient steam pressure/temperature
• check traps, valves, accumulator pressure
• steam flow restricted
Note: by monitoring both steam pressure and steam temperature, you’ll know your
steam quality.
– Insufficient steam times
– Too high a throughput through expander
(continuous) gives raw material carry-over
– Wet material
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Expansion - Troubleshooting
• High Density (continued)
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Collapsed or over expanded prepuff
Low blowing agent content in raw EPS
Additive problem (block and shape EPS grades)
Increased drop weight
Electric eye level too low
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Expansion - Troubleshooting
• Low Density
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High steam pressure/temperature
Longer steam times
Reduced drop/charge weight
Wet material
Over dried material
High blowing agent content
Surface additives (block & shape grade EPS)
Electric eye level too high
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Expansion - Troubleshooting
• Density Fluctuations
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Inconsistent steam pressure/temperature
Inconsistent steam time
Erratic drop/charge weights
Inconsistent measuring techniques
Inconsistent blowing agent content
High static (affects electric eye)
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Expansion - Troubleshooting
• Density Fluctuations (continued)
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Purge valve sticking
Vacuum problems
Inconsistent water inject volume
Poor additive distribution (block & shape grade
EPS)
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Expansion - Troubleshooting
• Wet Prepuff
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Common on expander start up
Wet steam
Purge valve or vacuum not working
Poor air flow through fluid bed dryer
Too much material in the fluid bed dryer
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Expansion - Troubleshooting
• Bead Collapse
– Over-expanded
• Excessive steam pressure/temperature
• Excessive steam time
– Too high an expansion rate
– Thermal shock after expansion
– Blades too close to walls or bottom of expansion
chamber
– Wrong additive package (block and shape grade
EPS)
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Expansion - Troubleshooting
• Lumping
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Too much moisture (condensate)
Inadequate stirring
Excessive steam pressure/temperature
Anti-lumping agent level too low
Hot spots in expander
Excessive steam time
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Expansion - Troubleshooting
• Irregular Prepuff (size/appearance)
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Poor or irregular steam flow
Hot spots in expander
Insufficient time in expander
Contamination of prepuff in raw EPS (double
pass)
– Irregular raw EPS