4/25/2016
Understanding
Amorphous and Crystalline
Molding
Tuan Dao
Molding Workshop
SoCal SPE Section
April 21, 2016
Amorphous vs
Crystalline
Similarities and Differences
1
4/25/2016
Crystalline vs amorphous polymers
Similarities

Broad range of commercial formulation
– Amorphous: ABS, PC, PS, PMMA, etc
– Crystalline: PA, PE, PP, PET, PBT, etc..

Multiple Processing Techniques
– Injection, blow molding, extrusion

Environmental sensitivities
– Upper and lower use temperatures.
– Weathering, mechanical and electrical
properties
Crystalline vs amorphous polymers
Differences

Molecular Interaction of the molecular chain
– Amorphous: low order in solid phase
– Crystalline: High order in solid phase

Response to temperature changes
– Amorphous: Tg only
– Crystalline: Tg and Tm

Solvent resistance
– Crystalline polymers are generally better
2
4/25/2016
Plastic Processing
Why are different
processing conditions needed
to mold crystalline and
amorphous polymers?
Plastic Processing

The density (volume) of amorphous and crystalline
polymers change at a different rate under the
influence of changing temperatures and pressures.
Mold Fill and Pack Parameters

Resin viscosity at processing temperatures and
pressures
Mold Fill

The effect of resin modulus vs. temperature are
different
Ejectability
3
4/25/2016
Thermoplastic Polymer Types

Amorphous
Structure
A polymer characterized by random
entanglement of the individual polymer chains
Amorphous Thermoplastics

Polycarbonate (PC)

Polyphenylene Oxide (PPO)

Acrylonitrile Butadiene Styrene (ABS)

Polysulfone (PSU)

Polyetherimide (PEI)

Polystyrene (PS)

Polymethyl Methacrylate (PMMA)

Polyvinyl Chloride (PVC)
4
4/25/2016
Thermoplastic Polymer Types

SemiCrystalline
A polymer structure characterized by the
combination of amorphous & crystalline
arrangements
Semi-Crystalline Thermoplastics

Polyester (PET or PBT)

Polyamide (PA), nylon

Polyoxymethylene (POM), acetal

Polyphenylene Sulfide (PPS)

Polyethylene (PE)

Polypropylene (PP)

Polytetrafluoroethylene (PTFE), Teflon
5
4/25/2016
Density vs. Specific Volume

Density = Mass/Volume (g/cc, oz/cu.ft….)

Inverse Relationship
Density = 1 / Specific Volume
Specific Volume = 1 / Density

Density important to part weight

Specific volume important to molding conditions
Density, g /cc
Density vs. Temperature at
Atmospheric Pressure
Amorphous
Crystalline
Tg C
Tg ATm C
Temperature
Ta
6
4/25/2016
Modulus vs. Temperature
Crystalline and Amorphous Polymers
Modulus
High Crystallinity
Rigid
Amorphous
Low Crystallinity
TgC
TgA
TmC
Temperature
PVT Diagrams

Plot of specific volume vs. temperature at
different pressures

Easy to find
– Tg – Glass transition temperature
– Tm – Melting point
7
4/25/2016
PVT Data on Nylon 6,6 and PC
1.10
1.05
Nylon 6,6
Specific Volume, cc/g
Tm
1.00
Tg
0.95
PC
0.90
0.85
0.80
0
100
300
200
400
600
500
700
Temperature, °F
PVT Data on Nylon 6,6 and PC
1.10
P = 0
Specific Volume, cc/g
1.05
1.00
P = 15,000
Nylon 6,6
0.95
P = 0
0.90
P = 15,000
0.85
PC
0.80
0
100
200
300
400
500
600
700
Temperature, °F
8
4/25/2016
PVT Diagrams

Plot of specific volume vs. temperature at different pressures

Easy to find
– Tg – Glass transition temperature
– Tm – Melting point

Easy to see how
– Volume increases with increasing temperature
– Volume decreases with increasing pressure
– Crystalline polymers undergo a rapid volume change
at Tm which is missing in amorphous polymers
– Volume changes between RT and processing
temperatures are:
10 – 15% amorphous polymers
20 – 25% crystalline polymers
Amorphous PVT Diagram
Effects on Molding Rules
Injection Pressure
Pressure
Transducer
After filling time:
Constant volume
Time
Means:
• No movement through
gate
• Injection pressure led
by cavity pressure
(via transducer)
9
4/25/2016
Amorphous Polymer
Post-Molding Deformation
Internal stresses
Deformation
or stress cracking
Molding Amorphous Polymers

Overpacking is a major concern
– Parts stick in mold
– Parts crack during ejection
– Residual internal stress

Optimum molding conditions
– Inject using high pressure
– Pack pressure should decrease
with time – constant volume
– High mold temperatures reduce
internal stress
10
4/25/2016
Crystalline PVT Diagram
Melting
0
• During melting process
Specific Volume
(solid to liquid)
volume x 16%
Temperature
Crystalline PVT Diagram
Injection – Crystallization
0
Specific Volume
11,600
psi
Crystallization
• Shrinkage by ~14%;
Voids created have
to be filled with liquid
polymer
• Crystallization under
constant pressure
Temperature
11
4/25/2016
Important Implications
During the solidification process after
dynamic filling

The hold pressure is decreased with time for
amorphous but is maintained constant for
crystalline polymers.

The flow through the gate is stopped for amorphous
but it continues until the end of the crystallization
for crystalline polymers.
Crystallization PVT Diagram
Allegory
Room
Door
Corridor
12
4/25/2016
Crystallization PVT Diagram
Allegory
Filling
Crystallization PVT Diagram
Allegory
Crystallizationorder
necessary to put more
material in during
crystallization to avoid
internal voids.
Door and corridor have
to be kept open.
13
4/25/2016
Crystalline Polymer
Post-Molding Deformation
Cold mold
Bad crystallization
Post shrinkage
recrystallization with
time and temperature
Deformation
Controlling Crystallization
During Molding

Mold Temperature
– The higher the mold temperature,
the higher the crystallinity

Pressure
– Increases rate of crystallinity

Stress During Crystallization
– Can produce orientation
14
4/25/2016
Molding Crystalline Polymers

Underpacking is a major concern
– Sinks and voids
– Low part weight

Incomplete crystallization is a major concern
– Warpage
– Consistent shrinkage

Optimum molding conditons
– Inject using moderate pressure
– Pack with same pressure
– High mold temperatures – aid crystallization
Part Weight vs Packing Time
15
4/25/2016
Conclusions

Properties depend on structure
– Amorphous
– Crystalline

Processing depends on structure
– Amorphous – small volume change
with temperature: use constant
volume rule
– Crystalline – large volume change
with temperature: use constant
pressure rule
Thank You !
16