Transcript OUTLINE - David Kazmer

VALIDATION OF IN-MOLD
SHRINKAGE SENSOR FOR
DIFFERENT CAVITY THICKNESSES
By
Rahul R. Panchal, Ph.D.
(University of Massachusetts, Lowell)
Dr. David Kazmer, P.E., Ph.D
(University of Massachusetts, Lowell)
OUTLINE






Introduction
Shrinkage Theory
Sensor Research
Analysis
Results
Summary & Future Work
Sensor Research
•
Problem statement – Current closed-loop system
•
Poor estimate of part quality
•
No direct operability and controllability of shrinkage
•
•
Controlling shrinkage for tight tolerance & multi-cavity – “Nightmare” *
Relying on cavity pressure and part weight
•
Effect of other factors on shrinkage???
Screw Position
Screw Velocity
Limit
Switches
Clamp
Force
Hydraulic
Pressure
Behind Screw
Barrel
Temperature
Machine Feedback
Nozzle Pressure
Nozzle Temperature
Process Feedback
Melt Pressure
Melt Temperature
Flow Rate
State Feedback
*Nucleation, Cycle Time, and Properties, Michael Sepe, Injection Molding Magazine, Sept. 04 2007
Shrinkage Theory
In-mold
Post-mold
•
•
Material Type:
•Amorphous vs. Semi-crystalline
•Filler or reinforcement content
•Degree of moisture absorption
Mold/Tool Design:
Part Design:
•Gate locations
•Nominal wall thickness
Climatic Conditions:
•Wall thickness variation
•Room Temperature
•Overall part dimensions
•Relative humidity
•Tool tolerances
•Shrinkage restricting
features
•Air circulation/movement
•Ejector system design
•Draft angles
•Types and sizes of gates
•Runner system
•Mold cooling layout
•Elastic deformation of tool
Processing Conditions:
•Melt & mold temperature and
uniformity
•Filling, packing & holding pressures /
times
•Part temperature at ejection
•Clamp tonnages
•Post mold fixturing/ annealing
Strategies for shrinkage control
•
•
•
•
•
CAE packages
Steel safe mold design
Statistical methods (SPC)
Robust design & processes
Analysis: PvT Behavior
P-v-T behavior of polymers
•
•
•
•
•
Spencer – Gilmore equation
Double Domain Tait equation
1.350
P=0 MPa
P=50 MPa
P=100 MPa
1.300
P=150 MPa
P=200 MPa
1.400
1.070
P=0 MPa
P=50 MPa
P=100 Mpa
Specific Volume (cm^3/g)
Specific Volume (cm^3/g)
1.050
P=150 MPa
1.030
P=200 MPa
1.010
0.990
CVTE 
v
T
s   Teject  Tfinal 
Positive thermal expansion
P-v-T relationship
Increasing
Pressure
0.970
0.950
v o  b1,s  b2,s (T  b5 )
Compressibility,
Increasing
Pressure
B(T )  b3, s exp(b4, s (T  b5 ))
if T > Tt
Specific Volume,
Tg
1.200
CVTE 
v
T
vo b1, m b2, m (T  b5 )
Compressibility,
Tm (Increasing Pressure)
Tg (Increasing Pressure)
0.910
Tt (P) b5 b6 P
1.250
1.150
1.050
20
70
120
170
Temperature (C)
Amorphous
(Polystyrene)
220
270
20
70
RT
M
if T < Tt
Specific Volume,
1.100
0.930
( P  a)( v  b) 
120
170
Temperature (C)
Semi- crystalline
(Polypropylene)
220
270
B(T )  b3, m exp(b4, m (T  b5 ))


P 
   vT (T , P)
v(T , P) v o (T )1  0.0894ln1 
B
(
T
)



vT (T , P)  b7 exp(b8 (T  Tt ( P))  b9 P)
Analysis: PvT Behavior
Shrinkage prediction from P-v-T behavior
•
•
end _ flow
no _ flow
pack
end _ use
1.1
Specific Volume (cc/g)
100
0
300
200
P= 0 MPa
P=100 MPa
P=200 MPa
1.05
n
tio
ca
sti
Pla
1
&
tion
Ejec kage
n
shri
v
100
Filling
Ejection &
shrinkage
Cooling
Packing
1.15
Filling
200
Melt
pressure
(MPa)
•
Plastication
end _ flow
•
v(Tno _ flow , Ppack )
Isotropic
v(T
,P
)
  v(T
,P )  
Anisotropic

  1  (2  a) s  as
 
  v(T
,
P
)

 

s
Limited accuracy / validation
(2  a)
Baaijens, Kennedy, Jansen, Chang, Bushko, Fan &
Kazmer - Thermal stress models + Linear viscoelasticity
s  1 3
Melt
temperature
(C)
•
g
kin
Pa c
Cooling
0.95
0
0
10
Time
20
0.9
0
50
100
150
200
Melt Temperature (C)
250
300
end _ use
2
3
Sensor Design
•
In-mold shrinkage sensor
•
Button-cell type
•
•
Strain gages + piezoelectric elements (optional)
Underneath the ejector pin
•
•
•
Travel of the pin ≈ 0.5 mm, resolution of 0.5 µm
Dimension prediction after post-mold shrinkage
Auxiliary controller for process consistency & control
Sensor Design
•
Shrinkage instrumentation
•
Shrunken
molding
Fmelt pressure
Sensor beneath the ejector pin
•
Actuation of the protruded pin
•
•
Ejector pin
Strain and stress/pressure data
Return of the pin during cooling
•
Shrinkage data
Sensor
Factuation
Signals
(shrinkage & stress)
Analysis: Post-Mold Shrinkage
•
Real time shrinkage analysis – Sensor data
•
•
Bypass the major limitation of P-v-T behavior
Using real time sensor data with the melt & mold
temperature
•
•
Prediction of post-mold shrinkage
Prediction of post-molding annealing conditions
s final  sin  mold  s post  mold
sin  mold 

H
s post  mold   (T ejection Troom )
Validation Experiments
•
Methodology
•
•
16 Run DOE with HIPS
Data Collection
•
•
RJG eDart for machine and
sensors
MATLAB codes for analysis
•
Melt
Temp,
°C
240
240
240
240
240
240
240
240
255
255
255
255
255
255
255
255
Coolant
Temp,
°C
40
40
40
40
60
60
60
60
40
40
40
40
60
60
60
60
Cooling
Time,
sec
10
10
20
20
10
10
20
20
10
10
20
20
10
10
20
20
Hold
Pressure,
%
20
40
20
40
20
40
20
40
20
40
20
40
20
40
20
40
Hold
Time,
sec
15
10
10
15
10
15
15
10
10
15
15
10
15
10
10
15
From Machine – Hydraulic Pressure, Injection
Velocity, Clamping Unit, Other Triggers etc.
From Mold – Shrinkage and Cavity Pressure Sensors
•
Sensor position calibration
Synchronizing RJG data
DOE
RUN
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
RJG Sensor Adaptors
RJG eDART system
2.5 mm
1.5 mm
Results: Cavity Pressure
Shrinkage
 Sensor

End of Flow

pressure
 Cavity
Sensor
Near Gate
Gate
228.25


 

Results: PvT Analysis
Shrinkage

End of Flow
 Sensor
pressure
 Cavity
Sensor


Near Gate
Gate
228.25
 



Tt  



4


Tm  Tw
 T
w

 t. . 2   
 exp



2

 h




Results: In-Mold Shrinkage Sensor
Shrunken
molding
Fmelt pressure
Shrinkage
 Sensor

Ejector pin

pressure
 Cavity
Sensor
End of Flow
Near Gate
Gate
228.25
Sensor



Factuation
Signals
(shrinkage & stress)



Results: Different Thicknesses
T




T for thick cavity = 2.5 mm & T for thin cavity = 1.5 mm

Shrinkage
 Sensor


pressure
 Cavity
Sensor
End of Flow
Gate
228.25



T
 2T

t
h 2
where T  (t , h)






Room Temperature



Post Mold
Shrinkage



Near Gate




20

Results: Different Thicknesses
•
Regression analysis
•
Part thickness to shrinkage from sensor (HIPS)
•
R2 = 0.966 for cavity thickness, T = 1.5 mm
•
R2 = 0.939 for PP for cavity thickness, T =2.5 mm
THIN
THICK
Results: End of Flow Comparison
•
Regression analysis – At end of flow
•
Part thickness at end of flow location
2
• R = 0.821 for shrinkage sensor
2
• R = 0.774 for maximum cavity pressure
Shrinkage Sensor
Cavity Pressure Sensor
Not as good
as shrinkage
sensor
Good
correlation
Results: Multiple Regression
•
Regression analysis -
Part thickness to shrinkage from sensor
R2 = 0.735 for thick cavity with PP – 2.5 mm
Multiple regression
indicates that the
shrinkage sensor provides
correct main effects
compared to PvT models
Asymptotical shrinkage behavior after ejection
• Majority of part shrinkage is in-mold after P = 0
Shrinkage occurs asymptotically depending on the type of
the polymer
• Post-mold dimensions may grow or shrink depending
on the stress state and constraints of the molded parts
•
Shrinkage, %
Asymptotical shrinkage
P<=0
Ejection
Temperature
Time
Room
Summary
In-mold shrinkage is measurable and exceeds postmold shrinkage
• Shrinkage sensor exhibited very good performance
•
Sensor output 2.0 mV/V
• Provided correlation coefficient, R2, on the order of 0.95
• Outperformed cavity pressure correlations at all thickness
measurements
• No wear evidenced in sensor traces
•
Very useful for new polymeric materials, compounds
and alloys
•
•
•
No need for material P-v-T data
Very useful for process validation & quality control
Future Work
•
Experiments with more polymeric materials
•
•
•
Multiple shrinkage sensors
Constrained & unconstrained part geometry
Alternative designs – Multi mode sensor
Operation Notes:
 Plastic part pushes on sensor head
 Aluminum sleeve provides compliant deflection
 Steel rod on contact provides greater stiffness
 PZT ring outputs charge, Q, with stress, s
 Deflection, d, back calculated from joint rod and sleeve
compliance.
Q


Low deflection regime
for pressure measurement
High deflection regime
for shrinkage measurement
Q
•
•
•
•
•
Acknowledgments:
Plastics Engineering Dept., UMass- Lowell
RJG Inc.
Dr. Stephen Johnston
NSF - Grant No. 02-045309
Sukant Tripathy Memorial
100 cycles were tested with Instron
Instron data – stress-strain curve
Linear Region
Linearity and repeatability of sensor output
10
Load removal
9.9
9.8
Sensor Output, mV
Linear Region
9.7
9.6
Sensor diaphragm
deflection under load
9.5
9.4
Instron Probe comes in contact
with the sensor diaphragm
9.3
9.2
9.1
9
0
10
20
30
Time, sec
40
50
60
Capacitance type shrinkage sensor
Capacitance
Element on
Pin Base
L
Where,
A = w x L = 2πaL
d=b–a
Capacitance
Capacitance
Element on Pin
Displacement
1
Energy ,U  CV 2 Work ,W  Force, F x Displaceme nt , d
2
Force, F
 Cavity Pressure 
Area of Movable Pin, Ap