Quality Improvement of a Plastic Injection Molder
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Quality Improvement of a Plastic Injection Molder
Iowa State UniversityMarch 19,1999
Kevin Dodd
Salvador Neaves
Kendall Ney
Matt Raine
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Why An Injection Molding Corporation?
• Large industrial facility
• Injection Molding demands precision• Injection molding is the wave of the
future
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Objectives• Attempt a logical statistical quality analysis in
a real world situation
• Provide a useful assessment of the variability in an injection molding process
• Characterize the current process performance
• Work to improve unsatisfactory performance
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What ?•Analyze the variability of pre-form
weights for a 48-cavity injection molding
machine
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Inside an Injection Molding Plant
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Initial Analysis• We began by benchmarking the
current process to determine how the machine is currently running
• Based on these findings and past performance, four variables with the greatest potential impact on pre-form weight were chosen for an experiment
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Factors Analyzed in an InjectionMolding Process Experiment
•Hold Time (2.5 - 7sec)
•Hold Pressure (600 - 1400psi)
•Injection Time (1.5 - 3.5sec)
•Injection Pressure (60 - 100 % of 2800psi)
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Data Collection• Five cavities were selected to represent
performance throughout the mold
3 37
18
9 47
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Data Collection• Keeping these five cavities constant, the
mean and variability across cavities could be observed
• Pre-forms were taken from these same cavities for each of 33 different set-ups and weighed on the same scale
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Data Collection
• All 16 combinations of High and Low values
• 1 All-nominal run (combination of Medium values)
• 16 combinations of High-Medium-Low values
• Total number of set-ups = 33
Set-ups
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Data Collection
• 33 set-ups X 5 runs/set-up X 48 pre-forms/run
= 7920 pre-forms manufactured
• 33 set-ups X 5 runs/set-up X 5 pre-forms/run
= 825 pre-forms weighed
Total pre-forms analyzed
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Experimental Matrix(Partial)
Setup # Hold Pressure Hold Time Injection Time Fill Pressure Cavity 3 Cavity 9 Cavity 18 Cavity 37 Cavity 47 Y-bar Std Dev
1-1 1400 2.5 3.5 60 23.36 23.36 23.19 23.30 23.38 23.32 0.0776
1-2 1400 2.5 3.5 60 23.37 23.36 23.20 23.32 23.39 23.33 0.0760
1-3 1400 2.5 3.5 60 23.33 23.33 23.28 23.32 23.37 23.33 0.0321
1-4 1400 2.5 3.5 60 23.34 23.34 23.29 23.34 23.36 23.33 0.0261
1-5 1400 2.5 3.5 60 23.35 23.35 23.23 23.32 23.37 23.32 0.0555
2-1 600 7 3.5 60 23.56 23.56 23.57 23.55 23.59 23.57 0.0152
2-2 600 7 3.5 60 23.56 23.55 23.57 23.55 23.60 23.57 0.0207
2-3 600 7 3.5 60 23.56 23.56 23.58 23.55 23.60 23.57 0.0200
2-4 600 7 3.5 60 23.56 23.55 23.57 23.55 23.59 23.56 0.0167
2-5 600 7 3.5 60 23.55 23.55 23.58 23.54 23.59 23.56 0.0217
EXPERIMENTAL MATRIX PRE-FORM WEIGHT (grams) SUMMARIES
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Data Analysis
Tools Used in the analysis
•Minitab “DOE” quadratic regression used to identify the most influential variables and model response
•Minitab for contour and surface plots
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Example Regression AnalysisResponse Surface Regression
Estimated Regression Coefficients for Y-bar Term Coefficients T PConstant 22.2283 387.347 0.000Hold Pressure 0.0010 24.738 0.000Hold Time 0.2631 36.314 0.000Injection -0.1447 -3.327 0.001Fill Pre -0.0046 -3.834 0.000Hold Pre*Hold Pre -0.0000 -16.371 0.000Hold Tim*Hold Tim -0.0116 -17.368 0.000Injectio*Injectio 0.0303 3.614 0.000Fill Pre*Fill Pre 0.0000 2.263 0.025Hold Pre*Hold Tim -0.0000 -14.852 0.000
S = 0.01406 R-Sq = 99.7% R-Sq(adj) = 99.7%
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Summary of QuadraticRegression Analyses
•All 4 variables as predictors of Y-bar
R-Sqrd = 99.7%
•Hold Time - Hold Pressure as predictors of Y-bar
R-Sqrd = 97.7%
•All 4 variables as predictors of log(StDev)
R-Sqrd = 69.3%
•Hold Pressure as predictor of log(StDev)
R-Sqrd = 53.5%
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Contour Plot
23.023.423.6
14001300120011001000900800700600
7
6
5
4
3
2
Hold Pressure (psi)
Hol
d T
ime
(sec
)
Contour Plot for Predicted Y-bar
23.6grams
23.4grams(Target Value)
23.0grams
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Fitted Regression Equation for Log (StDev)
14001300120011001000900800700600
-3.1
-3.2
-3.3
-3.4
-3.5
-3.6
-3.7
-3.8
-3.9
-4.0
Hold Pressure (psi)
Pre
dict
ed
Lo
g(S
tDev
)
(measuring within-die variability)
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Results• From the contour plot for Y-bar we are able to choose values for Hold
Time and Hold Pressure to produce an ideal mean weight (23.4gr)
• From the regression analysis for Log(StDev) we found that within die variability is minimum around 880psi Hold Pressure
• Predicted Log(StDev) for Hold Pressure in the range 600 - 1160psi is not substantially larger the minimum possible (minimum is at 880psi)
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Results• The company prefers a small Hold Time, so for a target
value of 23.4 grams, using the contour plot we recommend:
Hold Pressure = 1140psi
Hold Time = 3.95sec
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Initial Verification Study
•Benchmarking: Average weight = 23.280gr•Verification Run: Average weight = 23.405gr
Run # Hold Pressure Hold Time Injection Time Fill Pressure Cavity 3 Cavity 9 Cavity 18 Cavity 37 Cavity 47 Y-bar Std Dev
1 1140 3.95 1.99 66 23.40 23.39 23.39 23.39 23.44 23.40 0.02172 1140 3.95 1.99 66 23.41 23.40 23.47 23.39 23.45 23.42 0.03443 1140 3.95 1.99 66 23.40 23.38 23.41 23.39 23.44 23.40 0.02304 1140 3.95 1.99 66 23.40 23.39 23.40 23.40 23.44 23.41 0.0195
5 1140 3.95 1.99 66 23.41 23.40 23.41 23.39 23.44 23.41 0.01876 1140 3.95 1.99 66 23.40 23.39 23.39 23.39 23.43 23.40 0.01737 1140 3.95 1.99 66 23.40 23.38 23.40 23.40 23.43 23.40 0.01798 1140 3.95 1.99 66 23.41 23.40 23.40 23.39 23.44 23.41 0.01929 1140 3.95 1.99 66 23.41 23.39 23.39 23.39 23.43 23.40 0.0179
10 1140 3.95 1.99 66 23.40 23.39 23.39 23.39 23.43 23.40 0.0173
VERIFICATION MATRIX PRE-FORM WEIGHT (grams) SUMMARIES
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Further Verification
• Compare historical machine output against weights produced using set-up taken from the contour plot (all 48 cavities)
(Routine process monitoring done on the basis of 6 randomly selected pre-forms each hour)
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Histogram of Historical Data
23.523.423.323.223.1
40
30
20
10
0
Pre-Form Weight (grams)
Fre
quen
cy
Histogram of Historical Production Data
Y-bar = 23.28gr.
StDev = 0.035gr.
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Histogram of Current Data
23.523.423.323.223.1
40
30
20
10
0
Pre-Form Weight (grams)
Fre
quen
cy
Histogram of Current Production Data
Y-bar = 23.38gr.
StDev = 0.037gr.
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Comparison Histograms
23.523.423.323.223.1
40
30
20
10
0
Pre-Form Weight (grams)
Fre
quency
Histogram of Current Production Data
23.523.423.323.223.1
40
30
20
10
0
Pre-Form Weight (grams)
Fre
quency
Histogram of Historical Production Data
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Recommendations• We suggest the company use contour plots
as a guide to setting the values of the Hold Time and Hold pressure and move Hold Pressure toward 880psi to whatever extent is possible (and is consistent with low cycle time goals)
-These values will help provide the company with an optimal set-up for pre-form weights near 23.4gr
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??? Questions ???