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Comparison of Switchover Methods for Injection Molding. David O. Kazmer, Sugany Velusamy, Sarah Westerdale, and Stephen Johnston Plastics Engineering Department University of Massachusetts, Lowell Priamus Users Group Meeting September 30 th , 2008. Agenda. Motivation

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Comparison of Switchover Methods for Injection Molding

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Comparison of switchover methods for injection molding l.jpg

Comparison of Switchover Methods for Injection Molding

David O. Kazmer, Sugany Velusamy, Sarah Westerdale, and Stephen Johnston

Plastics Engineering Department

University of Massachusetts, Lowell

Priamus Users Group Meeting

September 30th, 2008


Agenda l.jpg

Agenda

  • Motivation

    • Manufacturing competitiveness

    • Characteristics of highly productive molders

  • Switchover Methods

    • Overview

    • Experimental Setup

    • Results

    • Conclusions


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Is U.S. Manufacturing in Decline?


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Is U.S. Manufacturing in Decline?


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U.S. Manufacturing Productivity


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U.S. Manufacturing Productivity

Where is it going tocome from?

  • Manufacturers need 1.5% annual productivity gains to remain competitive


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Characteristics of Highly Competitive Molders

  • Highly systematized

    • Excellent layout

    • Consistent and often uni-directional flow of materials

    • Uniform internal planning processes

    • Uniform quality control processes.

  • Many highly productive facilities use only one primary supplier of plastics machinery.


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Characteristics of Highly Competitive Molders

  • Highly utilized

    • 24 x 7 operation

    • 90% plus machine utilization

  • Steady state strategy

    • Use fewer and better machines running continuously rather than more machines running fewer shifts


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Characteristics of Highly Competitive Molders

  • High yields

    • 95% typical

    • 99.8% not necessary

  • High quality assurance

    • Automatic: in-mold systems, vision, poka-yoke

    • Conservative rules to contain defects

      • Better to automatically reject 10 good parts than accept one bad part


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Characteristics of Highly Competitive Molders

  • Industry sector andapplication focus

    • Connectors

    • Gears

    • Syringes

  • Focus provides

    • Advanced application-specific knowledge

    • Market commitment and technology investment


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Obsolete vs. Competitive

  • Number of machines

ObsoleteCompetitive


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Obsolete vs. Competitive

  • Number of workers

ObsoleteCompetitive


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Obsolete vs. Competitive

  • Number of supervisors

ObsoleteCompetitive


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Obsolete vs. Competitive

  • Plant size

ObsoleteCompetitive


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Obsolete vs. Competitive

  • Energy usage

ObsoleteCompetitive


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U.S. Manufacturing Productivity

  • Manufacturers need 1.5% annual productivity gains to remain competitive


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Agenda

Motivation

Manufacturing competitiveness

Attributes of highly productive molders

Switchover Methods

Overview

Experimental Setup

Results

Conclusions


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Overview: Switchover Concept

Switchover is the point at which the filling phase ends and packing phase starts

From a controls perspective, there is a switch in the system’s boundary conditions and stiffness

Variances cause:

Dimensional errors

Part weightvariations

Back flow

Pressure

Velocity

Switchover

time

time


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Overview:Switchover Methods

Various methods for switchover:

Screw Position*

Injection Time

Injection Pressure

Cavity Pressure

Cavity Temperature

Nozzle Pressure

Tie Bar Deflection

Other studies have been conducted.

This study is more comprehensive with respect to number of methods and also long term variation.

Filling Stage

Packing Stage


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Experimental Setup

  • Molding Machine

    • 50 metric ton All Electric Machine

    • Make: Ferromatik Milacron

    • Model: Electra 50 Evolution

  • Plastic Material:

    • AMOCO Polypropylene

    • Grade 10-3434


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Process Monitoring & Control

  • Extremely well instrumented machine & mold

    • Screw position transducer

    • Nozzle pressure transducer

    • Ram load transducer

    • 3 barrel thermocouples

    • 4 in-mold pressure transducers

    • 2 in-mold temperature sensors

    • Nozzle infrared pyrometer

    • In-mold infrared pyrometer

    • PRIAMUS DAQ8102 acquisition

  • Custom machine override circuit

    • Internal or external voltage signal triggers the machine for switchover


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Seven Switchover Methods

Machine Controlled

Screw Position

Injection Pressure

Injection Time

Externally Controlled

Nozzle pressure

Runner Pressure

Tensile Cavity Pressure

Cavity Temperature

Six Measured Attributes

Impact Thickness (mm)

Impact Weight (g)

Impact Width (mm)

Tensile Thickness (mm)

Tensile Weight (g)

Tensile Width (mm)

Switchover Methods & Measured Attributes


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Single Cycle: Screw Position, Nozzle Pressure, & Cavity Pressure


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10 Consecutive Cycles


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100 consecutive molding cycles were monitored & data acquired

The average & standard deviation was calculated to measure of short term variation

Molding Machine Statistical Characterization


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Switchover values for each method were determined to provide same part weight

Switchover Settings


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DOE performed to impose long term variation

Design of Experiments (DOE)


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Analysis

  • The 90 cycle DOE was repeated for each of the seven switchover conditions

  • Parts weighed & dimensions measured

  • The data was analyzed in Matlab to provide:

    • Individual traces for each of 630 cycles

    • Overlaid traces for all cycles in a DOE run

    • Overlaid traces for all cycles in a switchover method

    • Regression coefficients & main effects plots


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90 Cycles across the DOE for Ram Position (Conventional) Switchover

Position

Switchover


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Main Effects on Impact Thicknessfor Ram Position Switchover

Good process robustness


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90 Cycles across the DOE for Filling Time Switchover

Time Switchover


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Main Effects on Impact Thicknessfor Filling Time Switchover

Very poor process robustness


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90 Cycles across the DOE for Cavity Pressure Switchover

Pressure

Switchover


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Main Effects on Impact Thicknessfor Cavity Pressure Switchover

Good process robustness


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90 Cycles across the DOE for Cavity Temperature Switchover

Temperature

Switchover


Main effects on impact thickness cavity temperature switchover l.jpg

Main Effects on Impact Thickness Cavity Temperature Switchover

Best process robustness


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Coefficient of Variation COV = σ / µ

Different switchovers are best for different attributes


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Switchover Performance:Short vs. Long Run Variation

Short Run Variation (%)

More robust

Long Run Variation (%)


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Switchover Performance:Long-Run Variation

Screw position

Injection time

Machine pressure

Nozzle pressure

Runner pressure

Cavity pressure

Cavity temperature


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Conclusions

Cavity temperature provided the most robustness against changes the process settings.

Place the sensor near but not at the very end of flow due to small control system delays (speed matters)

Cavity pressure provided reasonable switchover control but had susceptibility to changes in melt temperature and velocity.

Position control provided reasonable control but roughly twice the variation of cavity temperature.

Injection time is the least reproducible method for the transfer from fill to pack, with literally 10 times the variation of temperature control.


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Conclusions

  • Measured consistency is much better than SPI guidelines of 0.2%

    • Response time of the molding machine, controller and ram velocity are important to process repeatability.

  • Weight and thickness show higher COV than length and should be used for QC

In-mold instrumentation is vital to achieving process robustness, automatic quality control, and competitiveness.


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Acknowledgements

National Science Foundation grant numberDMI-0428366/0428669

Priamus System Technologies


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