Identifying and Correcting Problems in Blown Film

Coextrusion to Reduce Scrap and DowntimeAuthored by:

Dr. Karen XiaoExtrusion Development MgrBrampton Engineering

Common issuesCommon issues

Haze linesMelt stabilityGelsInterfacial instabilityPoor gauge

Contributing Factors Contributing Factors ---- Haze LinesHaze Lines

Processing temperaturesTemperature gradientScrew and barrel conditionsDie design

Considerations: Melt stabilityConsiderations: Melt stability

Resin selectionsExtruder sizingScrew design – Good melt temperature controlProduct layflat and blowup ratios

GelsGels

Identify type of gels (degraded or unmelts)Which layer(s) exhibit gels?Resin contaminationScrew and die design

Gels Gels -- UnmeltsUnmelts

Resin sourcesProcessing temperaturesResin blend compatibilityScrew design

Gels Gels -- Degraded GelsDegraded Gels

Resin contaminationProcessing temperaturesScrew and barrel conditions

Interfacial instabilityInterfacial instability

Zig-zag – viscosity mismatch

Interfacial instabilityInterfacial instability

Wave type – elongational viscosity differences between layers

Interface instabilityInterface instability

Identify the interface problem –possibly more than one locationChange only one parameter in one layer at a timeLook for layer percentage change and a temperature changeCan be minimized with proper resin selections and die design

Poor GaugePoor Gauge

Temperatures and outputsScrew and barrel conditionsDie designIBC and air ring design and conditionsLocal ambient temperatureHardware failure (Heater bands, T/C’s)

Case Study: 7 layer blown film lineCase Study: 7 layer blown film line

Stacked pancake die – inner layer at bottom of stack, outer layer at top of stackTrouble free operation for a few yearsRoutine die cleaningSudden appearance of carbon buildup on die, poor gauge – all after die cleaning

Information RequiredInformation Required

Recipe change? New structures?What materials in each layer?Layer percentage, outputProcessing temperatures of each layer

- setpoints and actuals if availableScrew RPM of each layerSignificant events –e.g. power outage, extruder failure

Assessment/EvaluationAssessment/Evaluation

Calculate shear rates and shear stresses- material flow, potential degradation location- premature hardware failure, e.g. screw and barrel wear or heater band failure

Problem identifiedProblem identified

New recipeOutput rate calculations

Output in one layer too lowShear rates 4-6sec –1

Residence time for that layer too longCarbon build up Change in flow behaviour

Action:ResultsAction:Results

Increase output of specific layer

Better gauge uniformityFaster changeover timesReduced downtime – less cleaning time

Case Study: 3Case Study: 3--layer Blown Film Linelayer Blown Film Line

Spiral mandrel die – 3-layerStarting gauge was too high at 12%, 2-sigmaGauge profile shows porting effect

Information RequiredInformation Required

Resin informationProcess conditions: output and temperaturesMelt temperature measurements

Assessment/EvaluationAssessment/Evaluation

To make sure that the die is operating under optimal conditions: check output rates and melt temperaturesMake sure that the temperatures are set up properly from the extruders to the die

Problem identifiedProblem identified

The die temperature setting was too low Caused a differential temperature difference in the melt stream in the diePorting effect was caused which adversely affected gauge

Action:ResultsAction:Results

Match the die temperature setpoint to melt temperatures coming off the extruders

Improved gauge uniformity from 12% to 7%, 2-sigmaMinimized porting effect

Common issues in coextrusionCommon issues in coextrusion

Processing Temperatures

Screw and Barrel

ConditionsResin

Selection Die

DesignScrew Design

Air Ring & IBC

ConditionsGels X X X X X Unmelts X X X Gauge X X X X Interfacial Instability X X X

Haze Lines X X X X X

Melt Stability X X X X

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PRESENTED BY

Jim StobieVP - SalesBrampton Engineeringjstobie@be-ca.com