injection molding fault and prevention

塑料成型

Injection molding faults in styrene

copolymers and their prevention

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I.Injection molding faults

1. Streaks

1.1 Moisture streaks/splay marks1.2 Burning streaks/silver streaks1.3 Dark streaks due to indrawn air1.3 Color streaks

81012141618202224262830323436

2. Peeling/delamination3. Weld lines

4.Air entrapments/bubble formation5. Sink marks6. Voids

7. Glossy spots or differences in gloss/mat spots8. Microcracks, crazing, stress whitening9.Diesel effect

10. Demolding problems

11. Push marks (“tiger stripes/tiger lines”)12. Record effect13. Short shot

14. Flash formation15. Jetting16. Cold slug17. Warpage

II.Overview: Control of injection molding faults by changing the processing parameters

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Introduction

Plastic moldings produced by the injection moldingprocess are usually long-lived consumer goods

whose suitability for the purpose in question dependsnot only on the properties of the finished part butalso highly on the surface quality.The complex inter-play among molding and mold design,processingconditions for the raw material and the parametersfor the actual process require a great deal of experi-ence for optimum results especially when focusingon the short-term removal of processing faults.Onthe basis of an individual case a decision has to betaken as to whether simple and rapid fault correction(by modifying the processing parameters for example)is possible or whether intervention in the design ofthe parts or the layout of the mold and gating is nec-essary.

In conclusion the surface faults occurring most com-monly in styrene copolymers are noted and described

and recommendations for their removal are set out.

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I.Injection molding faults

1. Streaks

1.1 Moisture streaks/splay marks

1.2 Burning streaks/silver streaks4

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1.1 Moisture streaks

Fig.1.1.1:Streaksdue to an exces-sively high mois-ture content in the

granules

Fig.1.1.2:Sectionthrough a torn streak;transmitted light,magnification 80:1

1.2 Burning streaks

Fig.1.2.1:Silverysurface streaks

Fig.1.2.2:Blackdiscolorations on

a transparent part

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1.3 Dark streaks1.4 Color streaksThe cause for the formation of streaks can be deter-mined in many cases only after costly investigations,especially as the appearance of burning and moisturestreaks is similar.Extensive knowledge of the plastic,the mold design and the processing are indispensablefor overcoming the problem.Before launching expen-sive and time-consuming investigations the followingpoints should be checked in the sequence given andif necessary optimized:

Melt temperatureInjection speed

Residence time in the cylinderBack pressure

Moisture content of granulesMold venting

Cleaning of the plasticising unitScrew recovery speed

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1.3 Dark streaksFig.1.3.1:Dark streaks caused by deposits of thermally damaged materialfrom the hot runnerFig.1.4.1:Color streaks (inhomogeneous distribution of colorant)1.4 Color streaks

Fig.1.4.2:Color streaks due to deposits of material due to “dead spots”in ahot runner system

Fig.1.4.3:

Color streaks in transmitted light; magnification 7:1

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2. Peeling/Delamination3. Weld line8

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2. Peeling/Delamination

Fig.2.1:Peeling and delaminationcaused by strange material in thegranules

Position 1

Position 2

Fig.2.2:Partial cross section through Fig.2.1:(sample thickness 1.5 mm)

Fig.2.3:Position 1

Fig.2.4:Position 2

3. Weld line

Fig.3.1:Weld line notch; vertical illumination,

magnification 11:1

Fig.3.3:Weld line (metallic effectcoloring)

Fig.3.2:Weld line notch; transmitted light,polished,magnification 560:1

Fig.3.4:Color marking in theweld line; vertical illumination,dark field,magnification 52:1

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4. Air entrapments/bubble formation10

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4. Air entrapments (bubble formation)(front elevation and section)

Fig.4.1:Air entrapment

Fig.4.2:Air bubblecut open

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5. Sink marks

6. Voids12

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5. Sink marks

(front elevation and section)

Fig.5.1:Sink markon the visible side(Cause:Materialaccumulation asshown in Fig.5.2)

Fig.5.2:Materialaccumulation(avoidable by suitable recessing)

6. Voids

Fig.6.1:

Voids at the end of the flow path in a cup

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7. Glossy spots or differences in gloss/mat spots14

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7. Glossy spots or differences in gloss/mat spots(Automobile mirror mounting)

Fig.7.1:Differences ingloss on accountof good (mat) orpoor (glossy) moldreproduction

matglossy

Fig.7.2:Highreproductionaccuracy(mat); scan-ning electronmicrograph,magnification50:1Fig.7.3:Highreproductionaccuracy(mat); scan-ning electronmicrograph,magnification200:1

Fig.7.4:Poorreproductionaccuracy(glossy); scan-ning electronmicrograph,magnification

50:1Fig.7.5:Poorreproductionaccuracy(glossy); scan-ning electronmicrograph,magnification200:1

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8. Microcracks, crazing and stress whitening16

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8. Microcracks, crazing and stress whitening

Fig.8.1:Microcracks produced by overloading and deforming under residual pressure

a)

b)

Fig.8.2:

a) Ballpoint pen with stress whitening in the thread region(undercut) made from pure polybutadiene ABS

b) The same ballpoint pen without stress whitening made from Terluran 877 T

Fig.8.3:Terluran 900-series (967 K)

Terluran 877 T

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