18

Click here to load reader

FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

  • Upload
    s-y

  • View
    212

  • Download
    0

Embed Size (px)

Citation preview

Page 1: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

This article was downloaded by: [Erciyes University]On: 21 December 2014, At: 18:09Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

Materials and Manufacturing ProcessesPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lmmp20

FRACTURE TYPES AND THICKNESS DISTRIBUTIONIN SUPERPLASTIC SHEETS FORMED WITH PLASTICINJECTION MOLDINGS. H. Parng a & S. Y. Yang ba Department of Mechanical Engineering , National Taiwan University , No. 1, Sec. 4,Roosevelt Road, Taipei, 10617, Taiwan Republic of Chinab Department of Mechanical Engineering , National Taiwan University , No. 1, Sec. 4,Roosevelt Road, Taipei, 10617, Taiwan Republic of ChinaPublished online: 07 Feb 2007.

To cite this article: S. H. Parng & S. Y. Yang (2001) FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTICSHEETS FORMED WITH PLASTIC INJECTION MOLDING, Materials and Manufacturing Processes, 16:4, 503-518, DOI: 10.1081/AMP-100108523

To link to this article: http://dx.doi.org/10.1081/AMP-100108523

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. Taylor and Francis shallnot be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

MATERIALS AND MANUFACTURING PROCESSES, 16(4), 503–518 (2001)

FRACTURE TYPES AND THICKNESSDISTRIBUTION IN SUPERPLASTIC SHEETS

FORMED WITH PLASTIC INJECTION MOLDING

S. H. Parng and S. Y. Yang*

Department of Mechanical Engineering, National Taiwan University,No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan 10617,

Republic of China

ABSTRACT

This paper investigated the fracture types and thickness ratio distribution insuperplastic Zn–22% Al sheets formed during a hybrid process combiningsuperplastic forming with plastic injection molding. Three types of sheet frac-tures (edge crack, central crack, and combined crack) were observed. Theeffects of using this approach on sheet molding and fracture window for vari-ous parameters, including melt temperature, injection pressure, and mold tem-perature, were investigated. They are presented and discussed as they relateto molding area and various fracture types. Central cracks occurred when su-perplastic sheets were formed by injection molding at higher melt temperature,whereas edge cracks occurred at higher injection pressure. When melt flowwas parallel to the sheet rolling direction, areas of edge crack were enlarged.The sheet thickness ratio distribution was obtained for various injection param-eters and rib depths. Observation of sheet thickness distribution for variationparameters, and the tendency for fracture can be generalized.

Key Words: Electromagnetic interference (EMI); Fracture types; Housingsfor electronics; Injection molding; Moldability; Molding area; Naturalelongation strain; Natural thickness strain; Processing conditions; Rib design;Rolling direction; Superplastic forming; Superplastic injection molding;Thickness ratio; Zn–22% Al sheet.

* Corresponding author. Fax: 886-2-83695574; E-mail: [email protected]

503

Copyright 2001 by Marcel Dekker, Inc. www.dekker.com

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 3: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

504 PARNG AND YANG

1.0 INTRODUCTION

Metallic materials provide physical protection and shield electrical compo-nents from electromagnetic interference (EMI). Magnesium has become a promis-ing material because it is lightweight and recyclable, and shields against EMI.However, porosity is unavoidable in die cast products (1). Kimura et al. (2) devel-oped a process to manufacture a hybrid housing in which resin is injected withaluminum adhered to it. This combined aluminum–resin housing offers excellentheat dissipation coupled with reasonable weight. However, the main restrictionon wider applications of this method is that the metal allows little deformationduring the process. Most features, such as ribs and bosses in housings, cannot bemolded using this design.

This research used superplastic material as a housing cover. Superplasticmaterial exhibits excellent elongation under proper conditions. Conventionally,superplastic sheets are formed by blow at very low strain rates of about 10�3 –10�1 sec�1. Forming superplastic sheets by injecting plastic faces presents newchallenges because of limited deformation and high strain rates. The superplasticmaterials formed by injection molding are called superplastic injection molding(SIM) in this paper. The SIM process is illustrated in Figure 1 in three stages: (i)superplastic sheets are prepared and fixed in the mold; (ii) the molds are closed,and superplastic sheets are preformed; and (iii) plastic is then injected into themold to drive complete superplastic sheet filling.

The products provide a continuous metal enclosure covering the plastic sur-faces, either on the outside or on the inside, which protects against EMI. A profile

Figure 1. Schematic showing the process of superplastic injection molding (SIM), a hybrid processcombining superplastic sheet forming with injection molding to mold plastic parts with metal sur-faces.

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 4: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

SUPERPLASTIC SHEETS FORMED WITH INJECTION MOLDING 505

conformity to the mold is used for quality inspection in the process. For superplas-tic thin sheet forming, Chandra et al. (3) compared the theoretical membrane ele-ment method with experimental data for several sheet thicknesses and for variousgas pressures for complex shapes. In addition, the grain size of Zn–22% Al forthe different procedures was determined (4). Fracture of cavitated superplasticmaterials is considered to be a process of neck development in the ligament be-tween two cavities. Mohamed (5) indicated that localized straining leads to cavityinterlinkage followed by failure of the overall material.

In the region of thin section, linking of the microvoids by shear bands promotesthe final fracture of the material (6). Large superplastic elongations of more than1000% were reported for superplastic IN9021 alloy deformed at extremely highstrain rates of 10 – 100 sec�1 and at a temperature of 823 K (7). Recently, themaximum tensile elongation of Zn–0.3%Al was obtained to be 1400% at roomtemperature at a strain rate of 2 � 10�4 sec�1 (8). However, few reports have dis-cussed types of superplastic fracture of thin sheet under high strain rate. In thisstudy, superplastic sheets were formed instantly by melting plastic at high strainrates. Therefore, the thin sheet fracture limit for various operation parameters is quitedifferent from the gas forming process. In this research, various complicated fracturemodels and operation windows were applied to form superplastic thin sheets.

To establish the sheet deformation profile and to investigate sheet fracturetypes during the process, the distribution of thickness ratio must be examined. Anumber of workers (9–11) have studied, both theoretically and experimentally,superplastic sheet forming by means of hydrostatic pressure. However, formingmetal sheets by means of plastic injection molding is complicated because of theuse of two entirely different materials, and also because of the variable parameterof driving force in the mold. Yang and Parng (12) investigated the SIM processand found that the melt temperature and injection pressure are the most criticalparameters. They also extended the concept of the moldability diagram from con-ventional injection molding to the SIM. To apply the SIM process, some funda-mental information about the forming of ribs must be obtained. This study strivesto understand the relationships between sheet fracture types and operation parame-ters. Furthermore, the thickness ratio provides a tool for finding the weakest pointsleading to sheet fractures.

The four objectives discussed in this paper are (i) determining the windowsof the molding area and various fracture type regions; (ii) examining thicknessratio after SIM formation and comparing it to the theoretical values; (iii) findingcorrelations between fracture types and the thickness ratio distribution; and (iv)observing mechanical properties and microstructures of the deformed sheet.

2.0 EXPERIMENTAL SETUP

2.1 Materials and Molds

The superplastic sheet used in this study was made of Zn–22% Al, manufac-tured by the Light Metals Laboratory of Ta-Tung University (13). The sheet thick-

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 5: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

506 PARNG AND YANG

ness was 0.15–0.3 mm with 0.05 mm intervals. The SIM experiments were con-ducted on a 50-ton injection molding machine (Green Pax G-50, Taiwan), witha maximum injection pressure of 2000 kg cm�2 and a maximum injection rate of112 cm3 sec�1. An injection grade polystyrene, 951N (TAI-TA, Taiwan), was usedin this study.

Ribbed plates (90 mm long, 20 mm wide, and 2 mm thick) covered with asheet of superplastic zinc–aluminum were molded in this study. Deformed(ribbed) zinc–aluminum sheet enclosed the bottom surface of the transparent part.A mold was designed and constructed for the SIM experiments. Plastic inlets wereplaced at the middle of the rib. The inserts for the movable mold plate could beexchanged with the molding parts for three different rib depths as shown in Figure2. For development of the molding and fracture window, rib type A was used.

The Zn–22% Al sheet was cut 100 mm long and 30 mm wide. The sheetwas then placed into the bottom of the insert in a movable mold plate. A convexpart was used to compress the sheets tightly after the mold was closed, to preventwrinkling and shrinkage of the sheets during plastic filling. The cross section ofthe part was semicircular with a C1 fillet around the rib. Each mold had coolingchannels for purposes of mold temperature control. As the melting proceeded, thesheet was deformed and a geometry feature, a rib, was formed as well. Deformedsheet profiles for various rib depths are shown in Figure 3. In this study, no adhe-sion was applied on the sheet.

2.2 Theoretical Values Research and Observation of SheetDeformation

Forming of superplastic material from rectangular sheet is based on the as-sumptions that (i) there is no change in volume, (ii) there is uniform pressureduring the process, (iii) the isotropic property is retained, and (iv) the bulge profileat any instant is nearly circular. To analyze the thickness variation of the sheet,the equation governing the thickness ratio for a uniform pressure P is

h

h0

�b0

Rθ�

2b0H

b20 � H 2 �tan�1 � 2b0 H

b20 � H 2��

�1

(1)

where b0 , θ, R, and H are the symbols of the apparent description, and h0 and hare the thickness of the sheet before and after forming, respectively (14). Thethickness ratio, defined as h/h0, is obtained based on the forming geometry asshown in Figure 4.

To understand the variation of thickness in deeper molds, the natural thick-ness strain of the rib can be described as

ε t � ln(t0/t) (2)

where t0 is original thickness and t is current thickness.

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 6: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

SUPERPLASTIC SHEETS FORMED WITH INJECTION MOLDING 507

Figure 2. Mold construction for superplastic injection molding. (a) The geometry of the insertfor a stationary mold plate. (b) The geometry of the insert for a movable mold plate.

Furthermore, the natural elongation strain in the transverse direction is de-fined as

ε l � ln(l0/l) (3)

where l0 is original length and l is current length.Figure 5 reveals the measurement positions along the flow direction and also

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 7: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

508 PARNG AND YANG

Figure 3. Photos showing the profile of the formed sheets with various rib depths.

on the transverse cross sections for thickness ratio observation. A sheet thicknessof 0.2 mm was used in the molding and fracture window study. Two kinds ofsheet rolling direction were observed in this process, as shown in Figure 6. Meltflow was parallel to the sheet rolling direction for most cases, whereas melt flowwas vertical to sheet rolling direction for the melt temperature and injection pres-sure window. Furthermore, a sheet thickness of 0.25 mm was used for thicknessratio observations. In the filling process analysis, the following three processingparameters were fixed. The values in each stage, for injection rate and injectionpressure, were held constant and were as follows: time for filling and packing, 1and 3 sec, respectively; injection rate, 20% (% setting).

Figure 4. Geometry of deformation during free bulging of a rectangular sheet.

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 8: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

SUPERPLASTIC SHEETS FORMED WITH INJECTION MOLDING 509

Figure 5. Schematic representation of the thickness ratio measurement. (a) Cross section measure-ment positions on the rib. (b) Thickness measurement points on cross sections. Note that umbersare used for thinning ratio measurement points and letters for hardness.

Figure 6. Sheet rolling direction parallel and vertical to the melt flow direction.

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 9: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

510 PARNG AND YANG

Figure 7. Photograph showing three types of sheet fractures. The arrows indicate the location ofinitial crack tips and crack propagation paths.

To study the changed hardness of the deformed rib, H1, H2, and H3 wereselected as the measure points on the rib as shown in Figure 5.

The hardness and shape of Zn–22% Al and aluminum sheets could be di-rectly measured with a microhardness testing machine (model MVK-EII, Akashi,Japan) using a 50-g load. The hardness values were of the Vickers type. Themicrostructure observations in this study were made using a scanning electronmicroscope.

3.0 RESULTS AND DISCUSSION

3.1 Sheet Fracture Observation in This Process

According to the initial crack tip and the propagation directions, three typesof sheet fractures were formed, as shown in Figure 7. In type I fractures, initialcrack tips appeared at the end of the rib. Crack extensions cut through the filletedge of the rib or deflected to the rib center. A prominent characteristic of thetype III fracture was crack tips at the center of the rib. Crack growth propagatedall the way through the center line of the rib. Furthermore, in type II fracture,

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 10: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

SUPERPLASTIC SHEETS FORMED WITH INJECTION MOLDING 511

Figure 8. Molding windows of superplastic sheet formed with melt flowing parallel to sheet rollingdirection. Within the molding area the metal covered plate with rib A is moldable. To the right ofthe boundary is the area of type III fracture (central crack). As the injection pressure increases, thefracture becomes a type I fracture (edge crack). The boundary between type III and type I fracturesis a type II fracture (hybrid crack).

crack tips initiated at both the fillet edge and center rib, and were almost alwayslinked together.

Figure 8 shows the molding area and fracture windows for injection pressureand melt temperature as the operation parameters. A type III fracture emerged onthe right side of the line of the molding area, and a type I fracture occurred besideit. At a higher melt temperature, sheet fracture resulted with crack propagatingto the center rib. However, for a lower melt temperature, the sheet was cut onthe fillet edge of the rib. A type II fracture occurred occasionally in the area be-tween type III and type I fractures. This is a transitional fracture. Figure 9 indicatesthat the sheet rolling direction was perpendicular to the melt flow front. The per-pendicular direction sheet was more sensitive to the type of fracture. The type IIIfracture area was minimal and the type I fracture area was fairly large. However,this did not change the molding area. For various sheet thicknesses, Figure 10shows the sheet fracture types. On the right side of each line, the sheet wouldbreak following plastic injection into the mold. For a thin sheet and high melttemperature, the sheet fracture was of the type III model. However, for lower melttemperatures and thicker sheet, the sheet ruptured with type I fracture. That impliesthe thicker sheet would change to type I fracture. The mechanisms are furtherelaborated in the following discussion.

At high melt temperature or with a thin sheet, the superplastic sheet can bedeformed easily. More than half of the rib depth was deformed at the first 80%

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 11: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

512 PARNG AND YANG

Figure 9. Molding windows of superplastic sheet formed with melt flowing vertical to sheet rollingdirection. Within the molding area the metal covered plated with rib A is moldable. Note that thearea of type III fracture is small compared with that for sheet formed with melt flowing parallel tosheet rolling direction.

Figure 10. Effects of sheet thickness on molding window. Molding window expands with sheetthickness.

filling stage (12). At the final 20% filling stage, the rest of the rib profile wasdeformed suddenly, as the cavity pressure increased sharply. If the increase ofcavity pressure is so fast that the superplastic sheet could not respond in de-forming, the sheet would endure tensile stress in the center rib, even resultingtype III fracture. On the other hand, at low temperature or with a thick sheet, the

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 12: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

SUPERPLASTIC SHEETS FORMED WITH INJECTION MOLDING 513

Figure 11. Distribution of thickness ratio in cross sections along the flow direction, comparedwith the conservation volume model.

sheet deformed little at the first filling stage. Moreover, superplastic formabilitydecreases at the lower temperature. As the cavity pressure increases instantly, thesuperplastic sheet fails to respond fast enough to form the rib. The sheet wouldendure shear stress. If the stress were too high, the crack tips would initiate andpropagate along the fillet edge, resulting in the type I fracture.

3.2 Thickness Ratio Distribution with Various OperationParameters and a Compression with the Ragab Model

Figure 11 shows the thickness ratio distribution in three transverse cross sec-tions along the flow length. The thickness ratio distribution of these cross sectionsapproached a good fit. The values of the thickness ratio could be obtained as afunction of Eq. (1). Neglecting the fillet corner of the rib for the theoretical curve,there was an apparent divergence compared to the experimental data for the filletof the rib. With respect to assumptions on symmetrical profile, the thickness ratiodistribution shown in Figure 11 indicates that material with the anisotropy propertyand plastic forming pressure did not act uniformly on the rib of the sheet.

With an increase in injection pressure, the sheet thickness decreased bothin the center area and fillet, and is shown in Figure 12. However, for a lowerinjection pressure, there was no apparent variation between the two areas. Figure13 presents the variation of thickness ratio profile for various melt temperatures.The thickness ratio profile at higher melt temperature exhibited a gradual decreasein the central rib. Comparing the values of thickness ratio for various melt temper-

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 13: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

514 PARNG AND YANG

Figure 12. Distribution of thickness ratio for various injection pressures. P and T are the moldingconditions of injection pressure and melt temperature, respectively.

Figure 13. Distribution of thickness ratio for various melt temperatures. Major variation of thick-ness ratio profile is in the central rib for various melt temperatures.

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 14: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

SUPERPLASTIC SHEETS FORMED WITH INJECTION MOLDING 515

Figure 14. Distribution of thickness ratio for various rib depths. Deeper formed rib provided athickness ratio reduction in the central rib.

atures and injection pressures, it was concluded that the pressure increased as thethickness decreased on the fillet edge. The thickness ratio of the sheet, particularlyin the central and fillet areas of the hemispherical rib, decreased with increasingmelt temperature and injection pressure. When crack tips emerged in these areas,they propagated along the thinnest regions of the sheet, terminating in fracture.This is the reason for a larger pressure for the type III model. Furthermore, itis significant that the approximate thickness ratio distribution profile serves as athreshold for forming the rib profile. On the other hand, the driving force mustovertake the lowest sheet thickness distribution profile to form the rib.

Compared to the conventional free bulging of circular diaphragms formedby gas, a representation of the natural thickness strain of 2.3 was obtained byJohnson (15). Figure 14 shows the thickness ratio distribution for various rib depthprofiles. As the depth of the rib mold increased, the sheet thickness on the ribgradually decreased. The thickness ratio on the center of rib C almost reached0.2. For example, the sheet thickness was 0.25 mm for rib C, and the smallestthickness was about 0.05 mm in the central rib. The maximum natural thicknessstrain reached almost 1.44 from Eq. (2). To successfully mold plates havingthicker ribs, strict operation parameters are necessary. To compare different ribdepths, Table 1 presents the operation conditions and the natural strain in theseribs. These data indicate that for molding plates having thicker ribs, stepped con-trol of the filling pressure and speed is essential. Furthermore, the maximum natu-ral thickness strain is larger than the elongation of transverse cross section for thedeeper ribs.

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 15: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

516 PARNG AND YANG

Table 1. Operation Conditions and Maximum Natural Strain for Various Rib Depths

Rib A (3 mm) Rib B (5 mm) Rib C (7 mm)

Sheet thickness (mm) 0.25 0.25 0.25Tmelt (°C) 240 240 240Tmold (°C) 50 120 150First stage injection pressure (%) 17 17 2.9Second stage injection pressure (%) 17 11.3 2.9First stage injection speed (%) 20 50 10Second stage injection speed (%) 20 10 10εmt (max. of natural thickness strain) 0.36 0.77 1.44ε l (natural elongation strain in section I) 0.34 0.58 0.83

3.3 Mechanical Property

Table 2 shows the hardness in the top (point H1), middle (point H2), andbottom parts (point H3) of the samples. For Zn–22% Al, different positionsshowed the same tendency with H1 � H2 � H3. As the temperature decreased,the difference in hardness was obvious. However, aluminum produced a differenttendency, with H3 � H2 � H1. This indicates that pure Al becomes hardenedduring the process. Therefore, crack tips propagated along the weakest centralline of the rib for the Zn–22% Al sheet fracture in the normal case. However,this occurred along the edges for aluminum.

A microstructure of the Zn–22% Al sheet is shown in Figure 15. The grainsize is about 0.5–1 µm. A fine microstructure is a fundamental condition for struc-tural superplastic materials having good formability.

4.0 CONCLUSIONS

Different types of sheet fractures in the SIM were observed in this study. Theeffects of parameters, such as sheet thickness, rolling direction, melt temperature,injection pressure, and mold temperature, were examined. The thickness ratio dis-tribution profile helped determine the weakest points of the sheet and the influ-ences of processing parameters. It also helped us analyze sheet fracture types andcontrol of the SIM process.

Table 2. The Hardness Values at Different Positions for Zn-22% Al and Al

Material Zn–22% Al Zn–22% Al Zn–22% Al Aluminum

Section Tmelt 210 240 260 240

Transverse cross- Point H1 36.4 27.6 31.1 27.4section Hv

Point H2 35.9 27.3 29.9 32.4Point H3 33.4 27.4 30.7 38.1

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 16: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

SUPERPLASTIC SHEETS FORMED WITH INJECTION MOLDING 517

Figure 15. Scanning electron micrograph showing grain size. The bright and dark grains corre-spond to Al-rich and the Zn-rich grains, respectively.

The conclusions are summarized as follows:

1. There are three major fracture types in the superplastic injection moldingprocess. Fracture windows emerge with a left-to-right sequence of typeIII, II, and I. Higher injection pressure and a vertical rolling directionare likely to cause a type I fracture window to grow. However, alongthe rolling direction and higher melt temperatures caused a type III frac-ture to grow. Type II with a combination of central and edge crack isa transition state between type III and type I fractures.

2. Thickness ratio observations for various melt temperatures revealed thatthe central line could be the weakest point during the forming process.This leads to type III fracture. Nevertheless, the fillet edge of the ribcould become a fragile area under higher injection pressure, causingtype I fracture.

3. Thickness ratio distribution predicted by the conservation volume issimilar to the experimental profile that approaches symmetry at the cen-ter. The sheet thickness decreased conspicuously for the thicker ribs.The geometry of rib C restricts sheet forming within severe limitingconditions.

4. Microstructure observation reveals that the grain size of a superplasticZn–22% Al sheet is limited to about 0.5–1 µm. A pure aluminum sheetstrengthens after forming, but a Zn–22% Al sheet weakens in this process.

5.0 ACKNOWLEDGMENTS

The authors would like to express thanks to National Science Council,R.O.C., for financial support under Contract No. NSC 89-2622-E-002-011; to pro-fessor T. H. Chuang of National Taiwan University for organizing the research

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 17: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

ORDER REPRINTS

518 PARNG AND YANG

team and initiating the idea of SIM; to professor C. F. Yang of Ta-Tung Universityfor supply of Zn–22% Al sheets; to the coworkers in the Grace Laboratory forPolymer Processing for stimulating discussion and experimental assistance; andto POLYPAX Machinery Company for help in machine acquisition and modifica-tion.

6.0 REFERENCES

1. Huang, Y.J.; Hu, B.H.; Pinwill, I.; Zhou, W.; Taplin, D.M.R. Effects of ProcessSettings on the Porosity Levels of AM60B Magnesium Die Castings. Mater. Manuf.Processes 2000, 15 (1), 97–105.

2. Kimura, K.; Nishi, K.; Ishizuka, M.; Miyahara, S. Sub-notebook Computer CoolingTechnology via Hybrid Housings. The Sixth Intersociety Conference on ThermalPhenomena, 1998; 35–42.

3. Chandra, N.; Chandy, K.J. Superplastic Process Modeling the Plane Strain Compo-nents with Complex Shapes. Mater. Shaping Technol. 1991, 9 (1), 27–37.

4. Furukawa, M.; Ma, Y.; Horita, Z.; Nemoto, M.; Valiev, R.Z.; Langdon, T.G. Micro-structural Characteristics and Superplastic Ductility in a Zn-22% Al Alloy with Sub-micrometer Grain Size. Mater. Sci. Eng. 1998, 241A (1), 122–128.

5. Mohamed, Z. Metall. Micronecking and Fracture in Cavitated Superplastic Materi-als. Mater. Trans. 1996, 27A (4), 1043–1046.

6. Higashi, K.; Mabuchi, M. Experimental Investigation of Cavitation Fracture at VeryHigh Strain Rates in Superplastic Aluminium Alloy Matrix Composites. Mater. Sci.Eng. 1994, 176A (1,2), 461–470.

7. Higashi, K.; Okada, T.; Mukai, T.; Tanimura, S. Positive Exponent Strain-RateSuperplasticity in Mechanically Alloyed Aluminum IN9021. Scr. Metall. Mater.1991, 25 (9), 2053–2057.

8. Ha, T.K.; Lee, W.B.; Park, C.G.; Chang, Y.W. Room-Temperature Superplasticityin a Zn-0.3 Wt Pct Al Alloy. Metall. Mater. Trans. 1997, 28A (8), 1711–1713.

9. Bellet, M.; Massoni, E.; Chenot, J.L. Numerical Simulation of Thin Sheet FormingProcesses by the Finite Element Method. Eng. Comput. 1990, 7 (1), 21–31.

10. Bonet, J.; Wood, R.D. Incremental Flow Procedures for the Finite-element Analysisof Thin Sheet Superplastic Forming Processes. J. Mater. Processing Technol. 1994,42 (2), 147–165.

11. Xing, H.L.; Wang, Z.R. Finite-element Analysis and Design of Thin Sheet Super-plastic Forming. J. Mater. Processing Technol. 1997, 68 (1), 1–7.

12. Yang, S.Y.; Parng, S.H. Injection Molding of Ribbed Plastic Plates with a Superplas-tic Zn-22% Al Sheet. Adv. Polym. Technol. 2001, 20 (3), in press.

13. Yang, C.F.; Chiu, L.H.; Sheu, Y.P. Effects of Thermomechanical Treatments onSuperplasticity of Zn-22% Al Alloy. Mater. Manuf. Processes 1997, 12 (2), 199–214.

14. Ragab, A.R.; Thermoforming of Superplastic Sheet in Shaped Dies. Met. Technol.1983, 10 (9), 340–348.

15. Johnson, W.; Al-Naib, T.Y.M.; Duncan, J.L. Superplastic Forming Techniques andStrain Distributions in a Zinc-Aluminum Alloy. J. Inst. Met. 1972, 100, 45–50.

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14

Page 18: FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

Order now!

Reprints of this article can also be ordered at

http://www.dekker.com/servlet/product/DOI/101081AMP100108523

Request Permission or Order Reprints Instantly!

Interested in copying and sharing this article? In most cases, U.S. Copyright Law requires that you get permission from the article’s rightsholder before using copyrighted content.

All information and materials found in this article, including but not limited to text, trademarks, patents, logos, graphics and images (the "Materials"), are the copyrighted works and other forms of intellectual property of Marcel Dekker, Inc., or its licensors. All rights not expressly granted are reserved.

Get permission to lawfully reproduce and distribute the Materials or order reprints quickly and painlessly. Simply click on the "Request Permission/Reprints Here" link below and follow the instructions. Visit the U.S. Copyright Office for information on Fair Use limitations of U.S. copyright law. Please refer to The Association of American Publishers’ (AAP) website for guidelines on Fair Use in the Classroom.

The Materials are for your personal use only and cannot be reformatted, reposted, resold or distributed by electronic means or otherwise without permission from Marcel Dekker, Inc. Marcel Dekker, Inc. grants you the limited right to display the Materials only on your personal computer or personal wireless device, and to copy and download single copies of such Materials provided that any copyright, trademark or other notice appearing on such Materials is also retained by, displayed, copied or downloaded as part of the Materials and is not removed or obscured, and provided you do not edit, modify, alter or enhance the Materials. Please refer to our Website User Agreement for more details.

Dow

nloa

ded

by [

Erc

iyes

Uni

vers

ity]

at 1

8:09

21

Dec

embe

r 20

14