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Precision Engineering

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Injection by sheet-bulk forming

J.P. Magrinho, M.B. Silva, P.A.F. Martins∗

IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal

A R T I C L E I N F O

Keywords:Sheet-bulk formingSheet injectionMetalPolymerExperimentationFinite element method

A B S T R A C T

This paper is focused on injection by upsetting of sheet-formed side walls. The presentation draws from fun-damental aspects of material flow to typical failures caused by buckling and by the development of materialflaws. Results show that the utilization of two-piece blank holders in conjunction with an adequate selection ofthe die cavity thickness and the creation of relief-holes to allow divided material flow are effective and easy-to-implement strategies to increase process formability and ensure the production of sound sheet-bulk parts. Thework is carried out in two completely different materials (polyvinylchloride and aluminium AA1050) and itsoriginality stands not only for the investigation on sheet injection but also for the utilization of thermoplastics insheet-bulk forming. Numerical modelling with finite elements gives support to the overall results and discussion.

1. Introduction

Sheet-bulk forming (SBF) (also known as ‘plate forging’) is a newmanufacturing process that combines bulk forming operations such asupsetting, extrusion and ironing with general sheet forming operationssuch as shearing, bending, stamping and deep drawing. The rationalebehind the development of SBF is to meet the growing demand forcomplex sheet metal parts resulting from integration of multiple func-tional features in one piece in order to minimize the number of com-ponents and reduce the overall costs of assembly and maintenance.

Maeda and Araki [1] were the first to propose the combination ofsheet and bulk forming operations to produce complex sheet metalparts. They patented a process based on the use of drawing and up-setting to fabricate circumferential gears with concave thick teeth. Theutilization of drawing and upsetting operations was subsequently ap-plied by Suzumura et al. [2] to increase the flange thickness of drawnstep cups. Nakano et al. [3] extended the overall concept by combiningthree different operations (upsetting, drawing and ironing) to producegear drums for automotive engines.

The keynote paper by Merklein et al. [4] and the state-of-the-artreview by Mori et al. [5] provide a comprehensive overview of differentSBF processes. One conclusion that may be drawn from these two pa-pers is that sheet metal parts with geometric features such as solidbosses and teeth positioned outside the plane of the blanks from whichthey were produced, encompass significant amounts of sheet thinningand/or thickening.

As known, these material flows give rise to formability problemsbecause thinning is limited by fracture and thickening and local

thickening are constrained by the occurrence of buckling and/orfolding during the upsetting stage of a SBF process [6]. Buckling andfolding are schematically illustrated in Fig. 1 for the wall thicknessupsetting of a sheet metal part.

Another problem associated to SBF is the large applied forces. Werbset al. [7] suggested to heat the blanks prior to forming in order to re-duce the force and improve formability. Heating the sheets prior toforming also helps reducing the risk of excessive tool wear and failuredue to high contact pressures but at a cost of increasing the processchain runtime.

Another approach to improve formability and prevent the occur-rence of folding and buckling is based on the utilization of tailoredblanks with customized variations of sheet thickness [8]. The objectiveis to employ customized sheet thickness profiles to compensate andavoid the defects resulting from material underfilling in critical dieregions such as, for example, the cup corners of deep drawn sheet metalparts. However, this solution also comes at a cost of increasing theprocess runtime.

Merklein et al. [8] and Schneider and Merklein [6] proposed designguidelines to modify the geometry and dimensions of the active toolcomponents and to improve the frictional conditions and the initialgeometry of the blanks in an effort to prevent folding, buckling andmaterial underfilling in processes that combine drawing and upsettingoperations.

Wang et al. [9] proposed the utilization of hydraulic pressure toensure a permanent contact between the tool and the wall cup in orderto eliminate the risk of buckling during upsetting. Punching a hole inthe bottom of the cup was also suggested to reduce the final upsetting

https://doi.org/10.1016/j.precisioneng.2019.06.003Received 7 March 2019; Received in revised form 4 May 2019; Accepted 4 June 2019

∗ Corresponding author.E-mail addresses: joao.magrinho@tecnico.ulisboa.pt (J.P. Magrinho), beatriz.silva@tecnico.ulisboa.pt (M.B. Silva), pmartins@tecnico.ulisboa.pt (P.A.F. Martins).

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T

force.Another solution to improve formability and prevent folding at the

corner of deep drawn sheet metal parts was recently disclosed byMagrinho et al. [10] who proposed a two-piece blank holder concept.The main advantage of the two-piece blank holder design is that itenhances workability without compromising the process chain runtime.

In contrast to sheet thinning and thickening that have been ex-tensively investigated in the past years, the work on sheet injection islimited to that included in a previous publication by the authors [10].The paper introduces a new flexible SBF demonstrator capable of fab-ricating representative sheet metal parts with thickening, local thick-ening and injection features. Selected experimental test cases provedthe effectiveness of the overall SBF demonstrator concept but the at-tention given to sheet injection was limited.

In fact, as far as the authors are aware, there is no research work inthe field and, therefore, it makes sense to focus the aims and scope ofthis investigation on sheet injection. Fig. 2 shows potential applicationsin which is possible to identify ribs that can be produced by combina-tion of deep drawing or bending with sheet injection.

Two completely different materials (polyvinylchloride and alumi-nium AA1050) and various shapes and sizes of tools were utilized toinvestigate material flow, to identify typical defects and to fosterguidelines to prevent failing by buckling and by the occurrence of flawsat the vicinity of dead metal zones. In connection to this, it is worthmentioning that utilization of polyvinylchloride at room temperature isby itself an innovation because the feasibility of using polymer blanksin SBF has never been investigated by other researchers in the field.

The experimental work on sheet injection is performed on the SBF

demonstrator that was designed and fabricated by the authors [10] andthe overall results and conclusions are supported by numerical model-ling with finite elements.

2. Experimentation

2.1. Stress-strain curve

The work on sheet injection was performed on commercial poly-vinylchloride (PVC) and aluminium AA1050-O sheets with 3mmthickness. The stress-strain curves of both materials were determined bymeans of tensile and stack compression tests in specimens that weremachined out from the supplied sheets. The AA1050-O tensile testspecimens were prepared in accordance to the ASTM E8/E8M standard[11] whereas the PVC tensile specimens followed the ASTM D638-02standard [12]. For both materials, the stack compression test specimenswere prepared by piling up four circular discs with 12mm diameter and3mm thickness.

The stress-strain curves were determined at room temperature on ahydraulic testing machine (INSTRON SATEC 1200 kN) with a constantmoving cross-head speed of 5mm/min. The results for both tensile (T)and stack compression (SC) tests are shown in Fig. 3 and allow con-cluding that the stress response of PVC is dependent on hydrostaticpressure, giving rise to differences between tensile and compressiveflow stress at a given strain. This phenomenon is designated as the‘strength differential effect’ and is typical of the cold forming of poly-mers.

2.2. Methods and procedures

The work on sheet injection was carried out on the flexible SBFdemonstrator developed by Magrinho et al. [10]. The SBF demonstratorwas installed in the same hydraulic testing machine where the stress-strain curves were determined and the experiments made use of ‘unitcells’ that approximate circular deep drawn slices by straight bendedslices. The objective is to replicate the combined utilization of deepdrawing and sheet injection by means of the testing procedure shown inFig. 4.

Although the approximation of the circular deep drawn slice by astraight bended slice does not consider the reduction in thickness at thebottom of the cup and only accounts for the reduction in thickness atthe corner, it can still be seen as an effective and easy-to-implementsolution to study SBF processes based on the combination of deepdrawing and sheet injection. This is because; (i) the bottom of the cup isnot relevant for sheet injection, which is exclusively performed withmaterial of the bended side wall, (ii) the reduction in thickness at thecorner of a circular deep drawn slice is similar to that of a straight

Fig. 1. Schematic illustration of buckling and folding in a sheet-bulk formingprocess that combines deep drawing and upsetting operations. Both defectsappear in the upsetting stage.

Fig. 2. Sheet-bulk forming by combination of deep drawing and injection. (a)Potential applications (belt pulleys and heat sinks); (b) Schematic representa-tion of sheet injection with three possible defects (buckling, flaws and cracks).

0

50

100

150

200

250

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Stress(MPa)

Strain

AA1050-O - T

AA1050-O SC

PVC - T

PVC - SC StackCompression

(SC)

Tensile (T)

Fig. 3. True stress-true strain curves of polyvinylchloride (PVC) and aluminumAA1050-O obtained from tensile (T) and stack compression (SC) tests.

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bended slice and (iii) the axisymmetric constrain along the circumfer-ential direction of the circular deep drawn slice is similar to the planestrain constraint along the width of the straight bended slice.

For this purpose, the SBF demonstrator was equipped with thebending and sheet injection setups that are shown in Fig. 4c and e,respectively. The main tool components utilized in both setups are theupper driver plate (1), the lower fixed plate (2), the pressure pad (3),the die block (4), the punch holder (5) and the compression gas springs(6) to apply force on the blank holder (7). In case of bending (Fig. 4c),the basic setup is modified to include a wiping punch (8) for bendingthe free length of the straight slice unit cell. The remaining portion ofthe unit cell is kept fixed between the blank holder and the pressurepad. In case of sheet injection (Fig. 4e), the basic setup is modified toinclude an upsetting punch (9) and several dies (10) with differentgeometries and sizes to fulfil the entire working plan.

The pressure pad (3), blank holder (7), punches (8–9) and dies (10)were made from a C265 steel (DIN X 155 CrVMo 12-1) and were heattreated to ensure a surface hardness of approximately 60 HRc. After theheat treatment, the tool components were grinded to ensure high ac-curacy and low surface roughness ≅R 0.145a μm. The compression gassprings were selected to heavy duty loading conditions and can providemaximum forces of 5 kN for bending and 15 kN for sheet injection.

Table 1 shows the two different sheet injection variants that werefabricated by changing the die, labelled as (10) in Fig. 4e. Appropriatenotation for identifying the die cavity thickness td, the die cavity lengthld and the die cavity height hd are included. The initial bending stage isnot shown in Table 1 because it is identical to that shown in Fig. 4c.

The experimental work plan is shown in Table 2. Several test caseswith two different materials (PVC and aluminum AA1050-O), two dif-ferent initial dimensions of the straight rectangular unit cell, and dif-ferent die (10) geometries and sizes were tested to investigate materialflow, identify typical defects and foster basic guidelines to preventfailing by buckling and by the occurrence of flaws at the vicinity ofdead metal zones. All tests were performed with a constant moving

cross-head speed of 5mm/min.

3. Numerical modelling

Numerical modelling of bending and sheet injection operationswere performed with an in-house computer program built upon thefinite element flow formulation. Classical von Mises yield function wasutilized for modelling the stress response of the aluminium AA1050-Ounit cells whereas a pressure modified version of the von Mises yieldfunction F σ( )ij due to Caddell et al. [13] was utilized for modelling thestress response of PVC,

= − ⋅ + − =F σ σ σ σ σ σ σ( ) ¯ ( ) 0ij C T C T kk2 (1)

In the previous equation, which explicitly accounts for the differ-ences between compressive and tensile flow stresses of polymers, σ̄ isthe effective stress and =σ δ σkk ij ij. The tensile σT and compressive σCflow stresses are obtained from material characterization (refer toSection 2.1). In case =σ σT C equation (1) corresponds to the classicalvon Mises yield criterion.

The finite element computer program is built upon the followingvariational principle that considers the velocity field ui to be de-termined to correspond to the absolute minimum of the total potential

Fig. 4. Schematic representation of the overall experimental procedure. (a) Theunit cell concept; (b) Deep drawing with approximation of the radial deep-drawn slice by a straight bended slice; (c) SBF demonstrator with identificationof the major components utilized in the bending stage; (d) Sheet injection byupsetting of the side wall; (e) SBF demonstrator with identification of the majorcomponents utilized in sheet injection.

Table 1Sheet injection variants with identification of the unit cells to be fabricated andinformation on the corresponding setup and associated notation.

Table 2Experimental work plan and initial geometry of the unit cells (nomenclatureaccording to Table 1).

Case Material Initial dimension ofthe unit cell

× ×t w l0 0 0(mm3)

Die cavitythicknesstd(mm)

Diecavitylengthld(mm)

Die cavityheighthd(mm)

1 PVC × ×3 50 150 5 1 102 PVC × ×3 50 150 5 2 103 PVC × ×3 50 150 2.5 12 104 PVC × ×3 50 150 2.5 12 05 AA1050-O × ×3 50 150 5 1 106 AA1050-O × ×3 50 150 5 2 107 AA1050-O × ×3 50 150 5 12 108 AA1050-O × ×3 50 150 2.5 12 109 AA1050-O × ×3 50 150 10 2 1010 AA1050-O × ×3 50 150 5 12 011 AA1050-O × ×3 50 150 2.75 12 012 AA1050-O × ×3 50 100 2.75 12 0

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energy rate Π ,

∫ ∫ ∫ ∫ ∫= + − + ( )Π σ ε dV K ε dV T u dS τ du dS¯ ¯̇ 12

˙V V

vS

i iS

uf r

20

| |

T f

r

(2)

where ε̇̄ is the effective strain rate, K is a large positive constant im-posing the incompressibility constraint, V is the control volume limitedby the surfaces SU and ST , Ti are the surface tractions on ST and τf arethe friction shear stresses on the contact interface Sf between materialand tooling.

Friction along the material-tool interface was modelled as a tractionboundary condition and the additional power consumption resultingfrom the rightmost term of the variational principle (2) is determinedby the utilization of the law of constant friction =τ mk. The frictionfactor m was set to 0.1 after checking the finite element predicted forcesthat best matched the experimental results.

The numerical simulation of bending and sheet injection was per-formed in two-dimensional plane strain conditions because the unitcells have a constant width of 50mm. The unit cells were discretized bymeans of approximately 4000 quadrilateral elements and the tools weremodelled as rigid objects and discretized by means of linear contact-friction elements.

The overall central processing unit (CPU) time of a typical sheetinjection stage requiring several intermediate remeshings was ap-proximately equal to 90min on a computer equipped with an Intel i7-6700HQ Processor (2.6 GHz). Fig. 5 shows the initial, intermediate andfinal computed meshes in a sheet injection stage of a SBF process.

4. Results and discussion

4.1. Bending and sheet injection of PVC

The preliminary investigations performed in PVC were aimed atevaluating the overall performance of the process without running therisk of overloading the SBF demonstrator due to the high contactpressures resulting from injection by upsetting of the straight bendedside walls of the unit cells. The correspondent experimental test casesare labelled as 1 to 4 in Table 2 and were all performed with straightunit cells having an initial length =l 1500 mm. Both tool setups ofTable 1 were used.

Fig. 6 shows the experimental and finite element predicted evolu-tion of the bending (first operation) and sheet injection (second op-eration) forces with displacement. The overall agreement is good and,in case of bending (Fig. 6a), differences are attributed to frictionalconditions between the wiping punch (8) and the die block (4). Inparticular, the decay of the experimental force occurs at a lower ratethan that of the numerically predicted force because small differencesin the normal force and bending moment applied by the bended side

wall over the wiping punch (8) have a major influence in the frictionalconditions and, therefore, in the evolution of the total applied force.Still, the maximum force (0.95 kN) predicted by finite element analysisis close to that experimentally measured (1.10 kN).

In case of sheet injection, the force-displacement evolution allowsidentifying three different zones. The zone labelled as ‘A’, correspondsto the amount of displacement during which the moving part of thetwo-piece blank holder remains in its uppermost position (refer to theleft inset in Fig. 6b). The zones labelled as ‘B’ and ‘C’ correspond tosubsequent punch displacement during which the moving part of thetwo-piece blank holder applies pressure on the bottom region of theunit cell. The steep increase in force during the final punch stroke (zone‘C’) is caused by localized three dimensional material flow of thestraight bended side wall into the small die cavity up to ensure itscomplete filling. Material flow in the direction perpendicular to thewall thickness is typical of sheet injection and gives rise to high contactpressures that need to be carefully controlled in order to preventoverloading the SBF tools.

The differences in the experimental and numerical forces at the endof the punch stroke are due to small compressibility of PVC that be-comes relevant to very high pressures [14]. This effect was not takeninto consideration in the numerical model, which assumed PVC to beincompressible.

The actual and finite element predicted geometries at the end ofsheet injection are shown in Fig. 7a. The agreement is very good asproves that PVC can successfully be employed in SBF processes invol-ving cold forming operations.

The other test case in Fig. 7b, shows a typical failure by bucklingbecause the die cavity provides enough free volume for the material tobuckle and create a fold at the die cavity entrance. This type of defect inconjunction with the development of material flaws in test cases withlarger die cavity lengths are further analysed in the following sectiondealing with aluminium AA1050-O unit cells.

4.2. Bending and sheet injection of aluminium AA1050-O

Fig. 8a shows the experimental and finite element predicted evo-lution of the bending force with displacement for straight aluminiumAA1050-O unit cells having an initial length =l 1500 mm (test cases 5 to11 of Table 2). The difference between the experimental and numeri-cally predicted evolutions is similar to that previously observed in PVC(Fig. 6a) apart from a small mismatch at the beginning, which is notvisible for PVC due to its low elasticity modulus.

The reason for this mismatch is attributed to the fact that the initialcontact between the wiping punch and the actual unit cell does notoccur uniformly along the entire width as in case of numerical model-ling [10].

The force-displacement evolution disclosed in Fig. 8b is more in-teresting and reveals three different trends at the end of punch stroke(refer to zone ‘C’). Case 5 presents a steep increase in force at the end ofpunch stroke, similar to that previously observed in PVC. As earlier saidthis is due to material flowing in the direction perpendicular to the wallthickness in order to completly fill the small die cavity. The highpressures of the sheet injection process and the contact with toolingallow obtaining a final surface roughness ≅R 0.151a μm.

Fig. 9 shows the computed and actual cross section geometries ofthe unit cells at the beginning, intermediate and final stages of the sheetinjection operation.

The second trend observed in the force-displacement evolution ofzone ‘C’ (Fig. 8b) is that resulting from the experimental and numericalsimulation of case 9 of Table 2. As seen, the force drops slightly at firstas a result of buckling and then grows steeply from a lowered level.

The normalized velocity vector plot v v/| |0 shown in Fig. 10, where v0is the velocity of the upsetting punch, shows the mechanism underlyingthe occurrence of failure by buckling. The subsequent increase in forcetakes place once the side wall gets in contact with the end of the die

Fig. 5. Sheet injection of an aluminum AA1050-O unit cell (case 7 of Table 2).(a) Computed finite element mesh at the end of the bending stage; (b) Com-puted finite element mesh in sheet injection at the time when the two-pieceblank holder starts to exert pressure on the bottom of the unit cell; (c) Com-puted finite element mesh at the end of a sheet injection process with in-complete filling of the die cavity.

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(caption on next page)

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cavity and the two counter facing surfaces of the inner side wall get incontact with each other (see also Fig. 10).

As it is obvious, failure by buckling is more likely to be triggeredwhen the thickness td of the die cavity is larger. Materials with smallelasticity modulus like PVC are also more likely to develop failure bybuckling (refer to buckling in case 2 versus no-buckling in case 7 ofTable 2).

In contrast to the two previous test cases, force-displacement evo-lution in zone ‘C’ for case 7 discloses two different patterns (Fig. 8b).Initially and similarly to case 5, force increases steeply as a consequenceof localized three dimensional material flow of the straight bended sidewall into the die cavity. This initial pattern corresponds to non-steadystate material deformation and is subsequently replaced by steady-statematerial deformation under an applied force of 50 kN, which prevailsuntil the end of punch stroke. The steady-state deformation is triggeredwhen material from the straight bended side wall starts filling the diecavity, which is now much longer than that of case 5 (Fig. 11).

The force-displacement evolution of case 7 is similar to that of case5 up to the development of steady-state deformation. But the mainproblem with steady-state deformation is that force stabilization in case7 is linked to the development of a dead metal zone (DMZ) and to theformation of an inadmissible material flaw. The latter is clearly ob-served in the normalized velocity vector plot included in the rightmost

Fig. 6. Experimental and numerical evolution of the force with displacement for a SBF forming process resulting from combination of bending and sheet injection.The unit cell is made from PVC (case 1 of Table 2). (a) Bending operation (first stage of the SBF process); (b) Sheet injection by upsetting (second stage of the SBFprocess).

Fig. 7. Computed finite element geometries and longitudinal cross sectionphotographs of PVC unit cells fabricated by combination of bending and sheetinjection. (a) PVC sheet-bulk formed part without defects (case 1 of Table 2);(b) PVC sheet-bulk formed part with buckling (case 2 of Table 2).

Fig. 8. Experimental and numerical evolution of theforce with displacement for a SBF process resultingfrom combination of bending and sheet injection. Theunit cell is made from aluminum AA1050-O (cases 5,7 and 9 of Table 2). (a) Bending operation (first stageof the SBF process); (b) Sheet injection by upsetting(second stage of the SBF process).

Fig. 9. (a) Finite element predicted geometries and (b) longitudinal cross sec-tion photographs of sheet injection of aluminum AA1050-O straight bendedslices at the beginning, intermediate and final stages (case 5 of Table 2).

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finite element image of Fig. 11a.The same problem occurs when the die cavity moves from the

middle to the bottom of the side wall (Fig. 12). In fact, the steady-statedeformation conditions that prevail in both cases 7 and 10 are similarand require injection forces that are almost identical apart from thefinal values under steady-state deformation, which are higher in case 10as a result of the additional friction contact area along the side wall.This overall similarity of the force-displacement evolution is due to thefact that both die cavities have equal thickness td.

Under these circumstances, the main objective of the next section isto discuss how the formation of DMZ's can be prevented and longer ribsbe formed without the development of material flaws.

4.3. Sheet injection with divided flow

The strategy utilized by the authors to prevent the formation ofDMZ's is inspired in the divided material flow concept that was pro-posed by Ohga and Kondo [15] to reduce forces and ensure completefilling of dies in precision forging.

For this purpose, authors create a relief-hole (RH) in the middle ofthe deep drawn part to allow material to flow simultaneously inwardlyand outwardly with respect to a neutral axis located in the middle of theoriginal wall thickness and adjacent to the die cavity entrance. Theexistence of a relief-hole and a neutral axis (NA) is replicated in the SBFdemonstrator by using straight bended slices with a bottom length

Fig. 10. Finite element predicted geometries and longitudinal cross section photographs of sheet injection of aluminum AA1050-O straight bended slices at the (a)beginning and (b) final stages (case 9 of Table 2). The normalized velocity vector plot at the intermediate stage of the process shows evidence of failure by buckling.

Fig. 11. (a) Finite element predicted geometries and(b) longitudinal cross section photographs of sheetinjection of aluminum AA1050-O straight bendedslices at the beginning, intermediate and final stages(case 7 of Table 2). The normalized velocity vectorplot at the end of the stroke shows evidence of a deadmetal zone (DMZ) and a material flaw.

Fig. 12. SBF process resulting from combination of bending and sheet injection.The unit cell is made from aluminum AA1050-O (cases 7 and 10 of Table 2). (a)Finite element predicted geometry with normalized velocity vector plot andlongitudinal cross section photograph of a unit cell with a rib positioned at themiddle of the original side wall (case 7); (b) Finite element predicted geometrywith normalized velocity vector plot and longitudinal cross section photographof a unit cell with a rib positioned at the bottom of the original side wall (case10); (c) Experimental and numerical evolution of the force with displacementfor cases 7 and 10.

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smaller than the total length available in-between the blank holder andthe pressure pad (Fig. 13a and b).

As seen in Fig. 13c and d the final length of the rib is larger for theunit cell replicating the relief-hole concept because its bottom lengthincreased by approximately 6% in order to allow divided material flowand ensure filling of the die cavity. This also means that material soughta divided flow pattern to extend the non-steady-state deformationconditions of region ‘C’ and prevent the development of DMZ's andmaterial flaws. This last conclusion is confirmed by observing the dif-ference between the steady-state force-displacement evolution of case11 and the non-steady state force-displacement evolution of case 12,which replicates the relief-hole concept in the SBF demonstrator (referto zone ‘C’ of Fig. 13e).

5. Conclusions

Injection by upsetting of side walls can be successfully applied to

produce ribs in deep drawn or bended sheet parts. Appropriate choiceof die cavity thicknesses combined with the utilization of two-pieceblank holders, which remain in their uppermost positions at the be-ginning of the operation and then move downward to compress sheetsagainst pressure pads, can effectively prevent the risk of failure bybuckling. Creation of relief-holes in sheet-formed parts to allow dividedmaterial flow can successfully improve formability by avoiding thedevelopment of material flaws. This enables larger ribs to be formedthan those obtained in sheet parts without relief-holes.

The investigation also proves the effectiveness of PVC (a thermo-plastic material) in sheet-bulk forming processes. The main advantageof thermoplastics against metals is the smaller required forces whereasthe main drawback results from its small elasticity modulus, whichincreases spring back and the risk of buckling during upsetting of sheet-formed side walls.

Acknowledgments

The authors would like to acknowledge the support provided byFundação para a Ciência e a Tecnologia of Portugal and IDMEC underLAETA-UID/EMS/50022/2019 and PDTC/EMS-TEC/0626/2014.

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Fig. 13. SBF forming process resulting from combination of deep-drawing andsheet injection. The unit cell is made from aluminum AA1050-O. (a) Unit-cellutilized to replicate conventional sheet injection in the SBF demonstrator (case11 of Table 2); (b) Unit-cell utilized to replicate sheet injection with dividedmaterial flow in the SBF demonstrator (case 12 of Table 2); (c) Finite elementpredicted geometry with normalized velocity vector plot and longitudinal crosssection photograph of a unit cell corresponding to (a) (case 11 of Table 2); (d)Finite element predicted geometry with normalized velocity vector plot andlongitudinal cross section photograph of a unit cell corresponding to (b) (case12 of Table 2); (e) Experimental and numerical evolution of the force withdisplacement for cases 11 and 12 of Table 2.

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