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Failure analysis of H13 working die used in plastic injectionmoulding

D. Papageorgiou a ,b , C. Medrea a ,⇑ , N. Kyriakou a

a Technological Educational Institute of Piraeus, Department of Physics, Chemistry and Materials Technology, 250 Thivon and P. Ralli Str. 12244 Aigaleo, Greeceb Stassinopoulos-Uddeholm Steel Trading S.A., 20 Athinon Str. 18540 Piraeus, Greece

a r t i c l e i n f o

Article history:Available online 6 March 2013

Keywords:Failure analysisPlastic injection mouldingFatigue-corrosion crackingH13 tool steel

a b s t r a c t

The present study is focused on the failure of a die used in plastic injection moulding. Thedie was made from AISI H13 steel and was intended for the production of plastic cups usedfor the outer closure of cylindrical aluminium cans in coffee packaging. The appearance of the die provides a clear picture of degradations. Extended corrosion damage on variousareas of the metallic part and a wide crack can be observed by the naked eye. Hardnessmeasurements and chemical analysis eliminated the probability of faulty material selec-tion or improper heat treatment. Visual inspection, macro-examination and microscopicobservations of representative failed parts revealed that the failure was caused by corro-sion that led to the total cracking of the die. The design deciency and improper coolingconditions generated a complex fatigue-corrosion cracking mechanism that lead to thedamage of the die after half of it’s predicted service life.

2013 Elsevier Ltd. All rights reserved.

1. Introduction and background information

Hot-working dies are used for applications in which process temperature is an important parameter for the manufactur-ing of the tooling material. During operation, the die is subjected to repeated temperature cycles, increased compressivestresses and plastic deformation. Consequently, it should incorporate specic mechanical properties [1] . The toolmakers pro-vide a wide range of high quality steels. AISI H13 is the most common steel used for hot working dies. It is chromium–molyb-denum–vanadium-alloyed steel attaining high purity and very ne structure, if produced by special processing techniquesand progressive quality control [2] . H13 tool steel is characterised by high hardenability, strength and toughness. These spe-cic mechanical properties, along with its moderate cost, have led to extensive use of the steel in hot work applications [3] .In plastic moulding, the tool experiences a complex combination of high mechanical and thermal stresses. Therefore, thermalfatigue and corrosion/oxidation resistance are also important characteristics that shorten the working life of the die and de-crease the reliability of the material [4,5] .

The failure of these dies has been studied for more than 20 years. As the technology of the injection moulding is contin-uously evolving, leading to more severe operating conditions, failures keep occurring to the die tooling. The failure of dieproles made of AISI H13 steel can be attributed to fracture (43%), wear (26%), deection (19%), mixed failure mode(45%), miscellaneous (2%), and to mandrel-related (3%) [6] . However, design deciencies [7] or aggressive environmentalconditions [8] can lead to unexpected die damage. Continuous improvement of die, error correction and careful elaborationof materials used for their manufacture is continuously changing the percentages above.

1350-6307/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.engfailanal.2013.02.028

⇑ Corresponding author. Tel.: +30 210 7219685; fax: +30 210 7253534.E-mail addresses: dim_papg@yahoo.gr (D. Papageorgiou), cmedrea@teipir.gr (C. Medrea), kyriakou_nikos@hotmail.com (N. Kyriakou).

Engineering Failure Analysis 35 (2013) 355–359

Contents lists available at SciVerse ScienceDirect

Engineering Failure Analysis

j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c at e / e n g f a i l a n a l

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Service life increasing and die design improvement lead to cost reduction. This is one of the signicant concerns that theplastic injection end-users face. Failure mode of hardened and nitrocarburized H13 steel as well as tensile properties of thedie were studied, in order to increase wear and fatigue resistance [9] . Surface coatings were considered to increase die life byimproving thermal fatigue [10,11] . Understanding corrosion/oxidation behaviourof H13 steel was helpful to improve servicelife and die performance [5] . Results of failure analysis investigations can be extremely useful to designers of the same orsimilar products.

In the paper, a study has been performed to investigate the failure of H13 die used in plastic injection moulding. The typeof failure and the principal factors that caused it were studied. Furthermore, some suggestions are presented, in order to pre-vent its root causes.

2. Experimental

Data were collected regarding the material selection, manufacturing and operational history. The die was optically in-spected and photographed. Chemical analysis was performed by optical emission spectroscopy using an OE SpectrometerThermo ARL. Rockwell hardness measurements were carried out on the backside of the part. Samples were collected andprepared for optical and electron microscopy. Metallographic examination was conducted using an optical microscope withreversed lenses, OLYMPUS GX51 and microscopic observation was carried out by scanning electron microscopy (SEM) in a JEOL-JMS-5600 LV microscope. Prior to the analysis, the samples were cleaned using ultrasonic method. The failed partswere carefully examined and recorded before any surface cleaning was performed. Local chemical analysis of the sampleswas made by EDX analysis (EDX spectrometer, Oxford Instruments, INCA 200 soft.

3. Results and discussions

An injection mould designed for the production of plastic cups for cylindrical aluminium cans, used in coffee packaging.The cup was engraved with customer’s trademark. The mould consisted of four dies for simultaneous production. The dies’working life was predicted to 10 millions (10,000,000) cycles. The failure occurred in one of the dies after 5 millions(5,000,000) working-cycles ( Fig. 1 ).

The selected steel was Premium AISI H13 and was delivered by Uddeholm Company as Orvar Supreme. The chemicalcomposition of the steel ( Table 1 ), was found in compliance with steel’s producer typical chemical analysis [12] as wellas Standards Specications [13] .

Recorded history indicated that dies were manufactured by the same machine shop. Coarse machining was carried outwith maximum cutting depth of 0.5 mm and the holes were manufactured by milling. After machining the pieces were heattreated to nal hardnessof 50–53 HRC. The following heat treatment cycle was applied: rst preheating at 650 C for30 min,second preheating at 850 C for 30 min, austenitizing at 1030 C for 40 min, mar-quenching in a salt bath at 500 C for10 minand then was let freely in air to 50 C. Three tempering stages at 550, 620 and 590 C respectively for 2 h each. After heattreatment, the dies were returned to the manufacturer for the nal dimensioning process: ne machining with maximumcutting depth of 0.2 mm and grinding. The logotype of the end-user was engraved and the hole for the plastic ow was man-ufactured by Electrical-discharge Machining (EDM). Finally, the pieces were polished by hand.

Fig. 1. General aspect of die’s pressing surface.

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Plastic is injected into the die at a temperature of 170–180 C and the mould is water cooled. The cooling water is sup-plied by bore and was found rich in salts. Salt reduces cooling capacity signicantly and is favoured to corrosion.

The tool has a cylindrical complex shape with several changes in diameter ( Fig. 2 a). Extended corrosion damage can beobserved on various areas ( Fig. 2 b and c). The lateral side of the die has a peripheral cooling waterway Fig. 2 c). This consid-erable cross-section change was not followed by appropriate radii. It is not yet claried, if lack of radii was a result of poordesign or machining malfunction. Radius in heavy diameter changes prevents from the accumulation of stresses to sharpcorners, avoiding stress augmentation. A wide cracking can be observed by naked eye. The crack initiated to the bottomof the cooling area, propagated around the circumference, reached the edge of the piece and through the stamp grooves,ended up to the central hole ( Fig. 2 d).

Hardness measurements were carried out on corrosion-free points to the backside of the tool (i.e. the injection area). Thetool had uniform hardness of 50 HRC; this is the expected value according to designer’s hardness specication.

Optical metallographic specimens revealed a microstructure of ne tempered martensite and carbides. The structure istypical for this type of tool steel and proves that the heat treatment sequence was carried out properly ( Fig. 3 ).

Poor quality cooling water on the cooling area, could have led to corrosion initiation ( Fig. 4 ). The surface irregularities canconvert the regular liquid ow to turbulence. During this type of ow, the cooling agent (i.e. water) may remain stable for aperiod of time (i.e. poor circulation). As a result, the cooling of the component is insufcient and the local temperature incre-ment causes material expansion [14] . In case of regular water ow, the samearea is cooled downrapidly and it shrinks. Obvi-ously, during operation the component was subjected to continuous expansion and shrinking cycles that caused extensivematerial strain (because of the thermal shocks) and led to crack initiation ( Fig. 4 a). Contact of cooling water with poor sur-face promotes surface corrosion and favours crack development. Crack grow and spread due to expansion and shrinkagestresses that developed because of the trapped water at local notches due to corrosion ( Fig. 4 b). Both processes co-existedand caused tool failure. Corrosion increases the surface roughness, hinders the regular water ow, intensies the thermalshocks and removes material from the particular area, reducing locally the strength.

Scanning electron microscopy on the main crack cross section showed the coalescence of propagating crack and the ex-panded corrosion on die surface. Crack was developed inside the mould, tearing the material. Furthermore, smaller branchesof secondary cracks emanated from the main crack ( Fig. 5 a). Intergranular fracture surfaces are predominantly observed tothe crack propagation area ( Fig. 5 b). These cracks were the new grooves for the cooling water, and led to the development of the crack. On the surface, around the crack, corroded stripes can be observed ( Fig. 5 c).

Chemical qualitative analysis was consistent with the typical chemical composition of the die. The main elements wereiron (Fe) and chromium (Cr), while carbon (C), vanadium (V) and molybdenum (Mo) also existed. High oxygen concentrationwas revealed as typical of intensive corrosion presence.

Table 1

Chemical composition of the die compared with relevant specication limits.

Tool’s composition Orvar Supreme Uddeholm Standard DIN composition

Component (wt.%)C 0.34 0.39 0.35–0.42Si 0.85 1.10 0.80–1.20Mn 0.41 0.40 0.25–0.50Cr 5.19 5.20 4.80–5.50

Mo 1.20 1.40 1.20–1.50V 0.80 0.90 0.90–1.10

Fig. 2. General aspect of the side view of the die: (a) detail of the hole used for the assembly; (b) the rest of the mould; (c) detail of the cooling area; (d)detail of the moulding face.

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4. Conclusions

Poor design and improper cooling agent are responsible for the die failure. The water corroded the cooling surface leadingto increased surface roughness. The surface irregularities converted the laminar liquid ow into turbulence. In the coolingarea, the local turbulent ow induced high stresses that led to crack initiation. Corroded crack tip worsens the stress contri-bution around it and accelerates crack propagation. The crack propagated towards the mould-surface and ended at the injec-tion hole. It is a usual case of corrosion fatigue failure.

The most effective method to prevent similar failures is the reduction of the stress contribution. Furthermore, the inten-sive corrosion could have been avoided by using salt-free cooling water. In addition, selection of corrosion fatigue resistantsteels is desirable. Some properties and characteristics of H13 tool steel hard coatings may inhibit corrosion fatigue.

Fig. 3. Optical micrographs showing the steel microstructure.

Fig. 4. SEM micrographs: (a) initiation of the corrosion fatigue crack; (b) crack propagation in association with corrosion.

Fig. 5. SEM micrographs on cross section of the main crack: (a) crack coalescence; (b) internal oxidation zone consisting of intergranular fracture; (c)expanded oxidation around the cracks.

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