THE EFFECT OF A NITRIDE LAYER ON THE ... · The results for gas nitriding are shown in Fig. 11, while the results for plasma nitriding are shown in Fig. 12. As can be seen from these

THE EFFECT OF A NITRIDE LAYER ON THETEXTURABILITY OF STEELS FOR PLASTIC MOULDS

R. Rahn and R. UferEschmannStahl GmbH & Co. KG

Dieringhauser Straße 161–183

D-51645 Gummersbach

Germany

Abstract This study investigates the dependence of the texturability of 40 CrMnMo7 steel (1.2311), which is used to produce moulds for plasticparts, on thepresence of a nitride layer.

For this purpose, two sets of nine specimen plates were nitrided, usinggas nitriding for one set and plasma nitriding for the other set. In order todetermine the effect of the nitride layer, the surfaces of the plates were groundto various depths, ranging from 0.01 mm to 0.5 mm.

The specimen plates were textured using three different texture patternsand three different etching media.

It was seen that if a suitable etchant (nitric acid) is used, the nitride layerhas no effect on the appearance of the texture. This applies to both nitridingprocesses. The only noticeable effect is that the degree of gloss increases asmore of the nitride layer is ground off.

OBJECTIVE OF THE STUDY

Generating a surface structure on moulds used for injectionmouldingplastic parts, which is also called texturing or etching, isbeing used increas-ingly often. This gives the moulded part an attractive surface, which is ofgreat importance for the appearance and ’touch’ of the workpiece. Thismeans that the surface texture is an important feature of thedesign of aninjection-moulded part. Many injection moulds are surface-hardened usinga nitriding process. This increases the abrasion resistance of the surface and

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1076 6TH INTERNATIONAL TOOLING CONFERENCE

improves the ejection characteristics of the injected partfor release from themould. Although nitriding of the mould should take place following textur-ing, it is not uncommon for moulds to be supplied for texturing subsequentto nitriding.

The objective of this study is to determine whether the desired textureappearance can be obtained in the presence of a previously applied nitridelayer. Besides leading to higher costs, under certain conditions this can resultin the failure of the mould if the mould thickness is thereby significantlyaltered.

ESCHMANNSTAHL AND ESCHMANN TEXTURESINTERNATIONAL

EschmannStahl provide end-to-end service for three product groups: hot-work steel, cold-work steel and mould steel for plastic moulds. This serviceextends from sawing the raw material through machining mould assembliesto texturing of the surfaces of injection moulds.

DESCRIPTION OF THE PHOTOCHEMICALTEXTURING OF INJECTION MOULDS

Etching, which is also called structuring or texturing, is nowadays pre-dominantly performed using a photochemical process.

The process of photochemical texturing can be described as follows:A photosensitive lacquer is applied to the inner surface of the mould that

is to be textured. Following this, a film with the appropriatedesign is placedon top of the surface. Exposure to intense light causes the lacquer below thetransparent areas of the film to harden. The unexposed, soft portions of thelacquer film are then washed away using a special developer. This producesareas of the steel surface that are covered by the lacquer filmand other areasthat are free of lacquer. In the areas that are free of lacquer, steel is removedfrom the surface in an etching bath, with the amount removed dependingon the concentration of the acid and the duration of the etching. The actualetching times are relatively short and lie at around 60 seconds for an etchingdepth of 0,01–0.02 mm. After the rest of the lacquer has been removed,additional etching is possible.

Using this multistage etching process, it is possible to obtain a variety ofdifferent etching effects.

The Effect of a Nitride Layer on the Texturability of Steels for Plastic Moulds1077

Figure 1 shows the etching process in schematic form [3].

Figure 1. Representation of the working process for producing photoetched textures.

STEEL PROPERTIES RELATED TO TEXTURABILITY

Fundamentally, all common types of tool steel can be textured. The steelmust have the following properties to allow a texture with a homogeneousappearance to be achieved:

a fine, uniform microstructure,

a good level of purity with regard to sulphide and oxide content,

few segregations.

As a rule, steels that have been tempered and hardened are used fortexturing. For injection moulds, EschmannStahl employs the EST (Extra-Structure) grade, which has an excellent purity level (a maximum K4 value


of 20 per DIN 50 602) and reduced sulphur content (< 0.005 %)) as theresult of special metallurgical treatment.

Figure 2 shows the homogeneous martensitic microstructureof the steelgrade 40CrMnMo7.

Figure 2. Martensitic microstructure of steel grade 40 MnCrMoV7 (1.2311) for mouldsfor plastic parts.

NITRIDING MOULDS FOR PLASTIC OBJECTS

The objective of a nitriding treatment is to form a diffusionlayer havinga nitride hardening depth of up to 0.9 mm, depending on the material.

A prerequisite for a successful nitriding treatment is the use of an alloysteel containing nitride-forming elements, such as aluminium, chromium,vanadium, molybdenum or manganese.

Moulds for plastic objects are frequently nitrided, due to the increasedabrasion resistance and better mould ejection characteristics of nitridedmoulds.

A distinction is made between gas nitriding and plasma nitriding.

GAS NITRIDING

Gas nitriding takes place at a temperature of 490 to 530℃.


In gas nitriding, gaseous ammonia (NH3) is used as the nitrogen carrier.This gas dissociates, and the atomic nitrogen present in thegas diffuses intothe surface of the steel and combines with the iron to form iron nitrides(Fe2−3N & Fe4N). As the duration of the treatment is extended, the nitrogendiffuses deeper into the steel. This leads to the formation of special nitrides[2].

PLASMA NITRIDING

In plasma nitriding, the component to be treated is placed ina vacuumretort and brought in contact with a treatment gas consisting of a nitrogen-hydrogen mixture. The treatment temperature lies between 250 and 600℃. Afixed or pulsating dc voltage (300–800 V) is applied between the component(which forms the cathode) and the vacuum retort (which formsthe anode)to generate a plasma by means of a gas discharge.

The ionised nitrogen molecules strike the surface of the component withhigh kinetic energy and are split into atoms and initiate thenitriding process,which then continues using the previously described diffusion process. Sinceatoms are released from the surface at the same time (by sputtering), thisprocess can also be used to nitride corrosion-resistant steels, since the passivechromium layer can be destroyed [2].

FORM OF THE NITRIDE LAYER

For both nitriding processes, the nitride layer consists of:

an outer compound layer made up of iron and nitrogen compounds,and

a diffusion layer underneath the outer layer, in which the nitrogen isembedded in the iron matrix

The compound layer, which has thickness of 0.002–0.008 mm, exhibitsextreme hardness combined with good corrosion resistance [1].

The considerably thicker diffusion layer is less resistantto corrosion.Figure 3a and 3b shows the structure of the nitride layer, both schemati-

cally and in the form of a micrograph.


(a) schematic (b) micrograph

Figure 3. Structure of the nitride layer.

EXPERIMENTAL PROCEDURE

Experiments were carried out on 18 specimen plates. The plates weresawn from a forged steel plate with raw dimensions of 205×600×3500 mmand then machined.

In order to minimise the effect of the material used, all plates were takenfrom the same block. The material grade is 40CrMnMo7 (material number1.2311 on the Stahleisen list). It corresponds to the American grade P20.The chemical composition of the steel is shown in Table 1 . Thebar was pre-tempered to a hardness of 285 Brinell. Of the 18 plates, 9 weregas nitridedand 9 were plasma nitrided. Gas nitriding took place at a temperature of510℃ with a duration of 36 hours. The depth of the nitride layer is here 0.44mm. Plasma nitriding took place at a temperature of 530℃ with a durationof 24 hours. The depth of the nitride layer was 0.30 mm.

Table 1. Chemical composition of 40CrMnMo7,wt%

C Si Mn P S Cr Ni Mo V W

0.40 0.32 1.51 0.016 0.001 1.90 - 0.21 - -

After the nitriding, the surfaces of the plates were ground to variousdepths. The grinding depths for each of the plates were 0.01 mm, 0.10


mm, 0.30 mm and 0.50 mm, Fig. 4. The grinding depths were chosen inaccordance with the depths of the nitride layers.

Figure 4. Cross section of the specimen plates after nitriding.

The prepared plates were taken to the etching works of Eschmann TexturesInternational, where they were etched using three different texture patterns.As a further parameter, three different acid baths were used: nitric acid,hydrochloric acid and ferric chloride. The process parameters, such as thecomposition of the acid, the bath temperature and duration of the etching,corresponded to the operating conditions developed and fixed by EschmannTextures for the material and texture patterns used.

Figure 5 shows the experimental set-up for the specimen plates. Figure 6describes the texture patterns. The K09 texture is a coarse leather texture,while the F215 texture represents a finely figured texture. The F108 textureis a very finely structured ’powder’ texture.


Figure 5. Experimental set-up for the specimen plates.

Figure 6. Description of the texture patterns.

EXPERIMENTAL RESULTS

The 18 textured specimen plates were evaluated in terms of the appearanceof their textures.

Figure 7 shows a specimen plate with the K09 texture. The various grind-ing depths can be seen on this plate.


The evaluation scale is reproduced in Fig. 8. It ranges from No. 3 fora good texture with a uniform appearance to No. 1, ’texture appearanceunusable’. Figures 9 and 10 show the appearance of the K09 texture withgood and unsatisfactory results. An evaluation sheet was generated for eachspecimen plate.

The results of the evaluations of the texture appearances are listed sepa-rately according to the nitriding process used. The resultsfor gas nitridingare shown in Fig. 11, while the results for plasma nitriding are shown inFig. 12.

As can be seen from these results, the texturing is well formed in all caseswhen nitric acid is used as the etchant, independent of the nitriding layer.This is true for both nitriding processes and all three texture patterns.

With the hydrochloric acid and ferric chloride etchants, a trend towardbetter texturing results with increasing grinding depth can be seen, Fig. 13and Fig. 14.

However, here an optimum texture appearance is not achievedeven whenthe base microstructure is reached at a grinding depth of 0.5mm. This isparticularly the case with the fine F108 texture.

The evaluation of the degree of gloss shows a matt gloss for the nitridedsurface for both nitriding processes. With increasing grinding depth, thegloss increases for all etchants and texture patterns.

Figure 7. Photograph of a textured specimen plate.


CONCLUSIONS

In summary, it can be said that good texturing results can be obtainedeven with nitrided surfaces with the use of the appropriate etchant (in thecase of this study, nitric acid).

The nitriding process and the texture pattern are not important in thisregard.

If the proper etchant is used, the nitride layer has no effecton the textura-bility of steels for plastic moulds. However, the level of gloss is affected bythe nitride layer.

ACKNOWLEDGMENTS

We would like to thank the following for their support and assistance incarrying out this work: Dipl.-Ing. H.J. Grasemann at Messrs, Carl Gom-mann, Hardening works, Remscheid, Germany also Mr A. Scherer-Eurichand Mr M. Lennartz, Eschmann Textures International GmbH, Neuenstadta. K., Germany. Our special thanks goes to Mr H.D. Eschmann, who madethis study possible.

REFERENCES

[1] Klöckner Ionon GmbH ’Ionitrieren ... denn auf die Oberfläche kommt es an’ [Ion ni-triding… since the surface is what counts]

[2] Carl Gommann ’Nitrieren und Nitrocarburieren von Stahlbauteilen’ [Nitriding and ni-trocarburising steel components]

[3] Bayer Leverkusen Technical Application Data ’Die strukturierende Formnestoberflächeund deren Abformung durch Thermoplaste’ [The structuring cavity surface and itsdeformation by thermoplasty] Dieter Schauf


Figure 8. Evaluation scale for texturing results.

Figure 9. Photograph of the K09 texture pattern with a good result.


Figure 10. Photograph of the K09 texture pattern with a poor result.

Figure 11. Results of the texturing experiments for the gas-nitrided specimen plates.


Figure 12. Results of the texturing experiments for the plasma-nitrided specimen plates.

Figure 13. Effect of grinding depth on texturing results (gas-nitrided specimen plates).


Figure 14. Effect of grinding depth on texturing results (plasma-nitrided specimen plates).

Documents

THE EFFECT OF A NITRIDE LAYER ON THE ... · The results for gas nitriding are shown in Fig. 11, while the results for plasma nitriding are shown in Fig. 12. As can be seen from these