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

Transcription

1 THE EFFECT OF A NITRIDE LAYER ON THE TEXTURABILITY OF STEELS FOR PLASTIC MOULDS R. Rahn and R. Ufer EschmannStahl GmbH & Co. KG Dieringhauser Straße D Gummersbach Germany Abstract This study investigates the dependence of the texturability of 40 CrMnMo 7 steel (1.2311), which is used to produce moulds for plastic parts, on the presence of a nitride layer. For this purpose, two sets of nine specimen plates were nitrided, using gas nitriding for one set and plasma nitriding for the other set. In order to determine the effect of the nitride layer, the surfaces of the plates were ground to various depths, ranging from 0.01 mm to 0.5 mm. The specimen plates were textured using three different texture patterns and three different etching media. It was seen that if a suitable etchant (nitric acid) is used, the nitride layer has no effect on the appearance of the texture. This applies to both nitriding processes. The only noticeable effect is that the degree of gloss increases as more of the nitride layer is ground off. OBJECTIVE OF THE STUDY Generating a surface structure on moulds used for injection moulding plastic parts, which is also called texturing or etching, is being used increasingly often. This gives the moulded part an attractive surface, which is of great importance for the appearance and touch of the workpiece. This means that the surface texture is an important feature of the design of an injection-moulded part. Many injection moulds are surface-hardened using a nitriding process. This increases the abrasion resistance of the surface and 1075

2 1076 6TH INTERNATIONAL TOOLING CONFERENCE improves the ejection characteristics of the injected part for release from the mould. Although nitriding of the mould should take place following texturing, it is not uncommon for moulds to be supplied for texturing subsequent to nitriding. The objective of this study is to determine whether the desired texture appearance can be obtained in the presence of a previously applied nitride layer. Besides leading to higher costs, under certain conditions this can result in the failure of the mould if the mould thickness is thereby significantly altered. ESCHMANNSTAHL AND ESCHMANN TEXTURES INTERNATIONAL EschmannStahl provide end-to-end service for three product groups: hotwork steel, cold-work steel and mould steel for plastic moulds. This service extends from sawing the raw material through machining mould assemblies to texturing of the surfaces of injection moulds. DESCRIPTION OF THE PHOTOCHEMICAL TEXTURING OF INJECTION MOULDS Etching, which is also called structuring or texturing, is nowadays predominantly 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 appropriate design is placed on top of the surface. Exposure to intense light causes the lacquer below the transparent areas of the film to harden. The unexposed, soft portions of the lacquer film are then washed away using a special developer. This produces areas of the steel surface that are covered by the lacquer film and other areas that are free of lacquer. In the areas that are free of lacquer, steel is removed from the surface in an etching bath, with the amount removed depending on the concentration of the acid and the duration of the etching. The actual etching times are relatively short and lie at around 60 seconds for an etching depth of 0, 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 of different etching effects.

3 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 steel must have the following properties to allow a texture with a homogeneous appearance 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 for texturing. For injection moulds, EschmannStahl employs the EST (Extra- Structure) grade, which has an excellent purity level (a maximum K4 value

4 1078 6TH INTERNATIONAL TOOLING CONFERENCE of 20 per DIN ) and reduced sulphur content (< %)) as the result of special metallurgical treatment. Figure 2 shows the homogeneous martensitic microstructure of the steel grade 40CrMnMo7. Figure 2. Martensitic microstructure of steel grade 40 MnCrMoV7 (1.2311) for moulds for plastic parts. NITRIDING MOULDS FOR PLASTIC OBJECTS The objective of a nitriding treatment is to form a diffusion layer having a 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 alloy steel containing nitride-forming elements, such as aluminium, chromium, vanadium, molybdenum or manganese. Moulds for plastic objects are frequently nitrided, due to the increased abrasion resistance and better mould ejection characteristics of nitrided moulds. A distinction is made between gas nitriding and plasma nitriding. GAS NITRIDING Gas nitriding takes place at a temperature of 490 to 530.

5 The Effect of a Nitride Layer on the Texturability of Steels for Plastic Moulds1079 In gas nitriding, gaseous ammonia (NH 3 ) is used as the nitrogen carrier. This gas dissociates, and the atomic nitrogen present in the gas diffuses into the surface of the steel and combines with the iron to form iron nitrides (Fe 2 3 N & Fe 4 N). As the duration of the treatment is extended, the nitrogen diffuses 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 in a vacuum retort and brought in contact with a treatment gas consisting of a nitrogenhydrogen mixture. The treatment temperature lies between 250 and 600. A fixed or pulsating dc voltage ( V) is applied between the component (which forms the cathode) and the vacuum retort (which forms the anode) to generate a plasma by means of a gas discharge. The ionised nitrogen molecules strike the surface of the component with high kinetic energy and are split into atoms and initiate the nitriding process, which then continues using the previously described diffusion process. Since atoms are released from the surface at the same time (by sputtering), this process can also be used to nitride corrosion-resistant steels, since the passive chromium 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 is embedded in the iron matrix The compound layer, which has thickness of mm, exhibits extreme hardness combined with good corrosion resistance [1]. The considerably thicker diffusion layer is less resistant to corrosion. Figure 3a and 3b shows the structure of the nitride layer, both schematically and in the form of a micrograph.

6 1080 6TH INTERNATIONAL TOOLING CONFERENCE (a) schematic Figure 3. Structure of the nitride layer. (b) micrograph EXPERIMENTAL PROCEDURE Experiments were carried out on 18 specimen plates. The plates were sawn from a forged steel plate with raw dimensions of mm and then machined. In order to minimise the effect of the material used, all plates were taken from the same block. The material grade is 40CrMnMo7 (material number on the Stahleisen list). It corresponds to the American grade P20. The chemical composition of the steel is shown in Table 1. The bar was pretempered to a hardness of 285 Brinell. Of the 18 plates, 9 were gas nitrided and 9 were plasma nitrided. Gas nitriding took place at a temperature of 510 with a duration of 36 hours. The depth of the nitride layer is here 0.44 mm. Plasma nitriding took place at a temperature of 530 with a duration of 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 After the nitriding, the surfaces of the plates were ground to various depths. The grinding depths for each of the plates were 0.01 mm, 0.10

7 The Effect of a Nitride Layer on the Texturability of Steels for Plastic Moulds1081 mm, 0.30 mm and 0.50 mm, Fig. 4. The grinding depths were chosen in accordance 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 Textures International, 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 the composition of the acid, the bath temperature and duration of the etching, corresponded to the operating conditions developed and fixed by Eschmann Textures for the material and texture patterns used. Figure 5 shows the experimental set-up for the specimen plates. Figure 6 describes the texture patterns. The K09 texture is a coarse leather texture, while the F215 texture represents a finely figured texture. The F108 texture is a very finely structured powder texture.

8 1082 6TH INTERNATIONAL TOOLING CONFERENCE 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 appearance of their textures. Figure 7 shows a specimen plate with the K09 texture. The various grinding depths can be seen on this plate.

9 The Effect of a Nitride Layer on the Texturability of Steels for Plastic Moulds1083 The evaluation scale is reproduced in Fig. 8. It ranges from No. 3 for a good texture with a uniform appearance to No. 1, texture appearance unusable. Figures 9 and 10 show the appearance of the K09 texture with good and unsatisfactory results. An evaluation sheet was generated for each specimen plate. The results of the evaluations of the texture appearances are listed separately according to the nitriding process used. 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 results, the texturing is well formed in all cases when 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 toward better texturing results with increasing grinding depth can be seen, Fig. 13 and Fig. 14. However, here an optimum texture appearance is not achieved even when the base microstructure is reached at a grinding depth of 0.5 mm. This is particularly the case with the fine F108 texture. The evaluation of the degree of gloss shows a matt gloss for the nitrided surface for both nitriding processes. With increasing grinding depth, the gloss increases for all etchants and texture patterns. Figure 7. Photograph of a textured specimen plate.

10 1084 6TH INTERNATIONAL TOOLING CONFERENCE CONCLUSIONS In summary, it can be said that good texturing results can be obtained even with nitrided surfaces with the use of the appropriate etchant (in the case of this study, nitric acid). The nitriding process and the texture pattern are not important in this regard. If the proper etchant is used, the nitride layer has no effect on the texturability of steels for plastic moulds. However, the level of gloss is affected by the nitride layer. ACKNOWLEDGMENTS We would like to thank the following for their support and assistance in carrying out this work: Dipl.-Ing. H.J. Grasemann at Messrs, Carl Gommann, Hardening works, Remscheid, Germany also Mr A. Scherer-Eurich and Mr M. Lennartz, Eschmann Textures International GmbH, Neuenstadt a. K., Germany. Our special thanks goes to Mr H.D. Eschmann, who made this study possible. REFERENCES [1] Klöckner Ionon GmbH Ionitrieren... denn auf die Oberfläche kommt es an [Ion nitriding since the surface is what counts] [2] Carl Gommann Nitrieren und Nitrocarburieren von Stahlbauteilen [Nitriding and nitrocarburising steel components] [3] Bayer Leverkusen Technical Application Data Die strukturierende Formnestoberfläche und deren Abformung durch Thermoplaste [The structuring cavity surface and its deformation by thermoplasty] Dieter Schauf

11 The Effect of a Nitride Layer on the Texturability of Steels for Plastic Moulds1085 Figure 8. Evaluation scale for texturing results. Figure 9. Photograph of the K09 texture pattern with a good result.

12 1086 6TH INTERNATIONAL TOOLING CONFERENCE 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.

13 The Effect of a Nitride Layer on the Texturability of Steels for Plastic Moulds1087 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).

14 1088 6TH INTERNATIONAL TOOLING CONFERENCE Figure 14. Effect of grinding depth on texturing results (plasma-nitrided specimen plates).