glossyCAST: mirror-gloss cast surfaces without the use of release agents

Thanks to its mechanical properties and the possibility of surface finishing, zinc die casting is ideal for the production of everyday objects. High quality standards apply in particular for decorative applications, such as in the automotive sector or for bathroom fittings. Defects can be caused by release agents in the casting process or post-processing, which makes electroplating more difficult and increases the number of rejects. A research project by Fraunhofer IFAM and fem has developed a new release coating for die casting molds that does not require release agents. Polished mold surfaces produce high-quality castings that require less post-processing. The improved surface quality shortens the pre-treatment time and saves material, time and waste water during electroplating.

Introduction

Today, zinc die-cast components are high-tech products that are used for a wide variety of applications and are used in many areas of everyday life, in automotive, mechanical and apparatus engineering, in electrical engineering and electronics as well as in construction and furniture making. Around 2 million tons of zinc die-cast parts are produced worldwide every year, of which around 70,000 tons are produced in Germany [1]. The reason for the market success of zinc die casting is that it offers an ideal technical basis for the cost-effective production of perfect surfaces and allows the tightest tolerances for the production of lightweight, thin-walled components. The process is therefore always in demand where large quantities and high precision are required. Precise surface structures, such as fine threads or inscriptions, can also be integrated during the casting process. Zinc die-cast components are ideal for subsequent surface treatment. Depending on the intended use and requirement profile, this can be a decorative or functional galvanic coating, passivation or painting. Around 50 % of the die-cast parts produced are electroplated, with the coating costs accounting for around a third of the total costs [2]. In the production of zinc die-cast components, release agents and lubricants must be applied to the mold cavity before each casting cycle so that the zinc die-cast parts are safely demolded and thus not damaged when ejected from the mold. However, due to the use of these additives, the cast parts must first be elaborately reworked in order to guarantee the surface quality expected by the market. In addition, the evaporation and pyrolysis of the additives during the casting process can lead to the formation of pores near the surface, one of the biggest quality problems in zinc die casting. Typical casting defects caused by the use of release agents include pores, streaks, roughness, flow lines and release agent residues on the surface of the casting. In the case of decorative castings, defects near the surface are only exposed during the downstream process steps of grinding and polishing. Some defects only become visible on the shiny, reflective surface after electroplating and only lead to rejects, the so-called "precious scrap", at the very end of the complex process and value chain [3,4].

Fig. 1: SEM images of zinc die-cast components a) before polishing the mold, b) after polishing the mold

Mold coatings for die casting molds

To overcome the limitations described above, permanent wear protection coatings currently available on the market for die casting molds, such as those produced using CVD and PVD processes, represent a partial solution. CrN, TiAlN, _Al2O3, _ZrO2, _TiO2 and other nitrides and borides are used here. They already allow a significant reduction in release agent consumption. However, it is technically challenging to apply these coatings to complex shapes with good adhesion, as they are usually brittle and exhibit high residual stresses. In addition, the coefficient of thermal expansion differs significantly from that of the substrate material. This requires the additional application of complex and expensive adhesion promoter layers. In this respect, there are technical limitations and economic obstacles to the widespread use of such coatings. Plasma polymer release coatings offer a promising alternative to conventional PVD coatings in zinc die casting. In contrast to the state of the art, these coatings can be gently removed if necessary, so that a new coating can be applied several times without any loss of quality. Plasma polymer coatings not only offer excellent wear protection, but also act as release coatings due to their low surface energy. This dual function makes them particularly valuable in applications where both protection against mechanical wear and effective separation of materials are required. Thanks to their special structure and composition, plasma polymer coatings can extend the service life of die casting molds and at the same time increase the efficiency of production processes.

Fig. 2: Polished mold with (a) high gloss, (b) technical gloss

Surface treatment processes for zinc die casting

Depending on the intended use, the resulting requirement profile for the surface of a zinc die-cast component can be implemented using a large number of process variants, such as painting, application of conversion coatings or electroplating. An overview of typical finishing processes is shown in Table 1. Purely functional zinc die-cast components are not electroplated, but passivated directly. Depending on the appearance and corrosion protection requirements, galvanizing is followed by passivation. This makes it possible to match the appearance of combined galvanized steel parts. Alternatively, zinc die-castings can be painted after appropriate pre-treatment. If high demands are placed on the zinc die-cast parts in terms of corrosion resistance, appearance and feel, galvanic processes are generally chosen. For decorative applications, the coating sequence of copper, nickel and chrome is often used. The undercoats are used to ensure good adhesion, to level the surface and to optimize corrosion protection, while the top coats are used to create a visual impression. The total layer thickness is in the range of 25-50µm, which requires a relatively large amount of material and long process times and makes dimensional accuracy more difficult.

Materials and methods Release agent-free zinc die castings

The advanced release and easy-to-clean coating developed at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) is applied as a gradient layer using a cold plasma process and is suitable for materials such as tool steel, stainless steel and aluminum. Due to the special physical properties of the coating process, all molded parts and even mirror-finish surfaces can be coated with a thin, homogeneous layer. In practical tests, die-casting molds were polished to a mirror finish in parts of the cavity and the entire die-casting mold was coated with a plasma-polymer release coating. In the casting process, the zinc die-cast parts were produced using standard die-casting parameters in a hot chamber process. By dispensing with the spraying process, it was shown that process time can be saved and the output of cast parts is correspondingly higher.

Parameters of electrochemical deposition

The zinc die-cast samples (Fig. 3a + b) were first cathodically degreased, then decapped/activated and then coated in a cyanide copper electrolyte. Finally, the components were nickel-plated in a bright nickel electrolyte (Fig. 7a + b). The electrolytes and treatment parameters used are listed in Table 2.

Results and discussion: Microscopic examination and evaluation of the casting quality of the zinc die-cast parts

As part of the research work at the Research Institute for Precious Metals and Metal Chemistry (fem), the focus was placed on analyzing the samples produced with and without release agents, whereby the quality was examined both before and after electroplating. Investigations on zinc die-cast parts using SEM and confocal microscope initially showed that the morphology (Fig. 1 left) and the surface roughness (Fig. 8a) of the cast samples are not sufficient to dispense with post-processing methods such as grinding and polishing. To reduce the surface roughness and to produce high-gloss surfaces, the surfaces of the interchangeable cores were first polished in the mold and then coated. The surfaces of the interchangeable cores were polished to a technical high gloss (B2,_Ra = 0.05 µm) and high gloss (B0,_Ra = 0.02 µm) and then coated with the plasma-polymer release layers (Fig. 2a + b). The microscopic images of the surface of the component after mechanical polishing of the mold show clear changes in morphology and a reduction in surface roughness (Fig. 8b). Figures 3a + b show zinc die-cast components (pigs) that were manufactured using the hot chamber process before electroplating. The microscopic examinations and roughness measurements (Fig. 1b + 8b) show a smooth surface with low roughness.

Examination in cross-section using an optical microscope

The examination of zinc die-cast samples in cross-section enables the cast skin to be assessed. The zinc die-cast components were etched with 10% NaOH and examined in cross-section with an optical microscope in order to characterize the microstructure, grain boundaries, grain surfaces and casting skin of the samples. It is often difficult to determine the exact thickness of a casting skin, but in this case the thickness of the casting skin was assumed to be 30 to 45 µm (Fig. 4). A high-quality casting skin for zinc die casting, which is produced without the use of release agents, is characterized by a fine-grained structure and low porosity. However, it should be noted that it is practically impossible to completely avoid pores and blowholes in die castings [7]. As shown in Figure 4, the microscopic analysis carried out confirms the presence of small and small pores in the sample examined. Due to the high-quality zinc die-cast surfaces, post-processing (grinding and polishing) can be dispensed with. As a result, the casting skin remains intact.

Pore analysis with 3D X-ray computed tomography:

Zinc die-cast samples normally have many pores and blowholes. This is due to turbulence caused by the flow of the molten metal during the die casting process and the use of release agents. This makes it difficult to avoid pores and blowholes. However, this undesirable effect can be reduced by optimizing the casting parameters and avoiding the use of release agents. To check the casting qualities, a pore analysis was carried out using 3D X-ray computed tomography. The results show that zinc die castings produced without release agents have fewer pores and blowholes compared to those produced using conventional methods. Figure 5 presents the pore analysis of the sample produced without release agent.

Electroplating

The conventionally manufactured zinc die-cast components usually have a high roughness. In order to reduce the surface roughness during electrodeposition, a bright copper layer made of an acidic copper electrolyte is used between the copper layer made of a cyanide electrolyte and the bright nickel layer (Fig. 6a).

Fig. 6: a) Typical layer sequence for zinc die casting coating, b) new layer sequence (omission of acidic copper, reduction of the cyanide copper and bright nickel layer thicknesses)

The main advantage of the project is the production of components with low roughness values, which makes it possible to dispense with the deposition of a conventional bright copper layer. As a result of the low surface roughness, the effect of reducing the copper and nickel layer thickness was also taken into consideration.

The exposure time and the current density of the galvanic processes were selected in the project so that 5 and 10 µm were deposited from the cyanide copper electrolyte and 5 and 10 µm nickel from bright nickel electrolyte. As the surface of the zinc die-cast components, which were produced without a release agent, has a low roughness, a bright copper layer (acid copper) is not deposited (Fig. 6b).

The next step was to investigate the effect of reducing the coating thickness on corrosion resistance and roughness. The following illustrations show the zinc die-cast components after the electroplating process (Fig. 7).

Roughness measurement using a confocal microscope

The zinc die-cast samples were examined with a confocal microscope before and after the coatings in order to determine the surface roughness (_Ra, _Rz). The roughness measurements showed that the surface roughness of the zinc die-cast components was reduced after polishing the mold (technical gloss and high gloss) (Fig. 8b). The roughness measurements of the samples were carried out again following the coating processes (cyanide and bright copper, nickel). The coatings resulted in an additional slight reduction in the roughness values of the samples (Fig. 8c).

Fig. 8: Measurement of the sample roughness (mean value) using a confocal microscope (a) before polishing the mold, (b) after polishing the mold and (c) after the copper/nickel coating.

Corrosion tests (CASS test)

Fig. 9: Zinc die-cast components (after 4 hours of corrosion testing), a) 10 µm coating thickness with release agent, b) 5 µm coating thickness without release agent, c) 10 µm coating thickness without release agent

Fig. 10: Zinc die-cast components (after 20 hours of corrosion testing), a) 10 μm coating thickness with release agent, b) 5 μm coating thickness without release agent, c) 10 μm coating thickness without release agent

The salt spray test is the most commonly used method for corrosion testing. It is used both to examine protective coatings and to assess the corrosion behavior of different materials. The copper-accelerated salt spray test (CASS test) is another test method in accordance with DIN EN ISO 9227 [7]. In the CASS test, copper salts are introduced into the test solution to intensify the corrosive attack. To evaluate the corrosion resistance, the zinc die-cast components were tested for a period of 4 and 20 hours (Fig. 9 + 10). The results of the corrosion test show that the corrosion resistance of the samples produced without release agent is higher than that of the substrates produced with release agent. This could be due to the fact that release agents can impair the adhesion of protective coatings or surface treatments. If residues of the release agent remain on the surface of the zinc die-cast part after production, this can impair the adhesion of protective coatings or surface treatments and thus negatively affect corrosion resistance. Some release agents can enter into chemical reactions with the zinc or other alloying elements. These reactions could lead to corrosion or impair the protective layers of the material. As expected, the corrosion tests show that reducing the copper and nickel coating from 10 to 5 microns reduces the corrosion resistance of the components. However, this thinner coating is possible for decorative applications.

Literature

[1] Initiative Zink im Netzwerk der WVMetalle: Zinc die casting - range of applications and innovations (2016). https://www.bdguss.de/fileadmin/content_bdguss/BDG
[2] Pfeifer-Schäller, I.; Klein, F.: Defects in the surface finishing of zinc die-cast parts; 23rd Aalen Foundry Symposium, Lecture 10, 2002
[3] International Zinc Association: International Zinc Die Casting Conference Vienna, September 22-24, 2010
[4] Walkington, W. G., North American Die Casting Association, DIE CASTING DEFECTS - CAUSES AND SOLUTIONS, Wheeling, Illinois, 2007
[5] N.N.: DE 10 2017 131 085 A1 "Plasma-polymeric solid, in particular plasma-polymeric layer with hydrocarbon network formation, its use and method of manufacture
[6] Brenner, T.; Vissing, K.: New insight into organosilicon plasma-enhanced chemical vapor deposition layers and thear crosslinking behavior by calculating the degree of Si-networking. Plasma Process Polym. Vol. 17 (2020)
[7] Corrosion testing in artificial atmospheres - salt spray testing. (2022). Standard