Description

The present invention relates to a mould for injection moulding, especially for injection moulding of optical components of automotive lighting devices, comprising a mould body with a coating applied on a surface of the mould body.

PRIOR ART

Moulds for injection moulding comprise a mould body with a cavity, which represents the negative form of the parts to be manufactured. The surface of the mould body is subject to various requirements concerning e.g. surface quality, wear resistance or tribological properties. Therefore, the state-of-the-art comprises a large number of dif ferent approaches to tailor the properties of the mould body surface depending on the precise application, e.g. by mechanical treatments or by application of functional coat ings.

In the field of automotive lighting devices, many optical components are commonly manufactured by means of injection moulding, e.g. single lenses, micro-lens arrays, reflectors, diffusors, diffractive and holographic elements, anti-reflex structures, optical fibres, light guides, transparent housings or cover lenses. A crucial property of such optical components is their surface condition, which significantly determines the func tionality of the components in terms of light manipulation. Smooth surfaces are re quired e.g. for highly efficient lenses or reflectors, whereas a precise roughness level or dedicated surface patterns are mandatory for light diffusing or diffracting elements. The surface condition of the moulded optical components is directly determined by the surface condition of the respective moulds, which therefore has to meet high quality requirements. Additionally, since optical components for automotive devices are typi cally mass products, the tool life of the moulds, and especially their surface condition, should be very long-lasting.

Mould bodies are commonly machined from tool steel workpieces. A traditional treat ment to finish the surface of the mould body is manual polishing in order to attain a preferably smooth surface. Such manually polished surfaces are generally inferior to coated surfaces because the manual treatment suffers from limited precision and re producibility and may furthermore yield to undesired alterations of the surface contour. Coating technologies applied to mould surfaces in the state-of-the-art mainly comprise physical vapour deposition (PVD), chemical vapour deposition (CVD) or electroplating. Both PVD and CVD represent rather complex vacuum deposition methods and typi cally require a significant heating of the substrate in order to facilitate appropriate ad hesion of the coating. Depending on the precise alloy used to manufacture the mould body, such heating during deposition might be detrimental to the mechanical proper ties, especially the hardness, of the mould body. Furthermore, PVD represents highly directional deposition methods, which are thus inappropriate for the application of ho mogeneous coatings on mould bodies with complex, three-dimensional shapes or sur face patterns. Likewise, electroplating is also inappropriate to deposit coatings of ho mogeneous thickness on surfaces with complex topography or patterns due to corre sponding local electric field variations during the deposition process.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an alternative embodiment of a mould for injection moulding comprising a mould body with a coating applied on a surface of the mould body, which is especially appropriate for injection moulding of optical com ponents of automotive lighting devices, but not restricted to such applications.

This object is achieved by a mould as taught by claim 1 of the present invention. Ad vantageous embodiments of the invention are defined in the subclaims.

The invention discloses the technical teaching that the coating applied to the mould body comprises electroless nickel.

Electroless nickel coatings are deposited by means of electroless plating. Electroless nickel plating is an autocatalytic method in which the reduction of nickel ions in a solu tion and the nickel coating deposition are carried out through the oxidation of a chemi cal compound present in the solution itself, i.e. , a reducing agent like hydrated sodium hypophosphite, which supplies electrons. Unlike electroplating, it is not necessary to pass an electric current through the plating solution to form the nickel deposit. Electro less nickel plating creates homogeneous coatings regardless of the geometry of the mould body surface and can even be applied to non-conductive surfaces depending on the catalyst. Electroless nickel coatings exhibit a very dense microstructure and are thus appropriate as corrosion protection for the mould body. The composition of elec troless nickel coatings comprises apart from nickel typically also a certain amount of phosphorous. Furthermore, co-deposition and incorporation of hard particles, e.g. AI2O3 or SiC, can lead to improved wear resistance and extended tool life of the in ventive mould.

As a preferred embodiment of the invention, the coating features a thickness in the range of 2 pm to 50 pm. The precise choice of the coating thickness depends on the specific mould. If a smooth surface with minimum roughness is required, e.g. for injec tion moulding of reflector elements, high-gloss decorative components or cover lenses, a high coating thickness is appropriate to cover and flatten the roughness of the mould body surface, which is typically machined by milling. On the other hand, a thin coating is useful to preserve and protect a micro-topography on the mould body surface, e.g. a dedicated pattern to mould a part for a holographic application.

Advantageously, the surface of the coating features a lower roughness level com pared to the surface of the mould body. Roughness is generally defined as a surface deviation of third, fourth or fifth order, with characteristic length scales on the order of micrometers to nanometers. As stated above, a flat coating surface of low roughness level is mandatory for injection moulding of optical components with smooth surfaces for reflective applications or low-loss light transmission. Furthermore, mould body sur faces with a desired microscopic topography can additionally feature an overlaid nano- scopic roughness, which yields to undesired light loss by diffraction in the moulded op tical components. Such nanoscopic roughness level can be reduced by the applied electroless nickel coating, even if the coating features only a thickness of few microm eters and preserves microscopic patterns of the mould body surface. Therefore, the surface of the coating advantageously features a lower nanoscopic roughness level and/or a lower microscopic roughness level compared to the surface of the mould body.

Advantageously, the surface of the coating features a root-mean-squared profile roughness in the range of 0,001 pm to 0,02 pm. The root-mean-squared profile rough ness R _q is defined as follows. A roughness profile is filtered from the raw profile data and the mean line is. The roughness profile contains

N equally spaced data points along the trace, and y, is the vertical distance from the mean line to the i ^th data point. Then:

Advantageously, the surface of the coating features an average absolute profile slope in the range of 0,01 ° to 0,5°. The average absolute profile slope Rda is defined as: where D _Ϊ is a Savitzky-Golay filter smoothed yi data set calculated according to ASME B46.1

The range of profile roughness levels stated above especially yields appropriate mould surfaces for the injection moulding of various optical components, e.g. of auto motive lighting devices.

Advantageously, the surface of the coating features a lower waviness level compared to the surface of the mould body. Waviness is defined as surface deviations of second order, i.e. with characteristic length scales higher than roughness. Especially a coating with a high thickness is appropriate to reduce the waviness of the as-machined mould body surface.

According to another preferred embodiment of the invention, the electroless nickel comprises phosphorus at a portion in the range of 3 at.% to 14 at.% and/or 6 at.% to 9 at.%. Phosphorus is typically incorporated in electroless nickel coatings by using sodium hypophosphite in the bath solution. High phosphorus content in electroless nickel yields an amorphous microstructure of the coating with high corrosion-protec tion capability. Lower phosphorus content coatings feature higher hardness and wear resistance, both of which can be further enhanced by a heat treatment of the coating, which converts the amorphous coating microstructure into crystalline nickel and a hard nickel phosphide phase. Coatings of medium phosphorous content provide a suitable balance of corrosion-protection properties and hardenability. In industrial practise, cor rosion issues in moulding tools arise e.g. from leakage of tempering hoses or chan nels, or during transport from the manufacturer to the user.

Advantageously, the coating features a hardness in the range of 30 HRC to 80 HRC and/or 55 HRC to 70 HRC. Increased hardness level can be reached by appropriate heat treatment of the coating and provides a high resistance of the mould against me chanical wear and thus a prolonged tool life.

As a preferred embodiment of the invention, the mould body comprises a metallic ma terial, e.g. a tool steel or an aluminium alloy or a copper-beryllium alloy. In case of a projected heat treatment of the electroless nickel coating, a metallic material of appro priate heat resistance has to be chosen. Furthermore, the chosen material preferably features a high corrosion resistance. Advantageously, the mould body features a hard ness in the range of 20 HRC to 70 HRC. Increased hardness level provides a suitable persistence and durability of the mould body for use in mass production of mould in jection components

PREFERRED EMBODIMENT

Additional details, characteristics and advantages of the object of the invention are disclosed in the following description of the respective figures - which in an exemplary fashion - shows preferred embodiments of the mould according to the invention.

Fig. 1 a cross-section of a first embodiment of the inventive mould, and

Fig. 2 a cross-section of a second embodiment of the inventive mould. Fig. 1 and Fig. 2 show cross-sections of segments of inventive moulds 100 each com prising the mould body 1 with the electroless nickel coating 2 applied to the surface 11 of the mould body 11. The surface 11 of the mould body 1 features both microscopic asperities 12 and nanoscopic asperities 13 representative of different orders of the surface roughness. Such different roughness orders stem for example from different machining processes applied to the mould body 1, e.g. milling or electrical discharge machining and/or subsequent grinding or manual polishing. Alternatively, the micro scopic asperities 12 can be part of a dedicated surface pattern, which is intendedly structured into the surface 11 in order to generate a corresponding surface pattern on the moulded part, e.g. for optical elements with light diffusing or light diffracting proper ties. The coating 2 yields a mirroring, high-gloss surface appearance of the mould 100.

The thickness 20 of the coating 2 is chosen according to the desired, application-spe cific surface topography of the coating 2 and/or the functionality of the coating 2 re garding the protection of the mould body 1 from mechanical wear and/or corrosion.

The embodiment of the mould 100 shown in Fig. 1 features a comparably thick coating 2, e.g. with a thickness 20 of about 10 pm to 50 pm, which exhibits a basically flat sur face 21 , thus smoothing the roughness of the underlying surface 11 of the mould body 1. The surface 21 of the coating essentially defines the surface quality of the parts in jection-moulded by the inventive mould 100 and the smooth surface 21 depicted in Fig. 1 is therefore especially appropriate for the manufacturing of optical components with even surfaces, e.g. base bodies for reflectors and decorative elements with thin metallic mirror coatings or for highly transparent lenses or covers panes.

Fig. 2. shows an embodiment of the mould 100 with a coating 2 of much lower thick ness 20, e.g. about 2 pm to 20 pm. The coating 2 flattens the nanoscopic asperities 13 of the surface 11 of the mould body, but essentially preserves the contour of the microscopic asperities 12. The mould 100 is therefore dedicated for injection moulding of optical components with patterned surfaces, e.g. diffusors or diffractive or holo graphic elements. The present invention is not limited by the embodiment described above, which is rep resented as an example only and can be modified in various ways within the scope of protection defined by the appending patent claims.

List of Numerals

100 mould

1 mould body

11 surface (of the mould body)

12 microscopic asperity

13 nanoscopic asperity

2 coating

20 thickness (of the coating)

21 surface (of the coating)