Surface Quality of Moulds Flattening with diamonds optimises hardened steel surfaces of tools and moulds

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Germany — Researchers at the TU Berlin have been looking at how the surfaces of components used of injection moulds could be optimised. Here are some of the results.

Tool and mould making is a key technology for the replicative production of economically important plastic components in the field of medical technology and automotive engineering. Hardened steel materials are used for the injection moulding process of these components. These materials are subject to constantly increasing requirements in terms of mould accuracy G_F and surface roughness characteristics [BEL13, REI13]. Of crucial importance here is, among other things, micro injection moulding, which makes applications in small, medium and large series possible [KLC05].

State-of-the-art high-precision machining with cutting materials made of polycrystalline diamond (PCD) or coated carbide tools is used to manufacture special mould inserts. However, this technology places high demands on regard to finishing in order to produce low surface roughness characteristics and high shape accuracies G_F. The surface roughness parameters that can be achieved during machining are fundamentally dependent on the feed rate “f”, which is visible on the workpiece surface to be machined.

For the development of dedicated finishing technologies in particular, a targeted adaptation of the process parameters with reduced feed rates f becomes necessary, which, however, considerably increases the machining times t_B and significantly reduces the economic efficiency. Moreover, residual stresses σ are released by the machining process, which can result in component distortion and the achievable shape accuracies G_F are limited as a result. One way to meet these challenges is to develop a specific flattening technology on milling machines for the economic machining of hardened steel materials.

Experimental set-up and execution

Based on the existing challenges for current technologies, research was carried out on the reworking of the hardened steel material Elmax Superclean type PMX170CrVMo18-3-1 to produce technical surfaces with arithmetic centre-line roughness values of Ra ≤ 80 nm at feed rates of v_f ≥ 6000 mm/min. In addition to the selection of the cutting material and the material to be machined, the result is largely determined by the use of the process parameters. The flattening tool used for the manufacturing process consists of a carbide shank, a steel housing with integrated spring core and a flattening head made of PCD. The functional principle of the flattening tool is based on a pretensioned spring with a spring constant c_F = 34,4 N/mm, which enables a defined adjustment of the process force F_Pr.

The process force F_Pr is set via a force measuring platform and an axial infeed a_P of the PCD smoothing head which is movable in the Z-axis. This is of crucial importance for the forming process, as the necessary material-specific surface pressure p_B is achieved and can be adapted to the respective materials.

For experimental testing, the tool developed in cooperation with Baublies from Renningen, Germany, was used with a PCD flattening head of type PCD 001 and a sphere radius of r_Sp = 3,0 mm.

The entire test was carried out on the 5-axis high-precision machine tool PFM 4024-5D from Primacon, Peißenberg, Germany. In its initial state, the material used exhibited a hardness of H_v,w = 693,8 HV according to Vickers. The metrological analysis of the manufactured surfaces as well as the hardness H in the near-surface edge layer was carried out using the Hommel-Etamic nanoscan 855 tactile roughness and contour measuring device from Jenoptik. Prior to the investigations, a defined initial surface AA with an arithmetic centre roughness value of Ra = 1.2 µm was created with the aid of milling.

A ball milling tool with a diameter of D = 1 mm and a number of cutting edges of z = 2 was used. For a detailed analysis of the surface quality, the influence of the process force F_Pr, the feed rate v_f and the lateral infeed a_e on the arithmetic centre-line roughness Ra was examined. The results are summarised in Fig. 1-1.

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