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Getting melt into shape: tools and moulds in diecasting technology

A central issue regarding the diecasting process is the mould, it determines the contours of a die cast part and affects its properties. Mould making still has a huge development potential, particularly in view of using additive manufacturing processes.

Diecasting is a forming process for the mass production of parts made from aluminium, zinc, magnesium, copper, lead, tin and their alloys. The casting process takes place in either hot chamber or cold chamber diecasting machines. The main difference being that in hot chamber casting machines the container with the molten metal is located inside the machine, whilst in cold chamber the container is placed outside the machine. In both types of machines, the molten metal is pressed from a casting chamber through one or more casting channels into the cavity of a permanent steel mould where it takes the shape, which is determined by the die, and solidifies. These diecasting moulds consist of two halves in order that the cast part can be removed from the mould. The feed side mould half is mounted on a fixed plate on the rigid side of the diecasting machine, while the ejector side mould half is mounted on a movable plate placed on the other side. Before closing, the halves are sprayed with a release agent so that later the cast part can be easily released from the mould and the plates do not overheat. Depending on the size of cast parts, up to 300 casting cycles per hour can be achieved.

Extreme loads

When the mould is closed, the melt is pressed into the mould under a pressure of up to 1,200 bar, achieving maximum mould filling speeds of 150m/s (540km/h)(1). High closing and clamping forces are required to press the mould halves against each other and keep the moulds closed: up to 8,000kN (800t) in hot chamber diecasting machines and up to 45,000kN (4,500t) in cold chamber diecasting machines. By using such high forces, large-sized cast parts can be manufactured. Concerning material and design, the moulds that are used for this purpose must be designed in such a way that they can permanently withstand the loads related to the large melt quantities. When the metal has solidified, the mould halves open and the cast part is ejected by bolts or removed by a robot and conveyed for further processing.

High performance steels

The mould is a central issue as it determines the contours that have to be transferred to the cast part and should also enable the cast part to solidify as quickly as possible. In this way, the formation of a fine-grained microstructure is promoted, which is beneficial to the casting quality. To achieve optimal cooling, the moulds are cooled in certain parts. Another effect is that the production time is shortened, which provides economic advantages. The design of diecasting tools is described in the standard DIN 16760-1(2). The tools used in the diecasting process are inevitably exposed to high thermal and mechanical loads and must be able to withstand them permanently. For example, moulds for zinc diecasting reach service lives of 500,000 to two million cycles. To achieve such performances, the diecasting tools, which besides the above mentioned moulds include mould inserts, cores, slides and ejectors, are made of high-strength hot-work steels such as X40CrMoV5-1 (1.2344) or special materials, for example hard metals. Properties that play a very important role in these tools are high wear resistance, high ductility, high heat resistance, high hot tearing and hot wear resistance and good thermal conductivity. When choosing the materials, their technological properties, the design of the tools, their heat treatment and, last but not least, the complex interactions between the tools and the metal to be cast must be considered. For this purpose, manufacturers and suppliers of the appropriate steels offer informative brochures and consulting services(3).

CAD/CAM systems

In the past, tools for diecasting technology were manufactured on the basis of drawings, today designers work with 3D CAD data and use state-of-the-art IT technologies. In the design of casting moulds, both the casting process – and thus the melt flow and the cooling – and the geometry and the dimensions of the die cast parts to be manufactured must be considered. The cast parts should be characterised by a uniform, fine-grained microstructure, high dimensional accuracy and dimensional stability and a high surface quality. Computer-aided simulation calculations help to make the tools optimally adapted to the die cast parts. Toolmakers and mould makers use CAM systems for manufacturing. CNC controlled milling machines as well as die sinking and cut erosion machines are used to incorporate the forming contours into the moulding material with high precision. The manufacture of the moulds is very complex and therefore expensive. The tooling cost is in the order of up to 20 per cent of the total cost of an aluminium die cast part(3). For the production of components in large series, however, this is more cost-effective from a certain lot size onwards than manufacturing the parts in other ways, for example by machining processes. In addition, the manufacturing time for each part is shorter. A standardised, time-saving procedure for the design of diecasting tools has been developed at the Institute of Machine Design at the University of Magdeburg, Germany(4).

Still a lot of potential

The design diversity of die cast parts and the demands placed upon them are continually growing. Therefore, the demands on the properties of the tool steels used for diecasting and the constructive design of the tools and moulds manufactured from these steels are increasing too. These steels, the software programs foreseen for design and simulation and the capability of the machining systems are subject to constant development. The topics ‘digitisation’ (Industry 4.0) and ‘3D printing’ are gaining increasing importance. Trade fairs are responding to this trend. EUROGUSS pays special attention to this topic with the special show ‘Additive Manufacturing’. By using digital technologies, processes can be controlled more efficiently and the optimisation potential can be better recognised. With 3D printing processes (additive manufacturing), it is possible to make parts which cannot be manufactured by conventional processes, for example inserts for diecasting moulds with a complex shape and integrated cooling channels which are close-to-contour and curved. According to Dr-Ing Ioannis Ioannidis, president and CEO of the diecasting machine manufacturer Oskar Frech, chairman of the Foundry Machinery Association and member of the board of the Additive Manufacturing Association in the German Machinery and Plant Manufacturers Association VDMA, in this area there is still a lot of potential for mould making: “The complete heat management in the mould can be affected in such a way that, for example, the mould is better protected against wear and the quality of the part to be cast can be affected,” he says(5).

EUROGUSS 2020

An insight into state-of-the-art in pressure diecasting and suggestions as to how diecasting foundries can strengthen and expand their market position, but also around resource efficiency and environmental protection, will be provided at the EUROGUSS 2020International Trade Fair for Die Casting: Technology, Processes, Products, to be held in Nuremberg, Germany on 14th to 16th January 2020.
The EUROGUSS family includes EUROGUSS trade fair as well as the non-European diecasting trade fairs China Die Casting, Alucast in India, EUROGUSS Asia Pacific in Thailand and EUROGUSS Mexico. www.euroguss.de/en

Image Source: Gebru¨der Nelles Werkzeugbau GmbH, Sankt Augustin, Germany

References

1. Herfurth K, Ketscher N, Ko¨hler M, ‘Foundry technology compact. Materials, processes, applications’. (In German), Hrsg. Verein Deutscher Giessereifachleute. Du¨sseldorf 20132. S. 109 f.

2. DIN 16760-1: Tools for moulding – Part 1: Machined, undrilled plates. Berlin 2008.

3. Uddeholm Tool Steels for Die Casting. Hagfors, Sweden 2016, S.16. https://www.uddeholm.com/files/AB_die_casting_eng.pdf

4. Berkau A, Birke C, Brockop S J, ‘Procedure for the computer-controlled design of rapid tooling tools on the example of a diecasting tool’. (In German), Magdeburger Maschinenbautage. Magdeburg, September 2001, S. pp195-203. http://www.pemos.de/artikelsammlung/beitraege01/bebibr.pdf

5. Additive Manufacturing offers diecasting foundries great opportunities. Interview with Ioannis Ioannidis. (In German), Giesserei 11/2017. https://www.giesserei.eu/magazin/interview/2016/interview- ioannidis/?L=0zudem