Keywords

1 Introduction

The increasing demand for energy, climate change and scarcity of resources are major challenges of our time. As a reaction to the drastically increasing environmental problems, this topic has developed into an independent policy area and demands corresponding ecology-oriented information and decision-making instruments [1].

DIN EN ISO 50001 is an instrument that provides the normative basis for the introduction, operation and continuous optimisation of an energy management system. The aim of this document is to assist companies in continuously improving energy-related performance, including energy efficiency, energy use and energy consumption. To this end, the standard requires the implementation of processes that ensure effective and measurable improvement. The organisational units shall collect, analyse and evaluate its energy data and systematically identify energy potentials from it. To ensure continuous optimisation, these processes are subdivided according to the Deming Circle, the Plan-Do-Check-Act (PDCA) cycle. In addition, there are other standards of the ISO 5000 family (Fig. 1) for a comprehensive explanation of the individual processes and for support in implementing the requirements of DIN EN ISO 50001 [2].

Fig. 1.
figure 1

The ISO 50000 family and the Plan-Do-Check-Act (PDCA) cycle.

Full size image

The standards are based on the principle of uniform standardisation, are formulated very generally and do not contain any industry-specific information. However, each industry has certain specifics that must be taken into account in order to set up and introduce an energy management system. For this reason, this article deals with the toolmaking industry, in particular with the production of press hardening tools. The production of thermo-mechanically stressed tools is associated with energy- and resource-intensive manufacturing processes. With the help of the developed method, the currently existing deficits regarding the use of energy and resources in tool production are to be shown comparatively between the conventional and an alternative route of tool production.

2 Production of Press Hardening Tools

For classical press hardening, the cooling effect of the thermal and mechanical highly stressed tool is of particular importance. In order to realise a stable process, the heat must be dissipated evenly and quickly from the component, so that the cooling structure should be arranged as close to the surface as possible. The insertion of the cooling channel structures can be done with different concepts: cooling holes, cast-in cooling tubes, sand core casting, additively manufactured cooling structures or shell structures.

2.1 Process Chain for Manufacturing the Tool with Drilled Cooling Channels

In industry, cooling with the use of deep-hole drilled cooling channels is most frequently used. This solution is comparatively simple to manufacture, robust in use and enables a high degree of automation in production. In order to map the geometry of the tool and the cooling system, these are divided into segments and provided with holes. The size of the individual segments is determined by the complexity of part and thus also the tool geometry as well as the maximum drilling depth. The disadvantage of this segmented construction is the required sealing of the segments, taking into account the temperature-related expansion. Another disadvantage is the formation of so-called hot spots with insufficient cooling in the case of very complex geometries of the tool, especially in radius areas. The manufacturing of tools with drilled cooling systems is characterised by a high proportion of machining. After milling, drilling and deburring, the tool is hardened to increase its mechanical resistance and then hard-milled to its final geometry and finished (Fig. 2) [3].

Fig. 2.
figure 2

Process chain for manufacturing the tool with drilled cooling channels, based on [4]

Full size image

2.2 Process Chain for Manufacturing the Tool with Cast Cooling Channels/sand Core Casting

Another manufacturing for cooling channels option is the sand core casting process. With this method, cooling close to the contour can be realised with high material utilisation. The process chain begins with pattern making and sand core production. The sand core represents the complete cooling system and thus reproduces the inner contours of castings. The core consists of sand and a chemical binder, has a high strength and should withstand the pressure as well as the high temperature until the shell solidifies. The model is moulded with the compacted sand, supplemented with the sand core and then cast. After cooling and demoulding, mechanical processing takes place. If necessary, the castings are further hardened to increase the strength of the tool. After a final grinding, hard milling and reworking, the tool parts are ready for further assembly (Fig. 3) [4].

Fig. 3.
figure 3

Process chain for manufacturing the tool with cast cooling channels/sand core casting, based on [4]

Full size image

3 Method Devised

The method developed is used to evaluate and identify energy-efficient process chains for the manufacturing of press hardening tools and identifies existing deficits in terms of energy and resource use. As a result, the objective is to increase energy efficiency and reduce energy costs. For this purpose, a comprehensive analysis of the energy consumption and the development of possible savings potentials is carried out. All the information collected is used to carry out the energy audit.

The aim of the method is to create an energy management structure based on existing organisational processes that can be used to evaluate manufacturing strategies for the manufacturing of thermally and mechanically highly stressed tools. When introducing the method, the framework conditions must first be created in order to be able to carry out an evaluation. For this purpose, the scope of application, the responsibilities, the energy-relevant data to be included and the available sources of information must be defined. Building on the basis created, the assessment and identification of the manufacturing process chains can begin. For this purpose, the PDCA cycle is used to achieve a continuous improvement of the set goals and target values (Fig. 4).

Fig. 4.
figure 4

Method for evaluating and identifying energy-efficient manufacturing process chains [ff. 2]

Full size image

4 Evaluating and Identifying Energy-Efficient Process Chains

4.1 Scope of Application

The investigations are to be carried out for the manufacturing of a thermal and mechanical highly stressed forming tool for the press hardening of sill components. The process chains for manufacturing a forming tool with drilled cooling channels and a forming tool in which the cooling channel structure was introduced by means of sand core casting are to be compared with each other (Fig. 2; Fig. 3). The tools were both made from a hot work tool steel 1.2367. The mechanical processing was carried out on a milling machine Hermle C 400 and a grinding machine ELB Perfekt 6 SPS (Fig. 5).

Fig. 5.
figure 5

Schematic representation of the scope of application

Full size image

4.2 Planning Phase

The first step is to define target values that can be used for an evaluation. It is important to pay attention to the available data:

  • The total energy consumption of the process, which represents the integration of the absorbed active power P(t) within a time period T [5].

    $${W}_{system/process}={\int }_{{T}_{0}}^{{T}_{n}}P\left(t\right)dt$$
    (1)

  • Energy efficiency, which describes the ratio or other quantitative relationship between an achieved output or yield of services, goods, commodities or energy \({E}_{id}\) and the energy used \({E}_{real}\) [2].

    $$\eta =\frac{{E}_{id}}{{E}_{real}}$$
    (2)

    The energy driver describes the plant/machine or a process that has the highest energy consumption \({W}_{system/process}\) along a manufacturing task or process chain [5].

    $${W}_{ET}=\mathrm{max}({W}_{system/process})$$
    (3)

  • The energy consumption peak/peak load \({P\left(t\right)}_{max}\) describes a high power demand occurring for a short time [7].

Furthermore, the measures for necessary adjustments of the energy demand of the manufacturing processes are defined in this section. This is influenced by process-specific data, such as the technology, the type, the year of construction or the manufacturer of the machine/plant. In addition, the states taken, their temporal relationships and the temporal sequence of the states are to be considered as factors influencing the energy demand and the temporal sequence of the states is relevant. The measures can include various instruments such as short-term or medium-term adjustment of process starts, machine occupancy, order sequence, break or shift times, storage of energy or even change of energy source. These have a wide variety of control variables such as shift times, break times, maintenance times, machining times, start-up times, set-up times, daily requirements or the number of variants at their disposal [8]. In addition, new processes or process chains can be introduced to achieve the objectives.

The concern of every company is the reduction of energy consumption. For this purpose, the energy consumption of two process chains for manufacturing of a thermal and mechanical highly stressed forming tool, by means of cast cooling channels/sand core casting and by means of drilled cooling channels, is to be determined and compared in this example. The process steps of both process chains are determined by the selected field of application (Fig. 2, Fig. 3).

4.3 Implementation Phase

The measures described are implemented in the company during this phase. All determinations and implemented measures must be precisely documented and communicated. The documentation of all activities carried out and the results achieved serve as evidence. In this way, it can be proven and understood what has been achieved. Within the framework of a management system, environmentally relevant records should be filed in an orderly and protected manner. Furthermore, the involvement of employees in the energy management system must be ensured through regular meetings. For internal and external communication, enquiries, complaints, communication with customers, suppliers and authorities must be included.

4.4 Check-Phase

In the check phase, the effectiveness of the measures is checked with regard to the achievement of target values or energy goals. This involves monitoring and measuring the defined target values, which should be carried out regularly. For this purpose, the energy-relevant data must be recorded and evaluated. The results should be documented in a comprehensible way and included in the performance evaluation.

A voltage divider, which was developed by the TU Chemnitz, was used to measure the voltage. The current was measured using current clamps such as ELCONTROL (clamp C 1000/I), HT97U (HT Instruments), MN71 (Chauvin Arnoux) and VOLTCRAFT AC 200. An NI USB6259 measuring amplifier was used for the measurements. The recorded energy data were evaluated in the jBEAM programme. The energy demand of the heat treatments was calculated via the amount of heat to be introduced. This is calculated from the specific heat capacity c, the tool mass m and temperature change ∆T.

$$Q=c\cdot m\cdot \Delta T$$
(4)

Table 1 summarises the process steps of the considered process chains and the results of the measurements. The overview shows the energy-intensive processes depending on the production plant. It is possible to reduce the energy consumption of these processes by adjusting the process settings. The process chain for manufacturing the press hardening tool with cast-in cooling channels has the lower energy requirement as a result.

Table 1. Comparison of the total energy demand of the processes, tool lower part
Full size table

4.5 Act-Phase

The Act phase serves to continuously improve energy consumption in manufacturing and the energy management system. The first step is to derive optimisation potentials. This enables an interpretative evaluation of collected data and the derivation of optimisation potentials. These suggestions form the basis for setting new goals for the next cycle to evaluate the application area under consideration. Furthermore, all necessary information/data for an energy management review and decision are compiled. This review is a presentation of the results, the analysis and evaluation of the considered application area as well as an interpretative interpretation of solution proposals. It represents an important basis for communication with the management and, if necessary, serves as a report for an external energy auditor.

As a result of the analysis and evaluation of the two production process chains considered, the process chain for manufacturing the forming tool using sand core casting shows the lowest energy consumption. It thus represents the potential for energy-efficient implementation and is presented as a proposal in the energy management review. The decision is incumbent on the management.

5 Summery and Outlook

In this article is presented a method for evaluating and identifying energy-efficient process chains for the manufacturing of press hardening tools based on the PDCA cycle. The method was used and validated for a comparison of the process chains with drilled and cast cooling channels. The result showed that by documenting the individual phases, the evaluation was recorded in a comprehensible and transparent manner for the energy management review and forms a good basis for decision-making. The described procedure using the PDCA cycle enables a standardisation and introduction of an energy management system with continuous improvement of the energy consumption of the considered application area.