**3.1. System technology impact: Sheet metal bending**

When compared to other manufacturing processes, such as chipping, coating or cutting, the effective loading time per production cycle during bending is relatively short and the specific rate at which the load is applied is not a significant process parameter. In turn, the analysis of the energy needs should focus on effective energy values in alternative to the time-dependent power parameter value.

The scans in Fig. 1 expose the referred discrete loading character inherent to bending, and the influence of machine-tool technology in the temporal evolution of power and energy consumed per operation cycle. In the case of bending, and according to the time scans presented in Fig. 1, the total energy consumption per bending cycle can be modelled according Eq. (3):

$$B\_{elect} = P\_{ldle} \Delta t\_{ldle} + B\_{approach} + P\_{bending} \Delta t\_{bending} + B\_{return} \tag{3}$$

where, P���� is the active power consumed during stand-by mode, being technology-related, and �������� is the active power consumed during loading, here proposed to be modelled according to Eq. (4):

$$P\_{bending} = P\_{0-bending} + c.F\_{bending} \tag{4}$$

**Figure 1.** Examples of active power and active energy per bending cycle recorded for: a) a hydraulic press-brake and b) an all-electric press-brake.

This model is built according to Eq. (1) proposed by Gutowski (Gutowski et al., 2006), but adapted for discrete loading manufacturing processes, where, *P0\_bending* stands for the power required at a zero load cycle, *c* is a constant in kW/t, corresponding to the positive power rate needed to sustain a theoretical continuous load increase, and *Fbending* would take the real load value pre-defined for production.

However, for such discrete loading operations, the process rate must be described as a function of the frequency of production cycles, instead of the amount of material removed or being processed. In this perspective, Eq. (2) should take the form proposed in Eq. (5):

$$B\_{elect} = \frac{p\_{idle}}{n} + q \tag{5}$$

Towards Benign Metal-Forming:

The Assessment of the Environmental Performance of Metal-Sheet Forming Processes 141

Machine A (50%) Machine C (50%) Machine B (40%) Machine D (6%) H110 ton Machines (avg) Machine E (50%)

throughput, in opposition to the loading time in the chipping processes. From this, it can be concluded that the actual energy and usage of each machine are essential to estimate the

The SPE as a function of the throughput was estimated for a set of hydraulic and all-eletric press-brakes based on real consumption data measured directly on the machines. Figure 2 shows the estimation models obtained for all machines, working at the highest used loading capacity during the study and with a maximum throughput value of 720 cycles/h, as this is the theoretical limit for a machine working continuously at the smallest cycle time observed

0 100 200 300 400 500 600 700 800

**E110 ton (Machine E) B (n) = 200/n + 3.422**

**Figure 2.** Specific process energy (SPE) during bending as a function of throughput, obtained from energy consumption data measured directly on a set of bending machines (n: throughput [cycles/h]; Hxxx: hydraulic technology and Exxx: all-electric technology, indicating the respective maximum

In this perspective, the following principles were proposed for the modelling of the SPE

From this, the categorization criteria proposed to be used for bending and similar discrete chipless-shaping manufacturing operations in general are the type of operation, technology, maximum loading capacity and usage scenario. Regarding the usage, typical throughput values for 3 main usage scenarios installed for bending (robot-assisted, manual-intensive and manual-discrete) have been appointed. Table 1 resumes the energy consumption values per bending technology category and usage scenario proposed to be considered for the estimation of electricity consumption related to the bending operation. These values can be


**Throughput [cycles/h]**

SPE required for bending operation or any other discrete loading operations.

**H110 ton (avg Machine B, C, D) B (n) = 2500/n + 2.958 ; R² = 0,9124**

bending capacity; xx%: capacity loading tested in a specific machine).


**H170 ton (Machine A) B (n) = 4800/n + 13.167**

(5 s).

**Specific Process Energy [W.h/cycle]**

during metal-sheet bending:

where *n* corresponds to the throughput in cycles/h.

Following the previous discussion on the fixed and variable contributions of the specific process energy, constant *q* represents the cycle peak energy obtained at a pre-defined process load and loading time, excluding the fixed contribution, while the variable contribution ����� � considers the total cycle time, which is dependent on the production throughput, in opposition to the loading time in the chipping processes. From this, it can be concluded that the actual energy and usage of each machine are essential to estimate the SPE required for bending operation or any other discrete loading operations.

The SPE as a function of the throughput was estimated for a set of hydraulic and all-eletric press-brakes based on real consumption data measured directly on the machines. Figure 2 shows the estimation models obtained for all machines, working at the highest used loading capacity during the study and with a maximum throughput value of 720 cycles/h, as this is the theoretical limit for a machine working continuously at the smallest cycle time observed (5 s).

**Figure 2.** Specific process energy (SPE) during bending as a function of throughput, obtained from energy consumption data measured directly on a set of bending machines (n: throughput [cycles/h]; Hxxx: hydraulic technology and Exxx: all-electric technology, indicating the respective maximum bending capacity; xx%: capacity loading tested in a specific machine).

In this perspective, the following principles were proposed for the modelling of the SPE during metal-sheet bending:


140 Metal Forming – Process, Tools, Design

press-brake and b) an all-electric press-brake.

load value pre-defined for production.

contribution �����

where *n* corresponds to the throughput in cycles/h.

**Figure 1.** Examples of active power and active energy per bending cycle recorded for: a) a hydraulic

This model is built according to Eq. (1) proposed by Gutowski (Gutowski et al., 2006), but adapted for discrete loading manufacturing processes, where, *P0\_bending* stands for the power required at a zero load cycle, *c* is a constant in kW/t, corresponding to the positive power rate needed to sustain a theoretical continuous load increase, and *Fbending* would take the real

However, for such discrete loading operations, the process rate must be described as a function of the frequency of production cycles, instead of the amount of material removed or being processed. In this perspective, Eq. (2) should take the form proposed in Eq. (5):

������ <sup>=</sup> �����

Following the previous discussion on the fixed and variable contributions of the specific process energy, constant *q* represents the cycle peak energy obtained at a pre-defined process load and loading time, excluding the fixed contribution, while the variable

� considers the total cycle time, which is dependent on the production

� � � (5)


From this, the categorization criteria proposed to be used for bending and similar discrete chipless-shaping manufacturing operations in general are the type of operation, technology, maximum loading capacity and usage scenario. Regarding the usage, typical throughput values for 3 main usage scenarios installed for bending (robot-assisted, manual-intensive and manual-discrete) have been appointed. Table 1 resumes the energy consumption values per bending technology category and usage scenario proposed to be considered for the estimation of electricity consumption related to the bending operation. These values can be

used in LCI databases, in alternative to the theoretical values typically adopted, often even associated to generic manufacturing work (Ecoinvent Centre, 2010). These data should be applicable to all types of material to be worked, as they are categorized on a bending capacity basis. The potential energy savings related to the selection of the driving system technology and the motor rated power installed were quantified from the estimated SPE values and are also here included.

Towards Benign Metal-Forming:

The Assessment of the Environmental Performance of Metal-Sheet Forming Processes 143

**Figure 3.** LCA results of the Assembly-phase and Use-phase (Electricity and Oil) inputs used for an

These results make evident that the contribution of the hydraulic oil used to the environmental impact of such machines should not be neglected. The relative contribution of the oil consumption to the global environmental impact of the machine is higher than 20%, in at least 2 of the 11 middle-point impact categories analysed. The main impact of the oil consumption, in absolute value of the indicator, is on the depletion of fossil fuels, and, in this category, the impact is similar to that of the total of assembly resources incorporated in

**Eco-Indicator 99 Assembly Use\_Electricity Use\_Oil**

In the case of a Laser-cutter, the Laser energy source and the cooling system determine its overall integration of components and energy consumption and, thus, their analysis is essential to characterize the environmental profile of the Laser cutting process. The recent Fiber Laser (FL) technology has been compared with the well-established CO2- Laser (CO2). While the technical benefits related to the integration of fewer components are intuitive to promote the environmental performance of the FL technology, the strategies for energy savings are not that evident and there is still some room for improvement (Oliveira et al, 2011). Besides the Laser and cooling units, other important electrically fed subsystems, such as the general control unit (including the motorized head positioning) and the exhaustion system, should not be neglected on the analysis of the energy demand of a Laser cutter, although this later has not been actually measured

Figure 4 presents the SPE results obtained from analysing 3 equipments of FL and CO2 technologies, in similar utilization conditions and for a similar job (1 mm steel sheet). The

hydraulic press-brake, per impact category (middle-point).

**EcoIndicator99 [Pt]**

the machine, which is considered quite significant.

technology influence on the SPE is also here made evident.

in this work.

**3.2. Sub-system approach on SPE accounting: Sheet metal cutting** 


**Table 1.** Specific process energy values related to bending operation, as a function of technology and usage scenario, and comparative analysis in the form of potential energy savings.

Particularly for irregular and/or low usage scenarios, the electric-based drive technology is to be recommended, as this might lead to energy savings of about 90% when compared to an all-hydraulic system, for a similar loading capacity machine, while the potential savings tend to be reduced for more intense usage scenarios. Nevertheless, even for the highest robot-assisted production scenarios, energy savings as high as 67% could be achieved with electrically-driven systems when compared to the hydraulic ones, for similar loading capacities installed.

As advanced, apart from the SPE, technology also determines the type and amount of other consumables during operation. In the case of a hydraulic press-brake, hydraulic oil is a technology-specific resource essential for its operation. The environmental impact profile related to the oil consumption is significantly affected by its no-renewable character, as this is a standard crude oil by-product, typically incinerated at the end-of-life. Figure 3 shows the contributions of the Assembly-phase and Use-phase inputs (Electricity and Oil) to the environmental profile of a hydraulic press-brake, described per different middle-point impact categories. A lifetime of about 15 years was assumed, and the contribution of an endof-life scenario has here not been accessed. The most probable machine-tool end-of-life scenario is reutilization as second-hand which, in practice, would represent an extension of the lifetime, reflected by an increase on the use-phase inputs, i.e. SPE and hydraulic oil contributions.

values and are also here included.

*Technology Category Manual-discrete*

*Specific Process Energy (SPE)* 

*Potential energy savings*

capacities installed.

contributions.

used in LCI databases, in alternative to the theoretical values typically adopted, often even associated to generic manufacturing work (Ecoinvent Centre, 2010). These data should be applicable to all types of material to be worked, as they are categorized on a bending capacity basis. The potential energy savings related to the selection of the driving system technology and the motor rated power installed were quantified from the estimated SPE

*(n=20)* 

I. Bending, hydraulic, 170 t 253.2 73.2 32.4 II. Bending, hydraulic, 110 t 128.0 34.2 13.0 III. Bending, electric, 100 t 13.4 5.9 4.2

Technology-related (III *vs* II) 90% 83% 67% Motor rated power-related (II *vs* I) 49% 53% 60%

**Table 1.** Specific process energy values related to bending operation, as a function of technology and

Particularly for irregular and/or low usage scenarios, the electric-based drive technology is to be recommended, as this might lead to energy savings of about 90% when compared to an all-hydraulic system, for a similar loading capacity machine, while the potential savings tend to be reduced for more intense usage scenarios. Nevertheless, even for the highest robot-assisted production scenarios, energy savings as high as 67% could be achieved with electrically-driven systems when compared to the hydraulic ones, for similar loading

As advanced, apart from the SPE, technology also determines the type and amount of other consumables during operation. In the case of a hydraulic press-brake, hydraulic oil is a technology-specific resource essential for its operation. The environmental impact profile related to the oil consumption is significantly affected by its no-renewable character, as this is a standard crude oil by-product, typically incinerated at the end-of-life. Figure 3 shows the contributions of the Assembly-phase and Use-phase inputs (Electricity and Oil) to the environmental profile of a hydraulic press-brake, described per different middle-point impact categories. A lifetime of about 15 years was assumed, and the contribution of an endof-life scenario has here not been accessed. The most probable machine-tool end-of-life scenario is reutilization as second-hand which, in practice, would represent an extension of the lifetime, reflected by an increase on the use-phase inputs, i.e. SPE and hydraulic oil

usage scenario, and comparative analysis in the form of potential energy savings.

*Manual-intensive (n=80)* 

*Robot-assisted (n=250)* 

*[W.h/cycle] Equipment Usage Scenarios* 

**Figure 3.** LCA results of the Assembly-phase and Use-phase (Electricity and Oil) inputs used for an hydraulic press-brake, per impact category (middle-point).

These results make evident that the contribution of the hydraulic oil used to the environmental impact of such machines should not be neglected. The relative contribution of the oil consumption to the global environmental impact of the machine is higher than 20%, in at least 2 of the 11 middle-point impact categories analysed. The main impact of the oil consumption, in absolute value of the indicator, is on the depletion of fossil fuels, and, in this category, the impact is similar to that of the total of assembly resources incorporated in the machine, which is considered quite significant.

#### **3.2. Sub-system approach on SPE accounting: Sheet metal cutting**

In the case of a Laser-cutter, the Laser energy source and the cooling system determine its overall integration of components and energy consumption and, thus, their analysis is essential to characterize the environmental profile of the Laser cutting process. The recent Fiber Laser (FL) technology has been compared with the well-established CO2- Laser (CO2). While the technical benefits related to the integration of fewer components are intuitive to promote the environmental performance of the FL technology, the strategies for energy savings are not that evident and there is still some room for improvement (Oliveira et al, 2011). Besides the Laser and cooling units, other important electrically fed subsystems, such as the general control unit (including the motorized head positioning) and the exhaustion system, should not be neglected on the analysis of the energy demand of a Laser cutter, although this later has not been actually measured in this work.

Figure 4 presents the SPE results obtained from analysing 3 equipments of FL and CO2 technologies, in similar utilization conditions and for a similar job (1 mm steel sheet). The technology influence on the SPE is also here made evident.

Towards Benign Metal-Forming:

The Assessment of the Environmental Performance of Metal-Sheet Forming Processes 145

components and guidelines for effective energy management during machine-tool processing are still not established. The examples given in the previous section support the two strategies generally proposed to improve energy efficiency: (1) the conversion of hydraulic to all electric systems and (2) the maximization of the rate at which the physical mechanism can perform the desired operation, i.e., the optimization of machine

It must be noted that awareness of the manufacturing end-user regarding the importance of energy management should be strongly enhanced. Although widely discussed in different areas, this is a topic that the manufacturing user tends to neglect, regarding each individual machine on his plant, particularly in what concerns technology, process and usage strategies. Independently on the many possible solutions targeting the automatic control of the machine-tool, the user's perception surely determines this optimization. Enabling the user to obtain detailed and real-time data about the energy consumption of the manufacturing process is essential to accomplish the optimization of the machine-tool environmental profile during the use stage, as the user must be actively involved in this process. It is on the side of the machine-tool manufacturer to preview and implement this. On the other hand, and apart from all criteria behind the selection of each individual subsystem on the machine, including its technology, it is on the manufacturer side to match the power demand profile of the main energy-consuming sub-systems integrated, in what concerns the power consumption of the sub-systems, as realised from the Laser cutting

However, as referred at start, the technology-related improvement potential of a manufacturing process towards benign metal forming must be sustained by an integrated perspective, mainly presenting energy-related technical solutions but notonly, as the contribution of the 3 types of resources listed above is affected, namely the assembly resources, the energy consumption during use and the other consumables related to machine operation, such as the influence of the hydraulic oil to the environmental impact of the bending process, as here demonstrated. Combining these perspectives, and in view of the discussed influence of the machine-tool technology on the environmental impact of the manufacturing process, special attention is here given to the assembled sub-systems of the machine-tool, and particularly to the materials incorporated on these, in which steel has traditionally been dominating. On the other hand, the change in steel pricing policy and current increasing steel cost are pressing overheads and margins at the machine-tools manufacturers and their components suppliers. As the need for alternative materials, less subjected to such market variations, becomes more evident, technical targets, process quality and environmental profile might be compromised. In addition, market has been specifically requiring performance increase, in the sense of higher stiffness, dimensional stability, ease of manufacturing, good dampening properties and high mass to avoid rigid body movements. Some examples of high potential actions enhancing benign metal forming currently being

developed and adopted are pointed out in the next sub-sections.

usage.

study followed.

**Figure 4.** Specific process energy (SPE) observed for the Laser cutting process, showing relative contribution of the main sub-systems per equipment type (FL: Fiber Laser technology; CO2: CO2-Laser technology). Reference is 1 mm carbon steel sheet processing for 1 h.

Regarding the individual contribution of each main sub-systems in CO2 equipment, the chiller unit of a 4.5 kW machine was seen to be responsible for more than 50% of the energy demand, contradicting the assumption that the Laser source dominates the total energy consumption of these machines (Devoldere et al, 2006). In what concerns to the FL equipment, the energy benefits were clear:


In resume, FL technology brings down the Laser and chiller energy needs, to the consumption level of standard control/motion units, or even exhaustion systems. In such a scenario of no predominant contributor, and targeting to maximize energy efficiency, all sub-systems apart from the Laser unit must be accurately specified.
