Application in the plastic and moulding sector

The use of plastic injection moulding as a production process has increased greatly over the years and is expected to continue this growth in the near future. Knowing this, it is important to understand where and how to control and minimize the environmental impacts associated with the injection moulding process, without contributing to an increase in costs [31].

According to Esteves et.al (2014), the overall Eco-efficiency of the injection moulding process is mostly dependent on two factors: the injection machine, which mostly determines energy consumption, and mould design, which influences the amount of material wasted and regulates the injection cycle time [31]. However, for Thiriez (2004), energy consumption is also influenced by the part design and mould design [21].

Considering environmental impacts, an important factor are emissions, where the majority are related to the polymer production stage and the remaining can be divided in energy related emissions

24 originated from the generation of electricity or processing emissions which arise from the processing sites. However the latter are smaller than the energy emissions [21].

As previously stated, to produce a part through this process it is necessary to first manufacture a mould, which induces environmental impacts. However, each mould is expected to produce a few million parts throughout its lifetime, so it is predicted to have relatively low environmental impacts, when compared to the impact of the parts produced throughout its life. Still, the environmental influence of the manufacturing processes, chosen for the mould’s production, is extremely important as well as the efficiency of the process itself since, for example, manufacturing a mould exclusively with a milling process can be more efficient but the energy consumption higher and so the environmental impact greater. However, it is increasingly uncommon for a mould, nowadays, to be produced exclusively by one manufacturing process [31].

The most important parameters in mould design are geometry, feeding system, injection capacity, sprue location, and geometry and temperature distribution [48], where the process design considerations regarding these parameters will influence process parameters such as clamping force, heating temperature, compression force and injection speed. These results determine the cycle time and overall maintenance and support costs which, in turn, affect product cost. To avoid moulding defects, which can render the product unusable, and ultimately increase costs and material usage increasing non-environmentally friendly emissions, the geometric characteristics of the product must be considered in addition to the mould’s design results [49]. Regarding the feeding system, the injection can be made through hot or cold runners. As previously explained, hot runners allow the material to be kept in the runner system ready for the next shot, improving quality and production rate, making the overall production more cost-efficient. Thus, hot runners reduce cycle time, when compared to cold runners, and labour time considering there is no sprue to separate from the part.

This system also implies energy savings considering it requires a smaller shot. However, the percentage of saved energy, as opposed to the cold runner system, depends on the percentage of regrind that is allowed in each shot. Hot runners also allow for smaller injection pressures, which contribute to decrease in the machine´s energy consumption [21].

“Even though hot runners are more expensive than cold runners, their cost will be amortized by the energy saving, labour saving, material savings, cost savings and faster production rates” [21].

Summing, considering the injection process, the use phase of the moulds life cycle carries out most of the environmental impacts due to the influence of the quantity of polymer wasted and the injection cycle time, depending on the engineering design of the mould [31] .

Adding to the mould aspect, the Eco-efficiency is also dependent on the injection moulding machine, as previously stated, greatly influencing the overall energy consumption of the process as higher cycle times or higher clamping forces will demand higher energy consumption [31]. Energy consumption for injection moulding machinery is dependent on several factors which include part shape, polymer to be processed, shot size, production rate and machine size which, in turn, influence the machine to be used [21]. As stated in the previous chapter, there are three sets of machines, categorized by their

25 power supply. The oldest and most common is the hydraulic machine which uses hydraulic pumps to power all the machine´s movements. However, there are two setbacks considering the pumps continue running even when the machine is in idle which used power, not applied in production, and such wasting energy. Secondly, a hydraulic machine requires an electrical motor to power each pump which, in turn, transfers work to all mechanical components. Considering each transfer has an intrinsic inefficiency, the overall efficiency of the machine is low, especially when compared to electrical machines which use electric motors to power the mechanical components directly. As the efficiency lowers the energy consumption rises however, electrical machines are not applicable to high clamping force applications due to instabilities in their configuration [50].

To study these aspects, a life cycle analysis should be performed taking into account not only the mould production but also the impact design throughout its use and end-of life, as well as machine alternatives. However, unlike other products, there are no guidelines or standards to support sustainable design of mould leading to the conclusion that only recently has the study of this plastic manufacturing process, on an environmental level, been considered, especially when considering the mould due to reasons explained earlier [31].

A final contributing factor to energy consumption is part design, including the choice in polymer, given that each polymer has its own specific heat and so the energy requirements to form the melt are different. Another influential parameter is the crystallinity which increases the necessity of energy to transform the crystal into melt. Finally the viscosity and hygroscopia, referring to the necessity for the polymer to be dried, also influence the energy consumption considering that a more viscous material will have an increased difficulty to be moulded and a polymer dried before injection will need less energy since it is entering at a higher temperature. Considering the part itself, the projected area determines the clamping force which, in turn, determines the size of the clamping unit and its energy consumption. Part thickness is also important for it controls the cooling time, which will represent a large percentage of the cycle time for large parts, and hence the production rate influencing the machine´s energy consumption. In addition, the thinner the part, the greater the injection pressure and temperature needed to fill the mould before the material solidifies [21].

According to these studies, it is possible to conclude that material and energy consumption have been considered the main contributing factor for the environmental aspect of Eco-efficiency, analysed in terms of mould design, injection machine and even part characteristics. As Eco-efficiency is not only dependent on the environmental influence but on economic factors and the relation between cost and environmental influence, it is important to understand some of the work that has been performed is the area.

For the injection moulding process, the costs which will determined the quoted price are driven mainly by material, labour, maintenance, electric energy and acquisition costs of both the machine and the mould. A typical cost structure is shown in Figure 3.7. However, it is important to understand that these figures may vary depending on different production specifications [51].

26 Figure 3.7 - Breakdown of different fixed and variable cost factors for a typical injection moulding process

[51]

The chart illustrates the strong relevance of material costs, which constitutes 50% of the overall production costs. In terms of the other costs it is possible to affirm, according to bibliography, that mould, machine and labour are static costs, when considering the same part, for there is no need to change any influencing parameters. However, maintenance and energy costs are difficult to predict beforehand for they depend on a number of factors that are difficult to control. Still, it is important to evaluate the possible improvements to these two costs for they contribute to the increase in the company’s turnover [51].

The conclusions of these authors, although valid, are only an example of the several studies performed regarding the injection moulding process. The main focus has been mould design and its machining processes, the energy and material consumption in plastic parts production and the environmental impact of different moulding machines, focusing on energy consumption. Despite the importance of these studies, there is a gap in this area regarding environmental impact and cost analysis for parameter analysis, meaning a study on how parameters are determined, considering mould design, machine specifications and part characteristics, and ultimately, how costs and environmental impacts are influenced by the different parameters [31].

27

4 Methodology for Model Development and Results Analysis

For better understanding of how the model created for this work was developed, the general methodology applied is presented in this chapter, introducing every main step from the Input Variables to the Eco-efficiency results for the different mould design alternatives.

Following the research made in terms of Plastic Injection Moulding and Eco-efficiency, this work starts by stating the need to develop a model where the process is defined and creating a resource inventory, ultimately resulting in costs and environmental impact.

The development of this model began with a PBCM, establishing every equation to define the injection moulding process. In this PBCM, it was first established what costs to calculate, namely material, energy, labour, machine, mould, building and maintenance costs, and working backwords until the input variables. For this, a great deal of research was made in order to guarantee as little simplifications as possible and its scientific validity. To validate the model, a case study was used, defined in a previous work, comparing the results with the cost results of this model, to guarantee the legitimacy of this work and to explain any differences.

As a resource inventory was created to calculate the costs, the environmental impact can also be found through this inventory, analysing the impact of material, mould, energy and wasted material.

With these steps, the model was developed and validated, and it was possible to start discussing the results in terms of mould design and Eco-efficiency.

To start analysing the results it was necessary to establish all input variables, defining the process and part, in terms of production volume, part dimensions, etc. As the focus of this work is to create a tool with which designers can compare mould design alternatives in an early production phase, it was also necessary to define a set of mould design alternatives with which to present the results. For this, the number of cavities, feeding system and machine type were taken as variables.

It is important to note that a database of machines and mould dimensions must be created, prior to the start of the analysis, since the model defines the best machine and mould to use in terms of clamping force and number of cavities, respectively.

Having established the part dimensions, process data and the mould design alternatives, for all variables presented in Annex A, it is then possible to present the results. As there are a few resources, namely material, energy and cycle time important in injection moulding, these parameters were chosen to start the result analysis, comparing the results between mould alternatives, and between consumption for one part and one cycle. Since these consumptions are not as straightforward as one might realize, each parcel of material, energy and cycle time is divided, to better understand their influence in the final result. From this point it is presented the costs and environmental impact, for all previous mould design alternatives, and discussed the influence of the different variables in the final result. With this, the direct results of the model developed are presented and it is now possible to start the analysis of Eco-efficiency.

28 From the research made on Eco-efficiency it was established that there are two types of indicators, General and Specific, with which to measure Eco-efficiency. It was also found that General Indicators are usually established as the ratio between the Added Value and the Total Environmental Impact, for a certain life cycle phase, and the Specific Indicators are used to demonstrate certain relations the company deems relevant, and are only significant for that specific company and industry. As such, these indicators can be defined as the ratio between any number of parameters. As the resources previously presented were material, energy and cycle time, it is chosen to present the Specific Indicators as the ratio between total material and energy consumption, for one year, and annual required time, relating to the cycle time, and the total amount of material injected, for the same year.

For the General Indicators, a study on economic indicators to translate Added Value is made where, as stated in literature, there are two types of economic indicators: traditional, which include GVA (gross value added) and NVA (net value added), and non-traditional, where it is worth highlighting EVA (economic added value), MVA (market added value) and EBITDA (earnings before interest, taxes, depreciation and amortisation) [52]. Although these may be important indicators, taking into account the non-traditional indicators are performance indicators more suited for analysts and investors, the GVA and NVA would be the more correct choices, considering the goals and parameters of this work. However, as standards often indicate, the EBITDA is an important economic indicator for value. As such, a brief analysis, based on the definition, will be performed to investigate its relevance in this study.

According to bibliography, GVA is the difference between value of total production and non-factor costs, where the value of total production is the revenues and the non-factor costs are the costs of purchase of raw materials and energy, additional components and services, such as contract work [53]. The equation for GVA is illustrated in Equation (4.1).

GVA = revenues - (non-factor costs) (4.1)

As for the NVA, it is defined as the difference between revenues and the sum of all internal and external costs, or, in other words, the liquid value [54]. Equation (4.2) illustrates NVA.

NVA = revenues - internal costs - external costs (4.2) Finally the EBITDA is defined as the difference between revenue, non-factor costs and operational expenses, such as salaries. According to this definition it is possible to conclude that EBITDA is the GVA with the additional administrative and operational costs [52]. The equation for EBITDA is presented in Equation (4.3).

EBITDA = revenues - (non-factor costs) - operational costs (4.3) For this work, the mould cost is considered a fixed cost, therefore a depreciable value not included in the non-factor costs, and the operational costs are translated into salary, which is considered a constant in this work. As such, it is possible to conclude that the EBITDA offers no additional information, when compared to the GVA, for this study, and will not be evaluated further.

29 As for the GVA and NVA, both indicators should translate important information, since material and mould cost must be the most important cost drivers, due to the large scale of this process and the high costs of the injection mould, hence both will be analysed further to determine their actual importance in this study. However, as there is no information on revenue, and this work is mainly to compare mould design alternatives, it is chosen to normalize the cost results, dividing the cost, for the alternative with the lower cost, by the cost of the alternative to study.

In terms of environmental impact, the results from the PBM are presented, multiplied by the specific impacts for each parameter found using the software SimaPro, for all mould design alternatives, and normalized using the same mould design alternative as reference, dividing the environmental impact for each mould design alternative by the reference environmental impact. With the Added Value and Environmental Impact Results normalized, it is possible to illustrate the Eco-Efficiency Ratios for all mould design alternatives, presenting the ratios with the previous results of Added Value and Environmental Impact, in order to better understand how each variable influences the final result.

However, it is concluded that the ratios are not as reliable as expected, where the highest ratio indicates the best alternative, since different mould design alternatives, with different results for cost and environmental impact, can present a close or equal ratio. To counteract this effect it is created a graph, placing each alternative in terms of cost and environmental impact, with its respective Eco- efficiency Ratio. All these results are discussed when presented, analysing the influence of each mould variable and the machine type in the final result.

For Specific Indicators, the ratios are presented are previously discussed, for the resources discussed in the PBM results, material, energy and cycle time, and for all mould alternatives. Furthermore, as there is no need for company based financial information, there is no need to normalize the results.

The macro flowchart for the development of this work is presented in Figure 4.1.

Input Variables

Resource Inventory

Costs Environmental Impact

Model Validation

PBM Results

Cost Results

Environmental Impact Results

Resource Results

Normalized Results

General Eco- efficiency Indicators

Specific Eco- efficiency Indicators

Analysis of Eco- efficiency Results

Case Study

Part and Process Data

Mould Design Alternatives

Figure 4.1 – Methodology for Model Development and Result Analysis PBCM PBM

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5 Process Based Model

In this chapter will be explained the development of the PBCM for this work, combining the explanation of Chapter 2, for plastic injection moulding, with the knowledge of the most important parameters and how they can affect the process, through a process flowchart. With the understanding of all the phases, or models, which compose a process based cost model, and the environmental impact factors, the model for this work created, allowing to analyse the influence of all variables in this process and the resulting effect those variables have in cost and environmental impact.

The difference between these phases, and all additional factors, will be explained and the model developed, describing all equations and simplifications made, as well as illustrating the complexity of each individual cost through a flowchart, divided in all models defined for this PBCM.

Having constructed the model, its validation is performed with the aid of a case study, presented in [32], and which its results were confirmed by Celoplás, a Portuguese mould making and injection moulding company.

Finally, the environmental impact for this model is explained, following the goal of this work which is to create a tool to study Eco-efficiency in the mould design phase, presenting the reasoning behind these calculations and what variables, from the PBCM, will contribute to the environmental impact. This component of the model will not be validated, since there is no basis in which to compare results and the resources inventory is intrinsically validated when the costs for both models are discussed.