Authors: Žiga Jelen, dr. Branka Viltužnik
Plastika Skaza d.o.o., Selo 20a, 3320 Velenje
Abstract: Plastic injection moulding is one of the most commonly used manufacturing processes applied to produce plastic products. The process is relatively energy efficient, since it takes less energy to melt or soften plastics than it does metals. However, injection moulding is a multifaceted process which has many variables that many forget to account for. This may lead to financial losses and a larger environmental impact than necessary. This paper is a brief introduction of the steps we at Plastika Skaza take and the tools, we use to ensure that our product have a minimal environmental impact. We have taken our Bokashi Organko 2 as a case study to present the actions we took during the products life time, from first concepts to active production. By using Computer aided simulations, modern machinery, recycled materials and statistical methods, such as the design of experiments, Taguchi model and Life cycle impact assessment, we achieved an average reduction of 39,4 % in the products environmental impact.
Keywords: Injection moulding, Bokashi Organko 2, Computer-aided simulations, Design of experiments, Life cycle impact assessment
Nowadays, plastic products are essentially unavoidable. They are common in all areas of our lives, from food packaging, hi-tech devices, aerospace, commodity product and medicine. Since they are so frequent, an increased awareness for environmental responsibility has sprung up in the last 10 years. Even though plastics have a wide range of practical applications and many advantages, they also have a certain environmental impact. As a result, environmentally responsible manufacturers have taken it upon themselves to do their part in reducing the environmental impact of plastics. To achieve this, companies have started using tools, such as life cycle assessments (LCA), CAD simulations, green materials and modern processing machines, to optimize their processes. The goal of this paper is to briefly introduce some of the key aspects, steps and tools that we have used on one of our high-end products, the Bokashi Organko 2 (BO2), which enables and enriches sustainable organic waste management in every household. The waste is stored in an airtight container. As the inflow of air is smaller, so is the probability of rotting. There is no unpleasant odour even when opening the container, as we put bran with effective microorganism on the organic waste. They ensure that a fermentation process occurs instead of rotting. The waste preserves all the important nutrients, which would otherwise be lost. If we close a filled composter and leave it for 14 days at room temperature, we get an excellently fermented mixture for storing as compost, for digging into garden soil or for disposing of composted waste in the brown organic waste bins. Such fermented waste is the basis for first-rate compost.
BO2 is made from recycled polypropylene (PP) using an injection moulding process. Plastic injection moulding is an energy intensive process. While it requires significantly less energy than metal processing, it still uses a lot of energy. Since energy (in this case electrical energy), has an environmental and financial cost, it is an important place to start. For instance, a typical injection moulding company uses approximately 66 % of its energy for polymer processing (Mobil, Energy Saving Guide) and the remaining 26 % for all the peripheral systems needed to support the injection moulding process. This is where the most significant energy savings can be made. An approximate distribution of energy use in a typical injection moulding plant is shown in Figure 1.
Figure 1: Energy use distribution in a typical injection moulding plant
Process optimization starts at the development phase. It is essential, that products are designed according to the principles of sustainability with the main goal: maximize resource efficiency. This means using as little material, energy, manpower and time to produce a product as possible. However, the crucial part is to find a balance that must be struck between resource efficiency and quality. While it is easy to make a high-quality product, you can simply use more material than is needed, making your product bigger, stronger, tougher and heavier.
Designing a cost effective, high-quality product with low environmental impact is a fine balance, we strive for. The tools to achieve this have been around since 1971, with the public release of NASA’s NASTRAN finite element analysis (FEA) program (ESRD, Brief History of FEA). Simulation programs have been improved, expanded and diversified since then. They are a powerful tool, that we used during the design phase of BO2. While designing a product, computer simulation can be roughly divided into the structural design phase, where the key dimensions, thickness, potential reinforcements, etc. are determined, and the manufacturing simulation phase.
The injection moulding of plastic parts is influenced by a wide range of different parameters, that must be set manually. While simulation programs give a rough estimate of some of the parameters, they are limited. To set these parameters correctly and efficiently, it takes an expert that relies on his experience and statistical methods, such as the design of experiments (DOE), with an emphasis on the Taguchi approach. The latter was developed in the late 1940 by Dr. Genechi Taguchi, whose effort was to make the method more user-friendly in order to improve the quality of manufactured goods (K. Roy, 2001). The key benefit of using DOE are that the method allows the optimization of products and processes, by studying the influence of individual factors. The method can help the user determine which factors have a larger influence and which ones have a smaller one. By doing this in an organized statistical fashion it takes away most of the guesswork that can be associated with injection moulding. There is however no replacement for a seasoned process engineer and his experience.
The benefits of applying an organized approach and optimizing the processing parameters are savings in energy, higher quality of products, better reproducibility and increased production (Godec et al, 2013).
Injection moulding machines have advanced significantly since their beginning as hand operated contraptions. Nowadays, state of the art machines are computer controlled with high accuracy servo motor drives, or hybrid hydro-electric systems (Zhang et al, 2017). Since plastic processors rely on their machines running nonstop, to make a profit, the machines are built to a high standard. It is not uncommon to see machines that are 20 years or older and still in use. These old machines usually use hydraulic systems, which are robust and reliable, yet they are notoriously energy inefficient.
In recent years, there has been an increase in the sales of electric injection moulding machines, that instead of using hydraulics use servomotor to function. In our experience, all-electric machines demand a 10–20 % higher initial investment when compared to hydraulic machines. Nevertheless, they offer many benefits:
- higher reproducibility, stemming from the high accuracy electric motors,
- the machines are capable of doing multiple operations at the same time,
- they are quieter,
- no hydraulic fluids,
- less maintenance,
- faster start-up,
- up to 70 % energy savings (Socks, 2005).
With all this talk of the benefits that electric machines can bring, they do have certain drawbacks. Currently, they are only available in the sub 1000t clamping force range. For larger machines, there is no replacing hydraulics, for certain parts of the machine. As a result, hybrid machines exist, that employ the accuracy and repeatability of electric motors, where the largest benefit is seen mainly in the injection side of the machine and hydraulics in the clamping side of the machine, where large forces are required. These hybrid machines also incorporate variable frequency electric motors and improved hydraulic pumps to reduce the losses in the hydraulic system, by only using the pump when pressure is needed. This has an added benefit of increasing the lifespan of the electrohydraulic drive (Zhang et al, 2017).
Measuring environmental impact
The key tool when assessing a products environmental impact is ‘’Life cycle assessment’’ (LCA). The latter allows users to asses all the key factors, that add up for a products or a company’s total environmental impact. This gives companies an opportunity to (European Commission, 2010):
- show their environmental strategy and stance,
- obtain the results of environmental impacts using standardised methods,
- analyse the impact of the product beyond the scope of CO2 emissions,
- identify the potential to reduce the environmental impacts within the scope of technological processes,
- identify the major factors on the overall environmental impact in all life cycle stages of a given product.
Case study: Manufacturing optimization of Bokashi Organko 2
During the tool, design phase is where the largest impact in regards to energy usage can be made. In injection moulding it is common to use software to simulate the production or moulding of parts. This entails making a digital approximation of the object you wish to produce and the tooling with which you intend to produce the part. By using these simulation tools, it is possible to optimize the injection moulding process beforehand. It is common to simulate the moulding of most parts that are intended to be produced by injection moulding. There is however a difference in the number of results you can obtain. This is dependent on the type of simulation you run and the complexity of the object you wish to produce. During the development of BO2 we encountered few issues with certain parts of the BO2. The inner container was shown to be a relatively simple part to produce, however, the simulation showed us that certain errors might occur and be visible on the finished part, such as weld lines. We encountered more issues with our ‘’Press cover’’. The cover is a complicated part that is manufactured using the two component (2K) process, which is capable of producing a single product from 2 different materials. To determine the optimal tool design, it was necessary to run several simulations before we found a satisfactory tool design. The key issue was the tool filling, which is influenced by the injection location and hot runner design. A hot runner system is a series of heated channels that distribute molten plastic to the tool cavity in order to fill it. This is a prime example of why the injection moulding simulation programs were first developed. By simulating the different iterations of hot runners, as shown in Figure 2, we were able to produce a functioning tool on the first attempt, with only minimal fine adjustments needed after the tool was delivered. Before the advent of these programs, it was not uncommon for tools to be repaired multiple times before a satisfactory product was produced.
Figure 2: Different hot runner designs for the two different components
It should be noted that simulation programs are not flawless. It takes a great deal of expertise to get relevant results from them and even in the best of cases, these results do not completely represent reality. Besides the results we have shown, injection moulding software offers many more different results, that help you troubleshoot potential problems before they occur as well as giving a general idea of the processing parameter that should be used during production.
By producing one of our parts on a hydraulic and electric machine, we were able to see a 61 % improvement in energy use and a 1.4 second cycle time reduction on the electric machine, due to the electric drives of electric injection moulding machines, which offer improved energy efficiency and higher reproducibility that results in a more stable process and efficient process. Results are shown in Table 1.
Table 1: Comparison of an electric and hydraulic machine in Plastika Skaza d.o.o.
|Hydraulic machine||Electric machine||Unit|
|Measured energy usage||17.86||7.169||kW|
|Energy usage per part||0.241||0.094||kWh/part|
|Price of electricity per part||0.016||0.006||€/part|
The LCA (life cycle assessment) study of our Bokashi Organko 2 includes all energy and mass flows for production of materials. The methodology used comprised the analysis of life cycles more commonly known as the LCA methodology. The environmental impact was assessed using the CML2001 methodology which includes global, regional and local environmental indicators and thus allows an in-depth analysis of the results. The Gabi Thinkstep software environment was used to set-up an LCA model and to calculate the environmental balance of Bokashi Organko 2 under different scenarios: made from petroleum-based materials compared to recycled materials. The results were analysed using global, regional and local environmental indicators. The functional unit is 1 piece of Bokashi Organko 2 without packaging. To show how the measures in the production process of the company Plastika Skaza affect the environment, an environmental analysis of two scenarios was made:
- Bokashi Organko 2 made of petroleum-based PP,
- Bokashi Organko 2 made of recycled PP and the implemented environmental measures within Skaza, that were briefly described earlier.
Bokashi Organko 2 consists of an external and internal container, presser, lid, plug, carrier strap, two seals and a dosing trowel. The total weight of BO2 is 1742.8 grams, of which 96 % are recycled PP.
Figure 3: Improvement of LCA markers, by the usage of recycled PP
The results in Figure 3 show that on average after the introduction of recycled material and all the measures in the production of Bokashi Organko 2, there is a relative reduction of environmental indicators of 39.4 %.
Since recycled materials were already produced from oil, their reuse gives a better picture. They require less energy intense processing to make them suitable for use again. Recycled plastics are first sourced from landfills and companies that process plastics, afterwards they are cleaned and compounded to produce plastic granules. Companies can also add their own rejected parts back into the system thus reducing their impact. Because recycled plastics are commonly of a lower quality it is commonplace to mix them with virgin materials to improve their properties and appearance. Figure 4 shows the reduction in environmental impact by increasing the % usage of recycled materials. Results are displayed in table 2.
Figure 4 Improvement of LCA markers, by increasing the % usage of recycled PP
Table 2: LCA analysis results of increasing % usage of recycled PP
| ||0% Recycled||30% Recycled||50% Recycled||70% Recycled||100 Recycled|
|ADP elements [kg Sb eq.]||0,0000004450||0,0000003580||0,0000003000||0,0000002420||0,0000001550|
|ADP fossil [MJ]||66,00||47,30||34,90||22,40||3,77|
|AP [kg SO2 eq.]||0,00249||0,00194||0,00157||0,00121||0,00066|
|EP [kg Phosphate eq.]||0,0003100||0,0002450||0,0002020||0,0001580||0,0000931|
|FAETP inf. [kg DCB eq.]||0,019300||0,013700||0,009930||0,006190||0,000593|
|GWP 100 years [kg CO2 eq.]||1,680||1,380||1,180||0,977||0,674|
|HTP inf. [kg DCB eq.]||0,1580||0,1170||0,0900||0,0629||0,0223|
|MAETP inf. [kg DCB eq.]||75,30||60,40||50,40||40,50||25,50|
|POCP [kg Ethene eq.]||0,000447||0,000326||0,000246||0,000166||0,000045|
|TETP inf. [kg DCB eq.]||0,000576||0,000475||0,000408||0,000340||0,000240|
By utilising several improvements that include improved design, statistical methods, updated machinery, material choice, and finally quantifying our impacts with LCA methodology, we at Skaza were able to achieve an average of 39,4 % reduction in our environmental impact.
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