CO₂ Footprint of Machine Elements Made of Fiber-Reinforced Polymer Concrete Compared to Steel Components

Abstract

Energy efficiency and resource economizing are the drivers for the development of new types of material-hybrid design approaches for machine tools. Polymer concrete has been used for machine beds in machine tool design for many years. The good thermal and dynamic properties of the material are particularly convincing in this context. The good damping properties for structural components are also interesting, as this reduces for example tool wear and at the same time the high damping compared to steel structures has a positive effect on the surface quality of the machined workpiece. Current research in the field of structural dynamics is dealing with the substitution of steel and cast components with hybrid, actively preloaded polymer concrete parts. This allows the use of the positive damping properties of polymer concrete and the positive tensile strengths of the integrated fiber-reinforced structures for dynamically loaded machine components such as machine arms or machine stands. The focus of the study is to replace the arm of a bed-type milling machine, which is currently a welded design, with a component made of prestressed carbon fiber-reinforced polymer concrete. Based on the first results of the volume ratios of the structures, conclusions are drawn about the life cycle assessment (cradle to gate) of the components. The results will contribute to a design recommendation for the carbon fiber reinforcement in the polymer concrete arm to achieve a better structural efficiency on the one hand and a better life cycle assessment on the other.

1 Advantages and Disadvantages of Polymer Concrete for Machine Elements

The selection of materials used in machine tool design is always based on the occurring load collectives. This is especially true for approaches to structural optimization, which aim to reduce the component mass while optimizing stiffness but should not change the basic properties of the component. Because of its properties, such as very good damping and low thermal expansion [1], polymer concrete offers the best prerequisites for use as a material for highly stressed components in machine tools. Polymer concrete, also known as mineral cast or reactive resin concrete, is a composite material consisting of inorganic mineral filler particles and stone particles (quartz, granite, basalt) of up to 93% by volume and a binder, mostly epoxy resin or unsaturated polyester resin, of approx. 10% by volume [2, 3]. In moving components or structures that are exposed to high, possibly even alternating loads, especially in tension or bending, mineral casting quickly reaches the load limit [3]. But the properties of polymer concrete are dependent on several criteria: Binder, mineral aggregates, type of reinforcement and mass proportions of the individual components.

The chosen approach aims at the development of methods for prestressing such polymer concrete components with the help of textile reinforcement structures, in order to be able to permanently withstand occurring tensile loads. The enormous potential of high-performance fibers, which are successfully used in conventional plastic composites, has not yet been systematically exploited in polymer concrete. Nevertheless, fiber reinforcement is not a fundamentally new concept: fibers made of glass and carbon [4, 5], have already been used in the form of filler material or as reinforcement bars [6] with a low volume fraction.

In addition, it has been shown that the use of polymer concrete also offers advantages regarding the mass of components due to its lower density and thus has a direct influence on the energy required to move the components through lightweight design approaches [7]. The tensile strength of the material, however, is too low for use in places that are exposed to high and changing loads. Therefore, a hybrid approach of mineral casting and carbon fibers can be a target-oriented solution in lightweight design for machine tools. This opens new application possibilities for mineral casting in machine tool design and a step forward in the development of a general reduction of CO₂ emissions in manufacturing and the use of machine tools. Within the scope of this paper, a first investigation of the substitution of a welded steel boom against a carbon fibre-reinforced polymer concrete boom is examined regarding the CO₂ balance in the manufacturing process of the components. Based on an estimation of the volume-% of the different ingredients, the advantages, and disadvantages of fibre-reinforced polymer concrete with regard to CO₂ savings are discussed.

2 Optimization of the Machine Component “Extension Arm” Through Topology-Optimized Fiber-Reinforced Polymer Concrete

In this example, the extension arm of a machine in console bed design is considered. The extension arm currently consists of welded 4-sided steel tubes with a total weight of 28 kg. The steel framework is mounted on the movable frame and the spindle via two 20 mm thick flange plates. Figure 1 shows the current structure of the machine. The load spectrum used as a boundary condition for the redesign of the machine cantilever is based on a pocket milling process in aluminium AlMg₁. According to Victor-Kienzle, a cutting force Fc of 1318 N results for a cutting depth ae of 12 mm with full engagement of an end mill with 8 mm diameter and 3 cutting edges. A total force of 2000 N is assumed as a safety factor for the further design. The resulting moments M1 and M2 are time-variable depending on the force application vector and are applied in the FE simulation over several load steps.

Fig. 1.

Force directions of a machine arm in a machine tool in console bed design

The integration of the estimated possible damping constants via harmonic analysis is used as a further boundary condition for the design of the machine component. To achieve the general goal of minimizing the compliance of the system component, a topology optimization of the extension arm is carried out. The material-specific parameters for the analysis were taken from the manufacturer's specifications of Rampf EPUMENT 130/3 A3 as well as the carbon fibers of the manufacturer Mitsubishi Chemical Carbon Fiber and Composites - GRAFIL 34–700.

Fig. 2.

Topology-optimized, fiber-reinforced machine extension arm

By analyzing the resulting force paths, it is possible to pre-tension the Carbon fibers in the loaded direction. Figure 2 shows the results of topology optimization and weight minimization with a Carbon fiber content of only 1.58%. With a total weight of the polymer concrete extension arm of 23.33 kg, this corresponds to a total fiber weight of 368 g divided into 28 fiber bundles. Figure 3 shows a comparison of the results of the maximum displacement of the steel extension arm compared to the reinforced polymer concrete extension arm. The optimized fiber-reinforced polymer concrete arm has a 60% lower displacement under load than the steel body.

Fig. 3.

Displacement of the steel extension arm on the left and the prestressed polymer extension arm on the right when loaded in negative x-direction during pocket milling

With the objective of making the manufacturing of a polymer concrete arm even more efficient, the amount of carbon fibers was reduced further in a second step. With an average value of 15, 5 kgCO₂e/kg, carbon fibers cause very high CO₂ emissions in their manufacturing process. With the help of a parameter study, the used amount of carbon fibers was reduced until the polymer concrete extension arm achieves the same theoretical compliance as the steel extension arm. This reduced the volume of fibers to 0.58%, corresponding to 135 g of carbon fibers. For further consideration of the CO₂ footprint, the calculated mass proportions of the steel extension arm, a polymer concrete extension arm without carbon fibers and the structurally optimized polymer concrete extension arm with carbon fibers are summarized in Table 1.

Table 1. Overview of the mass ratios of the extension arms

3 Carbon Footprint Analysis

Fig. 4.

Carbon footprint of steel, polymer concrete and fiber-reinforced polymer concrete extension arms by country of production

To compare the three components, a carbon footprint calculation was carried out (cf. Fig. 4). The results show that in all cases the manufacturing of the raw materials of steel cause lower CO₂ emissions than those of polymer concrete. The manufacturing process of steel, however, causes high CO₂ emissions due to the required high energy input [8]. Polymer concrete is advantageous here due to its low primary energy demand and can thus reduce CO₂ emissions by up to 35%.

For all extension arms, Germany as a production site shows itself to be the most favourable regarding the CO₂ footprint, especially for the steel arm. In this case, the CO₂ footprint of the steel arm is with 25.57 kgCO₂e only 9% higher than the extension arm made of unreinforced polymer concrete and 15% higher than the fiber-reinforced polymer concrete.

Due to its high energy-related CO₂ emissions of 555 gCO₂ /kWh [8] (compared to 366 gCO₂/kWh in Germany [9]), China is the most unfavorable production site for all versions of the extension arms. For the steel extension arm, there is a 49% increase in the CO₂ emissions compared to the manufacturing site in Germany. The extension arm made of fiber-reinforced polymer concrete shows an increase of 34% and achieves almost the same CO₂ balance for the production site China as the heavy extension arm made of solid polymer concrete.

For a realistic comparison of the three components, the most likely countries of origin are determined for all individual parts. It turns out that China is the largest supplier of steel products [10], and the USA is the main supplier of carbon fibers with a market share of 30% [11]. Following manufacturers’ information, Germany assumed to be the production location for polymer concrete [8]. When comparing the extension arms under these assumptions, the arm made of fiber-reinforced polymer concrete has the best CO₂ balance with 24.82 kgCO₂e. The steel extension arm achieves the highest CO₂ emissions with 38.2 kgCO₂e. It is shown that the use of fiber-reinforced polymer concrete as a material for the extension arm can reduce CO₂ emissions by up to 35% compared to steel.

In the following, the CO₂ emissions of the extension arms made of unreinforced polymer concrete in Fig. 5 and fiber-reinforced polymer concrete in Fig. 6 will be analyzed in more detail, assuming a mixed country of origin.

Fig. 5.

Carbon footprint composition of an extension arm made of polymer concrete (production country: Germany, production country for the steel parts: China)

For the polymer concrete extension arm (Fig. 5), 53% of the carbon dioxide emitted can be attributed to the manufacturing process. The casting of the integrated steel parts is the largest part of this process (95%). This also represents the largest part of the overall balance. Other items are the drying of the minerals and the mixing of minerals, resin and hardener. The casting process does not produce any CO₂ emissions.

A look at the CO₂ balance of the ingredients shows that the epoxy resin provides the largest share here. The steel parts represent the second largest share. The mineral fillers contribute only slightly to the overall balance with 0.07 kgCO₂e.

The carbon footprint for the fiber-reinforced polymer concrete extension arm in Fig. 6 is made up almost equally of the carbon dioxide produced in the manufacturing process and the carbon dioxide produced in the production of the ingredients. Like with the unreinforced extension arm, the production of the steel parts represents the largest share.

Among the ingredients, epoxy resin is again the largest component in the CO₂ footprint. With a mass share of 4.4%, it accounts for over 50% of the ingredient-related greenhouse gas balance. The mass-related CO₂ equivalent for the material is 5.9 kgCO₂e/kg. Among the materials considered here, only carbon fibers have a higher mass-related CO₂ equivalent of 15.5 kgCO₂e/kg.

Fig. 6.

Carbon footprint composition of a extension arm made of fiber-reinforced polymer concrete (production site: Germany, production site for the steel parts: China, production site for the carbon fibres: USA)

If the aim in the design of the extension arm made of fiber-reinforced polymer concrete is to achieve a better CO₂ balance than with the steel extension arm, the maximum quantity of carbon fibers is limited due to the high mass-related CO₂ equivalent. As Fig. 7 shows, the extension arm made of fiber-reinforced polymer concrete achieves with a mass of 1.02 kg of carbon fibers the same CO₂ balance as the steel arm.

Fig. 7.

CO₂-equivalent of the extension arm made of fiber reinforced polymer concrete depending on the mass of carbon fibers compared to the CO₂-equivalent of the steel extension arm

4 Conclusion

In this paper, it was shown that replacing a steel extension arm with a fiber-reinforced polymer concrete arm can improve the carbon footprint in the manufacture of machine tools while maintaining the same or even a better compliance. It was also shown that adding more carbon fibers can further improve the mechanical properties of the fiber-reinforced extension arm. As long as the mass of the carbon fibers remains below 1.02 kg (4, 2% of the total weight), the carbon footprint of the extension arm is 34% better than the steel extension arm.