1. Introduction
The International Maritime Organization (IMO) set a strategy to force the marine industry to reduce ship-induced greenhouse gas (GHG) emissions by at least 50% by 2050 [
1]. This strategy leads to a continuous demand to improve ship performance and reduce the environmental impact during the lifecycle, which can be achieved holistically. Distinctly, this comprehensive approach addresses research areas, especially hull form optimization, novel propulsion systems applications such as energy-saving devices, new regulations, energy management systems, structural optimization, including life cycle improvements, sustainable materials, and social perceptions [
2,
3,
4]. In these areas, it has always been desired to produce ship components with sustainable lightweight materials with high specific strength, remarkable specific stiffness, and high corrosion resistance and to make life cycle improvements.
Lightweighting, which is the only way to achieve maximum efficiency with minimum consumption, has turned into a discipline under the pressure of increasing sustainability concerns. This discipline has three basic components: lightweight materials, lightweight manufacturing, and lightweight structures.
When it comes to lightweighting materials, polymer-based composites, with their applications developing day by day since the 1950s, come to mind before their metal counterparts, such as aluminum. Carbon fiber’s performance and price, which are constantly improved by material science and engineering, attract attention. Applications that historically started with the hand lay-up method turned towards faster-capability modding processes such as injection molding, compression molding, liquid molding, and thermoforming, especially in the automotive field, in the 1980s.
Efforts to reduce the cycle time by improving the process have required the development of vacuum-assisted methods, which have also achieved improvements such as better surface quality and the elimination of in-mold coatings.
Today, resin transfer molding and structural reaction injection molding methods are used for composites in which high-performance components are used, and these processes have advantages such as reducing fiber scrap, easily coping with the form complexity of the part, even better control of the part thickness, and a relatively high processing rate.
Again, today, there is an increasing trend towards production in which hybrid multiscale composites are used (for example, using carbon nanotubes in carbon fiber composites) in terms of lightweighting [
5].
However, all improvements, such as new cutting technologies, new welding technologies, additive manufacturing, and memories in general, are within the scope of lightweight manufacturing.
Additive manufacturing is also very suitable for creating lightweight structures. For example, by this method, lattice structures with varying internal density are an example of newly developed lightweight structures whose many properties are superior to those of solid materials. Additionally, with this method, topological optimization that will provide the best path for force transmission can also be easily performed [
6].
Manufacturing ship components made from sustainable and lightweight polymer materials such as thermoset plastics will enable the above-mentioned improvements to be achieved. In addition to the use of lightweight materials such as aluminum and polymer-based composites, the production of complex geometries without waste is also important in this context. The prominent technology today for the manufacturing of complex geometries with the help of polymers and/or polymer composites is additive manufacturing, which has become widespread with the use of 3D printers (3DAM). Producing ship components (e.g., rudders, propellers) with sustainable and lightweight materials and 3DAM is one of the research areas that has attracted attention in recent years.
One of the main problems in the marine industry today is the unsustainability of polymer-based composites consisting of thermoset plastics, which are the most widely used materials in this industry. Because of their complex internal structure, a cost-effective end-of-life alternative has not yet been developed for them, especially in terms of recycling [
7,
8,
9,
10]. The marine industry, where thermoset composites are widely used, faces two important challenges: (1) creating end-of-life alternatives for vehicles that will not have a negative impact on the environment and (2) rapidly finding new sustainable materials and production methods due to legislation requiring the appropriate reuse or recycling of all engineering materials and products [
11]. Relatively recent environmental legislation, such as EU directives for end-of-life vehicles [
12] and waste electric and electronic equipment [
13], requires sustainable end-of-life alternatives to thermoset plastics. By 2050, in the European Commission’s Plastics in a Circular Economy Strategy, it is stated that all plastics and composite wastes should be reused or recycled [
14].
As is known, due to technological and economic difficulties, the recycling of thermoset plastics almost entirely consists of incrimination to obtain energy, resulting in no or little fiber recovery. However, although mechanical, chemical, and thermal energy conversion methods have been extensively researched for these materials, there is no widespread commercialization in this field yet.
For this reason, on a global scale, the increasing amount of unhandled end-of-life composites is directing the marine industry towards thermoplastic composites that are easily recycled by thermal methods. In the literature, there are studies on the environmental effects of composites containing new thermoplastic resins, especially on the ease of recycling and the behavior of recycled composites. For example, in a study conducted by Allagui et al. [
15], it was observed that after recycling a composite produced from Elium, an innovative resin, and flax, a natural fiber, the elasticity modulus of the new composite improved while the failure properties and lifespan decreased. Additionally, in a study conducted by Sam-Daliri et al. [
16], the optimization of filament and product production from glass fiber-reinforced polypropylene composite waste for material extrusion using the 3D printing method was studied.
Among the thermoplastic counterparts, high-density polyethylene (HDPE) stands out for its compatibility with marine environmental conditions, such as the following:
- -
Resistance to moisture and the corrosive effect of the seawater;
- -
Not allowing marine microorganism growth on surfaces in contact with the sea;
- -
High UV stability;
- -
Endurance under cycling loads (high fatigue strength);
- -
Due to the mentioned advantages, the use of HDPE in the manufacturing of many products in the marine industry, such as underwater pipes and cables, piers, small work boats, geomembranes, and cages in aquaculture farms, is becoming widespread. The most popular manufacturing methods for these products are injection molding, hot-press and material extrusion printing, and, as of recently, vacuum-assisted resin transfer molding for Elium thermoplastic resin. Since it is possible to use HDPE in 3DAM in the forms of filament, powder, and pellets, these types of production encountered are limited to some experimental studies rather than large-scale industrial applications [
18,
19].
One of the main reasons for the inadequacies in the use of HDPE in 3DAM is its poor printability using the material extrusion (MEX) method. In previous studies on the improvement of the printability of HDPE, it has been stated that HDPE filaments have poor adhesion to the printing surface due to their low surface energy, leading to weak interlayer bonding, warping complications during printing, and stiffness limitations compared to other engineering plastics such as PA and ABS [
20]. In a study by Schirmeister et al. [
19], it is noted that the printing temperature of HDPE should be kept appropriate with the help of a closed chamber, and the correct adhesive should be used in the print bed. Similarly, in a study by Jagannathan et al. [
21], the printer setting, the quality of HDPE’s material properties, and the regulation of material flow were given as the key elements of HDPE’s printability and achieving a smooth product surface.
In studies carried out to improve the printability of polymer filaments, the addition of macro- and microfibers and nanoparticles comes to the forefront. In this context, glass fiber, which is affordable in cost and provides relatively moderate strength, and carbon fiber, which is relatively high in cost but provides high strength and stiffness, are preferred in new filament products, especially in industries such as the aerospace and automotive industries, where the goal is to achieve lightweight and durable structures such as car chassis, aircraft wings, frames, stringers, etc. [
22,
23,
24,
25].
Furthermore, lighter structures for the same product can be achieved thanks to the material extrusion method, which allows for thicker and more flexible blades, improving hydrodynamic performance by raising cavitation inception speeds [
26]. Most sustainable material research focuses on the structural design of a plate, beam, or aircraft wing [
27,
28] with a mechanical performance analysis [
29], failure analysis [
30], optimization [
31], and impact damage assessment [
32]. Research on 3DAM for wing structures is focused on aircraft wings [
33,
34,
35,
36] rather than ship propellers, whose geometry is more complicated. Only Herath et al. [
37] have used hydrofoil geometry in their study, but some modifications were made to the wing geometry since the geometry was not suitable for production with composite material.
In this study, a ship propeller geometry is selected as a case study that focuses specifically on sustainable materials and manufacturing systems since it is one of the fundamental components of a ship and is operated in challenging conditions such as under heavy loads and in corrosive and erosive environments. Carbon-reinforced HDPE and its coated version as sustainable polymer composites and the Fused Filament Fabrication (FFF), also known as Fused Deposition Modeling (FDM), of 3DAM as a sustainable option in the production of products with complex geometry were chosen. Experiments and numerical analyses were also performed to investigate the load-dependent deformation behavior of the blades, whose geometry was chosen to be more suitable for 3DAM [
38,
39].
The experimental work presented here was conducted at the Emerson Cavitation Tunnel (ECT) at Newcastle University. The results show the promising hydromechanical performance of the composite ship propeller model studied.