1. Introduction
Polymers offer versatility, durability and affordability, making them essential across industries, from packaging to automotive. Robust plastic components are vital for function and longevity, necessitating resistance to external forces without deformation. Additionally, lightweight polymers enhance fuel efficiency, aiding the environment. Processing techniques are optimised to improve strength, durability, and sustainability. Traditionally, augmenting part strength meant more material, yet harmonising strength and lightness is complex for lightweight elements. Optimal strength-to-weight balance emerges through design and simulation. Polymer choice markedly affects both attributes, influenced by processing and application needs. Flexibility, stiffness, toughness, thermal stability and impact resistance vary based on processing. Ultimately, product design considers application, method and cost.
Foam polymer is an evolving lightweight structural material that exhibits remarkable characteristics such as high specific strength, exceptional energy absorption, damping and thermal properties, all coupled with a low density (approximately 10–15 times less dense than its original volume) [
1,
2,
3,
4,
5,
6,
7]. In the 1980s, the Massachusetts Institute of Technology (MIT) introduced microcellular processing to the polymer industry. The objective was to reduce material usage, decrease the weight of final parts and modify material properties by incorporating small spherical cells into polymer-based products [
8]. The initial publications and theses resulting from this research laid the foundation for the technology, proving its concept and advancing fundamental theories. The original work focused on batch processing and extrusion, leading to the granting of the first U.S. patent in 1984 [
9]. Commercialisation of microcellular technology began in 1998 when Axiomatics Corp developed the first reciprocating screw microcellular injection moulding (MIM) machine. Before microcellular technology, conventional foam was utilized to create polymer parts with a cellular structure. However, the limitations of cell densities and poor mechanical properties were overcome through MIM. Microcellular foaming processes have the capability to produce smaller cell structures compared to regular chemical blowing agent methods [
10]. While chemical blowing agents typically achieve cell structures as small as 250 mm, microcellular foaming can produce cells in the range of 3–100 mm [
11]. MIM has also demonstrated its ability to reduce polymer content, decrease the weight of final parts and lower processing energy requirements. Various processing variables, such as injection speed, gate flow resistance, blowing agent content, melt flow rate, and talc addition, have been investigated in the context of MIM [
12,
13,
14,
15]. The unique structure of microcellular foamed polymers offers several advantages over traditional solid polymers, including the following:
Lightweight: Microcellular polymers are much lighter than traditional solid polymers, making them ideal for applications where weight reduction is important.
Improved Mechanical Properties: Microcellular polymers have improved mechanical properties, including increased stiffness, toughness and impact resistance. These properties make them ideal for many products.
Reduced Material Usage: Microcellular polymers use less material than traditional polymers to achieve the same level of strength and durability. This makes them an environmentally friendly choice as less material is used.
Better Insulation: The air bubbles in microcellular polymers act as insulators, making them ideal for use in applications where insulation is important.
Cost-effective: Microcellular foaming is a cost-effective manufacturing technique as it allows for the use of less material, resulting in lower costs for manufacturers.
In recent times, additive manufacturing (AM) of polymers has unlocked a vast array of possibilities for design innovation, enabling the creation of intricate and complex shapes [
16]. A growing number of industries are embracing AM technologies to expedite product development while enhancing cost-effectiveness [
17]. Through strategic material modification, process optimisation and print optimisation, polymers, blends and composites [
18] have evolved to exhibit novel properties that can meet or surpass the performance of traditional products [
19]. This has paved the way for exciting opportunities in 3D printed products.
The fusion of AM and microcellular foaming is a compelling route for designers. AM’s appeal lies in crafting intricate structures with ease [
20]. Utilizing DLP, polymer foams with controlled porosity are additively manufactured using expandable microspheres. This technique creates resilient bio-inspired materials, sustaining modulus and energy dissipation, even during cyclic loading. In addition, these 3D-printed foams demonstrate sustained values of modulus and energy dissipation, even under repeated loading at large deformations [
21]. Applications of this thermoplastic are already being utilised in fields such as aviation [
22,
23] and robotics [
24,
25] for their light weight and ability to construct complex structures.
Another approach involves the production of cellular thermoplastic structures with multiscale porosity, achieved through an integrated approach of 3D printing using fused deposition modelling (FDM). This method enables the creation of 3D porous structures that can be tailored for various applications, including drug delivery, energy storage, microfluidics and tissue engineering, where tuneable multiscale porosity is highly sought after [
26]. The choice of unit cell size is crucial in these processes, as smaller cells offer advantages in applications requiring high strength, energy absorption and dissipation, while larger unit cells can be utilized for superior damping performance. These materials find potential use in applications such as protective equipment, shoe midsoles and vibration dampers [
27].
Despite the progress made and the promising advantages arising from the combination of 3D printing and foaming, this technique has yet to be applied to any specific application. Nonetheless, the potential for innovation is an area of research and development [
28].
Fused filament fabrication (FFF), a popular 3D printing technique, is widely used for manufacturing plastic parts. It offers cost-effectiveness and efficiency in the production process. FFF provides extensive design flexibility, allowing designers to create intricate shapes and structures. However, the production of foamed structures using FFF remains challenging due to the time-consuming process and the difficulty of creating internal micro-porous structures using existing printing techniques [
29].
With the growing popularity of FFF in both industrial applications and hobbyist settings, there is increased awareness regarding the materials used and their sustainability [
30]. Research has shown that 3D printing with foam filaments can achieve comparable results to traditional methods, using up to 60% less polymer volume. Furthermore, the possibility of recycling makes the process more environmentally friendly than using natural polymer filaments [
31].
Efforts to address environmental concerns in 3D printing include designing experiments that focus on materials, energy consumption, production and waste generation [
32]. It has been demonstrated that reductions in energy consumption and waste contribute to sustainable long-term product development in this technology [
33]. Recent research has shown that printer and material modifications can significantly reduce polymer consumption from 55.8% to 56.4% and decrease power consumption from 29% to 38% [
34].
To achieve the goal of making plastic parts both strong and lightweight, a comprehensive approach is required. This involves utilising advanced materials, optimising design and simulation tools and employing efficient processing techniques. Ongoing research on microcellular foamed polymers for 3D printing is expected to further expand the range of applications for this technology by advancing materials and manufacturing processes.
The objective of this paper is to reduce part weight using a design of experiments approach when printing with a lightweight (LW-PLA) filament, incorporating a foaming agent. In addition to printing LW test parts, the study focused on evaluating material properties such as tensile strength (σ), stiffness (k) and strain to failure (εf). The paper is structured as follows: the experimental section is presented first, followed by the results and subsequent discussion. Finally, conclusions are drawn regarding the influence of the manufacturing process on weight savings and the observed relationship with material strength properties.
4. Conclusions
Continuous efforts are being made to develop lightweight materials with improved stiffness, strength and energy absorption properties for a variety of multifunctional applications. Microcellular foamed polymers offer many advantages, including being lightweight, having improved mechanical properties, reducing material usage, providing better insulation and being cost-effective. These benefits make them an attractive material choice for a wide range of applications across various industries. This study reveals that microcellular polymers offer a solution for designers when there is a trade-off between part strength and weight. The key findings are as follows:
For all nine experiments, microcellular foaming was achieved and the LW-PLA parts are significantly lighter than standard PLA parts. The ANOVA was performed to assess the processing parameters’ contribution to weight, which found that Lh was the most important factor. Using the predicted result for this setting, it is possible to produce a part with a weight of 8.15 g. This is 21.22% below the average weight of a test part made from original PLA.
For each combination of controlled parameters, Tensile strength (σ) Stiffness (k) and Strain to failure (εf) measurements were obtained. For all of the responses, the distributions are compared to the original PLA strength results. In all cases, the original PLA has higher results compared to the LW-PLA and there is a significant difference between the means. The chosen factors have an influence on each of these strength measurements. In particular, for σ there is a difference of >50.06% between the different experiments. For k, a difference of >37.40% is observed and εf measurements differ by >16.6%. This result shows the importance of optimisation during the manufacturing trials.
To improve manufacturability, the contribution of each processing parameter to the resulting strength results was investigated. For σ and k, T is the most important process parameter, followed by Lh and S. The mean results show that after reducing the T setting from 260 °C to 240 °C, the σ increased by 24.2% and the k increased by 16.5%. For εf, the S setting is the dominant factor with a change of 6.6% measured.
Using the DOE predicted results, it is possible to produce parts optimised for increased mechanical strength properties. This approach identified that for σ, k and εf, increased strengths of 17.5%, 15.4% and 7.5%, respectively, are achievable.
Optimisation for part weight and strength can be achieved. The compromise for reduced weight can be achieved with a medium σ and high εf, and high σ and low εf is achievable. For k, εf and reduced weight, the results are very similar. For the optimisation of σ, k and reduced weight, it is possible to achieve a low weight with a high σ strength and a high k.
The use of microcellular polymers in FFF presents an opportunity to achieve parts with reduced weight and improved strength. Research in 3D printing is often centred on industrial applications. More recently, the uptake of this disruptive technology has moved beyond industry, and it is important not to underestimate the energy and material footprint that these machines have. The adoption of material-saving polymers could provide marked savings in energy consumption and lead to more sustainable manufacturing practices in 3D printing. The findings in this research are important for material selection and further research and development are needed to fully exploit the potential of microcellular polymers in 3D printing and enhance their application across different industries.