1.1 Size effects in 3D printed materials
The method of producing a three-dimensional object from a computer-created design is known as 3D printing. The object is built up layer by layer, creating a three-dimensional object. The layers are thinly cut cross-sections of the object, this is an additive process. 3D printing enables more complex shapes to be produced, using less material than traditional manufacturing processes. With an estimate of $55.8 billion by 2027, the market for 3D printed materials has surged [1]. 3D printing's current market size is $16.5 billion. The advantages of 3D printing, such as its adaptability for rapid prototyping and the method's flexibility, which allows for the production of increasingly sophisticated designs, have raised demand. As a result, 3D printing may be used in a variety of industries, including automotive, aerospace, medicine, consumer goods, and many more.
Additive manufacturing technology comes in a variety of forms. The most popular technology is Fused Filament Fabrication (FFF), also known as Fused Deposition Modelling (FDM). Due to the simplicity and low cost, this is one of the most widely used 3D printing methods. The printing process begins with the melting of a filament, which is then built-up layer by layer with the nozzle. After the first layer is deposited, the nozzle advances along the z-axis to build the next layer, and so on until the object is produced. The FFF method is depicted in figure 1 as a schematic diagram.
The most popular 3D printing materials are the thermoplastic polymers Polylactic acid (PLA), Acrylonitrile butadiene styrene (ABS), and Polyethylene terephthalate glycol-modified (PETG). The materials are low-cost and widely available. Due to its low cost and ease of use, PLA is the most extensively used 3D printing material. PLA is a biodegradable plastic manufactured from cornstarch that is both environmentally friendly and helps to conserve petroleum resources by reducing pollution. Plastic is less energy-intensive to produce, and it takes less energy to alter the material throughout the printing process. ABS is a typical 3D printing thermoplastic that is also inexpensively priced. ABS has a lot of disadvantages, including the fact that it is not biodegradable and components are inclined to warp during printing. The material is manufactured from non-renewable resources, meaning it contributes to pollution. Given the current impact of global warming and climate change on the planet, demand for biodegradable materials and more sustainable production processes is at an all-time high. PLA and other biopolymers can be applied in a variety of technological fields. The applications of each material, as well as the benefits and drawbacks of using that material to 3D print, are listed in Table 1.
Table 1: A summary of PLA and ABS
|
Applications
|
Advantages
|
Disadvantages
|
Polylactic Acid (PLA)
|
- Plastic cups
- Medical
implants
|
- Biodegradable
- Relatively Cheap
- Easier to print with
|
- Can lose structural integrity at higher temperatures
|
Acrylonitrile
Butadiene Styrene
(ABS)
|
- LEGO
- Automotive industry
|
- Affordable
- More suitable for higher temperature applications
|
- Non-Biodegradable
Emits toxic fumes during printing
|
3D printed plastic materials currently have limited uses due to the naturally poor mechanical characteristics of the material [3]. The build orientation, layer thickness, printing temperature, printing speed, and other process parameters can all affect the material's mechanical properties. Researchers have studied the effect of varying these parameters to improve the properties and qualities of the printed samples
The purpose of this research is to investigate the size effects of 3D printed materials. Different materials and thicknesses will be explored to gain a greater understanding of how mechanical properties are affected.
1.1.1 Build Orientation
The build orientation is a vital factor that can influence the overall printing time as well as the material's mechanical properties. We have previously reported on the effect of build orientation and raster orientation in the build plane as key parameters affecting the mechanical properties of an FFF 3d printed PLA part [5, 6]. In addition, work performed by Zieman et al [4] and Cassovola et al [2] compared build orientations of 0o, 45o,90o, and +45o/-45o on the effect of ABS samples. The studies all resulted in the same conclusion that the greatest mean yield and ultimate strengths were obtained with a build orientation of 0o. The weakest strengths were at an orientation of 90o.
1.1.2 Printing Speed
Another process parameter that can affect the material's mechanical properties is the printing speed. The printer's motors, including the X and Y axis control and the extruder motor, move at a set speed specified by the printing speed.
Miazio [7] investigated the effect of print speed on the strength of PLA samples manufactured using FDM technology. From 20 mm.s-1 to 100 mm.s-1, the printing speed was increased by 10mm. s-1 intervals. The mean breaking force was calculated using tensile tests. The highest breaking force was found to be at the slowest printing speed of 20 mm.s-1. The strength of the samples deteriorated as the speed increased, with the mean breaking force being remarkably similar between 50 and 80 mm.s-1. Above a speed of 80 mm.s-1, the material's strength significantly decreased. This is because the quality of the sample produced decreases as the printing speed increases. The optimum printing speed of 60 mm.s-1 was determined to be the greatest in terms of durability and printing duration. A slower printing speed may result in better mechanical qualities for the material, but it comes at the expense of a substantially longer printing time.
1.1.3 Printing Temperature
The printing temperature, also known as the extrusion temperature, is another process parameter that has a substantial impact on the material's mechanical characteristics. The extrusion temperature is the temperature at which the extruder is heated during the printing process. The temperature differs depending on the printing material used, such as PLA or ABS, as well as other variables like printing speed.
Hsueh et al (2021) [8] investigated the effect of printing temperature on the thermal and mechanical properties of 3D-printed PLA and PETG. To measure the influence, tension, compression and bending tests were conducted on the materials at various printing temperatures. PLA was heated to a temperature of 180°C to 220°C, whereas PETG was heated to a temperature of 225°C to 245°C. The study identified when the printing temperature is increased, the mechanical properties of both materials improved. This is because the viscosity of the PLA and PETG melts decreases, and the fusion between the polymer fuse and the layers becomes stronger. As a result, the tensile properties of PLA and PETG improve.
1.1.4 Layer Thickness
The mechanical properties of printed samples are greatly influenced by the layer thickness. As earlier mentioned, samples are printed layer by layer, with the thickness of these layers customizable in the printing software. The layer height indicates the vertical resolution of the z-axis. Greater surface smoothness is achieved by printing at a thinner layer thickness, however, the printing time increases. Choosing a suitable balance between production time and material strength is crucial in industry. The majority of printers have an adjustable layer thickness of 0.1 to 0.4 mm.
In a critical review by Syrlybayev et al (2021) [9] the following statement was made “it was found from the literature that the layer thickness is the most important process parameter influencing the mechanical characteristics”.
Sharma et al [10] investigated the influence of layer thickness on the tensile and compressive strength of ABS. Layer thicknesses of 0.1, 0.2, and 0.3 mm were investigated. At a layer thickness of 0.1mm, the highest tensile strength of 30 MPa was recorded, while at a thickness of 0.3mm, the lowest tensile strength was obtained, giving a reading of 18 MPa respectively. At a smaller thickness, the layers are closely deposited over each other together resulting in better bonding between layers. Thus, resulting in greater tensile strength. The compressive strength of specimens had the opposite outcome. The compressive strength of the samples was increased as the layer thickness was increased. At a layer thickness of 0.3 mm, the greatest compressive strength was obtained, resulting in 42 MPa. At a layer thickness of 0.1mm, the lowest compressive strength (33 MPa) was achieved. This was further supported by Rankouhi et al [11] where the effect of layer thickness on the strength of 3D printed ABS was investigated. The research found that specimens with a thickness of 0.2 mm had higher ultimate tensile strength than specimens with a thickness of 0.4 mm, with 32.2 MPa compared to 26.0 MPa.
Research conducted by Samykano et al [12] investigated the influence of printing parameters on the mechanical properties of ABS. One of the parameters investigated was the layer height, using values of 0.35, 0.4, and 0.5 mm. The average ultimate tensile strength is the highest one (28.01 MPa) at the lowest layer height (0.35mm). Following the conclusions Sharma et al [10] and Rankouhi et al [11] made, it would be expected that at the largest layer thickness (0.5mm) the average ultimate tensile strength would be the lowest. However, this did not occur, as the lowest average ultimate tensile strength was obtained at a layer thickness of 0.4 mm. From the results, it is evident there are fluctuations of ultimate tensile strength when the layer thickness is increased. A criticism of this research is that only one sample was used for each layer thickness meaning that the results were not validated, this may explain why there were fluctuations. To improve this further samples should be tested to see if the results follow a trend.
These fluctuations were also present in the research performed by Vicente et al [13] where ABS samples were analysed through the implementation of tensile tests. Three different values of layer thicknesses were used 0.06, 0.1 and 0.17 mm respectively. The study found that there was a small increase in ultimate strength when increasing the thickness of the layer from 0.06 mm to 0.1mm. The largest layer thickness of 0.17mm had the lowest tensile strength. One reason for this is the distortion layers can form at thinner layers, which can counteract the improvement of the mechanical characteristics caused by the lower layer thickness.
The change in layer thickness on the mechanical properties of PLA has also been studied by Jatti et al [14] The layer thicknesses investigated were 0.08, 0.16, 0.24, 0.32 and 0.4 mm. It was found that when increasing the layer height, the tensile strength decreased. At a greater thickness, fewer layers are causing less adhesion, resulting in a smaller tensile strength. The impact strength and flexural strength were also tested, and it was found that the increase in layer thickness caused an increase in both impact and flexural strength.
Research conducted by Alafaghani et al [15] contradicted the findings by Jatti et al (2019) [14]. The research investigated the effect of layer height on PLA samples made using PLA. Layer heights of 0.1, 0.25, 0.3 and 0.4 mm were studied. The study found that increasing the layer height improved the mechanical properties of the specimens. These findings oppose the conclusions made by Jatti et al [14].
A comparative study by Rodríguez-Panes et al [16] investigated the influence of process parameters on the mechanical behaviour of samples of ABS and PLA. The variables investigated were tensile strength, modulus of elasticity and nominal strain. The layer thicknesses used were 0.1 and 0.2mm. For PLA, the increase in layer height from 0.1 to 0.2 mm caused an 11% decrease in the maximum tensile strength (falling from 38.47 MPa to 34.37 MPa respectively). The increase in layer height for samples of ABS causes an 8% drop in maximum tensile strength (dropping from 26.40 MPa to 24.40 MPa respectively), showing that there are more substantial reductions in tensile strength for PLA compared to ABS. Overall the results show that for both materials a thinner layer thickness leads to a higher tensile strength.
From the literature reviewed it was found that an increase in layer thickness caused the tensile strength of PLA and ABS samples to decrease (Sharma et al., 2019 [10]; Jatti et al., 2019 [14]). When comparing the work explored by researchers, it was found that the layer thickness had a greater effect on PLA samples compared to the ABS samples [16]. It must be noted that the other process parameters such as infill density, printing temperature etc may have small effects on the strengths so it is important to keep these parameters consistent. There were however some contradictions between researchers, a study by Akafaghani et al [15] concluded that increasing the layer thickness increases the tensile strength of the material. Further research should take place to further explain and validate the results.
1.2 Criticisms and Gaps in the literature
Overall, after reviewing the various literature sources, certain criticisms of the sources and minor gaps in the literature were discovered. In the study conducted by Samykano et al [12], one critique was found: the study only examined one sample for each layer thickness, implying that the results were not validated. Due to the lack of a clear pattern in the results, no clear conclusions could be drawn. If more samples were analysed, it would be possible to see if there were any variations in the results. Another criticism is that in the study by Rodríguez-Panes et al [16] only two different layer thicknesses were tested, 0.1 mm and 0.2 mm. The study should have also tested samples at a layer thickness of 0.3 mm to see if the same pattern was present when increasing the thickness from 0.2 mm to 0.3 mm.
The materials used were brand-new ABS and PLA filaments. Recycled PLA could also have been examined to see if it followed the same relationship between layer thickness and tensile strength as new PLA. This is significant, the reason is that as previously indicated, the desire for more sustainable manufacturing methods is growing, implying that recycled PLA with the same mechanical qualities as new PLA might be immensely advantageous to industry. Companies would be able to reuse filaments, saving money and reducing waste. Another gap found in the literature is that the ratio of nozzle diameter to layer thickness may be a factor in affecting the mechanical properties of the printed samples [9]. There is currently limited material available on this meaning there is a need for further research in this area.
1.3 Aims and Objectives
From the criticisms and the gaps identified in the literature review, it is evident that there are some conflicting statements on how layer thickness affects the mechanical properties of 3D printed samples of ABS and PLA. This project aimed to evaluate the performance of ABS and PLA samples at a range of layer thicknesses, also the influence of layer thickness on the mechanical properties could be further understood.