Three-dimensional (3D) printing technologies, also known as additive manufacturing (AM), first appeared on the market in the 1980s. However, a rapid growth of interest in AM techniques has taken place in the last decade. Increased interest in 3D printing is observed both from industry and home users. AM is more and more often being used to design and manufacture functional 3D parts and objects. 3D printing involves the production of parts by selectively adding material in successive layers of the cross-section, thus creating an object. AM covers a variety of technologies and is an alternative to traditional subtractive manufacturing (machining, abrasive and erosion treatment). The rise in popularity of AM is due to its many advantages over other traditional manufacturing processes [1–4]. The main reason is the widespread use of inexpensive, stationary 3D printers on the market and quite a large variety of materials. The basic advantages are comprehensiveness and simplicity of design, as well as the ability to produce structures with very complex shapes from technological point of view. Importantly, these structures may also contain moving parts. In addition, the lack of additional tools (extrusion heads, injection moulds, etc.), the high speed of introducing a new product to the market and the lack or minimal amount of waste make AM an environmentally friendly technique. It is also possible to use not only plastics or metal alloys as a material, but also concrete, chocolate or living tissue, which gives a wide range of applications of these technologies [5–7].
On the other hand, many aspects of AM processes still need improvement [8, 9]. The high cost of large-scale production, a smaller choice of materials and colours, the limited size of 3D printers and the limited accuracy of printed structures caused by the layer-by-layer structure are the basic elements limiting the wider development of this technique on the market. In addition, printed parts have reduced mechanical strength compared to parts with the same geometry produced from bulk material by traditional manufacturing techniques. It follows that there are many areas in the AM technique that require further research. The development of 3D printing techniques, understood as the study of the impact of its process parameters and material type on product properties, is crucial for the continuous increase in the importance of the AM technique on the market [10].
Many 3D printing technologies have developed with the progress of knowledge. Sometimes the difference between them is small. Often a new technology is referred to a new name, mostly in order to circumvent patent rights or to improve a given technology, after a slight modification. The breakthrough was the expiry of key Stratasys patents for Fused Deposition Modelling (FDM) technology (patent US5121329A, which expired in 2009), which significantly affected the cost of production of 3D printers [5, 11]. It made it possible for the development of small and cheap printers produced mainly in China for individual use, which had a huge impact on the greatest spread of this 3D printing technology. A similar future may await the Selective Laser Sintering technology, whose patent (US5695707A) expired in 2014 [5]; although it is still a relatively expensive technology.
FDM is currently still the most popular technology on the 3D printing market [12, 13]. This AM technique creates products by melting a thermoplastic polymer and applying it layer by layer on a heated bed. This process takes place in a closed chamber maintaining a higher temperature (for most materials) or without the presence of a chamber (mainly PLA printing). The raw material for the process is a polymer fiber (filament) with a standard diameter of 1.75 or 2.85 mm. The fiber is fed into a heated extrusion die, where the polymer is plasticized. The molten polymer is laid down in layers on a heated bed. As the polymer leaves the extruder nozzle, it begins to solidify. The printer applies the polymer layer by layer until the entire product is obtained.
Many researchers have focused on methods to improve the effects of interlayer adhesion in printed objects. Interlayer adhesion is a key factor related to the mechanical properties of components produced with material extrusion (ME) processes. Research aimed at improving layer adhesion mainly included optimisation of printing process parameters [14–17], thermal post-processing to increase strength [18, 19], physical and chemical cross-linking of the material polymer [20, 21] and chemical post-treatment of manufactured parts [22].
Polymers such as PLA, PS, PET, PP or TPU and various copolymers and blends such as ABS, ASA and PC/ABS can be used for 3D FDM printing. Polylactic acid (PLA) is most commonly used in the FDM process. This polymer can exist in various polymorphs. Depending on the lactic acid monomers used during the synthesis (L-lactic acid and D-lactic acid), the following can be obtained: poly(L-lactic acid) (PLLA), poly(D-lactic acid) (PDLA) and poly(DL-lactic acid) (PDLLA). Both L and D forms of lactic acid can be cyclized to form dimers: homochiral lactides (L,L- and D,D) or heterochiral lactide (meso-lactides), from which PLA can be obtained. Lactic acid from agricultural crops is a source of L or D stereoisomers. Racemic mixtures are obtained during synthesis [23]. Depending on the crystallisation conditions, different types of PLA crystallites can be obtained [24].
PLA has a semi-crystalline structure and is a glossy, rigid and colourless polymer with some, though limited, light transmission. Additives (pigments) are added to the material to colour PLA. They can change its various properties, mainly rheological [25], but also mechanical and thermal [4, 13, 26]. This is also confirmed by the results of tests performed for the ABS filament [27, 28].
Considering the latter issue, recent studies have shown that some seemingly insignificant differences in filaments, such as the type of pigment and the degree of crystallinity, can significantly affect the mechanical, thermal and dimensional properties of printed products [13, 26, 29]. Given the continuous development of AM, it now seems that one of the main goals of the development in this field is the standardisation of raw materials and processing conditions [30]. On the other hand, in order to fully exploit AM by engineers, still more research is required. These studies could link significant relationships between the material construction and the properties of the ME. This paper is devoted to such type of analysis.
Experimental studies were carried out on 12 types of PLA filaments obtained from one manufacturer in order to determine how differences in the type of pigment affect the material properties of the products. In this study, a series of standard mechanical and thermal tests were performed on a set of 12 commercially available PLA filaments. Similar studies have been conducted before [4, 13, 26, 29]. However, they included a smaller amount of materials and a smaller scope of material testing. This work had two main research goals. The main goal of this work was to quantify how the choice of filament colour can lead to differences in the mechanical and thermal properties of the resulting products. The second goal was an attempt to create the correlation between the measured material properties and the structure and characteristics of the material (degree of crystallinity, glass transition temperature), as well as the macroscopic structure of the products.
PLA filament was purchased from 3D Printers Company (Poland). Eleven filaments with different pigments and one filament without pigment were tested, which was a reference to the other samples. PLA samples were printed in the following colours:
Test samples were printed using the HBOT 3D F300 printer. The accuracy of its printing (provided by the manufacturer) is +/− 0.1 mm. The working area of the printer is 300x300x300 [mm]. The working platform is made of borosilicate glass. The diameter of the PLA filament was 2.85 mm. All samples were printed with the same printing parameters as shown in Table 1. After the printing process, the samples were not subjected to any finishing treatment. They were left for 72 hours in order to season.
Parameters for printing test samples set on the HBOT 3D F300 printer
parameter | size |
---|---|
Working table temperature [°C] | 60 |
Nozzle temperature [°C] | 235 |
Nozzle diameter [mm] | 0,60 |
Layer height [mm] | 0,30 |
Sample filling degree [%] | 100 |
Filling type | rectangular |
Printing direction | horizontal |
Print speed [mm/s] | 50 |
Bending tests are used primarily to determine the properties of rigid polymers (such as PLA), which are characterised by a relatively large modulus of elasticity
The compressive strength
Impact strength determines the brittleness of the material, that is, its resistance to impact fracture. It can be defined as the work necessary to dynamically break the sample related to its cross-section at the fracture site. Impact tests can be performed using notched or unnotched specimens. PLA is a brittle material, and there was no need to make a notch before testing (unless the test itself is intended to determine the sensitivity of the material to notch action). A Charpy hammer manufactured by Werk-stoffpruefmachinen Leipzig was used to measure the impact strength according to the PN-EN ISO 179-2 standard. Eight rectangular beams of each colour with dimensions of 80 x 10 x 4 [mm] were used for the test.
The softening point of polymeric materials can be determined and thus also their static heat resistance can be determined using the Vicat apparatus. Six samples from each series were tested. The Monagraph Studio Vicat/HDT Automatic apparatus was used for the tests. Its main part is a bath filled with silicone oil, connected to a heater that heats the entire system at a preset speed. During one measurement, three identical samples are placed in the apparatus. The load acting on the needle is 10 N. The rate of temperature increase in this test was 50°C/h.
The tests of the thermal properties of the samples were carried out using differential scanning calorimetry using DSC Polyma 214 from NET-ZSCH. PLA samples weighing approximately 10 mg were placed in aluminum vessels, using an empty vessel as a reference sample. They were subjected to dynamic analysis in a nitrogen atmosphere at a constant heating or cooling rate of 10 K/min.
Thermal analysis was carried out using three temperature programs:
I - heating from 20°C to 200°C; then the samples were kept at 200°C for 2 minutes; II - cooling from 200°C to 50°C; then the samples were held at 50°C for 2 minutes; III - second heating from 50°C to 200°C.
In order to determine the degree of crystallinity ( Δ Δ Δ
The content of additives was determined by calcination of PLA samples weighing about 1g in porcelain crucibles in an oven at 400°C for 6 hours.
Below are the results of material tests with a brief analysis. Error bars in the graphs indicate the standard deviation of the mean value taking into account the critical coefficient of the t-Student’s distribution.
Figure 2 shows the bending curves of
Analysing the flexural modulus, the highest values have the
The obtained differences in the results of flexural strength and flexural modulus for individual materials are similar to the values of tensile strength and Young’s modulus. The results of tensile strength tests were not presented in detail due to their very similar nature and analogous differences for individual materials. The modulus values for tensile and bending are very similar (for tensile they are higher by approximately 10%), while the tensile strength values are approximately 40% lower than the flexural strength for individual materials.
Figure 5 shows the obtained compression curves for
Analysing the compression modulus of tested materials, the
Figure 8 shows the dependence of the unnotched Charpy impact strength as a function of the type of pigment. The impact strength value for pure PLA is approx. 18 kJ/m2. Different effects of particular pigments can be observed in the case of impact strength, although the addition of pigment reduces impact strength in general. The
Figure 9 shows the dependence of the softening temperature as a function of the type of pigment. The highest value was obtained by
The melting temperatures (
Tables 2 and 3 summarise the basic thermal parameters obtained from the first and second heating of the samples, respectively, and the degree of crystallinity (
Summary of the data produced from DSC testing of first heating scan for the PLA filaments considered including heat capacity (
I heating | ||||||||
---|---|---|---|---|---|---|---|---|
J/(g*K) ΔCp | °C Tg | °C Tc | J/g ΔHc | °C Tm1 | °C Tm2 | J/g ΔHm | % X | |
natural | 1,463 | 63,4 | 106,4 | 29,24 | 162,5 | 153,9 | 43,34 | 13,2 |
0,884 | 61,8 | 98,2 | 24,55 | 160,6 | 149,6 | 40,98 | 15,4 | |
0,930 | 65,2 | 103,1 | 25,91 | 152,8 | 35,95 | 9,5 | ||
0,555 | 64,0 | 101,8 | 24,33 | 153,0 | 38,10 | 13,0 | ||
0,752 | 64,6 | 101,4 | 18,31 | 155,0 | 148,0 | 36,20 | 16,8 | |
1,622 | 64,6 | 106,0 | 28,63 | 152,5 | 35,25 | 6,2 | ||
1,461 | 65,5 | 103,4 | 19,98 | 151,8 | 39,67 | 18,5 | ||
1,425 | 65,7 | 100,2 | 21,74 | 153,7 | 34,39 | 11,9 | ||
1,327 | 64,8 | 101,3 | 27,88 | 155,2 | 148,3 | 32,67 | 4,5 | |
1,149 | 66,2 | 95,0 | 21,60 | 161,9 | 38,88 | 16,2 | ||
0,933 | 66,1 | 96,0 | 28,05 | 162,8 | 38,99 | 10,3 | ||
0,995 | 64,3 | 97,1 | 30,12 | 162,8 | 40,17 | 9,4 |
Summary of the data produced from DSC testing of second heating scan for the PLA filaments considered including cold crystallisation temperature (
II heating | |||||
---|---|---|---|---|---|
°C Tc | J/g ΔHc | °C Tm1* | °C Tm2** | J/g ΔHm | |
natural | 131,4 | 3,575 | 158,6 | 3,564 | |
124,3 | 44,900 | 155,7 | 159,2 | 42,360 | |
126,3 | 15,340 | 152,9 | 16,430 | ||
124,3 | 25,780 | 152,2 | 25,130 | ||
102,7 | 17,240 | 150,4 | 153,9 | 42,500 | |
126,6 | 14,210 | 152,6 | 13,570 | ||
105,9 | 20,850 | 149,2 | 38,360 | ||
122,2 | 33,090 | 152,1 | 28,490 | ||
127,7 | 13,560 | 153,8 | 13,140 | ||
116,5 | 12,110 | 159,4 | 7,293 | ||
113,1 | 47,870 | 155,2 | 161,3 | 46,310 | |
118,4 | 52,560 | 155,5 | 160,9 | 45,020 |
All samples had similar curves (Figure 10) during the first heating from 20° C to 200° C, in which the glass transition temperature (
The first heating process reveals thermal effects related to the thermal history of the sample, such as sample preparation (thermal and mechanical impacts related to the production of the sample – the production of filaments and the 3D printing process in this case), aging or annealing the sample. In the case of samples that are 3D-printed products, their properties may be affected by the temperature of the nozzle (235°C in this case) as well as the storage conditions of the samples. It should also be noted that during the printing process, the temperature of the platform was 60°C, and the printing process itself is time-consuming. Therefore, 3D printed PLA products can be treated as partially aged/annealed, because during their production they are subjected to ambient temperature, close to the range of glass transition temperatures of the tested samples. During this time, further spatial ordering of polymer chains may occur (system with lower energy, more thermodynamically stable), and thus an increase in the proportion of crystal structures. The presence of double temperature peaks during PLA melting was explained in the work [35], where the following theories were presented:
the lamellar thickness model – the double thermal peak in the endothermic transformation is associated with the presence of two types of lamellae of different thickness. Thinner lamellae have a lower melting point, and thicker ones – a higher one; melting recrystallisation model – during lamella melting, part of the melted crystalline fraction is recrystallised to a more ordered structure with a higher melting point than the melting point of the crystalline fraction that melted earlier; and a new model proposed by [35] suggesting the simultaneous occurrence of two processes of melting and crystallisation – this model assumes the existence of three crystal structures, where during the melting of the crystal part, recrystallisation occurs to a new, more ordered form, which also melts and recrystallises.
The difference between the total enthalpy of endothermic melting and exothermic cold crystallisation processes is the confirmation of the last model assuming the existence of three crystal structures. The presence of the
In order to analyse the obtained results in terms of possible correlations between the results obtained from DSC measurements and the mechanical and thermal properties of the tested materials, the linear correlation coefficients R were determined. The coefficients of determination R2 are presented in Tables 4 and 5. Coefficients (I) and (II) indicate the heating number.
Table of R2 coefficients between the parameters determined in the DSC study
1,00 | |||||||||||
0,00 | 1,00 | ||||||||||
0,03 | 0,06 | 1,00 | |||||||||
0,16 | 0,11 | 1,00 | |||||||||
0,01 | 0,03 | 0,00 | 1,00 | ||||||||
0,00 | 0,02 | 0,14 | 0,19 | 1,00 | |||||||
0,06 | 0,04 | 0,19 | 0,06 | 0,02 | 0,00 | 1,00 | |||||
0,06 | 0,01 | 0,16 | 0,21 | 0,03 | 1,00 | ||||||
0,28 | 0,17 | 0,01 | 0,00 | 0,25 | 0,01 | 0,02 | 1,00 | ||||
0,00 | 0,02 | 0,04 | 0,14 | 0,00 | 0,15 | 0,03 | 0,04 | 1,00 | |||
0,08 | 0,01 | 0,19 | 0,01 | 0,16 | 0,20 | 0,05 | 0,02 | 1,00 |
Table of R2 coefficients between the parameters determined in the DSC test and the mechanical properties of the tested materials
0,01 | 0,01 | 0,02 | 0,02 | 0,07 | ||
0,15 | 0,04 | 0,16 | 0,00 | |||
0,08 | 0,01 | 0,00 | 0,00 | 0,00 | 0,04 | |
0,03 | 0,00 | 0,30 | 0,04 | 0,03 | 0,02 | |
0,01 | 0,08 | 0,07 | 0,06 | 0,02 | 0,00 | |
0,09 | 0,04 | 0,17 | 0,04 | 0,03 | 0,02 | |
0,00 | 0,17 | 0,11 | 0,01 | 0,01 | 0,02 | |
0,10 | 0,03 | 0,08 | 0,00 | 0,01 | 0,11 | |
0,07 | 0,14 | 0,06 | 0,07 | 0,11 | 0,00 | |
0,00 | 0,00 | 0,01 | 0,03 | 0,01 | 0,11 | |
0,11 | 0,01 | 0,03 | 0,02 | 0,00 | 0,20 |
Observation of the results presented in Table 4 does not generally indicate any correlation between the degree of crystallinity
The analysis of the correlation of mechanical test results with the degree of crystallinity of the material also did not bring any effects, except for a small positive correlation between
It is worth noting that only Pearson’s linear correlation coefficients were measured. Therefore, it is not possible to exclude the occurrence of nonlinear correlations between the examined parameters, although linear correlation was the most expected here.
Thermal history is an important factor influencing the mechanical properties of polymers [24]. It includes sample aging [32], heating and cooling rates and sample processing [37]. An amorphous layer (interphase) can be distinguished in the semicrystalline structure of the polymer, directly adjacent to the crystalline structures. This layer is called rigid amorphous (RA), in contrast to the layer not directly adjacent to the crystal structures (mobile amorphous, [MA]). The RA layer partially crosses the crystalline-amorphous boundary, affecting the modulus of elasticity, which is similar to the modulus of the crystalline part. The RA layer is also strongly dependent on the crystallisation temperature ( Δ Δ Δ Δ
Determined
Δcp | X | w |
w |
|
---|---|---|---|---|
[J/gK] | - | - | - | |
natural | 1,463 | 0,13 | 0,73 | 0,14 |
0,884 | 0,16 | 0,44 | 0,40 | |
0,930 | 0,09 | 0,47 | 0,44 | |
0,555 | 0,13 | 0,28 | 0,59 | |
0,752 | 0,16 | 0,38 | 0,46 | |
1,622 | 0,06 | 0,81 | 0,13 | |
1,461 | 0,19 | 0,73 | 0,08 | |
1,425 | 0,12 | 0,71 | 0,17 | |
1,327 | 0,05 | 0,66 | 0,29 | |
1,149 | 0,17 | 0,57 | 0,26 | |
0,933 | 0,10 | 0,47 | 0,43 | |
0,995 | 0,09 | 0,50 | 0,41 |
The exothermic crystallisation peak was observed during the cooling stage from 200°C to 50°C only for the
The paper presents the analysis of the influence of the type of pigment in multi-coloured PLA filaments from one supplier on their selected mechanical properties (bending strength, impact strength) and thermal properties (softening point). The results of mechanical tests showed that PLA
Although the structural tests performed (DSC) did not directly explain the changes in the mechanical and thermal properties of the tested materials as a function of the type of pigment, the influence of the type of pigment on these properties is indisputable. Differences in mechanical properties between individual materials are very large: the range of results for bending and compressive strength is approximately 20% and impact strength – approximately 45%. Differences in the softening point are in the order of 5%. Such large differences in the properties of PLA filaments indicate that the selection of the wrong material may result in damage to the element during operation, which should be carefully analysed by designers. Although the difference in the degree of crystallinity of the
The test results show that the choice of material in the right colour affects the performance of the product with the desired mechanical and thermal properties. In order to correlate the results of these tests with structural properties, DSC test was performed. The analysis of DSC test results (analysis of R2 coefficients) showed no significant correlations between the structure (degree of crystallinity), physical properties of materials (such as melting point or enthalpy of melting) and the functional (mechanical) properties of materials. The correlation coefficients were either zero or very small. However, the differences in the mechanical properties of different coloured PLA filaments are so large (20% or even 50%) that these values must be taken into account as one of the critical parameters in the AM datasets. Only then can AM techniques be effectively used.
This article was created in cooperation between Wroclaw University of Science and Technology and Częstochowa University of Technology