Tensile strength of commercial polymer materials for fused filament fabrication 3D printing

TL;DR: In this article, the tensile strength of 3D printed parts using a commercial open-source 3D printer for a wide range of materials is investigated and conclusions are drawn about the mechanical properties of various fused filament fabrication materials.

Abstract: 3D printing functional parts with known mechanical properties is challenging using variable open source 3D printers. This study investigates the mechanical properties of 3D printed parts using a commercial open-source 3D printer for a wide range of materials. The samples are tested for tensile strength following ASTM D638. The results are presented and conclusions are drawn about the mechanical properties of various fused filament fabrication materials. The study demonstrates that the tensile strength of a 3D printed specimen depends largely on the mass of the specimen, for all materials. Thus, to solve the challenge of unknown print quality on mechanical properties of a 3D printed part a two step process is proposed, which has a reasonably high expectation that a part will have tensile strengths described in this study for a given material. First, the exterior of the print is inspected visually for sub-optimal layers. Then, to determine if there has been under-extrusion in the interior, the mass of the sample is measured. This mass is compared to the theoretical value using densities for the material and the volume of the object. This two step process provides a means to assist low-cost open-source 3D printers expand the range of object production to functional parts.

Summary

1. Introduction:

• Due to the open-source release of the RepRap (self-Replicating Rapid prototyper) [1-3] there was a distinct rise in popularity of 3D printing at the small scale [4].
• This growth is being fueled at the consumer level because 3D printers have been proven to be an economically beneficial purchase for the developed-world middle-class [8] and those in the maker community [11-13].
• As novel and affordable 3D printing technologies continue to develop the types of materials that may become common for FFF is expected to grow [22,23] and involve the use of additives [24] (i.e., strengthening agents) to common 3D printable materials [25,26].
• A follow up study [20] found that coloring agents altered the microstructure (percentage of crystallinity) and had an impact on the strength as is well established in the literature [35,36].
• These factors added to the inconsistencies found in a random sampling of RepRap users [19] making the strength of individual prints difficult for prosumers to determine.

2. Methods

• These temperatures and all the materials tested are summarized in Table 1.
• The density of the unextruded filament was determined by applying Archimedes principle: a small length (around 2”) of the filament was taken and the mass was measured in air (m1) and in water (m2) separately on a electronic balance with least count of 0.0001g.
• The rigid specimens were tested for tensile strength on INSTRON 4206 with a 10kN load cell for load measurement and cross head data was used for the extension measurement.
• The flexible materials were printed in two different orientations to compare the difference in flexibility between the orientations.

3. Results and Discussion

• The results of the tensile tests for the 3D printed materials are summarized in Table 2 and 3 for rigid and semi-flexible materials, respectively.
• Nylon materials were stronger than Ninjaflex and SemiFlex, and much more flexible than ABS, HIPS, T-Glase, and polycarbonate, providing a good balance between strength and flexibility.
• There may be a small difference in density among the various colors, which may explain the mass difference between the colors of a material.
• The material can also be printed with the length of the specimen being vertical on the printer and tensile strength can be tested.
• This dependence enables prosumers to solve the challenge of unknown print quality effects on the mechanical properties of a 3D printed part using a two step process to estimate the tensile strengths for a given material.

Figures

Table 3. The orientation of the print, average mass (g), average load at 60mm extension (N) and average stress at 60mm extension (MPa) for the semi-flexible materials.

Figure 6. Stress at 60mm extension (MPa) of Ninjaflex as a function of sample mass to filament mass percentage.

Figure 7. Stress at 60mm extension (MPa) of Nylon Bridge as a function of sample mass to filament mass percentage.

Figure 10. Under extrusion on exterior surface of 3D printed object (observable as dark lines).

Figure 1. The maximum stress (MPa) of ABS as a function of sample mass to filament mass percentage.

Figure 5. The maximum stress (MPa) of T-Glase as a function of sample mass to filament mass percentage.

Table 4. Correlation coefficients as a function of 3D printed material.

Figure 3. The maximum stress (MPa) of Nylon 618 as a function of sample mass to filament mass percentage.

Figure 2. The maximum stress (MPa) of HIPS as a function of sample mass to filament mass percentage.

Table 1. 3D printing materials, printing temperature and density of the filament.

Figure 4. The maximum stress (MPa) of polycarbonate as a function of sample mass to filament mass percentage.

Table 2. The average maximum extension (mm), average maximum load (N), average mass (g) and average tensile stress (MPa) for all the 3D printed rigid materials.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Tensile strength of commercial polymer materials for fused filament fabrication 3d printing" ?

This study investigates the mechanical properties of 3D printed parts using a commercial open-source 3D printer for a wide range of materials. The samples are tested for tensile strength following ASTM D638. The study demonstrates that the tensile strength of a 3D printed specimen depends largely on the mass of the specimen, for all materials. Thus, to solve the challenge of unknown print quality on mechanical properties of a 3D printed part a two step process is proposed, which has a reasonably high expectation that a part will have tensile strengths described in this study for a given material. This two step process provides a means to assist low-cost open-source 3D printers expand the range of object production to functional parts. 

Q2. What have the authors stated for future works in "Tensile strength of commercial polymer materials for fused filament fabrication 3d printing" ?

These limitations lead to several potential sources of future work. In addition, the impact of the geometry of the part need further study to determine the limitations of FFF for manufacturing [ 60 ]. 

Q3. What is the advantage of PLA for off-grid applications?

PLA has a relatively low melting point (150°-160° C), which requires less energy to print with than other materials and provides a distinct advantage for off-grid applications in the developing world [14-16]. 

Q4. What is the popular 3D printing material?

As novel and affordable 3D printing technologies continue to develop the types of materials that may become common for FFF is expected to grow [22,23] and involve the use of additives [24] (i.e., strengthening agents) to common 3D printable materials [25,26]. 

Q5. Why did the RepRap rise in popularity?

Due to the open-source release of the RepRap (self-Replicating Rapid prototyper) [1-3] there was a distinct rise in popularity of 3D printing at the small scale [4]. 

Q6. What were the other parameters used to produce acceptable print quality and uniformity?

Other printing parameters such as layer height, speed and custom controls were fine tuned for each material using the supplier's recommendations as a baseline to produce acceptable print quality and uniformity. 

Q7. What are the materials available for 3D printing?

there are many other materials available on the market for prosumer (producing consumer) FFF 3D printing including nylon, polycarbonate (PC), high-density polyethylene (HDPE), high impact polystyrene (HIPS), and others [21]. 

Q8. How was the density of the unextruded filament determined?

The density of the unextruded filament was determined by applying Archimedes principle: a small length (around 2”) of the filament was taken and the mass was measured in air (m1) and in water (m2) separately on a electronic balance with least count of 0.0001g. 

Q9. What was the test for the tensile strength of the specimen?

Ten printed tensile samples for each material/color combination were then subjected to tensile testing consistent with ASTM D638 standards [37]. 

Q10. What are the advantages of using flexible materials?

The use of flexible materials, such as SemiFlex, Nylon Bridge and NinjaFlex tested here, similarly open up other applications such as components that come directly in contact with humans such as hand grips, watch bands, shoes [65], belts and face mask rings. 

Q11. What software was used to test the extension of flexible materials?

The extension of flexible materials (Ninjaflex, SemiFlex, and Nylon Bridge) was found to be greater than allowed by the INSTRON 4206, hence flexible materials were tested on INSTRON 4210 using the same load cell and Bluehill 2 software [45]. 

Q12. What is the reason why the 3D printing process is so successful?

This points to the necessity of the open source developmental model, which has been so successful in 3D printing itself to be expanded beyond materials science software [46-51] to open source materials development [24,52,53]. 

Q13. What is the effect of coloring agents on the strength of printed parts?

A follow up study [20] found that coloring agents altered the microstructure (percentage of crystallinity) and had an impact on the strength as is well established in the literature [35,36]. 

Q14. What is the reason why the part should be reprinted?

If for example, under extrusions are detected on the outer surface as shown in Figure 10, then the part should be reprinted if mechanical stability is important for the specific application. 

Q15. Why is the growth of 3D printing being fueled at the consumer level?

This growth is being fueled at the consumer level because 3D printershave been proven to be an economically beneficial purchase for the developed-world middle-class [8] and those in the maker community [11-13].