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Studies on Extrusion – Based Additive Manufacturing of Poly-Ether-Ether-Keton (PEEK)

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Additive Manufacturing (AM) is a commonly used process in the delivery of complex parts with less lead time due to the absence of part-specific tooling that makes it even economical for the production of physical models or prototypes. The current work deals with the processing of PEEK material and manufactures components using the principles of the AM process.

Figure 1-1 Classification of AM processes [1]
Figure 1-1 Classification of AM processes [1]

Fused Deposition Modeling (FDM)

  • Materials used in the FDM process
  • Poly-Ether-Ether-Ketone (PEEK)
  • Applications of FDM
  • Advantages and limitations of the FDM process

With the desire for better mechanical properties, new materials have been identified for use in the FDM process, which include carbon fiber reinforced composites and the polyether group [2]. Parts manufactured using the FDM process are most often used in the medical, aerospace and automotive industries [2].

Screw Extrusion Process

Setup

Reasons why FDM is relatively more successful compared to other AM processes are due to its low cost and ease of use. Compared to other 3D printing processes such as Stereo Lithography Apparatus (SLA), which use high-energy lasers to solidify the photoresin, the FDM process uses a heated die to extrude material, making it very economical.

Various zones of the screw

The ratio of diameters at the beginning and end of the pressure zone is called pressure ratio and varies depending on the polymer being processed. As the material moves between the flanges of the screw, the solid bed is sheared with viscous melt causing friction between the interfaces.

Literature review

  • Parameter study of 3-D Printed PEEK parts
  • Comparison of 3-D printed PEEK and other thermoplastics
  • Study on temperature and crystallinity
  • Extrusion process

Layer thickness and filler ratio were reported to be the primary reasons for failure. It was also reported that the combination of low and high processing parameters resulted in poor mechanical properties. Their models reported that rapid cooling suppressed PEEK crystallization.

It was reported that nozzle temperature in the range of 370˚C -390˚C and bed temperature of 280˚C along with the pressure direction of 0˚ resulted in maximum tensile strength. It was also reported that the annealing process did not provide any improvement in the mechanical properties of the samples. It has been reported that samples deposited with lower press speed and higher nozzle temperature resulted in higher tensile strength.

Figure 1-6 shows print orientation used in their experiments.
Figure 1-6 shows print orientation used in their experiments.

Objectives

Introduction

Geometric model and material properties

The use of the total thermal energy (heat from band heaters and heat generated by the viscous shear effect). 𝑃𝐸 Represents the gain in potential energy of the polymer per unit mass as the material moves downward toward the nozzle tip. The energy balance of the screw extruder has been discussed to describe the boundary conditions imposed on the system.

Due to the complexity of the deposition process, only the print direction of 90˚ above 0˚ has been selected. Road width is one of the critical factors in determining the bond strength between passages. Greater road width leads to the minimum cross-sectional area of ​​contact between two passages, resulting in week strength of the bond.

On the other hand, less road width creates non-linear deposition between layers, which causes the uneven surface of the sample. Pitch distance refers to the distance between the tip of the nozzle and the deposition surface. One end of the spindle has been attached to the load cell, while the other end is free to move.

While the effect of road width is similar to that of previous experimental results, the extrusion temperature of Set-B with smaller road width resulted in the highest inter-road strength among all.

Figure 2-3 Boundary conditions used for studying temperature profiles
Figure 2-3 Boundary conditions used for studying temperature profiles

Thermal analysis of Screw extruder

Boundary Conditions and mesh generation

Ax symmetric model was created using the Abaqus software to examine the temperature profile at different locations in the extruder setup. The heat transfer to supporting components of the system was considered negligible, therefore heat flux of magnitude zero was assigned at contact regions between the barrel and supporting frame. As discussed from the energy balance of screw extrusion process, convection and radiation losses were assigned to regions exposed to the environment.

Numerical modeling is performed based on axisymmetric transient thermal analysis, so the corresponding mesh is generated using the quadrilateral convection-diffusion quadrilateral element (DCCAX4) with mesh size 2.5 mm x 2.5 mm.

Results

The numerical modeling is performed based on axisymmetric transient thermal analysis, so the corresponding mesh is generated using an axisymmetric quadrilateral 4-node convection-diffusion element (DCCAX4) with a mesh size of 2.5 mm x 2.5 mm. 23. a) b). Temperature variation with respect to time for different junction temperatures revealed that stabilization time varies based on location in the extruder setup, as shown in Figure 2-5. To conduct the experiments, the maximum stabilization time between all regions of the model was determined.

From the numerical analysis, it was found that node 5 has the maximum stabilization time of 90 minutes, as shown in Figure 2-5.

Figure 2-4 Temperature profile variation with time plots from Abaqus  a) 2 minutes b) 10 minutes c) 30 minutes d) 100 minutes
Figure 2-4 Temperature profile variation with time plots from Abaqus a) 2 minutes b) 10 minutes c) 30 minutes d) 100 minutes

Summary and conclusions

Introduction

The speed control of the XY table and the individual motor activation are done via programming inputs. The current The material in the pellet form moves by gravity from the hopper through a feed channel to the inlet on the barrel.

The feed channel is provided with an arrangement to stop or start the feed, but it should be noted that there is no specific control for material flow, so manual intervention is required for feed material. Through analytical calculation, the torque requirements for processing PEEK were estimated to be 24.5 N-m, Hence DC motor with a torque capacity of 10 N-m along with a gear ratio of 3:1 was selected to provide net torque of 30 N-m for the screw . . Since the melting temperature of PEEK was on the higher side, ceramic tape heaters with an operating range of 100 ˚C to 450 ˚C were selected to conduct experiments.

Process parameters and response variables

With the available group of process parameters and response variables, two different sets of experiments were performed as mentioned below.

Preparation of wire samples

Extrusion temperature

This is because the viscosity of the material decreases with an increase in temperature, allowing the screw to rotate freely and compress the material. Thus, based on the phenomenon, the minimum temperature at this region was set as 290 ˚C with an increase of 12.5 ˚C for each set of extrusion temperature. Apart from this, it is also important to keep the material in a semi-liquid state for a limited period of time (about 10 seconds) to allow it to settle and take shape before it rapidly cools and solidifies.

So, based on the mentioned requirements, a minimum temperature of 320 ˚C with an increment of 10 ˚C has been provided for each set. With the range for each zone, experiments were performed to study the mechanical properties of the extruded sample. Extrusion was repeated for each set temperature as previously discussed, and samples were collected for every 5 minutes of extrusion after the extrusion process reached a steady state.

Multi-pass sample preparation

  • Nozzle size
  • Extrusion temperature
  • Print Direction
  • Road Width
  • Standoff Distance

Preliminary experimental findings indicate that when using nozzle sizes below 1.5 mm, due to its smaller opening for the material to move out, the back pressure is increased, resulting in more strain on the motor for operation. From preliminary experiments, it was clear that the extrusion temperature corresponding to set-3 has shown better mechanical properties among all. Therefore, this set, along with adjacent sets, corresponded to slightly higher and lower temperatures.

To avoid any confusion with the wire samples, these sets will henceforth be referred to as Set-A, Set-B, Set-C. Since we extrude filament with a nozzle of 1.5 mm diameter, taking into account the effects of the swelling of the die along with experimentation, the minimum path gap selected was 1.6 mm, and the maximum was 1.65 mm after which there is a minimum contact area was between two corridors. Experimentally, it was found that lower standoff distance resulted in better accuracy of the sample, so a minimum stand of standoff of 3mm including 1mm for clearance was maintained to avoid material agglomeration near the nozzle end.

Figure 3-6 Samples Deposited with different nozzle diameters   a) 0.7mm b) 1.1mm c) 1.5mm
Figure 3-6 Samples Deposited with different nozzle diameters a) 0.7mm b) 1.1mm c) 1.5mm

Design of Experiments

Tensile testing of wire samples

  • Set-1 [220 ˚C, 290 ˚C, 320 ˚C]
  • Set-2 [235 ˚C, 302.5 ˚C, 330 ˚C]
  • Set-3 [250 ˚C, 315 ˚C, 340 ˚C]
  • Set-4 [265 ˚C, 327.5 ˚C, 350 ˚C]
  • Set-5 [280 ˚C, 340 ˚C, 360 ˚C]
  • Summary of wire testing results

The ultimate tensile strength of set-2 was highest for 35 minutes of continuous extrusion, the magnitude of which was 80.18 MPa, followed by 77.68 MPa and 74.57 MPa for 5 minutes and 20 minutes, respectively. Of all set 3 extrusion temperatures, this resulted in a higher tensile strength with a maximum of 83.7 MPa for 5 minutes and 35 minutes of extrusion, as shown in Figure 4-4. The graphs in Figure 4-6 show that the tensile strength results of set-5, compared to set-4 extrusion temperatures, are further lower, indicating that material degradation is more significant at a much lower tensile strength of 68.4. MPa for 5 minutes.

With a summary of the results that the set-2 set-3 and set-4 extraction temperature provided the necessary information through consistent results, further testing was conducted for 15 minutes and 25 minutes of extraction time as shown in Figure 4-7 and Figure 4-8 respectively. . It can be observed that among all the extrusion temperatures, the set temperature setting-3 gave the highest tensile strength of 83.7 MPa. The trend pattern suggests a gradual decrease in the tensile strength of the samples after the set-3 induced extrusion temperature.

Figure 4-2 Stress-strain plots for set-1 extrusion temperature wire samples  4.1.2  Set-2 [235 ˚C, 302.5 ˚C, 330 ˚C]
Figure 4-2 Stress-strain plots for set-1 extrusion temperature wire samples 4.1.2 Set-2 [235 ˚C, 302.5 ˚C, 330 ˚C]

Tensile testing of multi-pass specimens

  • Set-A [235 ˚C, 302.5 ˚C, 330 ˚C] with different road width
  • Set-B [250 ˚C, 315 ˚C, 340 ˚C] with different road width
  • Set-C [265 ˚C, 327.5 ˚C, 350 ˚C] with different road width
  • Summary of tensile testing results for multi-pass sample

All the specimens were deposited perpendicular to the loading direction, resulting in interpad strength being obtained as the response variable. It can be visualized that as the pad gap increases, the net cross-sectional area of ​​contact decreases, leading to failure in the specimen at lower stress levels. It can be observed from the results, as shown in Figure 4-13, that higher extrusion temperature affected the interbond strength in a similar manner to that of wire testing.

Among all the tensile samples, the Set-C temperature extruded samples recorded the lowest bond strength. It was interesting to note that the variation in the bond strength has decreased compared to the previous tests, indicating that as the extrusion temperature increases, the effect of material degradation is more prominent than the road width. This was because lower road width causes the material to settle densely, resulting in higher contact area between two passages.

Figure 4-11 Stress-strain behaviour for different road widths corresponding to Set-A  extrusion temperature
Figure 4-11 Stress-strain behaviour for different road widths corresponding to Set-A extrusion temperature

Summary

Conclusion

Future scope

Influence of thermal processing conditions in 3D printing on the crystallinity and mechanical properties of PEEK materials. Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies. Extrusion Deposition Process for Layered Fabrication.’’ Masters of Technology - Thesis, Indian Institute of Technology Kanpur.

Figure

Figure 1- 5 Viscous shearing mechanism     [5]
Figure 1-6 shows print orientation used in their experiments.
Figure 1- 7 Stress-strain curve for parts printed with infill ratio 100%,50%,20%
Figure 1- 8 Fracture after tensile test a), b) XY direction printed samples     c), d) Z direction printed samples      [6]
+7

References

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