Characterization of mechanical properties to Tension for FFF

The following are the codified guidelines for improving mechanical properties (MEC) based on case analyses

MEC-01. The percentage of filler must be increased to increase the mechanical strength.

The mechanical resistance increases with the percentage of filler, whether it is resistance to tension, bending, impact, or compression [1]–[4].

MEC-02. For 100% infill density, use +45°/-45° infill orientation for any loading, except creep, where 90° orientation is recommended.

At 100% infill density, infill orientation does not produce significant changes in tensile, flexural, and compressive strength, with sensitivity for impact strength, fatigue strength, and fracture toughness. For sensitive properties, it is recommended to use +45°/-45° infill orientation. For creep (slow creep deformation), a 90° infill orientation is recommended [1]–[4]

MEC-04. For optimal strength, it is recommended to fabricate parts oriented in the following order of highest to lowest strengths: edge, horizontal, and vertical.

The anisotropy of tensile strength, bending, and flexural fatigue is relatively low for edge and horizontal orientations, but very high when comparing edge and horizontal to vertical orientation [1], [5]–[7]

Figure 1-2 illustrates how it is preferable to manufacture the part so that the load is parallel or aligned with the manufacturing layer and not perpendicular to the build plane.

 

MEC-06. Before modifying parameters to enhance strength, to take a decision is recommended to characterize the strength of parts to printing speed, extrusion temperature, and layer thickness.

 

The effect of process parameters on strength varies depending on the material, interrelationships with other parameters, nonlinearities, and factors such as printer, material suppliers, build orientation, and infill orientation. While there is a general tendency to increase temperature, decrease speed, and reduce layer height to improve strength, there may be cases where the opposite is true, depending on the specific circumstances mentioned [6], [8], [9].

 

MEC-11. It is recommended to fabricate only one piece at a time. The fabrication time affects the mechanical properties of the parts; therefore, the number of replicas fabricated affects and reduces the strength of small-area parts (vertical 3D prints) [10].

MEC-12. Choose the type of support based on strength while paying attention to printing times, material waste, accuracy, and roughness to align with the design’s required objectives. The type of support used in cantilever, overhang, or bridge parts affects the accuracy and mechanical strength of the part [11].

MEC-13. The recommendation is to mechanically characterize the materials and the parts built with them [12], [13].  The mechanical properties reported by manufacturers do not correspond to those of mechanical characterizations of scientific articles and are above the scientific values. The properties of a material can vary from supplier to supplier, and mechanical properties vary from industrial to desktop machines for reasons such as the high chamber temperature that desktop printers lack. Mechanical properties may vary depending on the type of specimen used for testing, the size of the nozzle, or the number of perimeter layers.

MEC-14. Please select the color of the PLA carefully, as it can affect the mechanical properties. Color is not significant in mechanical properties except in semi-crystalline materials such as PLA [14].

MEC-15. Consider combining it with other processes to improve mechanical resistance (impact, toughness, flexural, tension, compression) , such as coatings [5], [15], [16], [17] infiltration [18],[19] and thermal treatments [20], [4].

 

Improvements in Mechanical strength to tension

 

Table 1 summarizes a partial overview of the results obtained after analysis of FFF mechanical property characterizations as a function of process parameters. Table 1 provides ranges of improvement (percentage) or changes, dependent on the interaction of factors.

To interpret the findings in Table 1, an example is provided:

  1. In the reference [6], improvements achieved in PLA FFF tensile characterization range from a minimum of 8% to a maximum of 77.3%, as indicated below the “Improvement (%)” column. The improvement corresponds to mechanical properties S, E, and AN, which are tensile strength, elastic modulus, and anisotropy, respectively; this is specified below the “Parameter” and “Output” columns, as well as in the table footer, which indicates the meaning of the table symbols. The input parameters modified to achieve these improvements are AC, VF, and TI, representing layer height, fabrication or printing speed, and fabrication or printing temperature, respectively. These are specified below the “Parameter” and “Input” columns. The “Material/process” column provides specific material and process details.

To use the figures of Table 1 or the database of failure theory and mechanical characterizations, from which these figures are derived, when designing a product, proceed as follows:

  1. Know the product’s properties a priori and compare them with the respective table or base until the material combination, output, and input parameters correspond to the desired property value.
  2. In case of redesign of parts outside of specifications, verify the percentages of improvement and the combination of material and parameters.
  3. Quantify times and costs associated with parameters to decide based on properties and costs.
  4. Consult on the specific base on the specific way to combine the parameters.
  5. In case of requiring clarification or expansion of specific information, consult the reference.

 

For more figures on improvements associated with characterization in tension, flexure, impact, compression, fatigue, wear and hardness, creep, and fracture toughness go to the database.

 

References

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