PSI - Issue 26

N.A. Fountas et al. / Procedia Structural Integrity 26 (2020) 139–146 Fountas et al. / Structural Integrity Procedia 00 (2019) 000–000

141

3

height, infill density, orientation angle and printing speed on the tensile strength (load, kN ) of PLA fabricated specimens designed according to the ASTM 638-10 type I standard. 2. Experimental With reference to the literature presented, five FDM parameters were considered for examining the tensile strength of standard specimens. The parameters are determined as follows:  Shell thickness: The thickness of the outer shell in horizontal direction. In combination with the nozzle size the number of perimeter lines is determined.  Layer height: Layer height is the thickness of a layer deposited by the extruder’s nozzle and is affected by the nozzle type.  Infill density: The density of material used for filling interion regions of the part. Infill density is given as a percentage of a solid layer whose infill density is 100%. A density value of 40% is enough to give almost all models good mechanical strength. A value of 20% is usually the minimum required to support flat ceilings.  Orientation angle: The part’s inclination angle on the printing bed, with reference to X and/or Y axes.  Printing speed: The extruder’s linear feed velocity. A set of other parameters controlling the FDM process were kept at fixed values. The values corresponding to the levels of FDM parameters selected for the experiments were determined regarding preliminary work and recommendations by the manufacturer of the equipment (3D printer and software) adopted. The FDM parameters for setting up the experiment and the constrained factors are given in Table 1.

Table 1. Experimental FDM parameters and corresponding levels. FDM constant parameters

FDM control parameters

Parameter

Value

Unit

Parameter

Symbol

Level Low (-1)

Center (0)

High (1)

Unit mm mm

1 st layer height

0.2 0.6 190

mm mm

Layer height Shell thickness Infill density Orientation angle Printing speed

A B C D E

0.050 0.300 15.00 0.000 26.25

0.125 0.500 52.50 30.00 35.00

0.200 0.700 90.00 60.00 43.75

Top/bottom thickness Printing temperature

o C o C %

%

deg.

Bed temperature Infill overlap

65 15

mm/sec

The experiment was a fractional factorial design with 16 runs (L 16 ). This design was selected to examine all interactions according to the number of FDM parameters. The design was then transformed to a customized response surface design to study the non-linear effect of all parameters to the response of tensile strength expressed through the load (kN) at breakage. Response surface design allows for fitting a second order polynomial regression relation among FDM parameters and the response of tensile strength and was preferred at that point of research owing to its ability of studying the curvature of the effects. Table 2 shows the factorial experiment and results for load (kN). Upper and lower levels for experimental parameters were coded to +1 and -1 respectively using Eq.1 and Eq.2 to examine them under the same range. Zero level for each parameter (center point) was coded to 0.

2   ij x x x      i   

ij 

 

(1)



2

x

ij

1

j



,

, 1, 2,..., ,

1, 2

x

x x x i    

K j





(2)

2

1

i

i

i

i

2