Important processing parameters

As shown in Figure 2-9, process variables which have an effect on SLS processing can be categorised into four types: controlled, machine specific and geometry-specific parameters, and material properties. Each of these factors has a different effect on sintering during the process of SLS component formation [Nelson, 1993; Agarwala et al, 1995].

Figure 2-9 Process variables which affect sintering in the SLS process [Nelson, 1993; Agarwala et al, 1995]

This section describes fabrication parameters directly related to the properties of SLS material. These are: part bed temperature, fill laser power, laser beam speed, scan

Controlled parameters Laser scanner

Laser power

Mechanical layering of powder Atmospheric control

Air flow

Heater (bed temperature)

Geometry-specific parameters Scan vector length

Machine specific parameters Laser type

Scan radius

Material properties Viscosity

Surface tension

Particle size distribution Particle shape Absorptivity/reflectivity Thermal conductivity Specific heat Emissivity Melting temperature

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spacing and slice thickness [Gibson and Shi, 1997; Cee Kai, 2003; Venuvinod, 2004; Noorani, 2006] as illustrated in Figure 2-10.

Figure 2-10 Processing parameters in SLS process

a. Part bed temperature (Tb)

The part bed temperature (Tb) is the temperature at which the powder material sits in

the part cylinder. In the part cylinder, powder material is heated to the part bed temperature before the laser scanners move, in order that laser power and distortion in the sintering process can be reduced. In other words, the need for laser power is reduced when the part bed temperature is higher, and the difference in temperature between pre- and post-sintering is reduced. Part bed temperature varies depending on the material. For an amorphous polymer, the temperature is normally set near the glass transition temperature (Tg), and for a perfectly crystalline polymer it should be

set 3-40C below its melting temperature (Tm), due to it having no glass transition

temperature [Gibson and Shi, 1997].

b. Fill laser power (P)

The fill laser power (P) is the power used to heat the powder material during scanning. The maximum laser power of machines is different based on the type and manufacturer.

Fill laser power is influenced by laser beam speed, the diameter of the laser beam, the density, specific heat, and latent melting heat of the powder [Gibson and Shi, 1997]. Scan spacing Layer thickness Scanning path Laser Sintered layer Beam speed

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In SLS technology, different kinds of lasers are used. CO2 and Nd-YAG lasers are commonly used in SLS machines. These operate at different wavelengths of 10.6µm for CO2 lasers and 1.06µm for Nd-YAG lasers. The absorption of laser light depends on material absorptivity, transmissivity and particle size [Nelson, 1993]. The laser wavelength should be adapted to the powder material to be sintered, due to the fact that laser absorption changes greatly according to the material and the laser light wavelength [Tolochko et al, 2000]. Normally laser absorption depends on laser wavelength, the condition of the sintered materials, surface geometry, ambient gas, and temperature [Tolochko et al, 2000]. Powder material can usually absorb laser energy better than solid material, due to multiple reflection and laser beam absorption trapped in the powder pores [Kruth et al, 2003].

CO2 lasers with a wavelength of 10.6µm are suitable for polymer powders. On the other hand, Nd-YAG lasers with 1.06µm wavelength are better for metals [Kruth et al, 2003].

c. Laser beam speed (BS)

Laser beam speed is defined as the linear velocity of the laser beam across the powder surface of the material [Nelson, 1993]. This machine-specific parameter can be adjusted to suit the material being processed.

d. Scan spacing (SP)

The distance between the two nearest parallel scan vectors is called the scan spacing (SP). Scan spacing influences laser beam size and energy density. The scan spacing must be smaller than the diameter of the laser beam on the part bed, in order that a cross-section of the sintered area can be built completely. Figure 2-11 illustrates scan spacing in the SLS process [Venuvinod, 2004].

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Figure 2-11 Scan spacing in the SLS process [Venuvinod, 2004] e. Slice/layer thickness (h)

Slice thickness (h) is another important parameter that must be considered, because it can cause a stair-step effect. Slice thickness is used to determine the powder material thickness of each layer in the part. The range of slice thickness is normally set from 0.01–0.5mm, depending on the type of SLS machine [Gibson and Shi, 1997]. Slice thickness will influence building time and surface roughness. Surface roughness can be reduced by using smaller slice thickness, but the building time of the part will then be longer. The penetration depth of the powder material limits the largest slice thickness. The penetration depth of powder material is influenced by laser power, energy density, particle size, powder density, specific heat and thermal conductivity [Venuvinod, 2004]. Therefore, a suitable slice thickness can be set depending on the material and the laser power available.

f. The energy density (ED)

One of the most important factors that influences the quality of laser sintering process is energy density. This indicates how the amount of energy per uniform volume delivered to the powders changes with process parameters.

The energy density is a function of fill laser power (P), beam speed (BS) and scan spacing (SP). Therefore, the important relationship between processing parameters can be defined as in equation 2.1 [Gibson and Shi, 1997]:

Energy density (ED) =

SP BS P * (Joule/cm 2 ) ... (2.1) where: P is fill laser power (watt)

BS is laser beam speed (cm/s) SP is scan spacing (cm)

SP

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From the energy density equation, two parameters are most commonly altered to gain greater energy density. These are fill laser power (P) and laser beam speed (BS) as they are the two most influential parameters. For Scan spacing (SS), as mentioned in section 2.3.2 (d) must be smaller than the diameter of the laser beam, or as an alternative, it is advised that scan spacing should be 3-6 times smaller. This is in order for each point on the powder bed to receive multiple laser exposures and thus the sintered area can be built completely.

Alterating the settings of these parameters will affect the ability of the material to be built successfully and the quality and properties of the parts manufactured.

For example, the density and tensile strength of an SLS fabricated part increase as a result of increasing fill laser power, decreasing laser beam speed and decreasing scan spacing. Beside of this, the viscosity of the material also decreases, which in turn lowers the porosity and this then increases density [Williams and Deckard, 1998; Caulfield et al, 2007]. On the other hand, due to increased liquid flow if the fill laser power is too high, shear stresses between layers are created and the parts may bend or become distorted [Beaman et al, 1997; Caulfield et al, 2007].

There is an optimum energy density that is needed to achieve the maximum density and strength, which is different for each different material. If the optimum energy density is exceeded, this results in the degradation of mechanical properties and an increase in porosity and the surface degradation of the resulting laser-sintered material [Ho et al, 1999; Caulfield et al, 2007].

Research has shown that varying the energy density can cause variations in the density and tensile strength of polyamide sintered material [Gibson and Shi, 1997; Tontowi and Childs, 2001]. This is supported by research conducted by Ho et al [1999], who investigated the effect of energy density on the mechanical properties of polycarbonate powder. Further discussion regarding the relationship between energy density and mechanical properties is given in section 2.3.3 below.