Effect of powder supply preheating on mechanical properties and moulding accuracy of carbon black/pinewood/polyethersulfone composites printed by image shape laser sintering

Abstract

Preheating in selective laser sintering plays a critical role in enhancing mechanical properties and dimensional accuracy. Due to equipment of image shape laser sintering limitations that prevent direct preheating of the build chamber, a thermal conditioning strategy was implemented in the powder supply chamber instead. To assess the impact of supply chamber preheating for image shape laser sintering on part strength and dimensional accuracy, this study systematically examined the effects of laser power (16–22 W), scanning speed (6–12 mm/s), layer thickness (0.1–0.25 mm), and preheating temperature (70°C–85°C) of carbon black/ pinewood/ polyethersulfone composites. Temperature calibration confirmed that the powder could be preheated to 85°C, maintaining a powder bed surface temperature of approximately 60°C during spreading. Orthogonal experiments, combined with a weighted performance analysis, showed that scanning speed had the most significant influence on mechanical and geometric outcomes, while layer thickness had the least. The optimal process parameters—20 W laser power, 85°C preheating temperature, 0.15 mm layer thickness, and 6 mm/s scanning speed—resulted in a twofold increase in tensile strength and a 1.5-fold increase in flexural strength compared to samples processed without preheating. These results highlight the effectiveness of indirect preheating and offer a practical solution for cases in which direct build chamber heating is not feasible.

Keywords

Image-shape laser sintering DMD heat powder supply polymer sintering orthogonal array design

Introduction

Selective Laser Sintering (SLS), a subset of Powder Bed Fusion (PBF) technology, is renowned for its high precision, broad material compatibility, and support-free fabrication,¹ making it indispensable in both consumer and industrial applications. In a conventional SLS process, a single CO₂ laser (λ = 10,600 nm) selectively sinters the cross-sectional areas of a polymer powder layer by layer, guided by sliced data derived from a Computer-Aided Design (CAD) model.² This additive manufacturing technique allows for the fabrication of intricate three-dimensional structures. However, traditional SLS systems face limitations due to CO₂ laser power attenuation, resulting in low production efficiency, high operational costs, and inconsistent process parameters.

To overcome these limitations, researchers have investigated the use of near-infrared (NIR) fiber lasers (λ = 1064 nm) as a viable alternative to conventional CO₂ lasers (λ = 10,600 nm). NIR fiber lasers offer several advantages, including superior beam quality, higher output power, enhanced thermal stability, and a more compact system design. Near-infrared lasers have found widespread applications in metal additive manufacturing,³ glass processing systems,⁴ and biomedical treatments such as bone tumor ablation.⁵ However, one significant challenge when applying NIR lasers to polymers is their inherently low laser absorption.⁶ This limitation arises mainly from two factors: the molecular bonds in most polymers do not interact strongly with NIR wavelengths, and many polymers are often white in appearance—exhibit poor intrinsic absorption. To enhance the NIR laser absorption of polymers, researchers have explored the incorporation of laser-absorbing additives such as carbon black,⁷ reduced graphene oxide,⁸ carbon nanotubes (CNTs),⁹ and infrared-absorbing dyes.¹⁰

Biomass materials, characterized by their wide availability, environmental friendliness, low cost, and relatively high mechanical strength, have been widely utilized in 3D printing, particularly in laser sintering.^11,12 Representative examples include sisal fibers,¹³ rice husks,¹⁴ peanut shells,¹⁵ bamboo,¹⁶ pine wood,¹⁷ seashells,¹⁸ and walnut shells.¹⁹ Incorporating biomass powders into polymer matrices can enhance the mechanical strength of the composite material and reduce warping during printing, thereby enabling the fabrication of dimensionally stable three-dimensional components. In the reviewed literature, most wood-plastic composites are composed of binary mixtures and are typically processed using conventional CO₂ point laser sintering techniques. In contrast, this study introduces a patterned sintering method using a fiber laser, enhanced by the addition of carbon black to improve laser energy absorption and processing efficiency.

DMD is a type of micro-electromechanical system (MEMS) that operates as an electronic input and optical output component. It comprises millions of individually addressable microscopic mirrors, each functioning as a single pixel in the projected image.²⁰ By modulating the tilt angle of each mirror, light can be precisely directed and dynamically controlled. DMDs are widely utilized in applications such as video projection,²¹ direct laser writing,²² and lithography.²³ Owing to their high resolution, rapid switching speed, and capacity to project programmable structured light with exceptional precision, DMDs are frequently integrated with nano-inks for micro-scale laser sintering applications.^24–26 However, conventional laser sintering processes face challenges at the microscale due to the high thermal conductivity of the powder, which can cause undesirable effects such as shrinkage during formation and secondary sintering.²⁷ As an alternative, employing a DMD-based projection system to produce large, programmable light spots presents a promising strategy. Although large-spot laser printing can improve efficiency, the power tolerance of the DMD limits the achievable power density. Therefore, a tailored powder formulation is required to accommodate this printing method.

Conventional laser sintering systems typically preheat the powder bed using infrared lamps installed above the build chamber.²⁸ However, this method is not applicable in our setup due to the short focal length of the optical system (70 mm), the height of the recoating mechanism (60 mm), and the presence of a protective window between them. Additional preheating methods include laser spot and build platform preheating. Laser spot preheating uses a secondary laser to project a large beam onto the processing area.²⁹ This method is effective for small build areas but unsuitable for large-scale applications due to limited coverage. Build platform preheating relies on thermal conduction through the powder bed. This method effectively reduces thermal stress and improves heat retention within the printed part. However, it is not suitable for bottom-feed systems and is better suited for top-feed powder delivery configurations.³⁰ To overcome this limitation, we employ an indirect preheating approach by heating the powder in the feed chamber before it is spread onto the build surface.

In summary, although Image-Shaped Laser Sintering (ISLS)³¹ represents an emerging and innovative additive manufacturing technique based on DMD, In the absence of a preheating system, the quality of printed components was largely influenced by ambient temperature conditions. To improve the preheating system for patterned laser sintering of CB/pinewood/PES composites, we implemented a strategy in which the powder is preheated within the supply chamber and subsequently delivered to the build surface via the recoating mechanism. This approach aims to raise the surface temperature as much as possible, thereby enhancing the dimensional accuracy and mechanical performance of the printed parts.

Experiment preparation

Hardware system

The sintering equipment is based on a compact patterned laser sintering prototype independently developed by the research team, as illustrated in Figure 1. The system primarily comprises four components: an optical system, a mechanical system, a control system, and a heating system.

Figure 1.

ISLS system schematic.

The optical system comprises a fiber laser and a homogenizing lens. The lens, acquired from VISITECH, is integrated with a DLP650NIR digital micromirror device, offering a resolution of 800 × 1280 pixels. The fiber laser is supplied by GW Laser Technology.

The mechanical system includes two subsystems: a light engine motion mechanism and a powder delivery system. The light engine motion system enables movement along the XY plane, allowing the laser to accurately project patterned light onto the forming surface. The powder delivery system operates the powder roller, which transfers preheated powder from the right-hand feed chamber into the left-hand forming chamber.

The control system is primarily centered around an AS228T Programmable Logic Controller (PLC) from Delta, which oversees the operation of the laser and powder motion systems. The GTS-400 motion control card from Googoltech is responsible for controlling the movement of the light engine. Additionally, the LRSMCX-WX-NIR unit functions as the low-level controller for managing the status of the DMD. The heating system comprises two instantaneous heating lamps: one U-shaped and one I-shaped. The U-shaped lamp is strategically positioned closer to the forming chamber for optimal heating. The heating process is regulated by a PLC using Pulse Width Modulation (PWM) to control a relay, allowing precise on/off operation of the lamps. This arrangement ensures the efficient and uniform heating of the powder surface within the feed chamber. In conclusion, the overall structure of the equipment is presented in Figure 2, with the key technical specifications outlined in Table 1.

Figure 2.

ISLS equipment.

Table 1.

Parameters of apparatus.

name	Parameter	Name	Parameter
Laser wavelength (nm)	1064	Forming area (mm)	220 × 220
Maximum output power(W)	27	Powder movement accuracy (μm)	50
Maximum spot size (mm)	25.6 × 16	Optical smovement accuracy (μm)	10
Operating temperature of the optical system (°C)	21	Maximum power tolerance of the DMD (W)	160

Materials

Polyethersulfone (PES), a thermoplastic resin, is extensively used in high-performance engineering plastics due to its superior mechanical properties, excellent thermal stability, and strong corrosion resistance.³² PES is especially suitable for laser sintering applications that demand low energy density. PES powder, with a particle size of 80 μm, is supplied by Anhui Tiannian Materials Technology Company. The powder is characterized by its fine, white appearance.

Carbon Black (CB) is a black material composed of microscopic carbon particles, renowned for its excellent thermal stability, low thermal conductivity, strong coloring capacity, and favorable mechanical properties. CB, produced by Hebei Jinfengda Company, is chosen as an additive material, with an average particle size of 8 μm.

Pinewood powder, sourced from the wood industry, has an average particle size of 96 μm.

The photographs of the powder and the scanning electron microscope (SEM) images are shown in Figure 3.

Figure 3.

Morphology of the powder and its SEM images. (a) appearance of PES powder; (b) Microstructure of PES powder; (c) Appearance of CB powder; (d) Microstructure of CB powder; (e) Appearance of pinewood powder; (f) Microstructure of pinewood powder; (g) Appearance of Mixed powder and (h) Microstructure of Mixed powder.

Temperature calibration

After preparing the composite material and obtaining its DSC thermal properties, the next crucial step is calibrating the preheating temperature. In this experiment, a non-contact infrared temperature sensor is employed for temperature measurement. Although this method is suitable for the experimental setup, it is prone to several interfering factors, including measuring distance, surface emissivity, and fluctuations in ambient temperature. These factors may lead to discrepancies between the actual and set temperatures, which can undermine temperature control accuracy, thereby affecting the stability of the heating process and the quality of the final parts.

To achieve accurate and reliable temperature measurements, the preheating system must undergo calibration prior to part fabrication. A FLUKE Ti400 infrared thermal imager is employed to capture the actual surface temperature of the powder bed. The captured data is then compared with the temperature readings relayed to the PLC software by the infrared sensor. Calibration is performed by adjusting the software’s set temperature to correct for deviations caused by environmental factors, ensuring that the set value aligns with the material’s actual surface temperature.

The initial step in the temperature calibration process is measuring the actual temperature distribution across the powder bed. The objective is to compare the measured temperature with the control system’s set values, generate an error chart, and adjust the target preheating temperature for precise thermal control, establishing a stable foundation for the subsequent sintering process.

Once the temperature stabilizes at the target value, a thermal imager captures the actual surface temperature of the powder bed at various points. These thermal images offer real-time temperature data, which is compared with the system’s target values to identify and correct any measurement deviations. This ensures accurate calibration, creating a reliable thermal environment for sintering and minimizing the risk of defects caused by uneven heating.

Based on DSC analysis, the initial preheating target is set at 30°C, with increments of 5°C up to 65°C. After each adjustment, the thermal imager records the surface temperature of the powder bed, as illustrated in Figure 4.

Figure 4.

Powder surface temperature of Powder Supply. (a) 50°C; (b) 55°C; (c) 60°C; (d) 65°C; (e) 70°C; (f) 75°C; (g) 80°C and (h) 85°C.

Figure 5 demonstrates a clear linear relationship between the set preheating temperature and the actual measured temperature, with an average deviation of approximately 20°C. This suggests that, in practice, the measured temperature consistently exceeds the target temperature by approximately 20°C. Based on this observation, future preheating temperature settings can be adjusted by lowering the target powder bed temperature by 20°C. This adjustment ensures accurate attainment of the target preheating temperature, thereby improving both the convenience and precision of the temperature control process.

Figure 5.

Temperature deviation between set and measured preheating values.

Temperature test

Determination of the critical caking temperature of the materia

After determining the actual temperature distribution of the powder bed, the next essential step is identifying the critical caking temperature of the powder material. This temperature represents the maximum safe preheating limit under the given experimental conditions. Exceeding this temperature can cause the powder particles to partially fuse and form clumps, disrupting the uniformity of the powder layer and resulting in poor spreading, printing defects, or even part failure.

Based on Differential Scanning Calorimeter (DSC) analysis, as shown in Figure 6, the material’s initial melting point is approximately 108.10°C, indicating the onset of particle softening and potential agglomeration. To prevent premature sintering or loss of flowability, the critical caking temperature is tested close to this value. Experimental results indicate that noticeable caking begins at approximately 90°C. Beyond this temperature, the powder’s flowability significantly decreases, leading to uneven spreading and compromised sintering quality, as illustrated in Figure 7(a) and (b).

Figure 6.

DSC curve of composite powder.

Figure 7.

Powder surface temperature and bed caking. (a) Powder surface temperature and (b) Bed caking.

Consequently, in subsequent experiments, the preheating temperature is maintained below 90°C. This ensures that the powder remains free-flowing and uniformly distributed during spreading, thereby maintaining process stability and enhancing the quality and precision of the final part.

Determination of the preheating temperature effect on material

Based on the critical temperature threshold of 90°C, the preheating temperature of the powder supply box is set between 65°C and 85°C. After spreading a single layer of powder, the temperature distribution within the forming chamber is measured using a thermal imager, as shown in Figure 8. This step evaluates the performance of the temperature chamber, confirming its ability to ensure uniformity in the powder’s temperature distribution during the sintering process. It also estimates the heat loss of the preheated powder during the spreading process, providing guidance for subsequent tests. Additionally, the captured images assist in selecting the optimal sintering region, thereby improving the overall quality of the printed parts.

Figure 8.

Surface temperature of the powder bed. (a) 65°C; (b) 55°C; (c) 50°C and (d) 40°C.

As shown in Figure 8, when the preheating temperature is set between 65°C and 85°C, the average temperature of the forming surface remains between 40°C and 60°C. This indicates that the preheated powder in the powder supply box experiences a temperature drop of approximately 25°C after the spreading process. The temperature distribution on the forming surface is generally uniform, but the side closer to the powder supply box exhibits a slightly higher temperature. This results in a peak pattern with higher temperatures in the center and lower temperatures on both sides. This is primarily due to the U-shaped heating pipe installed above the powder supply box, which radiates heat towards that side of the powder. The heat at the center of the U-shaped pipe is more concentrated, while heat at the upper and lower edges is more diffuse, resulting in a slight temperature increase on that side of the forming chamber.

Mechanical property characterization

Tensile strength is used to evaluate the performance of molded parts under tensile loads. This test follows the national standard GB/T 1040.1-2008, with standard specimen dimensions of 150.0 mm × 10.0 mm × 4.0 mm.

This test follows the national standard GB/T 1040.1-2008, with standard specimen dimensions of 150.0 mm × 10.0 mm × 4.0 mm. This test follows the national standard GB/T 9341-2008: Plastics—Determination of Flexural Properties. The standard specimen dimensions are 80.0 mm × 10.0 mm × 4.0 mm.

Dimensional accuracy is a critical indicator of the quality of the formed part. Dimensional accuracy is measured by comparing the actual values of the part in three-dimensional space (X, Y, Z axes) with the preset standard values.³³ In this experiment, however, the focus is on determining whether the temperature difference around the part is minimized under the influence of the temperature chamber and if this reduction helps to mitigate warping. Therefore, this study primarily analyzes the dimensional accuracy of the part along the Z-axis. The calculation formula is as follows:

φ_{z} = (1 - \frac{h_{z} - h_{z 0}}{h_{z 0}}) \times 100 %

(1)

The density of the formed part indicates the compactness of the reaction-sintered component. Higher density typically correlates with improved structural strength and durability. In this experiment, the geometric measurement method was employed to determine the density. Specifically, a vernier caliper was used to measure the length $l$ , width $b$ , and height $h$ . The density $ρ$ was then calculated using the following formula:

ρ = \frac{m}{l \times b \times h}

(2)

This study evaluates the improvement in forming quality under the influence of a temperature chamber using four key indicators: tensile strength, flexural strength, Z-direction dimensional accuracy, and density. Each indicator was analyzed individually through single-factor orthogonal experiments. However, the optimal processing parameters obtained from each indicator were not entirely consistent. Therefore, to comprehensively assess the influence of each evaluation metric on the forming quality of the sintered part, a weighted analysis method is used. This method assigns weights to each indicator based on its relative importance, enabling an integrated analysis to determine the optimal set of process parameters.

In orthogonal experiments, it is necessary to apply a dimensionless transformation to the experimental results of each indicator (tensile strength, flexural strength, Z-axis dimensional accuracy, and density). The conversion formula is given as follows:

η_{l s} = \frac{σ_{l s} {- σ}_{l s m i n}}{σ_{l s m a x} - σ_{l s m i n}}

(3)

η_{w q} = \frac{σ_{w q} {- σ}_{w q m i n}}{σ_{w q m a x} - σ_{w q m i n}}

(4)

η_{z} = \frac{σ_{z} {- σ}_{z m i n}}{σ_{z m a x} - σ_{z m i n}}

(5)

η_{ρ} = \frac{σ_{ρ} {- σ}_{ρ \min}}{σ_{ρ \max} - σ_{ρ \min}}

(6)

Next, the weights for each indicator need to be determined. Tensile strength and flexural strength are key indicators for evaluating mechanical properties. Strong mechanical properties enhance the application potential of sintered parts and suggest that surface light-source laser sintering technology has promising future prospects. Z-axis dimensional accuracy reflects the precision of the laser processing. Accuracy is particularly crucial during the fitting process in the assembly of sintered parts and serves as a key evaluation criterion for validating surface light-source laser sintering. Therefore, subjective weights are assigned to tensile strength, flexural strength, and dimensional accuracy, with greater emphasis placed on these factors. Density, however, does not directly indicate the molding effect of sintered parts and is considered a secondary research focus in this paper. As a result, the weights for tensile strength, Flexural strength, Z-axis dimensional accuracy, and density are chosen as ( $λ_{l s}$ = $λ_{w q}$ = $λ_{z}$ = 0.3 ) and ( $λ_{ρ}$ = 0.1).³⁴ The comprehensive weighted scoring formula for the experimental results is expressed as:

Z = λ_{l s} \times η_{l s} + λ_{w q} \times η_{w q} + λ_{z} \times η_{z} + λ_{ρ} \times η_{ρ}

(7)

Orthogonal experiment design

Therefore, this study investigates the effect of the temperature chamber on the quality of sintered parts made from this powder under varying process parameters, with the condition that the temperature chamber ensures uniformity in the temperature field during powder preheating and part formation. To reduce the wear on the DMD, the output power is set within the range of 16–22 W, with four laser power parameters selected. Based on the processing conditions supported by the current experimental platform, the approximate ranges for each process parameter are set as follows: preheating temperature 70°C–85°C, scanning speed 6–12 mm/s, and layer thickness 0.1–0.25 mm. Accordingly, this experiment requires the development of a four-factor, four-level orthogonal test design based on the above process parameter ranges, with the specific levels for each factor presented in Table 2.

Table 2.

Factors and levels of the LS test.

Levels	A: Laser power(W)	B: Preheating temperature (°C)	C: Scanning spacing (mm)	D: Scanning speed (mm/s)
1	16	70	0.10	6
2	18	75	0.15	8
3	20	80	0.20	10
4	22	85	0.25	12

Based on the four key process factors selected for the experiment—laser power, scanning speed, layer thickness, and preheating temperature—an orthogonal test matrix was designed, as presented in Table 3. Three specimens were fabricated for each set of process parameters to ensure repeatability. The performance of the formed parts under different test conditions was evaluated to determine the optimal combination of process parameters.

Table 3.

Design of the orthogonal experimental scheme.

Levels	A: Laser power(W)	B: Preheating temperature (°C)	C: Scanning spacing (mm)	D: Scanning speed (mm/s)
1	1 (16)	1 (70)	1 (0.10)	1 (6)
2	1 (16)	2 (75)	2 (0.15)	2 (8)
3	1 (16)	3 (80)	3 (0.20)	3 (10)
4	1 (16)	4 (85)	4 (0.25)	4 (12)
5	2 (18)	1 (70)	2 (0.15)	3 (10)
6	2 (18)	2 (75)	1 (0.10)	4 (12)
7	2 (18)	3 (80)	4 (0.25)	1 (6)
8	2 (18)	4 (85)	3 (0.20)	2 (8)
9	3 (20)	1 (70)	3 (0.20)	4 (12)
10	3 (20)	2 (75)	4 (0.25)	3 (10)
11	3 (20)	3 (80)	1 (0.10)	2 (8)
12	3 (20)	4 (85)	2 (0.15)	1 (6)
13	4 (22)	1 (70)	4 (0.25)	2 (8)
14	4 (22)	2 (75)	3 (0.20)	1 (6)
15	4 (22)	3 (80)	2 (0.15)	4 (12)
16	4 (22)	4 (85)	1 (0.10)	3 (10)

Laser sintering was performed in each test group according to the specified process parameters. The tensile strength, flexural strength, mass, and dimensional accuracy of the formed parts were measured to assess their mechanical properties and dimensional stability. Scanning electron microscopy (SEM) was also employed to capture cross-sectional microstructures of the formed parts, enabling an analysis of their internal structure and sintering quality.

Results and discussion

Effect of composition ratios on sintered parts

The performance of molded parts under various process parameters was systematically evaluated through statistical analysis of orthogonal tests. By calculating the variance and range for each process parameter, the specific impact of these parameters on the quality of the molded parts can be better understood. The analysis of variance and range results established the relationship between forming performance and process parameters, clearly revealing the influence trends of laser power, preheating temperature, layer thickness, and scanning speed on the forming performance. This analysis not only reveals the mechanisms by which process parameters affect the mechanical properties, surface quality, and dimensional accuracy of the molded parts, but also provides an essential theoretical foundation for optimizing the laser sintering process.

Figure 9 illustrates the effects of key processing parameters on the tensile strength of sintered CB/pinewood/PES composites. As shown in Figure 9(a), tensile strength decreases significantly with increasing scanning speed. This trend is attributed to reduced laser-material interaction time at higher speeds, which limits energy absorption and results in incomplete melting, thereby weakening the bonding between layers.

Figure 9.

The relationship between tensile strength and other factors. (a) scanning speed; (b)preheating temperature; (c) Layer thickness and (d) Laser power.

Figure 9(b) shows that increasing the preheating temperature from 70°C to 85°C improves tensile strength. A higher preheating temperature reduces the thermal gradient between the laser and powder bed, minimizes thermal shrinkage during solidification, and lowers the energy required to reach melting. These factors contribute to more uniform sintering and enhanced mechanical properties.

In Figure 9(c), tensile strength declines as the layer thickness increases from 0.1 mm to 0.25 mm. Thicker layers hinder effective laser penetration and melting through the entire depth of the powder bed, leading to weak interlayer adhesion and reduced part integrity.

As illustrated in Figure 9(d), tensile strength increases with laser power from 16 W to 20 W, reaching a peak at 20 W due to increased energy density, which enables more thorough melting and the formation of stronger sintering necks. However, further increasing the power to 22 W results in a decline in tensile strength. This is likely due to overheating, which may cause powder warping, disrupt powder spreading, and reduce print quality.

As shown in Figure 10, laser power, layer thickness, and scan speed have a significant impact on the flexural strength of the sintered parts. The maximum flexural strength is achieved under the conditions of 85°C preheating temperature, 20 W laser power, 6 mm/s scan speed, and 0.1 mm layer thickness.

Figure 10.

The relationship between flexural strength and other factors. (a) scanning speed; (b)preheating temperature; (c) Layer thickness and (d) Laser power.

Comparing the results of both tensile and flexural strength tests, it is clear that scan speed has the greatest impact on mechanical performance, as indicated by the largest range of variation. Laser power ranks second in terms of its impact, also showing a significant effect on the mechanical properties. Preheating temperature follows, with a range slightly larger than that of layer thickness, though the difference from laser power is considerable. Lastly, layer thickness shows the smallest range, indicating the least influence on the mechanical strength of the printed parts.

As illustrated in Figure 11, the dimensional accuracy along the Z-axis decreases with an increase in laser power, while it improves with elevated preheating temperature, greater layer thickness, and increased scanning speed. The increase in laser power results in a higher energy absorption by the powder, thereby enlarging the heat-affected zone and deepening the melt pool. Moreover, when the temperature within the sintering zone reaches or exceeds the material’s critical caking temperature, unsintered powder surrounding the melt pool may begin to adhere to the surface of the printed part. This phenomenon causes an increase in the Z-axis dimension, thereby reducing the overall dimensional accuracy of the printed part.

Figure 11.

The relationship between dimensional accuracy and other factors. (a) Scanning speed; (b) Preheating temperature; (c) Layer thickness and (d) Laser power.

Figure 12(a) reveals that part density decreases with increasing scanning speed. Faster scan rates reduce the laser’s interaction time with the powder, limiting energy absorption and resulting in incomplete sintering. The insufficient consolidation of particles ultimately leads to a reduction in part density.

Figure 12.

The relationship between density and other factors. (a) Scanning speed; (b) Preheating temperature; (c) Layer thickness and (d) Laser power.

Figure 12(b) shows that higher preheating temperatures also contribute to increased density. Elevated temperatures reduce the energy barrier for powder melting, enabling more efficient sintering within the laser-irradiated zone. This enhances bonding among particles and minimizes voids, resulting in a denser structure.

As illustrated in Figure 12(c), increasing layer thickness from 0.1 mm to 0.25 mm leads to a steady decline in density. Thicker layers introduce more unmelted powder between successive layers. As the laser energy dissipates during transmission through the powder bed, the lower energy input becomes insufficient for full melting, thereby increasing porosity and lowering density.

As shown in Figure 12(d), increasing laser power from 16 W to 22 W significantly enhances part density. This is attributed to improved energy absorption by the powder, which promotes complete melting and enlarges the melt pool. A larger thermal influence zone allows more particles to fuse, reducing internal porosity and increasing the final density.

For tensile strength, scanning speed exhibited the most significant impact, followed by preheating temperature, laser power, and layer thickness. In the case of flexural strength, the order of influence was scanning speed > preheating temperature > laser power. Part density was primarily affected by scanning speed, followed by laser power, preheating temperature, and layer thickness. For Z-direction accuracy, laser power had the greatest effect, followed by layer thickness, scanning speed, and preheating temperature. From the perspective of part fabrication and practical application, tensile and flexural strength as well as dimensional accuracy are the most critical properties. Accordingly, a combination of low scanning speed and high laser power is recommended to achieve superior part quality and formability.

Multi-index synthetic analysis

The comprehensive weighted method was also utilized for processing the experimental data. As shown in Table 4, in the multi-criteria orthogonal test results for laser sintering with a surface light source temperature chamber, the scanning speed has the largest range, indicating that it has the most significant impact on the experimental indicators among the four factors. Laser power ranks second in terms of influence on the forming results, followed by preheating temperature, and finally, layer thickness has the least impact on the forming performance.

Table 4.

LS process scheme and sintering quality of sintered parts processed using different laser parameters.

Test no	Laser power (W)	Preheating temperature (°C)	Scanning spacing (mm)	Scanning speed (mm/s)	Test index
Test no	Laser power (W)	Preheating temperature (°C)	Scanning spacing (mm)	Scanning speed (mm/s)	Tensile strength (MPa)	Flexural strength (MPa)	Dimensional accuracy in the Z direction (%)	Density (g/cm³)	Compound weight method
1	16	70	0.10	6	3.31	9.05	0.945	0.790	0.503
2	16	75	0.15	8	2.79	8.01	0.952	0.755	0.424
3	16	80	0.20	10	2.33	6.25	0.967	0.654	0.339
4	16	85	0.25	12	1.85	4.21	0.987	0.637	0.301
5	18	70	0.15	10	2.44	6.75	0.951	0.854	0.365
6	18	75	0.10	12	1.93	4.65	0.942	0.890	0.292
7	18	80	0.25	6	3.56	9.55	0.954	0.743	0.581
8	18	85	0.20	8	3.1	8.30	0.961	0.699	0.497
9	20	70	0.20	12	2.28	5.8	0.965	0.828	0.373
10	20	75	0.25	10	2.59	7.8	0.962	0.735	0.438
11	20	80	0.10	8	3.21	8.95	0.946	0.877	0.527
12	20	85	0.15	6	4.48	11.45	0.940	0.808	0.686
13	22	70	0.25	8	2.87	8.15	0.942	0.800	0.411
14	22	75	0.20	6	3.79	10.32	0.937	0.782	0.552
15	22	80	0.15	12	2.09	5.3	0.944	0.883	0.389
16	22	85	0.10	10	2.51	7.4	0.931	0.876	0.412
K1	1.567	1.652	1.834	2.322	Optimal portfolio: $A_{3} B_{4} C_{2} D_{1}$
K2	1.735	1.706	1764	1.859
K3	2.024	1.836	1751	1.554
K4	1.864	1.896	1731	1.355
R	0.185	0.042	0.036	0.367

The optimal process parameters were determined based on the orthogonal test results. For laser power, the third level (20 W) was found to be the best choice; for preheating temperature, the fourth level (85°C) proved optimal; for layer thickness, the first level (0.15 mm) was ideal; and for scanning speed, the first level (6 mm/s) was regarded as the best option. After considering the weights of the forming performance indicators, the optimized process combination was identified as A3B4C2D1, where: laser power: 20 W, preheating temperature: 85°C, layer thickness: 0.15 mm, scanning speed: 6 mm/s. Figure 13(a) and (b) show the tensile and flexural specimens fabricated under the preheating system, respectively. Under this configuration, the tensile strength reaches 4.48 MPa, and the flexural strength reaches 11.45 MPa. This represents a significant improvement compared to the best tensile strength of 2.13 MPa and the best flexural strength of 7.58 MPa, which were achieved when printing without the temperature chamber.³¹ This demonstrates the effectiveness of the temperature chamber in enhancing the mechanical properties of the sintered parts.

Figure 13.

Printed samples. (a) Tensile samples and (b) Flexural samples.

Finally, to analyze performance differences at the microscopic level, cross-sectional microstructures of the best and worst-performing sintered parts were observed. As shown in Figure 14(a) and (b), the part with better performance exhibits fewer and smaller sintering pores, with thicker and more continuous sintering necks between particles. This suggests that the sintering process was more complete, leading to a denser microstructure and better overall part quality. In contrast, the poorly performing part shows a higher number of larger pores and thinner, more dispersed sintering necks. This indicates incomplete sintering, which results in a more porous microstructure and weaker interparticle bonding, thereby compromising the mechanical properties of the part. These microscopic observations further reinforce the importance of optimizing the process parameters to achieve high-quality sintering and superior mechanical performance.

Figure 14.

SEM of sample section. (a) The part with better performance and (b) The part with bed performance.

Conclusions

This study investigates the effects of key processing parameters on the mechanical properties and forming quality of CB/pinewood/PES composites using both single-factor and multifactor experiments based on a powder-supply heating system. The detailed experimental results are summarized below.

(1) Image Shape Laser Sintering systems often lack direct powder bed preheating due to mechanical and optical constraints. To address this, an indirect strategy was employed by preheating the powder in the supply chamber and transferring it to the build surface via the recoating mechanism. This method elevated the powder bed temperature and reduced the influence of ambient conditions. Thermal imaging confirmed that the powder surface in the supply chamber reached 85°C, while the temperature on the build surface stabilized at around 60°C after spreading, indicating a thermal loss of approximately 25°C. Although the nominal melting point of the powder is 108.52°C, the presence of carbon black, with its high thermal conductivity, enabled early sintering at approximately 107.3°C.

(2) A systematic study was conducted to assess the effects of laser power, preheating temperature, layer thickness, and scanning speed on tensile strength, flexural strength, Z-direction accuracy, and density. Scanning speed had the most pronounced influence on tensile and flexural strength as well as density, due to its impact on energy absorption. In contrast, laser power most significantly affected Z-direction accuracy by influencing sintering depth. An orthogonal experiment with four factors at four levels, combined with weighted comprehensive evaluation, identified scanning speed as the dominant factor affecting part quality, while layer thickness had the least impact. The optimal parameters were 20 W laser power, 85°C preheating temperature, 0.15 mm layer thickness, and 6 mm/s scanning speed. Under these conditions, tensile strength doubled and flexural strength increased by 1.5 times compared to samples produced without preheating.

The proposed method provides a new perspective on preheating in complex powder bed fusion additive manufacturing systems and facilitates the enhancement of mechanical properties, particularly part strength.

Footnotes

Acknowledgements

The listed authors are grateful to the National Key Research and Development Program of China (2024YFD2200700) and the Doctoral Innovation Fund for First-Class Discipline of Forestry Engineering (LYGC202314), the Research and Development Center of 3D Printing Material and Technology of Northeast Forestry University.

Author contributions

All authors have read and agreed to the published version of the manuscript.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the National Key Research and Development Program of China (2024YFD2200700) and the Doctoral innovation fund for first-class discipline of forestry engineering (LYGC202314).

ORCID iDs

Haoyu Zhang

Yanling Guo

Jiaming Dai

Yanwei Cai

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