Optimization of surface roughness and elastic modulus in screw-assisted additive manufacturing of polymer parts

Abstract

In this study, a granular extrusion system comprising of a screw and cylinder was used to fabricate 3D-printed parts with Acrylonitrile butadiene styrene (ABS) material. The aim was to analyze how extruder processing parameters affect two critical properties: surface roughness, an indicator of surface quality, and elastic modulus, indicating the mechanical performance of the parts. An optimization strategy was employed to determine the optimal settings for both objectives. The findings reveal that increasing the nozzle (output zone) temperature negatively impacts surface smoothness, whereas higher temperatures in the middle and input zones result in smoother surfaces. It was also obsrerved that elastic modulus is significantly affected by input and nozzle tempreratures; lowering the input temperature and increasing the nozzle temperature enhanced the elastic modulus. However, the effect of middle zone temperature on elastic modulus was not significant. The study identified two optimal conditions where the temperatures of the middle and nozzle zones were equal: 215°C for achieving the best surface smoothness and 230°C for maximizing the elastic modulus. In both cases, the optimal input zone temperature was determined to be 200°C.

Keywords

additive manufacturing screw assisted pellet surface roughness elastic modules optimization

Introduction

Additive manufacturing (AM) has rapidly expanded beyond prototyping into mainstream industrial applications, establishing its role in aerospace, automotive, medical, and consumer products.^1–3 The ability to produce complex geometries without conventional design restrictions has enabled new pathways for product development and lightweighting.^4,5 Among polymer-based AM techniques, fused deposition modeling (FDM) has become the most widely used, owing to its simplicity, low cost, and broad accessibility compared to other methods such as selective laser sintering (SLS).^6,7 In FDM, thermoplastic filaments—typically polylactic acid (PLA), ABS, and polycarbonate (PC)—are melted and extruded layer by layer to form components. While attractive for its ease of use, filament-based FDM suffers from limitations such as high filament production cost, restricted part sizes, and unavoidable material waste.^8–10

To address these challenges, pellet-fed extrusion systems have been developed as an alternative to filament-driven FDM. In this approach, a screw-assisted extruder processes polymer granules directly, eliminating filament manufacturing and thereby reducing costs and broadening material selection.^11,12 Valkenaers et al.¹² first introduced this concept, demonstrating that pellet-fed systems bypass the precision requirements of filament fabrication and open opportunities for a wider range of polymers. Screw-assisted extrusion further ensures stable, continuous material feeding, and reduces defects such as air entrapment, making it particularly advantageous for large-format printing.¹³ The ability to print directly from granules is also beneficial for temperature-sensitive and biocompatible polymers. For example, Kumar et al.¹⁴ investigated ethylene-vinyl acetate (EVA) granules, Leng et al.¹⁵ explored polyurethane using a conical screw system, and Whyman et al.¹⁶ designed an extrusion system specifically for biopolymer pellets. Similarly, Liu et al.¹⁷ and Zhou et al.¹⁸ demonstrated the feasibility of PCL and PVOH granules, highlighting improvements in elastic modulus and cost efficiency due to reduced thermal exposure. Together, these studies emphasize the potential of pellet-fed AM for materials that are difficult to process in filament form.

Despite these advantages, pellet-fed extrusion introduces new process complexities, particularly in thermal management. Unlike filament-based FDM, screw-assisted extruders typically consist of multiple thermal zones that sequentially melt and homogenize the polymer. Each zone’s temperature influences the stability of flow, bonding between layers, and the overall quality of the printed part. Yet, systematic investigations into the role of these thermal zones remain scarce. Existing optimization studies have largely focused on filament systems, where parameters such as layer thickness, infill density, build orientation, and nozzle temperature are key factors governing surface quality and mechanical performance.^19–25 For example, Ghaffar²⁶ showed that extruder temperature critically affects both tensile strength and roughness in PLA, while Mushtaq et al.²⁷ highlighted the sensitivity of Nylon-6 to temperature control. Surface roughness measurements have been reported using both mechanical and optical methods,^27–30 but most studies evaluate roughness perpendicular to the build direction, leaving the roughness along the layer deposition direction less explored, despite its practical significance for visible surfaces.

ABS, one of the most widely studied engineering polymers in AM, has attracted greater attention than PLA due to its superior thermal and mechanical properties, making it suitable for functional parts.²¹ Research on ABS within pellet-fed systems, however, is still limited. In particular, the combined effect of input, middle, and nozzle zone temperatures on both surface roughness and elastic modulus has not been systematically examined. Granular extrusion enables the use of larger nozzles and faster build rates,¹³ but this intensifies the importance of achieving optimal surface quality, as poor finish can negate the productivity gains. Similarly, ensuring mechanical reliability through elastic modulus optimization is essential for practical adoption.

This study addresses these gaps by systematically investigating the influence of thermal zone temperatures on ABS parts fabricated using a screw-assisted pellet extruder. Surface roughness was measured along the layer deposition direction using an optical method, and elastic modulus was determined from tensile testing. By applying a Pareto-front optimization approach, the study identifies optimal temperature configurations that balance surface quality and mechanical performance. The findings provide both a deeper scientific understanding of thermal management in pellet-fed extrusion and practical guidelines for producing high-quality, mechanically reliable polymer components.

Materials and experimental tests

In the experiments, granular ABS material was employed to fabricate components. The printing process utilized an extruder head consisting of a screw, a cylinder, and a nozzle. This extruder featured three distinct thermal zones, where the raw material was progressively heated as it moved through the zones. Eventually, the material exited in a molten state, ready for the printing process (Figure 1).

Figure 1.

Specifications of the extruder and its thermally controlled zones for material processing.

For part fabrication, a screw-assisted extruder head specifically designed and manufactured for this study³¹ was utilized. The extruder head was installed on an iDesign Vandar350 printer, which includes an enclosed chamber capable of reaching and maintaining temperatures up to 80°C. This feature is critical for producing polymer components with materials such as ABS. Figure 2(a) presents a view of the printer with the custom-built extruder head in action, fabricating a component.

Figure 2.

Granular 3D printing process (a) Enclosed 3D printer utilized in the experiments, (b) Internal structure of the extruder showing three thermal zones: input zone, middle zone, and nozzle output zone.

Figure 2(b) depicts the components of the extruder, which is designed with three independently controllable thermal zones. Each zone enables precise temperature regulation, ensuring optimal processing conditions. Additionally, the system is equipped with a planetary gearbox to supply the necessary power for the extrusion process.

Design of experiments (DOE)

The objective of this experiment is to evaluate and identify optimized parameters to fabricate components based on surface quality and mechanical strength. The assessment involves measuring the surface roughness of the printed parts and determining their elastic modulus. While several factors can influence the output, this study focuses primarily on the impact of variables related to material flow within the extruder. Specifically, the temperatures of the three thermal zones in the extruder are treated as the key variables, with all other parameters held constant, as detailed in Table 1.

Table 1.

Constant parameters used in printing parts.

Parameter	Value
Nozzle diameter	0.8 mm
Layer height	0.4 mm
Chamber temperature	70°C
Bed temperature	80°C
Flow rate	33 mm³/s
Extrusion multiplier	1

The extruder has three thermal zones, each with independently adjustable and controllable temperatures. This configuration provides three variables for investigation. According to La Gala et al.³² the range of temperature for extruder is between 210°C to 250°C so the temperature levels selected for these three zones are summarized in Table 2. To ensure optimal performance and prevent clogging at the input, the temperature in the entry zone must be maintained at a lower level.

Table 2.

Levels of granular extruder variables for fabricating test parts.

Parameter	Description	Level (°C)
$T E_{0}$	Set point temperature for nozzle (exit zone of extruder)	215-230-245
$T E_{1}$	Set point temperature for middle zone	215-230-245
$T E_{2}$	Set point temperature for entry zone	190-200-210

For the design of experiments, a full factorial approach was adopted, with the temperatures of the three extruder zones as the primary parameters. Given the relationship between the thermal zones in the extruder, the temperature of each zone increases progressively along the direction of material flow. Consequently, the constraints $T E_{0} \geq T E_{1} \geq T E_{2}$ must be observed.

To make the experiments more practical, the research team excluded certain experimental sets and proceeded with the parameters listed in Table 3. While they could have chosen a parameter range to include all sets, this would have missed some critical conditions, and the results supported this decision. For each experiment, the extruder zones in were set to specific temperatures and allowed sufficient time to reach thermal equilibrium, ensuring stable processing. Prior to fabrication, the extruder was operated independently to homogenize the material, enhancing flow uniformity and part quality. To optimize the outcomes, the research team employed a Pareto front strategy to balance Elastic modulus and surface roughness, prioritizing higher Elastic modulus and lower surface roughness for desirable results.

Table 3.

Experimental design based on specified variable constraints for different extruder zones in fabricating test parts.

Test number	$T E_{2}$	$T E_{1}$	$T E_{0}$	Test number	$T E_{2}$	$T E_{1}$	$T E_{0}$
1	190	215	215	10	200	230	230
2	190	215	230	11	200	230	245
3	190	215	245	12	200	245	245
4	190	230	230	13	210	215	215
5	190	230	245	14	210	215	230
6	190	245	245	15	210	215	245
7	200	215	215	16	210	230	230
8	200	215	230	17	210	230	245
9	200	215	245	18	210	245	245

Tests characterization

The elastic modulus was measured through tensile tests conducted in accordance with ASTM D638³³ type III, as illustrated in Figure 3(a), with a sample shown in Figure 3(b). The 3D model of the specimen was designed in SolidWorks and exported to Cura software, which sliced the model to generate the toolpath for printing with the screw-assisted FDM system. Specimens were tested using a SANTAM STM-5 machine at a crosshead speed of 5 mm/min (strain rate of $3.4 \times 10^{- 4} [\frac{mm}{mm}] [\frac{1}{s}] [\frac{1}{s}]$ ), with the Young’s modulus determined from the linear region of the stress-strain curves. To evaluate surface roughness, a 3D optical profilometer (LPM-D1 model) from Kahroba was used to measure the surface profile. For this experiment, a cuboid shell with dimensions of 40 mm × 40 mm × 20 mm was printed, as shown in Figure 3(c). The scanned surface area measured by the device was 3 mm × 5 mm, as marked in the figure. To ensure the reliability of our findings, each experimental condition was performed at least twice under identical settings. For tensile testing, one representative specimen per condition was evaluated to determine the elastic modulus, whereas surface roughness was measured at multiple locations on each printed specimen to capture local variability and reduce random error. Across repeated trials, the variations remained within an acceptable range, and the observed trends were consistent, confirming the robustness of the results.

Figure 3.

Standard specimens for measuring elastic modulus and surface roughness (a) ASTM D638 tensile type III. (b) Tensile test specimen fabricated for elastic modulus evaluation. (c) Cuboid shell specimen designed for surface roughness assessment within the defined area.

For each experimental design specified in Table 3, tensile test specimens were fabricated and tested to measure their elastic modulus. Additionally, under each condition, a cuboid shell was printed using the granular extruder, and the surface roughness of its walls, oriented along the layer deposition direction, was assessed using an optical profilometer. Surface roughness data, acquired via the profilometer, were processed in Gwyddion software to calculate roughness parameters based on the scanned area. Initially, noise was filtered to isolate the primary surface profile, followed by separation of waviness and roughness using a cut-off frequency. The roughness is expressed as the mean surface roughness ( $S_{a}$ ), as defined in equation (1).

S_{a} = \frac{1}{N} \sum_{n = 1}^{N} | z_{n} - \bar{z} |

(1)

In this equation, $z_{n}$ represents the height of the points, $\bar{z}$ denotes the mean height, and $N$ indicates the total number of points.

Results and discussion

The surface profiles and surface roughness values for each experiment, along with the elastic modulus measurements from the fabricated tensile test specimens, are summarized in Table 4.

Table 4.

Results of surface roughness and elastic modulus of parts produced using the granular 3D printer.

Test 1	Test 2	Test 3	Test 4	Test 5	Test 6

S_a = 49.34 µm	S_a = 40.65 µm	S_a = 54.26 µm	S_a = 46.80 µm	S_a = 42.51 µm	S_a = 45.66 µm
E = 1643.9 MPa	E = 1575.6 MPa	E = 1556.7 MPa	E = 1553.8 MPa	E = 1522.1 MPa	E = 1389.9 MPa
Test 7	Test 8	Test 9	Test 10	Test 11	Test 12

S_a = 37.54 µm	S_a = 44.45 µm	S_a = 58.99 µm	S_a = 43.50 µm	S_a = 45.80 µm	S_a = 47.63 µm
E = 1588.7 MPa	E = 1448.8 MPa	E = 1609.1 MPa	E = 1792.5 MPa	E = 1393.4 MPa	E = 1471.3 MPa
Test 13	Test 14	Test 15	Test 16	Test 17	Test 18

S_a = 44.68 µm	S_a = 43.94 µm	S_a = 45.50 µm	S_a = 46.23 µm	S_a = 47.46 µm	S_a = 37.79 µm
E = 1042.9 MPa	E = 1318.8 MPa	E = 1575.1 MPa	E = 1352.4 MPa	E = 1423.4 MPa	E = 1423.5 MPa

Based on the experimental results, the influence of input parameters—specifically, the temperatures of the different extruder zones—on the surface quality and mechanical strength of the printed parts was evaluated. Subsequently, the optimal conditions for 3D printing with ABS granules were identified.

Analysis of the effects of input variables on surface roughness and elastic modulus

Figure 4, along with the experimental results, demonstrates the influence of each input variable on surface roughness, highlighting the relationship between these variables and the resulting outcomes.

Figure 4.

Effect of temperature variables in different extruder zones on surface roughness (a) Nozzle output zone temperature, (b) Middle zone temperature, and (c) Raw material input zone temperature.

As depicted in Figure 4(a), increasing the nozzle temperature results in a rise in surface roughness and greater variability in the results. This variability is likely caused by significant changes in the viscosity of the extruded material and reduces it, which reduces controllability because the material becomes more flowable. However, at a nozzle temperature of 230°C, the results show improved consistency and lower surface roughness. Figure 4(b) illustrates that surface roughness decreases as the temperature of the middle zone increases. Variability in the results is observed at both extremes of the temperature range, which can also be attributed to viscosity-related issues. At higher temperatures, controllability diminishes due to excessive material fluidity, while at lower temperatures, the material does not soften sufficiently for smooth flow. The mid-range temperatures demonstrate better consistency and appear to provide optimal conditions in surface roughness. For the raw material input zone (Figure 4(c)), the slope of the graph is relatively flat, indicating a less pronounced effect on the output. However, a slight reduction in surface roughness is observed with increasing temperatures in this zone. This improvement could be attributed to the raw material being better conditioned to reach the optimal printing temperature across the extruder cylinder’s short length, thereby reducing sharp temperature fluctuations.

Figure 5 illustrates the effects of input parameters on the elastic modulus of tensile test specimens. In Figure 5(a), an increase in nozzle temperature enhances the strength of the material and produces more consistent outcomes. This improvement can be attributed to reduced temperature fluctuations in the molten material, enabling it to reach thermal equilibrium and display uniform elastic properties. Figure 5(b) depicts the influence of the middle zone temperature on the elastic modulus. The relatively constant slope of the graph suggests no significant trend, despite minor fluctuations in the results. This indicates that the middle zone temperature has minimal effect on the elastic modulus. In contrast, Figure 5(c) reveals a substantial decrease in the elastic modulus with increasing input zone temperature. This decline may be due to prolonged heat exposure during the printing process, which could compromise the mechanical integrity of the final part.

Figure 5.

Analysis of the effect of temperature variables in different extruder zones on elastic modulus (a) Nozzle output zone temperature, (b) Middle zone temperature, and (c) Raw material input zone temperature.

Optimizing printing conditions

Determining the optimal printing conditions to minimize surface roughness and maximize elastic modulus can be approached as a optimization problem. To facilitate this, all experimental results are presented in Figure 6, which visualizes the feasible optimization space. The horizontal axis represents the elastic modulus, while the vertical axis corresponds to surface roughness. In this chart, optimal conditions are located where the elastic modulus is highest, and the surface roughness is lowest. Accordingly, the lower-right region of the graph serves as the criterion for identifying these optimal conditions, highlighted by the Pareto front.

Figure 6.

Evaluation of surface roughness and elastic modulus outputs for optimization and pareto front analysis.

As shown, experiments 7 and 10 form the Pareto front, indicating that these results are non-dominated and outperform others in at least one criterion. Experiment 7 is optimal with respect to surface roughness, while experiment 10 is optimal in terms of elastic modulus. Therefore, the choice between these experimental settings depends on the specific requirements of the final component. Figure 7(a) presents a comparison of elastic modulus and elongation between experiments 7 and 10 based on tensile test results in the form of comparative column charts,³⁴ while Figure 7(b) and (c) provides a 3D surface roughness analysis corresponding to these tests.

Figure 7.

Results for the tests with optimal conditions (a) Comparison of tensile test results for experiments 7 and 10, (b) Surface roughness profile for Experiment 7, and (c) Surface roughness profile for Experiment 10.

From these results, it can be concluded that the quality of the printed ABS parts is highly sensitive to the extrusion temperature at the nozzle, highlighting a narrow range of optimal settings to achieve the best results. For both surface quality and mechanical strength, the optimal input zone temperature was found to be 200°C. However, the optimal nozzle temperature differed based on the objective: 215°C for superior surface quality and 230°C for higher mechanical strength. The findings suggest that higher nozzle temperatures lower the melt viscosity, improving bonding and enhancing strength, however at the cost of increased surface roughness in the solidified part. Another key observation is that the optimal temperature settings for the middle and nozzle zones are the same in both cases, indicating that additional heating of the melt in the nozzle zone is unnecessary.

Conclusions

The article investigated the influences of extruder barrel temperature in different zones on the quality of 3D printing of ABS granule-based parts. In particular, an investigation into melt polymer temperature variations in the nozzle (outlet), middle zone, and input was carried out as key variables in the process, examining their impact on minimizing surface roughness and improving elastic modulus.

Results showed that increasing nozzle zone temperature improved the elastic modulus however caused a higher degree of surface roughness in specimens, whereas increasing temperature in the middle zone generally did not have an observable effect on elastic modulus, however, the increase of this temperature factor decreased surface roughness in the product. In contrast, input zone temperature seemed to hold great significance concerning the elastic modulus. Much more significant reductions in elastic modulus, with respect to other parameters, were observed at higher input temperatures, which simultaneously provided improvement in surface roughness.

The experimental executions were performed within the feasible design space, while their results were analyzed with a view to determining optimum conditions. Optimization involved a problem comprising the minimization of surface roughness responsible for surface quality and maximization of elastic modulus responsible for the mechanical strength of the printed parts. two optimum scenarios were identified as non-dominated solutions, forming the Pareto front.

Results show that the temperature in the input zone must be maintained at 200°C. On the other hand, the best surface smoothness was achieved at a nozzle temperature of 215°C, and the value of elastic modulus reached its maximum under conditions where the nozzle temperature was 230°C. Furthermore, it can be noted that to obtain the best results, it is necessary to provide equal temperatures for the middle and nozzle zones, as additional heating of the polymer melt beyond the middle zone is not required.

Footnotes

Acknowledgements

The authors acknowledge the “iDesign 3D Printing Company” for cooperation in doing experiment tests and using the tools.

Handling Editor: Stefano Guarino

ORCID iDs

Mohammad Mahdi Salehi

Mohammad Reza Movahhedy

Author contributions

All authors contributed to the study conception. Theoretical analysis, Material preparation, Test design and setup, data collection and analysis, and first draft of the paper were performed by Mohammad Mahdi Salehi. Mohammad Reza Movahhedy supervised the research and contributed to analysis, and extensively edited the manuscript. All authors read and approved the final manuscript.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Mohammad Mahdi Salehi received a student grant for building experimental setup from center for development of Photonics and laser technologies and advanced materials and processes, Tehran, IRAN.

Declaration of conflicting interests

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

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Test number	$T E_{2}$	$T E_{1}$	$T E_{0}$	Test number	$T E_{2}$	$T E_{1}$	$T E_{0}$
1	190	215	215	10	200	230	230
2	190	215	230	11	200	230	245
3	190	215	245	12	200	245	245
4	190	230	230	13	210	215	215
5	190	230	245	14	210	215	230
6	190	245	245	15	210	215	245
7	200	215	215	16	210	230	230
8	200	215	230	17	210	230	245
9	200	215	245	18	210	245	245

Test number	$T E_{2}$	$T E_{1}$	$T E_{0}$	Test number	$T E_{2}$	$T E_{1}$	$T E_{0}$
1	190	215	215	10	200	230	230
2	190	215	230	11	200	230	245
3	190	215	245	12	200	245	245
4	190	230	230	13	210	215	215
5	190	230	245	14	210	215	230
6	190	245	245	15	210	215	245
7	200	215	215	16	210	230	230
8	200	215	230	17	210	230	245
9	200	215	245	18	210	245	245

Test number	$T E_{2}$	$T E_{1}$	$T E_{0}$	Test number	$T E_{2}$	$T E_{1}$	$T E_{0}$
1	190	215	215	10	200	230	230
2	190	215	230	11	200	230	245
3	190	215	245	12	200	245	245
4	190	230	230	13	210	215	215
5	190	230	245	14	210	215	230
6	190	245	245	15	210	215	245
7	200	215	215	16	210	230	230
8	200	215	230	17	210	230	245
9	200	215	245	18	210	245	245