3.1. The Effect of Molding Temperature Parameter
Figure 6 presents the microstructure of composite specimens at various molding temperatures. The findings reveal that the void content exhibited a noticeable increase at 220 °C, with a starting point for increased void content at 210 °C, as depicted in
Figure 6(d). Additionally,
Figure 7 provides magnified cross-section areas that reveal the positions of jute fiber, glass fiber, and void content. It is clear that higher molding temperatures lead to an increase in void content, while the un-impregnation of resin decreases as the molding temperature rises. The presence of void content in the specimens may result from trapped moisture within the jute fiber, which is released at higher molding temperatures [
5]. From the microstructure results, the void content and un-impregnation analysis with mechanical properties are shown in
Figure 8 (a), (b), (c) and (d). The higher temperature has affected the speed of moisture evaporation from the jute fiber.
The results regarding the correlation between un-impregnation, void content, and molding temperature can be found in
Figure 8(a), whereas the relationship between molding temperature and tensile modulus, as well as tensile strength, is depicted in
Figure 8(b). In the un-impregnation results, there was a clear tendency for un-impregnation to decrease with an increase in molding temperature. Resin exhibited a greater ease of impregnation into the jute fiber compared to the glass fiber, primarily due to the jute fiber having more filament space within each bundle than the glass fiber. Notably, there was a significant issue with resin un-impregnation at a molding temperature of 190 °C for the glass fiber, with a rate as high as 13.81%. As temperatures increased, resin impregnation into the fiber became more effective compared to lower temperatures. This indicates that the resin viscosity decreases as the molding temperature increases, making it easier for the resin to impregnate. Effective resin impregnation into the fibers is crucial for enhancing mechanical properties. The results of void contents presented in
Figure 8(a) demonstrate that void content begins to increase at 210 °C, with the highest void content at 220 °C being approximately 13.28% and the lowest void content at 200 °C being about 1.44%. The presence of void content in the composite results from moisture in the jute fiber [
5]. In this case, voids appear when the molding temperature exceeds 210 °C, possibly due to interactions between the cellulose in the jute and water molecules [
18].
The presence of void content directly affects the mechanical properties of the composites [
19]. Both tensile modulus and tensile strength exhibit a decline as void content increases. The findings reveal that tensile modulus increases as the molding temperature escalates up to 200 °C, but thereafter, it begins to decline once the temperature surpasses 210 °C. The highest tensile modulus value was 3.76 GPa at a molding temperature of 200 °C. Tensile strength followed a similar trend, with the highest value of approximately 118.13 MPa occurring at a molding temperature of 200 °C. This effect can be attributed to the decreased viscosity of the resin at higher molding temperatures, facilitating its easier impregnation into the reinforcing fibers [
6,
20,
21]. Typically, resin can easily impregnate when it exhibits low viscosity or when molding temperatures are high [
6,
22]. However, in this case, at 220 °C, the jute fiber experiences high un-impregnation, possibly due to the presence of voids within the jute bundle, which further contributes to increased un-impregnation. Moreover, the decrease in tensile strength and modulus at 210 °C is attributed to the degradation of the jute fiber, which initiates decomposition at 200 °C [
22,
23].
Figure 8(c) illustrates the relationship between un-impregnation, molding temperature, and tensile modulus, while
Figure 8(d) presents the relationship between un-impregnation, tensile strength, and molding temperature. These results demonstrate the impact of un-impregnation on tensile modulus and tensile strength. Un-impregnation of the resin in the fibers decreased with an increase in molding temperature. Lower un-impregnation resulted in higher tensile modulus and tensile strength. However, even at 220 °C, where un-impregnation was low, tensile modulus and tensile strength remained low due to other factors, including void content and the degradation of the jute fiber.
The void content of the specimens directly influenced their mechanical properties, with mechanical properties decreasing as void content increased. The void content at various molding temperatures was measured at 1.44%, 1.9%, 2.97%, and 13.28%, respectively. The relationships between (a) tensile modulus and void content and (b) tensile strength and void content are depicted in
Figure 9. It is evident that tensile strength decreases with an increase in void content, consistent with findings in the literature [
19,
23]. The presence of void content created spaces within the specimen, leading to a decrease in tensile strength and tensile modulus. These voids originated from moisture within the jute fiber.
The results, as depicted in
Figure 10, illustrate the connection between molding temperature, flexural modulus, and flexural strength, exhibiting a pattern akin to the tensile properties. A rise in the molding temperature led to an increase in both flexural modulus and strength, up to 210 °C, beyond which they showed a decline. The peak values for flexural modulus and strength, reaching 5.51 GPa and 69.27 MPa, respectively, were observed at a molding temperature of 200 °C.
Even though resin impregnation is good, the mechanical properties are not always favorable. This suggests that the mechanical properties at a molding temperature of 220 °C decreased, as they are also influenced by void content and the degradable temperature of the jute fiber. The results show that the highest void content at 220 °C of molding temperature had a significant impact on the previously low mechanical properties. Additionally, at 220 °C of molding temperature, the jute fibers began to degrade. Based on the microstructure analysis and the discussion of mechanical properties, a molding temperature of 200 °C was selected as the optimal temperature for further investigation into the influence of pulling speed.
The relationships between flexural modulus, flexural strength, void content, varied molding temperature, and pulling speed are presented in
Figure 11. The results indicate that flexural strength and flexural modulus exhibit a trend similar to that of tensile strength and tensile modulus. They both decrease with an increase in void content. Consequently, the presence of void content in the specimens contributes to a decrease in mechanical properties.
3.1. The Effect of Molding Temperature Parameter
After a study of the molding temperature parameter, the molding temperature was selected at 200 °C for the study of pulling speed parameters. The pulling speed is one parameter that is important in the pultrusion process. Normally, pulling speed parameters can affect resin impregnation [
24,
25,
26].
Figure 12 presents the analysis of the microstructure of composite specimens at various pulling speeds, ranging from 40 to 140 mm/min, through microscopic examination. The findings reveal that un-impregnation grows with an escalation in pulling speed, as seen in
Figure 12 (c). Following this, an assessment of void content and un-impregnation was carried out.
The results regarding the relationship between un-impregnation, void content, and pulling speed are presented in
Figure 12(a), while the relationship between tensile modulus, tensile strength, and pulling speed is depicted in
Figure 12(b). Similar to the un-impregnation results at different molding temperatures discussed previously, it is observed that the un-impregnation of glass fiber is higher than that of jute fiber. This difference can be attributed to the reduced space between filaments in glass fiber compared to jute fiber. As a result, the resin can impregnate the glass fiber bundle to a lesser extent than the jute fiber bundle, as shown in
Figure 12(c), in the magnified area illustrating un-impregnation between glass fiber and jute fiber. In the sections involving different pulling speeds, un-impregnation increased with an increase in pulling speeds [
14,
25,
26] because the resin can impregnate over a longer duration at lower pulling speeds. Consequently, higher pulling speeds allow for shorter impregnation times. This explains why the un-impregnation value can be used to confirm the decrease in mechanical properties at higher pulling speeds, in accordance with previous literature. The results also show an increase in void contents with higher pulling speeds. However, these variations in void content were relatively small, ranging from about 1% to 2%, with the lowest void value observed at 40 mm/min pulling speed, at approximately 1.44% of void content. These minimal differences had limited effects on the mechanical properties. The tensile modulus decreased with an increase in pulling speed because specimens remained in the mold for a longer time, allowing the resin to impregnate for an extended period. The results indicated a tensile modulus of 3.76 GPa at a pulling speed of 40 mm/min. Tensile strength in
Figure 13(b) exhibited a similar trend to the tensile modulus, with the highest tensile strength value of approximately 118.13 MPa observed at a pulling speed of 40 mm/min. These results align with existing literature [
25,
26].
The results regarding the relationship between flexural modulus, flexural strength, and pulling speed are presented in
Figure 14, illustrating a decrease in flexural strength and modulus with an increase in pulling speeds. Similar to the trends observed with tensile modulus and strength, these specimens experienced an increase in pulling speeds, resulting in less impregnation time for the resin, leading to lower flexural strength and modulus. Notably, the flexural strength and flexural modulus reached their highest values at a pulling speed of 40 mm/min, measuring 69.27 MPa and 5.51 GPa, respectively. As a result, a pulling speed of 40 mm/min yielded the highest mechanical properties in terms of tensile strength, tensile modulus, flexural strength, and flexural modulus.
For the analysis of the results, a window processing can be defined based on molding temperature and pulling speed, as shown in
Figure 15. A lower molding temperature leads to poor impregnation and high viscosity of thermoplastic, which in turn impacts the mechanical properties, given that they rely on impregnation quality. Furthermore, high viscosity of thermoplastic can cause jute fiber breakage before being pulled out from the hot die. Thus, the lowest feasible molding temperature for pultrusion is 190 °C. On the other hand, a higher molding temperature has a positive effect on impregnation and lowers viscosity. However, it’s important to note that jute fibers start to degrade at 220 °C of molding temperature. Therefore, the recommended molding temperature range for continuous composite pultrusion is 190 to 220 °C. Regarding pulling speed, lower pulling speeds result in better impregnation, but they can also lead to the degradation of jute fiber. Conversely, higher pulling speeds are associated with lower impregnation. The acceptable pulling speed range for composite pultrusion is 40 to 140 mm/min.