High injection molding welding strength and cross-sectional analysis of carbon fiber composite materials

In this study, carbon fibers, commonly used as reinforcement materials, were added to polyamide (nylon) PA6 to fabricate carbon fiber-reinforced thermoplastic composites (CFRTP), which were then subjected to injection molding welding. The aim of this research was to investigate the positive influence of carbon fibers on injection molding welding strength and explore the optimal injection conditions. Three different joining combinations and various injection parameters were examined. The tensile fracture surfaces after welding were observed using polarized microscopy and scanning electron microscopy. The reinforcing mechanism of carbon fibers was analyzed. Ultimately, the optimal conditions were determined as a mold temperature of 120°C, an injection temperature of 380°C, and an injection velocity of 150 mm/s. The addition of carbon fibers exhibited significant strengthening effects on the injection molded joints, and it also provided the potential for injection molding welding between resins with poor compatibility.


Introduction
In modern society, most developed countries are actively reducing carbon dioxide emissions to combat global warming.With the continuous improvement of economic levels and living standards, the automotive industry has encountered excellent opportunities for development.As a result, both the production and usage of automobiles have significantly increased.Vehicle exhaust emissions have become a significant source of air pollution in many countries.According to statistics, emissions from road transportation account for approximately 93% of the total emissions [1][2].Therefore, exploring ways to reduce exhaust emissions is a key measure in preventing the greenhouse effect.Due to the significant differences in performance between metals and resins, it is challenging to maintain a strong connection between them.Thermoplastic resins can be melted and effectively bonded together, offering more flexibility compared to thermosetting materials [3].Among them, PA6 is one of the most commonly used engineering polymer materials and has found extensive applications across various industries.There is a considerable amount of research on the interaction between PA6 and other materials.However, the relatively weak bonding strength remains a major obstacle to its substitution of metal materials [4][5].Therefore, studying ways to enhance the adhesive strength between reinforced resins becomes particularly important.Carbon fiber-reinforced plastic (CFRP) is a material with excellent specific strength and specific stiffness within the realm of resin materials.In recent years, significant progress has been made in the exploration of carbon fiber-reinforced thermoplastic plastics (CFRTP), which have found wide applications in various fields, particularly in the automotive industry.With its high productivity and ability to rapidly shape into various forms, CFRTP is suitable not only for high-end products but also for mass production [6].
Although there have been numerous studies on welding after incorporating carbon fiber, the strength achieved is not particularly satisfactory.Villegas [7] incorporated carbon fiber (CF) into polyetherimide (PEI) and utilized ultrasonic welding, resulting in a significant strength enhancement of 37.3 MPa.Senders et al. [8], on the other hand, introduced CF to polyphenylene sulfide (PPS) to enhance its properties.Senders et al. [8] focused on improving weldability by adding CF to a new resin called Elium.In the case of PEEK, a commonly used aerospace resin material, Tao et al. [9] also incorporated CF to enhance its ultrasonic connection strength, which reached 28 MPa.Bhudolia et al. [10] employed ultrasonic welding to connect carbon fiber-reinforced thermoplastic resin with PA6, achieving a maximum strength of 40 MPa.There is limited research specifically focused on welding methods designed for CFRTP (Carbon Fiber Reinforced Thermoplastic) using injection molding.The research aims to incorporate carbon fiber to enhance welding strength and explore optimal combinations through parameter modifications.

Materials and preparation
For this research, commercially available polyamide 6 (PA6) particles from Toray Industries, Inc. (Amilan CM1017) were used as the base resin for the carbon fiber reinforced plastic (CFRP).Mitsubishi Chemical Corporation's carbon fiber (CF) (Pyrofil chopped fiber TR06NE) was employed as the reinforcing agent for the thermoplastic resin.The basic physical properties of the TR06NE carbon fiber are presented in Table 1.To prepare the PA6/CF composite, PA6 and CF were subjected to a drying process at 80°C for 24 hours.Subsequently, the materials were mixed and kneaded using a twin-screw extruder (Technobel Co., Ltd., KZW15TW-30MG-NH(-700)-AKTP) at a temperature of 245°C.The extrusion process was conducted with a screw speed of 20 rpm and a coil feeder speed of 15 rpm.The resulting extrudate was then cut into pellets using a pelletizer, yielding pellets of the PA6/CF composite.The composite had a CF content of 30 wt%.

Welding principle and experimental apparatus
The injection connection process is shown in Figure 1: First, an injection molding machine (NP7-1F Co., Ltd., Nissei Plastics Industry Co., Ltd.) is used to shape a rectangular strip A (primary molded body) of 40mm × 10 mm × 4 mm.The molded body was taken out, burrs were removed, and after preheating at the same temperature as the mold temperature for 15 minutes, it was inserted into the mold cavity again.The mold was closed and held for 60 seconds, and injection molding was performed again to obtain a rectangular strip B (second molded body).Since the surface of the first molded body is melted by the high temperature when the second molded body enters the mold, the two parts of the resin are fused and connected.Finally, a connection molded body AB of 80 mm × 10 mm × 4 mm was obtained.
We chose three combination methods when connecting by injection molding.The first one is that the first molded body and the second molded body are pure PA6 without carbon fiber (hereinafter referred to as PA6-PA6); the second one is that the first molded body is pure PA6 without carbon fiber, and the second molded body is the reinforced thermoplastic composite material PA6/CF (hereinafter referred to as PA6-PA6/CF) with 30 wt% carbon fiber added to PA6; the third one is the reinforced thermoplastic composite with 30 wt% carbon fiber added to PA6 for both the first and second molded body (hereinafter referred to as PA6/CF-PA6/CF).Table 2 shows the test conditions of the second molded body.3, 30, 150

Welding tensile strength
From Figure 2, it can be observed that the addition of short carbon fibers has a significant enhancing effect on the welding strength.Furthermore, different injection molding conditions have a substantial impact on the welding strength.As shown in Figure 1(a), when the injection molding temperature is 245°C, the welding performance is very poor at a low injection speed (3 mm/s), resulting in low strength.However, when the speed is increased to 30 mm/s, a significant improvement in strength is observed.Further increasing the speed to high levels shows a decreasing trend in welding strength for PA6-PA6 and PA6/CF-PA6, although the change is not significant.On the other hand, when both sides are composed of PA6/CF, a notable increase in strength is achieved at a higher speed.Subsequently, when the injection molding temperature is increased to 380°C, a different scenario is observed.Firstly, for PA6-PA6 and PA6/CF-PA6, welding can be satisfactorily accomplished even at low speeds, exhibiting a noticeable improvement compared to lower temperatures.Moreover, a significant improvement is observed when the speed is increased to 30 mm/s.In the case of PA6/CF-PA6/CF, the performance is poorest at 380°C and low speeds.The results at lower temperatures do not show substantial improvement.However, increasing the injection speed leads to a considerable enhancement in strength.Additionally, the difference in strength between medium and high speeds is reduced.

Cross-section observation
To analyze the reinforcement mechanism of carbon fibers in the welding process, optical microscopy was used to observe the fracture surfaces of PA6/CF-PA under different conditions (Figure 3).As shown in the figure, a certain amount of carbon fibers can be observed on the fracture surface of PA6.At 245°C and a low speed, almost no carbon fibers can be found on the surface.This is because the slow speed causes the molten resin to cool down before it can smoothly melt the surface.With the increase in injection speed, the molten resin completes the welding process before cooling down, resulting in an increased number of residual carbon fibers on the surface.This also proves that the PA6 surface has been successfully melted by PA6/CF, with a certain amount of carbon fibers entering the PA6 matrix.Due to the embedding of carbon fibers and the adhesion between the fibers and the resin, the interfacial bonding area is increased, which is the reason for the improvement in strength.When the temperature rises to 380°C, a large number of carbon fibers remain on the fracture surface.At low speed, half of the area retains carbon fibers, because even though the speed is slow, there is a large cooling space that can melt the surface smoothly.After increasing the speed, the entire surface has residual carbon fibers, resulting in a significant increase in welding strength.For the combination of PA6/CF-PA6/CF, SEM observation was chosen as a means of analysis due to the presence of a large amount of carbon fiber in the cross-sections.Two typical conditions, 380℃ with a velocity of 3 mm.s - and 150 mm.s -1 , were selected for analysis.As shown in Figure 4, although the injection temperature was high, the low-speed condition exhibited a smooth and even cross-section of the first molded body, with only a small amount of residual carbon fiber matrix, indicating that it was not successfully melted.This is because the addition of carbon fiber to PA6 increases its surface hardness and makes it more difficult to be melted by the molten resin.Due to the excessively low velocity, the temperature dropped and the pressure was too low, resulting in low welding strength.Consequently, its strength is even worse than that of the first molded body without carbon fiber addition.However, when the velocity was increased to 150 mm.s -1 , the surface was successfully melted, and the cross-section showed a large amount of fibers encapsulated in the resin.The strong adhesive force between the fiber and resin significantly enhanced the welding strength.

Conclusion
Adding 30 wt% of short carbon fibers (CF) to PA6 resin can effectively improve the tensile strength after injection molding welding.The injection temperature and velocity have a significant impact on the strength, with the velocity not being too low and the temperature being higher being better.The tensile strength of the PA6 base material is 67 MPa, while the optimal welding tensile strength without carbon fiber is 35 MPa, achieving 52% of the base material strength.After adding carbon fibers, the optimal welding tensile strength is 59.3 MPa, reaching 88.5% of the base material strength.Based on this research, it can be observed that carbon fiber has a positive effect on enhancing welding strength.This approach is also applicable to other resin materials.The future research focus will be on surface modification of carbon fibers to enhance welding strength.Particularly, it offers possibilities for connecting dissimilar materials with poor compatibility.It has excellent prospects for practical applications.

Figure 1 .
Figure 1.Schematic diagram of Injection molding welding process.

Figure 3 .
Figure 3. Microscopic observation of cross-section under different conditions.

Figure 4 .
Figure 4. SEM images of the tensile section under different conditions (PA6/CF-PA6/CF: The images are the cross-section of the first molded body).

Table 1 .
Physical Properties of CF.