Morphological and mechanical properties of overmolded continuous/short glass fiber reinforced polypropylene composite for automotive components

Abstract

In recent years, overmolded continuous and short fiber reinforced polymer composite materials have gained much attention for use as structural automotive parts. The aim of the study is to demonstrate a practical and scalable method for manufacturing lightweight, impact-resistant automotive components using overmolded PP composites. It presents a novel integration of high-content continuous glass fiber (cGF) and short glass fiber (sGF) reinforcements within PP-based composites via FiberForm overmolding, enabling structural-grade automotive parts with tailored thicknesses (2 and 4 mm) and enhanced damage tolerance. In this study, 47% (w/w) continuous glass fiber (cGF) reinforced and 30% (w/w) short glass fiber (sGF) reinforced PP automotive prototype test parts were fabricated with 2 and 4 mm thickness values by means of an overmolding method. The mechanical and morphological properties of the cGF and sGF reinforced PP automotive prototype test parts were examined. The maximum contact force at puncture for PP based CFRT sheets with a thickness of 4 mm was measured to be 14.00 kN. As a result of this study, it was found that the mechanical properties of cGF and sGF reinforced PP automotive prototype test parts exhibited a more ductile deformational behavior and toughness compared to 47% (w/w) cGF reinforced PP composite.

Keywords

PP automotive parts composite overmolding

Introduction

In recent years, structural safety,^1,2 reduction of fuel consumption,^3,4 high-performance^5,6 and lightweight materials^7,8 have been emerging topics of interest in the automobile industry. For this reason, polymer composites are used instead of metals due to automotive industry targets^9,10 and environmental strategies such as European Green Deal,^11,12 circular economy^13,14 and carbon taxes^15,16 to reduce greenhouse gas emissions.^17,18 Thermoplastic matrix has replaced thermoset matrix in composite materials because of the recycling problem which causes environmental and economic concerns.^19,20 At the same time, thermoset composites which were widely used in composite production in the past years, require long cycle times and special storage conditions.^21,22 Thermoplastic based continuous fiber reinforced composites (CFRT) attract attention in terms of their suitability for mass production, their ability to be processed at low temperatures, the absence of paint and surface protection processes, their lightweight and high mechanical properties.^23,24

Polypropylene (PP) based CFRT sheets, which are used in many various engineering applications in the automotive industry, come to the fore especially with their advantages of high strength,^25,26 recyclability^27,28 and low cost.^29,30 Özdemir et al.³¹ examined the molding capabilities of continuous glass fiber (cGF) reinforced PP laminates. It is possible to easily process PP based CFRT sheets in short cycle times, suitable for mass production in the automotive industry. Joo et al.³² examined the experimental and finite element analysis of the bonding behavior of a cGF reinforced thermoplastic polymer composite rod and overmolded long chopped glass fiber reinforced thermoplastic polymer composite for automotive front bumper assemble component. While traditional thermoforming methods only give a shell-like form to CFRT sheets, they are insufficient in creating the detailed structures of the desired part. Alwekar et al.³³ reported the structural performance of cGF reinforced PP tape overmolded long fiber PP. The overmolding process combines the thermoforming of CFRT sheets and injection of short fiber reinforced polymer onto the surface of the CFRT insert due to their higher specific mechanical properties and short injection molding cycle time.^34,35

Jiang et al.³⁶ investigated the interfacial shear behavior of overmolded PP based hybrid composites by means of experimental and numerical techniques. Hirsch et al.³⁷ reported the numerical simulation of injection molded short and continuous fiber reinforced PP composite test structure. Fu et al.³⁸ reported the bonding mechanism of injection overmolded hybrid composites of cGF reinforced PP laminate and short glass fiber (sGF) reinforced polyamide 6,6 (PA 6,6). Anandakumar et al.³⁹ examined the low velocity impact behavior of injection overmolded sGF and cGF reinforced PP composites by means of drop-weight impact and compression after impact tests. Fu et al.⁴⁰ examined the effects of loading rate on the interfacial and failure properties of injection overmolded hybrid PP composites. The interfacial shear strength of the hybrid PP composites increased with loading rate. The deformation of the composites instigated at the interface area and continue to grow and propagate toward the top edge. Paramasivam et al.⁴¹ examined the effects of preheating on the process interface bond of the overmolded short and continuous fiber reinforced PP composites for automotive structural applications. The tensile, flexural and short beam shear strength of the preheated composites enhanced compared with non-preheated PP composites. Giusti and Lucchetta⁴² reported the effects of the injection overmolding process parameters on the welding properties of PP based hybrid composites. Budiyantoro et al.⁴³ reported the effects of overmolding processing parameters on the fiber flexural and impact strength of overmolding of long and short carbon fiber PP based hybrid composites. Tang et al.⁴⁴ examined the effect of plasma treatment on the interfacial behavior of overmolded continuous and short fiber reinforced thermoplastic polymer composites. The plasma treatment made the interface strength of the thermoplastic composites increased. Akpınar et al.⁴⁵ examined the effects of preheating time, hole diameter and the bonding strength of the overmolded the sGF and cGF reinforced PP composites.

Unlike many prior studies that focus on simplified specimens, we developed and tested a specially designed automotive prototype component with a rib-reinforced U-profile geometry, mimicking realistic load-bearing parts. The aim of this work is to study the fabricated PP based structural automotive parts as a prototype by means of an overmolding method. Because 30% sGF reinforced PP composite is widely used in the automotive industry. However, PP is a commercial polymer, its mechanical properties are often insufficient for many engineering applications. Therefore, the combination of sGFs and cGFs was aimed at improving the mechanical properties of PP parts. In this study, the PP prototype test part and their mold were specially designed. Then, the PP prototype test parts of different thicknesses (2 and 4 mm) were fabricated by reinforcing cGF and sGF. A mold suitable for both injection and thermoforming has been designed in which samples of different thicknesses (2 and 4 mm) can be formed. The use of FiberForm technology and an automated injection-overmolding line replicates near-industrial manufacturing conditions, enhancing the applicability of the results for real-world production environments. While the influence of thickness on mechanical behavior is generally recognized, this study systematically compares 2 and 4 mm overmolded structures under impact, drop weight and flexural testing. The results are correlated with scanning electron microscope (SEM) to provide a practical design for structural automotive applications.

Materials and methods

Material

Polypropylene based CFRT sheets reinforced 47% (w/w) cGF with a thickness of 2 and 4 mm were supplied. The weaving type of cGF was a twill 2/2. The twill weave structure was a type of weaving created by passing a weft thread under two warp threads. 30% (w/w) sGF reinforced PP granules were also supplied.

Design of PP test part for automotive structural application

This study combines a high content of continuous glass fiber (47% cGF) with short glass fiber-reinforced PP (30% sGF) through overmolding to fabricate structural prototype parts, a configuration not comprehensively explored in existing literature. The PP prototype test part was designed in accordance with the overmolding process. In order to ensure rigidity in the PP prototype test part, the thickness of cage structure to be added by injection around the PP based CFRT sheets was designed to be 4 mm. The U profile state samples of the designed part without the rib structure are given in Figure 1. The dimensions of the U-profile specially designed for this study are given in Figure 1(a). The prototype of the designed part in its manufactured form is shown in Figure 1(b). The U profile state samples of the designed part with the rib structure are given in to be added by injection and the final part images to improve the mechanical properties of PP based CFRT sheet are shown in Figure 2. Depending on the characteristics of the designed part, the part can be made in different thicknesses.

Figure 1.

(a) The U profile form of PP based CFRT sheet drawings without rib structure and (b) the fabricated U profile form of PP based CFRT sheet.

Figure 2.

(a) The top surface of designed PP prototype test part with the rib structure, (b) the bottom surface of designed PP prototype test part with the rib structure and (c) the fabricated PP prototype test part.

Design of mold of PP test part for automotive structural application

The mold, specially designed for this process, requires thorough study and feasibility analysis, as its design is a function that varies depending on the part geometry and the material used. The mold of the PP prototype test part was designed in accordance with the overmolding process. A modular mold suitable for forming both 2 and 4 mm thickness PP based CFRT sheet sheets was designed. The moving part of the mold is designed to give a “U” profile form to PP based CFRT sheets. The cavity where ribs will be added through the injection process is designed to be on the fixed side. The mold designed and used during production is shown in Figure 3.

Figure 3.

(a) Technical drawing of mold used for the preparation of the PP prototype test parts, (b) fabrication of the mold and (c) final form of the mold.

Fabrication of the PP composites

The fabrication of the PP prototype test part was carried out by injection overmolding method with so-called FiberForm technology. Kuka six axis robot model KR 120 R 2500 PRO, which has a specially designed robot gripper was used in the fiberform line, designed transfering the semi-molten PP based CFRT sheet to the mold. Krelus ınfrared (IR) heating system equipped with Siemens S7-1200 and KPT 400 heating control was used to heat PP based CFRT sheets for thermoforming process. The FiberForm process is shown in Figure 4, which includes the loading of the PP-based CFRT sheet loading by robot gripper, transferring the PP based CFRT sheet into the Infrared oven by the robot gripper and then heating the PP based CFRT sheet by the IR method, transferring the heated PP based CFRT sheet into the mold with the robot gripper, pre-molding the PP based CFRT sheet by means of the injection molding and overmolding sGF reinforced PP to formed PP based CFRT sheet, respectively. The thermoforming of CFRT PP sheets requires careful control of the heating temperature to prevent defects such as burning or shape deformation. During IR heating, the temperature must be maintained above glass transition temperature (Tg) but below melting temperature (Tm). Therefore, the Vicat softening temperature is used as a reference at this stage. For the PP sheets used in molding, the injection mold temperature ranges from 40°C to 80°C.

Figure 4.

(a) FiberForm technology and (b) PP prototype test parts on the production line.

The overmolding process consists of combining the cGF reinforced PP based CFRT sheets with sGFs and removing the prototype test part from the mold by the robot gripper. The designed structural automotive part was formed as a “U” profile form with 2 and 4 mm thickness of PP based CFRT sheets prototype. It is designed to be included in the cage structure through rib structures added to the PP based CFRT sheet by plastic injection process. In the injection process to be carried out on the “U” profile PP based CFRT sheet, 30% (w/w) sGF reinforced PP granules were used on the formed PP based CFRT sheets in order to have superior mechanical properties of the PP prototype test part. The injection molding process were carried out by means of a KraussMaffei brand injection molding machine. The injection molding machine has a clamping force of 600 tons.

Characterization

The Vicat softening temperature (VST) test was used to determine the softening temperature of PP composites under a constant load. The VST tests were performed on 10 mm × 10 mm samples in accordance with ISO 306. During the test, three specimens were used, and the average value was obtained. The temperature of the oil bath, initially heated to 50°C, was increased at a rate of 50°C/h. The VST was defined as the temperature at which a needle, loaded with 1 kg, penetrated 1 mm into a 1 mm² area of the test specimen.

Impact tests were performed to determine the impact strength of the PP composites under a suddenly applied load. Izod and Charpy impact tests were conducted according to ISO 180 and ISO 179 standards, respectively. During the impact tests, Zwick brand impact test device with a 25 kJ pendulum was used. Impact tests were carried out to PP based CFRT sheet samples with a dimensions of 80 mm × 10 mm × 4 mm samples without notche at room temperature. During the test, three specimens were used, and the average value was obtained.

Drop weight impact testing of PP composites was performed on the CEAST-Fractovis Plus device. The device has a mechanism (anti-rebound system) that prevents the weight from bouncing back after hitting the sample (for non-punctured samples) and hitting it again a second or more times. An impact tip with a spherical geometry with a diameter of 12.70 mm was used in the tests. The total weight dropped during impact (including impact tip, load-cell, etc.) is 5 kg. The square PP composite samples with a dimensions of 100 mm × 100 mm were used in drop weight impact tests at room temperature. During the test, three specimens were used, and the average value was obtained.

Three point bending tests of formed PP composites were performed according to ISO 178 standard. The tests were carried out using the Zwick Z010 brand universal testing device. Three point bending tests were carried out at room temperature using samples with a dimensions of 80 mm × 10 mm × 4 mm for both 4 and 2 mm thicknesses. The support span of three point bending device was set as 64 mm during the implementation of the tests. The thickness difference referred to here corresponds to the thickness of the CFRT layers used in the fabrication of the test specimens. The thickness of the samples during three point bending test was kept constant. Test results were compared only among specimens with the same thickness. During the test, three specimens were used, and the average value was obtained.

The three point bending test of the overmolded PP prototype test part was carried out on a Dartec universal testing machine. The three point bending test machine has a capacity of 600 kN. The support span of three point bending device was set as 280 mm during the implementation of the tests. The three point bending tests were carried out at room temperature with a test speed of 10 mm/min for both 4 and 2 mm thicknesses. The thickness of the samples during three point bending test was kept constant. The three point bending test set-up are given in Figure 5. It was performed to an overmolded PP prototype test part as the test sample. During the test, five specimens were used, and the average value was obtained. The results were compared only among specimens with the same thickness.

Figure 5.

(a) The three point bending test applied to PP prototype test part and (b) deformation of sample during the test of the PP prototype test part.

Microstructural analysis of PP prototype test parts were carried out using a scanning electron microscope (SEM). Before SEM analysis, all PP based composite samples were coated with an alloy (Au/Pd) of 80% gold and 20% palladium. Analyzes were carried out with a Philips SC 7620 brand SEM device under 2.5 kV voltage.

Results and discussion

Results of Vicat softening temperature tests

The Vicat softening temperature (VST) indicates the temperature at which PP based CFRT sheets begin to soften. The VST test is commonly used to evaluate the softening behavior of thermoplastic based composite materials. The Vicat softening temperatures were found to be 171.30°C for 2 mm PP based CFRT sheet, 172.30°C for 4 mm PP based CFRT sheet and 158.60°C for 30% glass fiber reinforced PP granules.

Results of impact tests

The performance and safety requirements of an automotive structural parts are characterized by the specific strength, stiffness and absorbed impact energy.^46,47 The effect of thicknesses of formed PP based CFRT sheet composite on impact behavior was examined with both Charpy and Izod impact tests. The Izod impact strength and Charpy impact strength of formed 2 mm PP based CFRT sheet were measured to be 79.40 and 98.00 kJ/m², respectively. The Izod impact strength and Charpy impact strength of formed 4 mm PP based CFRT sheet were measured to be 83.00 and 103.64 kJ/m², respectively. The impact strength of the cGF reinforced PP composite increased when the sample thickness increased from 2 to 4 mm. This shows that the mechanical properties depend on the thickness of the cGF reinforced PP composite. Similar mechanical results have been reported in the literature for PP based composites.⁴⁸ Morais et al.⁴⁹ reported the increase of the resistance to repeated impacts with the sheet thickness was also dependent on the fiber used, and on the spatial distribution of the fibers. Wang et al.⁵⁰ reported the impact properties of flax fiber reinforced polymer composites by means of Charpy and drop weight impact tests. The absorbed energy and impact resistance of the flax epoxy composite laminates were increased with an increase in thickness. And, the ductility indexes of the flax epoxy composite laminates were increased with the increase in thickness.

Results of drop weight impact tests

The effect of thicknesses of formed PP based CFRT sheet on drop weight impact behavior was examined to understand the damage resistance of PP composites. The damage resistance of a fiber reinforced polymer composite depends on various factors such as geometry of test sample, geometry of impactor, energy of impact, force of impact and velocity of impact.^51,52 The maximum amount of energy absorbed by the damage mechanisms was determined. The load and displacement curves of PP based CFRT sheets with thickness of 2 and 4 mm are given in Figure 6. The 2 mm sheet shows more localized damage and the bottom surface has more pronounced fiber pull-out. This suggests brittle behavior with less energy absorption and little plastic deformation-consistent with Figure 6(a) from before. The 4 mm sheet shows a larger damaged area with clear signs of fiber delamination and crushing, indicating higher energy absorption, delayed failure and progressive damage matching the behavior in Figure 6(b). The drop weight test images of PP based CFRT sheets with thicknesses of 2 and 4 mm are given in Figure 7. The force-displacement graph does not show a closed loop. The area under the curve is the deformation energy gradually transferred to the PP based CFRT sheets. When the load carrying capacity of the PP based CFRT sheet reaches saturation, puncture occurs. The maximum load Fmax was determined using these curves. The energy required to punture PP based CFRT sheet with a thickness of 4 mm was measured to be 93 J. And, the energy required to punture PP based CFRT sheet with a thickness of 2 mm was measured to be 33 J. As can be seen from the curves of PP based CFRT sheets with 4 and 2 mm thicknesses a similar situation also applies to the maximum contact force during puncture. The maximum contact force at puncture for PP based CFRT sheet with a thickness of 4 mm was measured to be 14.00 kN. The 2 mm thick PP-based CFRT sheet reached a maximum contact force of 6.30 kN. Lee et al.⁵³ reported the dependence of the kinetic energy for full perforation of armor-grade fiber reinforced composites on the panel thickness.

Figure 6.

The load and displacement curves of: (a) PP based CFRT sheet with thickness of 2 mm and (b) PP based CFRT sheet with thickness of 4 mm.

Figure 7.

Images of top and bottom surface of PP based CFRT sheets with thicknesses of 2 and 4 mm after drop weight test.

In the 4 mm sample, a more pronounced fiber separation and a wider distribution of damage are observed on the bottom surface compared to the 2 mm sample, indicating that the increased thickness enhances the structure’s ability to absorb impact energy more effectively. Thickness variations significantly affect stress distribution, strain localization and failure modes. Quantifying these effects is essential for accurate mechanical modeling and reliable design. Thicker specimens showed progressive damage mechanisms, while thinner sheets failed in a more brittle, localized manner. These results demonstrate the significant influence of thickness on the mechanical performance and damage resistance of PP-based CFRT composites. The current study demonstrates that thickness strongly governs the mechanical response and failure mechanisms of PP based CFRT composites (Table 1). Compared with Lee et al.,⁵³ who reported 45 J and 12.5 kN for 3 mm armor-grade CFRP, the 2 mm PP based CFRT sheet, yet the 4 mm PP based CFRT sheet not only surpassed CFRP in absorbed energy. These findings show that the thickness of PP based CFRT sheets is a critical parameter for optimizing mechanical performance and damage resistance in design.

Table 1.

Literature comparison of current mechanical performance datas.

Reference	Material type	Thickness (mm)	Impact energy (J)	Max load (kN)
Current study	PP-based CFRT	2	33	6.30
Current study	PP-based CFRT	4	93	14.00
Lee et al.⁵³	Armor-grade CFRP	3	45	12.5

Result of three point bending tests

The flexural behaviors of formed PP based CFRT sheets reinforced cGF and PP prototype test part reinforced cGF and sGF with added rib structure were examined. The results of three point bending tests are given in Figure 8 and Table 2, respectively. It was observed that a flexural load and deflection of formed PP based CFRT sheet with a thickness of 2 mm were measured to be 2.79 kN and 33.16 mm, respectively (Figure 8(a)). It was observed that a flexural load and deflection of formed PP based CFRT sheet with a thickness of 4 mm were measured to be 7.35 kN and 30.50 mm, respectively (Figure 8(c)). It was observed that a flexural load and deflection of PP prototype test part with a thickness of 2 mm were measured to be 6.20 kN and 18.78 mm, respectively (Figure 8(b)). It was observed that a flexural load and deflection of PP prototype test part with a thickness of 4 mm were measured to be 13.40 kN and 16.00 mm, respectively (Figure 8(d)). The mechanical properties of both the sheet and the prototype part improved as their thicknesses increased. This trend is consistent across both impact and drop weight tests.

Figure 8.

Force–displacement curves obtained from three-point bending tests of: (a) formed PP based CFRT sheet with thickness of 2 mm, (b) PP prototype test part with thickness of 2 mm, (c) formed PP based CFRT sheet with thickness of 4 mm and (d) PP prototype test part with thickness of 4 mm.

Table 2.

Result of three point bending tests.

Sample	Thickness (mm)	Force (kN)	Deflection (mm)
Formed PP based CFRT sheet	2	2.79 ± 0.04	33.16 ± 1.61
PP prototype test part	2	6.20 ± 0.15	18.78 ± 2.22
Formed PP based CFRT sheet	4	7.35 ± 0.25	30.50 ± 3.51
PP prototype test part	4	13.40 ± 1.86	16.00 ± 1.00

The maximum flexural force (F_max) and deflection at F_max of PP prototype test part with a 2 mm thickness were measured to be 6.20 kN and 5.44 mm, respectively. The force at break and deflection at plastic deformation of PP prototype test part with a 2 mm thickness were measured to be 2.83 kN and 12.91 mm, respectively. The maximum flexural force and deflection at F_max of PP prototype test part with a 4 mm thickness were measured to be 13.40 kN and 6.55 mm, respectively. The force at break and deflection at plastic deformation of PP prototype test part with a 4 mm thickness were measured to be 10.67 kN and 9.61 mm, respectively. After the addition of the fed structure with the injection phase, mechanical properties improved and deflection in PP prototype test parts decreased. Budiyantoro et al.⁴³ reported the maximum flexural strength value of overmolding of long and short carbon fiber PP based hybrid composite was obtained. These graphs demonstrate that the mechanical properties of the composites varies significantly depending on the type of reinforcement, fiber orientation, matrix material and manufacturing technique. In particular, the material represented in Figure 8(d) exhibits the highest performance in terms of both high strength and controlled damage progression. The flexural performance of PP based composite samples is significantly influenced by both its geometric form and thickness. PP prototype test part demonstrate elevated stiffness and maximum load-bearing capacity in comparison to formed sheets of the same thickness, signifying improved flexural strength. On the other hand, formed PP based CFRT sheet sheets are characterized by greater ductility, as indicated by larger displacements before failure. Increasing the specimen thickness from 2 to 4 mm contributes to substantial improvements in mechanical performance, particularly in terms of strength and energy dissipation. Due to experimental constraints, specimens composed solely of short glass fiber reinforced polypropylene were not fabricated in this study. The inclusion of sGF only test parts would enable a more comprehensive assessment of the mechanical reinforcement effect attributable to the overmolding process and is therefore identified as a key focus for future investigations.

Results of microstructure analysis

The fracture surface morphology of overmolded cGF and sGF reinforced PP prototype test parts were examined by means of SEM. The SEM micrographs of the PP prototype test part are given in Figures 9 and 10, respectively. Figure 9(a) shows the exposed weft and warp structure of the yarns after the fracture. Mechanical properties depend on the fiber and matrix interface in polymer composite materials. The PP matrix covered the cGF surface because of the good fiber–matrix interfacial interaction (Figure 9(b)). Jiang et al.⁵⁴ reported the interfacial bonding strength and fracture mechanism of cGF reinforced PP/sGF reinforced PA 6,6 hybrid composite was fabricated by means of overmolding method. Their study showed that the interaction between CFRT and injection molded part affected strongly the interfacial strength of overmolded cGF reinforced PP/sGF reinforced PA 6,6 hybrid composites. It was seen that there was a good interaction between glass fiber and PP in CFRT sheets. This is consistent with the three-point bending test results. The directional alignment of the fibers has been disrupted, and complex fiber breakages and fiber pull-outs are observed on the surface. Figure 9(c) confirms that the delamination mechanism, one of the most common types of damage in layered structures, is occurring. Fiorotto and Lucchetta⁵⁵ reported the mechanical performence of a prototype long glass fiber and short fiber reinforced thermoplastic structural element were fabricated by means of inmold forming. They applied three point bending tests on the thermoplastic composites to evaulate the interaction quality between the overmolded ribs and CFRT. The sGF orientation of PP prototype test parts in the injection phase affected the mechanical properties.

Figure 9.

SEM micrographs of: (a) characteristics of cGF reinforced PP composites, (b) the interface behavior between the glass fibers and the PP matrix and (c) the bending load affected the locations of the PP prototype test part.

Figure 10.

SEM micrographs of: (a) cross-section of cGF and sGF reinforced the PP prototype test part at 200× magnification and (b) the PP prototype test part at 800× magnification.

cGF and sGF as well as a region of fracture of the composites can be seen in Figure 10(a) and (b). This is further supported by the observation in Figure 10(a), where the PP matrix is seen to separate from the surrounding cGF. While the cGF maintains its structural integrity, the detachment of the PP matrix from the glass fiber indicates interfacial weakness and a localized concentration of cracks in these regions. This behavior corresponds well with the non-sudden, fluctuating load drops observed during the mechanical test. Furthermore, this failure pattern is corroborated by the fracture surface morphology shown in Figure 8, where separations between warp and weft yarns, as well as interlayer delamination, suggest that crack propagation was directionally concentrated along the fiber and matrix interface, leading to gradual damage accumulation throughout the structure (Figure 9(c)). The small load fluctuations following the peak and the eventual failure indicate the activation of progressive damage mechanism, such as micro-crack propagation and delamination. As presented in Figure 10(b), the fiber maintained its intact and smooth structure, suggesting that it was not directly affected by the damage and that the failure predominantly occurred within the matrix phase or at the fiber and matrix interface. The addition of cage and rib structures during the injection phase improved the mechanical properties of the PP prototype test parts. The improved performance in 4 mm thick samples is attributed not only to greater material volume but also to improved fiber orientation stability and enhanced interfacial bonding, as confirmed by SEM images. These features delay crack propagation and enhance energy dissipation during impact and flexural loading. Unlike thermoset composites which require longer cycle times and pose recyclability challenges, PP-based CFRT offers an optimal balance of manufacturability and performance, particularly when combined with short and continuous fibers as explored in this study. PP-based CFRT + sGF composite showed a 2.1× increase in flexural load capacity. This may be attributed to the higher rigidity of the ribbed cage structure and improved interface, as seen in SEM.

Conclusion

In this study, cGF reinforced PP based CFRT sheets were formed by the overmolding method. The FiberForm technology production conditions of integrated structures produced with PP based CFRT sheet are similar to mass production conditions. The drop weight impact behavior of PP-based CFRT sheets was evaluated for thicknesses of 2 and 4 mm. The 4 mm sheets absorbed 93 J of energy—almost three times more than the 33 J absorbed by 2 mm sheets—and exhibited a maximum puncture force of 14.00 kN, more than double the 6.30 kN measured for thinner sheets. The 4 mm PP based CFRT sheet exhibit a more balanced damage distribution on both the top and bottom surfaces by dissipating the impact energy more effectively. As a result of three point bending tests, it was determined that PP prototype test part with a 2 mm thickness could withstand a maximum load of 6.18 kN, while PP prototype test part with a 4 mm thickness resisted a maximum load of 13.38 kN. Based on the results from the various the force and deflection values of the samples subjected to the three point bending test is the changes in the fiber orientation. These results showed that the PP prototype test part are suitable for automotive structural applications to reduce cost and carbon emissions. We focused on thickness as a variable due to its relevance to automotive structural performance; future work will systematically vary fiber orientation and content to further optimize the composite architecture. The developed method holds promise for mass production of structural parts such as bumper beams, door modules or battery enclosures, offering a balance between performance, cost, and environmental sustainability.

Footnotes

ORCID iD

Mustafa Oksuz

Ethical considerations

Not applicable.

Consent to participate

Not applicable.

Consent for publication

Not applicable.

Author contributions

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research was supported by funding from the Ministry of Science, Industry and Technology of Turkish Republic the grant so-called SAN-TEZ (Project No:00940.STZ.2011-2). And, This research was supported by the Recep Tayyip Erdogan University Development Foundation (Grant number:02025005012478). The authors thanks to Farplas Automotive Inc. and Altan Yildirim.

Declaration of conflicting interests

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

Data availability statement

Not applicable.

Code availability

Not applicable.

References

Zhang

Advanced lightweight materials for automobiles: a review. Mater Des 2022; 221: 110994.

Wang

Material selection decision-making method for multi-material lightweight automotive body driven by performance. Proc IMechE, Part L: J Materials Design and Applications 2022; 236(4): 730–746.

Jasiński

Meredith

Kirwan

Sustainable development model for measuring and managing sustainability in the automotive sector. Sustain Dev 2021; 29(6): 1123–1137.

Yang

Tang

Research on optimization of injection molding process parameters of automobile plastic front-end frame. Adv Mater Sci Eng 2022; 2022: 1–18.

Ullah

Akhter

Palevicius

, et al. Natural fiber-based biocomposites for potential advanced automotive applications. J Eng Fiber Fabr 2025; 20: 15589250241311468.

Polat

Oksuz

Ekinci

, et al. The effect of injection molding processing parameters on chrome-plated acrylonitrile butadiene styrene-based automotive parts: an industrial scale. Polymers 2025; 17: 1787.

Shahzad

Cheema

. Low-carbon technologies in automotive industry and decarbonizing transport. J Power Sources 2024; 591: 233888.

Hartomacıoğlu

Oksuz

Ekinci

, et al. Optimization of produced parameters for PA6/PA6GF30 composite produced by 3D printing with Novel Knitting Method. Polymers 2025; 17: 1590.

Remigio-Reyes

Garduño

Rojas-García

, et al. Topology optimization-driven design of added rib architecture system for enhanced free vibration response of thin-wall plastic components used in the automotive industry. Int J Adv Manuf Technol 2022; 123(3–4): 1231–1247.

10.

Abedsoltan

Applications of plastics in the automotive industry: Current trends and future perspectives. Polym Eng Sci 2024; 64(3): 929–950.

11.

Haas

Sander

Decarbonizing transport in the European Union: emission performance standards and the perspectives for a European Green Deal. Sustainability 2020; 12(20): 8381.

12.

Thormann

Neuling

Kaltschmitt

Opportunities and challenges of the European Green Deal for the chemical industry: an approach measuring circularity. Clean Circ Bioecon 2023; 5: 100044.

13.

Garcia-Vazquez

Martinez-Hernandez

Trujillo-Barragan

, et al. Impact of thermomechanical reprocessing conditions on polypropylene composites made from automotive waste parts: a path to a circular economy. Polym Eng Sci 2023; 63(7): 2031–2042.

14.

El-Khodary

El Kadri

Dassouli

A comprehensive analysis of the inter-relationships of impact between automotive industry, economic growth, natural resources and environmental degradation: Morocco as an example. Environ Dev Sustain 2024; 27: 18837–18868.

15.

Zhu

Song

, et al. The choice of green manufacturing modes under carbon tax and carbon quota. J Clean Prod 2023; 384: 135336.

16.

Wang

Shang

Duan

, et al. Carbon quota allocation modeling framework in the automotive industry based on repeated game theory: a case study of ten Chinese automotive enterprises. Energy 2023; 279: 128093.

17.

Popp

Kleffel

Römisch

, et al. Fiber orientation mechanism of continuous fiber reinforced thermoplastics hybrid parts joined with metallic pins. Appl Compos Mater 2021; 28: 951–972.

18.

Ekinci

Öksüz

Ates

, et al. Thermal and mechanical properties of polypropylene/post-consumer poly (ethylene terephthalate) blends: bottle-to-bottle recycling. J Polym Res 2022; 29(10): 1–13.

19.

Wafai

Lubineau

Yudhanto

, et al. Effects of the cooling rate on the shear behavior of continuous glass fiber/impact polypropylene composites (GF-IPP). Compos Part A: Appl Sci Manuf 2016; 91: 41–52.

20.

Fang

Zhang

The combined effect of impregnated rollers configuration and glass fibers surface modification on the properties of continuous glass fibers reinforced polypropylene prepreg composites. Compos Sci Technol 2020; 197: 108259.

21.

Nikforooz

Montesano

Golzar

, et al. Assessment of the thermomechanical performance of continuous glass fiber-reinforced thermoplastic laminates. Polym Test 2018; 67: 457–467.

22.

Gibson

Torres

MEO

Browne

TNA

, et al. High temperature and fire behaviour of continuous glass fibre/polypropylene laminates. Compos Part A: Appl Sci Manuf 2010; 41(9): 1219–1231.

23.

Robert

Roy

Benmokrane

Environmental effects on glass fiber reinforced polypropylene thermoplastic composite laminate for structural applications. Polym Compos 2010; 31(4): 604–611.

24.

Lebrun

Bureau

Denault

Thermoforming-stamping of continuous glass fiber/polypropylene composites: interlaminar and tool–laminate shear properties. J Thermoplastic Compos Mater 2004; 17(2): 137–165.

25.

Paramasivam

Mallina

Ramachandran

, et al. Effect of interface temperature on low-velocity impact response of injection over-molded short/continuous fiber reinforced polypropylene composites. Polym Compos 2024; 45(2): 1165–1177.

26.

Yudhanto

Wafai

Lubineau

, et al. Revealing the effects of matrix behavior on low-velocity impact response of continuous fiber-reinforced thermoplastic laminates. Compos Struct 2019; 210: 239–249.

27.

Ekinci

Öksüz

Ates

, et al. Polypropylene/postconsumer recycled poly(ethylene terephthalate) hybrid composites: evaluation of morphological, mechanical, thermal and electrical properties. Iran Polym J 2022; 31(10): 1283–1295.

28.

Kroll

Meyer

Nendel

, et al. Highly rigid assembled composite structures with continuous fiber-reinforced thermoplastics for automotive applications. Procedia Manuf 2019; 33: 224–231.

29.

Bartkowiak

Kizak

Liebig

, et al. Fatigue behavior of hybrid continuous-discontinuous fiber-reinforced sheet molding compound composites under application-related loading conditions. Compos Part C: Open Access 2022; 8: 100265.

30.

Ning

, et al. Milling Parameter optimization of continuous-glass-fiber-reinforced-polypropylene laminate. Materials 2022; 15(7): 2703.

31.

Özdemir

Karatas

Kocak

, et al. Investigation of earing defect and thickness distribution in stamp forming of continuous glass fiber reinforced polypropylene composite laminates. J Thermoplastic Compos Mater 2023; 36(5): 2154–2168.

32.

Joo

Seock Kim

, et al. Design and manufacture of automotive composite front bumper assemble component considering interfacial bond characteristics between over-molded chopped glass fiber polypropylene and continuous glass fiber polypropylene composite. Compos Struct 2020; 236: 111849.

33.

Alwekar

Ogle

Kim

, et al. Manufacturing and characterization of continuous fiber-reinforced thermoplastic tape overmolded long fiber thermoplastic. Compos B Eng 2021; 207: 108597.

34.

Deng

Huang

chen

, et al. Injection over-molding warpage prediction of continuous fiber-reinforced thermoplastic composites considering yarn reorientation. Thin-Walled Struct 2022; 180: 109804.

35.

Matsumoto

Ishikawa

Tanaka

A novel joining method by using carbon nanotube-based thermoplastic film for injection over-molding process. J Reinforced Plast Compos 2019; 38(13): 616–627.

36.

Jiang

Zhang

, et al. Effect of thermal gradient on interfacial behavior of hybrid fiber reinforced polypropylene composites fabricated by injection overmolding technique. Polym Compos 2020; 41(10): 4064–4073.

37.

Hirsch

John

Leipold

, et al. Numerical simulation and experimental validation of hybrid injection molded short and continuous fiber-reinforced thermoplastic composites. Polymers 2021; 13(21): 3846.

38.

Zhang

Zhai

, et al. The influence of preheating temperature on the mechanical properties of injection-overmolded hybrid glass fiber reinforced thermoplastic composites. Polym Test 2022; 105: 107425.

39.

Anandakumar

Timmaraju

Velmurugan

Low-velocity impact behavior of injection over-molded short/continuous fiber reinforced polypropylene composites. Mater Today Proc 2023. http://doi.org/10.1016/j.matpr.2023.02.368

40.

Jiang

Ding

, et al. The effect of loading rate on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites. J Appl Polym Sci 2021; 138(47): 51390.

41.

Paramasivam

Timmaraju

Velmurugan

Influence of preheating on the fracture behavior of over-molded short/continuous fiber reinforced polypropylene composites. J Compos Mater 2021; 55(29): 4387–4397.

42.

Giusti

Lucchetta

Analysis of the welding strength in hybrid polypropylene composites as a function of the forming and overmolding parameters. Polym Eng Sci 2018; 58(4): 592–600.

43.

Budiyantoro

Rochardjo

HSB

Nugroho

Overmolding of hybrid long and short carbon fiber polypropylene composite: optimizing processing parameters. J Manuf Mater Process 2021; 5(4): 132.

44.

Tang

Shi

Ding

, et al. Experimental investigation and molecular dynamics simulations of plasma treatment on the interface strength of overmolded hybrid fiber reinforced polypropylene composites. Polym Compos 2022; 43(3): 1799–1808.

45.

Akpınar

Metin

Koçoğlu

, et al. Improving bonding strength of injection overmolded composites. Polym Eng Sci 2022; 62(10): 3206–3217.

46.

Yudhanto

Lubineau

Wafai

, et al. Monotonic and cyclic responses of impact polypropylene and continuous glass fiber-reinforced impact polypropylene composites at different strain rates. Polym Test 2016; 51: 93–100.

47.

Miller

Mansson

JAE

. Mechanical performance of highly loaded tow reinforced hybrid-molded composite structures. Compos Struct 2023; 322: 117311.

48.

Zhou

Lin

Dai

Studies on mechanical properties of discontinuous glass fiber/continuous glass mat/polypropylene composite. Polym Polym Compos 2002; 10(4): 299–306.

49.

de Morais

Monteiro

d’Almeida

JRM.

Effect of the laminate thickness on the composite strength to repeated low energy impacts. Compos Struct 2005; 70(2): 223–228.

50.

Wang

Chouw

Jayaraman

Effect of thickness on the impact resistance of flax fibre-reinforced polymer. J Reinforced Plast Compos 2016; 35(17): 1277–1289.

51.

ASTM International. Standard test method for measuring the damage resistance of a fiber-renforced polymer matrix composite to a drop-weight impact, 2012. https://doi.org/10.1520/d7136_d7136m-15

52.

Veerakumar

VGS

Shanmugavel

Paskaramoorthy

, et al. The influence of graphene nanoplatelets on the tensile and impact behavior of glass-fiber-reinforced polymer composites. J Mater Eng Perform 2021; 30: 596–609.

53.

Lee

Walsh

Won

, et al. Penetration failure mechanisms of armor-grade fiber composites under impact. J Compos Mater 2001; 35(18): 1605–1633.

54.

Jiang

Zhang

, et al. Molecular dynamics simulation on the interfacial behavior of over-molded hybrid fiber reinforced thermoplastic composites. Polymers 2020; 12(6): 1270.

55.

Fiorotto

Lucchetta

Experimental investigation of a new hybrid molding process to manufacture high-performance composites. Int J Mater Forming 2013; 6: 179–185.