The first generation of plastic tailgates used thermosetting SMC composite materials, commonly known as fiberglass. Advantages included high rigidity, good appearance, and low shrinkage; disadvantages included high density, limited weight reduction, and difficulty in material recycling. Landfill disposal resulted in long degradation times and soil pollution. Outer panel, made of fiberglass, was brittle and lacked plasticity, making it prone to breakage upon impact.
The second generation of plastic tailgates used SMC for inner panel, molded using compression molding, while outer panel was injection molded from thermoplastic materials such as PP or TPO. Inner panel, made of a composite material with good rigidity and strength, provided mechanical support; outer panel, made of thermoplastic, offered greater design flexibility and good elasticity, allowing it to recover from minor impacts and reducing maintenance costs. The second generation had significant advantages over the first generation, but inner panel still used SMC, resulting in non-recyclability and low production efficiency.
The third-generation plastic tailgate is an all-plastic tailgate. Inner panel uses long glass fiber reinforced PP (LGF+PP), while the outer panel and spoiler use PP or TPO, as shown in Figure 1. All components are injection molded and then assembled using a coating process. The third-generation plastic tailgate not only incorporates all advantages of first and second generations but also reduces the overall weight by approximately 30%. It also features more flexible styling, enhanced practicality (intelligent/human-vehicle interaction), faster production and assembly cycles, and more stable dimensional matching, representing a future trend in intelligent vehicle research and development.

 

Figure 1 All-plastic tailgate

Because plastic tailgate needs to meet high strength and high heat resistance requirements, and uses long glass fiber reinforced PP (LGF+PP) materials, added glass fiber, due to its orientation, causes varying degrees of warping deformation in molded part. Therefore, in initial design of tailgate inner panel mold, it is necessary to first arrange a reasonable gate location for deformation analysis, as shown in Figure 2. After obtaining deformation analysis results, it is necessary to combine theoretical data and accumulated experience from previous databases to determine scaling factor for pre-deformation of molded plastic part and design its pre-deformation. After pre-deformed plastic part design is completed, pre-deformation analysis needs to be performed again, and generated deformed data is compared with original data. If there is a large deviation, above steps need to be repeated until comparison result of warp analysis data and original plastic part data is within automotive assembly tolerance. Then, mold development is carried out based on pre-deformed data.

 

Figure 2 Gate location and plastic part deformation results

All-plastic tailgate inner panel uses a combination of a new material ratio plastic frame and metal parts, as shown in Figure 3. Subsequently, based on characteristics of plastic part, an advanced intelligent plastic tailgate mold integrating multiple processes was developed.

 

Figure 3 Inner panel plastic part

Since some areas of plastic parting surface are matching surfaces in vehicle body, some positions cannot be guaranteed to be accurate in initial deformation analysis and need to be adjusted according to actual vehicle body assembly situation later. These parting line locations need to be reserved in advance during initial design phase. During later adjustments, they are achieved through cavity plate machining, core welding, or inserts, as shown in Figure 4. This reduces later mold repair costs and risk of scrapping mold.

 

Figure 4 Parting Line Method

Due to high corrosiveness and abrasiveness of fiberglass to steel, coupled with high requirements for appearance of tailgate inner panel (generally with five textures, as shown in Figure 5), hardness of ordinary steel cannot meet requirements of mass production. To solve this problem, firstly, cavity plate undergoes tempering treatment after rough machining, followed by finish machining; secondly, before mass production, cavity plate is chrome-plated or coated to achieve a cavity wall hardness of over 1000 HV.

 

Figure 5 Appearance Requirements for Plastic Parts

To meet strength requirements, plastic part requires two metal inserts, approximately 1000 mm * 400 mm * 200 mm in size. During production, metal inserts need to be embedded into mold for integrated molding. First, metal inserts must be positioned in X, Y, and Z directions of mold, and then fixed in place using magnets, as shown in Figure 6.

 

Figure 6 Insert Positioning Method
To prevent misalignment of inserts after placement in mold, which could damage mold parts during molding, an insert sensing management device is installed in mold. If insert is not fully fixed in mold, sensing device automatically alarms and suspends production until process personnel adjust it.

To meet strength requirements, tailgate inner panel incorporates numerous reinforcing ribs in its plastic part. Due to design, these ribs are quite high. During injection molding, glass fiber content at high rib locations is insufficient, failing to meet strength requirements of plastic part. Furthermore, air entrapment causes sintering problems in some reinforcing ribs that cannot be resolved. A vacuum system was added to mold design, as shown in Figure 7. Before mold production, cavity is evacuated to create a negative pressure state before injection molding, solving aforementioned problems.

 

Figure 7 Vacuum System

An IoT management and process monitoring system is applied to mold. It utilizes cavity pressure and temperature sensors for monitoring, along with multi-point sequential valve hot runner technology, to control dimensional accuracy and surface defects of molded plastic part, achieving required assembly clearance and high-quality appearance between automotive plastic parts. Four pressure sensors, one integrated pressure and temperature sensor are installed in mold cavity, as shown in Figure 8. Real-time data on cavity pressure and fluid temperature are collected, curves showing changes in cavity pressure and melt temperature during production are output. By comparing simulated and actual pressure data, data is correlated with dimensional control of molded plastic part, providing a calculation basis for adjusting process parameters and enabling continuous injection molding quality monitoring.

 

Figure 8 Pressure and Temperature Sensor

Tailgate plastic part integrates tailgate handle, switch button, electric strut, rear window glass, and sealing structure. Due to complexity of plastic part, mold structure needs to be designed with various methods, including ordinary core pulling, large-stroke hydraulic cylinder core pulling, slider, ordinary angled push, and large-stroke angled push. To ensure strength, service life, smooth movement, cooling of these structures, detailed analysis and calculations are required during mold design to meet molding requirements of plastic part. A partial mold structure is shown in Figure 9.

 

Figure 9. Partial Structure of Mold

In addition to fulfilling conventional part picking requirements, robotic arm development also needs to accommodate insert installation. There is only a 0.2 mm gap between insert positioning hole and mold positioning pin. Robotic arm requires precise positioning to install insert correctly. Two positioning pins are designed for primary positioning on both robotic arm and mold, as shown in Figure 10. Furthermore, two interlocking positioning pins are installed on mold and robotic arm at positions of positioning holes on metal insert to ensure accurate insertion of insert into mold, as shown in Figure 11.

 

Figure 10. Main Positioning of Robotic Arm

 

Figure 11. Interlocking Positioning Pins of Robotic Arm