In recent years, automobile industry has developed rapidly, but ensuing energy shortage and environmental pollution have restricted further development of automobile industry. Automobile fuel consumption and emissions are closely related to automobile quality. According to relevant research, for every 10% reduction in automobile quality, automobile fuel consumption is reduced by 6% to 8%, and corresponding exhaust emissions are reduced by about 4%. Automobile lightweight has become one of development directions of major OEMs. Compared with conventional plastic parts, thin-walled plastic parts have advantages of small size, light weight, and easy assembly, which is of great significance to lightweight of automobiles. However, when wall thickness of plastic part is thin, it will cause defects such as high molding flow resistance, inability of melt to fill cavity, high molding pressure, serious flashing of plastic part, and warping deformation. Since difficulty of thin-wall filling is much greater than that of ordinary injection, conventional processes cannot meet requirements of thin-wall molding, which puts forward higher requirements on molding process and mold design of thin-walled plastic parts.
Many scholars and experts at home and abroad have conducted research on defects and molding process in molding process of thin-wall and ultra-thin plastic parts. Cai Houdao et al. used finite element simulation to study molding defects and process optimization of thin-walled LCD shell of liquid crystal display, which improved molding quality and reduced production cost. Meng Bing et al. used finite element method to analyze warping deformation of windshield deflector of automobile, believed that cause of warping deformation was "corner effect". Ding Yongfeng et al. used orthogonal experiment method and finite element software to simulate influence of different process parameters on warpage of thin-walled plastic part of back cover of mobile phone. S AIBA and others have carried out multi-objective optimization of plastic parts molding process parameters through experiments and simulations to minimize plastic parts warpage, molding time and clamping force. K C BIRAT and others applied Taguchi method to optimize injection molding process parameters to meet production requirements of automobile plastic parts. I IKY et al. studied injection molding process of thin-walled plastic parts through numerical simulation and experiment, analyzed its residual stress and warpage deformation.
Above studies have studied injection process of thin-walled plastic parts from different aspects. However, there are few studies on molding process of ultra-thin and large-scale plastic parts with poor wall thickness uniformity, optimization of plastic parts and mold structures. Research mainly focuses on finite element simulation. Now take ultra-thin wall automotive door trim as research object, combine various defects in trial production, through method of combining finite element simulation and experiment, plastic part structure, molding process and mold structure are optimized, structure of plastic part and mold is improved. After final trial production verification, produced plastic parts have good dimensional stability and meet actual production requirements.

 

Figure 1 shows a plastic door panel of an automobile, with an overall size of 931.04mm * 682.8mm * 116.33mm and a volume of 1184.3cm3. Initial wall thickness of plastic part is 2.5mm. After structural optimization, main wall thickness of door trim panel is reduced to 1.8mm. Because local area will be impacted by explosion of airbag, transition treatment of local thickening is required. As shown in Figure 2, wall thickness of plastic map pocket area is about 3mm, uniformly transitioning to 1.8mm outwards; wall thickness of top loading area is 2.5mm, uniformly transitioning downwards to 1.8mm. Size of plastic part is large, and aspect ratio (ratio of flow path length to thickness) is approximately 500:1, which is far greater than standard of 100:1 for ordinary thin-walled plastic parts. It belongs to ultra-thin-walled plastic parts. Molding die adopts 1 cavity layout.
When wall thickness of plastic part is thin, flow resistance of molding will be greater, and melt cannot fill cavity. Conventional processes and materials cannot meet requirements of thin-walled molding, and molding of thin-walled door panels requires relatively high material melt flow rate. According to requirements of door trim structure, performance and appearance, door trim material selects modified polypropylene with high flow, high modulus, and low crystallization rate. Parameters of modified polypropylene material are shown in Table 1.

 

Due to large size of plastic part, thinner wall thickness, poor wall thickness uniformity, and high injection pressure, plastic part has defects such as flash, push rod mark, gloss difference, warpage, and shrinkage during mold trial process, which cannot meet actual production requirements, as shown in Figure 3. In view of above-mentioned defects, scientific injection method is applied to analysis and solution of defects, injection process, structure of mold and plastic part are optimized by methods such as mold trial, following 5 technologies for production of thin-walled plastic parts have been developed.

Formation of flash is mainly related to clamping force, cavity plate deformation and cavity pressure. Due to thin wall thickness and large aspect ratio of plastic part, melt filling resistance is large. When injection pressure is low, melt cannot fill cavity. If injection pressure is forcibly increased, mold plate will be deformed, which will cause a large number of flashes to be formed during molding process of plastic part, which increases difficulty of melt filling, as shown in Figure 3 (a ). In order to avoid flash defects in melt molding process, it is necessary to develop mold technology that meets requirements of thin-walled parts molding. By improving accuracy of mold parting surface and overall rigidity of mold, mold plate will not be deformed under high pressure, clamping gap is unchanged to prevent flash. In addition, by appropriately increasing number of gates and reducing injection pressure during molding process, flash can also be avoided.

 

In order to improve rigidity of mold and avoid deformation of mold plate, following two schemes have been determined: ①Distance from plastic part formed in cavity to mold surface is increased from traditional 150~180mm to 180~200mm to improve rigidity of cavity plate and core, avoid uneven force that will reduce accuracy of clamping, as shown in Figure 4; ②Increase number of support columns to increase supporting force and avoid mold clamping gap from becoming larger due to thermal deformation during production process. In the case of satisfying mold structure, supporting columns and cooling channels should be arranged in a balanced manner. In theory, the more supporting columns, the more effective rigidity of mold will be improved.

Push rod mark is a common appearance defect in plastic molding. When plastic part is pushed out after being formed, push rod thrust exceeds yield strength of plastic part, which will form a push rod mark on plastic part. Reason for this defect is that thrust is too large, contact area between push rod and plastic part is small. Because plastic part is a large-scale thin-walled part, required demolding force is relatively large, and it is easy to produce push rod mark defects during ejection process, as shown in Figure 3(b).

 

According to forming principle of putter mark, in order to avoid it, it is necessary to increase contact area of putter and plastic part, reduce pressure at contact point between head of putter and plastic part. By calculating demolding force required for door trim, switch all push rods to push blocks, ensure that size of push blocks is above 30mm*30mm. Compared with φ16mm push rod, contact area between push block and plastic part is increased by more than 4.5 times, which reduces pressure at push position and eliminates push rod print defect of plastic part, as shown in Figure 5.

 

Wall thickness of plastic parts is thin and uneven, which brings greater challenges to molding. When conventional scheme is used for molding, number of gates is limited, melt filling distance is long, and filling is partially blocked. Combined with gloss difference in trial-produced plastic parts, MoldFlow simulation analysis found that during melt injection process, temperature in the front part of area dropped to more than 24°C, and there was a significant temperature difference, as shown in Figure 6. Research results show that gloss of plastic parts is closely related to temperature, and there are differences in gloss at locations with large temperature differences in simulation results.

 

In order to reduce temperature difference between various parts of plastic part during molding process, secondary speed adjustment hot runner technology is now introduced into mold to improve gloss difference of molded plastic part. Conventional needle valve gates are hydraulically controlled, which can only be opened or closed sequentially, and sudden opening of needle valve will cause instantaneous pressure relief of material flow, leading to stagnation of material flow front, temperature reduction and forming a temperature difference line, as shown in Figure 7. Figure 8 shows pressure curve of dual-speed needle valve system. After introduction of secondary speed hot runner technology, opening speed of needle valve can be controlled and opened slowly until material flow is smooth and opened to the maximum, which can avoid sudden pressure drop and eliminate appearance defects caused by ordinary valve needles.
In addition, by increasing number of gates to shorten flow length and reduce melt temperature difference, gloss difference of plastic parts can be further improved. Change 8 point gate arrangement to 11 point sequential needle valve gate, pressure and melt filling have been improved. After optimization, temperature difference of material flow front is <10℃.

When wall thickness of plastic part is unevenly distributed, parts with different thicknesses have different cooling rates. The thicker wall has a greater degree of shrinkage, causing area to shrink inward to form sink marks. Back of door trim has a complex structure, with a large number of Boss columns, ribs and other structures, thin wall thickness of plastic part exacerbates unevenness of wall thickness.

 

Optimization of Boss column structure is shown in Figure 9. Aiming at shrinkage defect on the back of Boss column, shrink-proof structure is designed at the bottom of Boss column to increase wall thickness of plastic parts around Boss column, and at the same time, wall thickness is uniformly transitioned to effectively eliminate this defect.

 

Ribs of conventional plastic parts are usually arranged in a cross, as shown in Figure 10. In order to avoid thick wall at intersection of ribs causing sink marks, cross-arranged ribs are staggered and converted into unidirectional ribs. At the same time, a shrink-proof structure is designed at the bottom of ribs to effectively avoid sink marks. In addition, different treatment methods can be selected according to different ribs: ①Thickness of bottom of rib is designed to be 0.8mm, and demolding angle is guaranteed to be 0.5° on one side; ②For rib with lower height, top thickness is ≥0.6mm, and it is difficult to fill when the thickness is less than 0.6mm; ③When top size of deeper ribs is less than 0.6mm, thickness of bottom end should be appropriately increased, and shrink-proof feature design is required.

Installation area of door trim is a critical area, and there should be no problems such as oversize. Due to uneven distribution of wall thickness of plastic part and inconsistent shrinkage at different positions, warpage and deformation of plastic part are serious. According to requirements of plastic part, X-direction deformation of upper part of plastic part is the most critical.

 

 

X-direction deformation trend of plastic part is shown in Figure 11. Maximum X-direction deformation of upper part of initial plan is -5.4mm, minimum is -1.5mm, and unevenness reaches 3.9mm. At this time, matching gap of plastic part is uneven and size is out of tolerance. serious. In order to eliminate size tolerance caused by inconsistent deformation of various parts of plastic part, local wall thickness is added as a pressure holding channel to better receive holding pressure from gate and transfer it to thinner wall area (1.8mm Zone), to achieve a more uniform holding pressure, as shown in Figure 12.

 

Finite element analysis of optimized scheme found that maximum deformation of same area of plastic part was -3.63mm, minimum deformation was -2.03mm, and unevenness was reduced to 1.6mm, which was significantly improved, as shown in Figure 13. Deformation unevenness of plastic parts after actual trial production is less than 0.5mm, which meets requirements of loading.
(1) Overall rigidity of mold is improved by increasing distance from molded plastic part in cavity to mold surface and increasing number of mold support columns. In addition, by improving precision of mold parting surface and appropriately increasing number of gates, occurrence of flash defects in molded plastic parts is avoided.
(2) For injection molding of large thin-walled plastic parts, a suitable push-out system should be designed according to required demolding force. Push-out of ultra-thin-walled door trim shall be in the form of a push block, and size of push block must be above 30mm*30mm to avoid occurrence of push rod printing defects.
(3) Gloss of plastic parts is closely related to temperature. By introducing secondary speed control hot runner technology into mold design and increasing number of mold gates, difference in gloss of plastic parts is significantly improved.
(4) In view of shrinkage defects of Boss column, ribs and other structures, structure of plastic parts was optimized, shrinkage resistance and rib staggered layout were designed to eliminate shrinkage defects of these parts.
(5) In view of size tolerance caused by inconsistent deformation of various parts of plastic part, by adding local wall thickness as pressure holding channel, uniform pressure is realized, unevenness of deformation of plastic part is reduced to 1.6mm, which is significantly improved.