Abstract: Based on structural characteristics and molding materials of automotive instrument panel frames, a large valve-type hot runner injection mold with one cavity was designed. Rationality of gating system (13-point valve-type hot runner + U-shaped cold runner + direct gate/overlap gate) of mold was verified by Moldex 3D software, and its molding defects were predicted. Based on encapsulation phenomenon observed in simulation results, venting inserts were added to cross rib positions of plastic part, and venting grooves were added to mold parting surface to further enhance mold's venting effect. For side hole features of plastic part, a combination of hydraulic side core pulling and large spring block side core pulling was designed for core pulling. For cross ribs and BOSS pillar structures on surface A, due to their different heights, spring block side core pulling mechanisms with different angles and strokes were used respectively. To prevent spring blocks from getting stuck and damaging plastic part during mold opening, mechanical hook devices were designed to provide initial power. Ejection mechanism consists of an ejector sleeve, ejector pins, and direct ejectors, and cooling system consists of a direct-flow water channel and a baffle-type water channel. After actual mold testing, all mold structures met design requirements, the overall operation was stable, assembly accuracy of plastic part met assembly requirements, and mass production was feasible.
Automotive instrument panel frame is a key structure in automotive instrument panel assembly, which can provide basic support and positioning for multi-functional display systems, air conditioning systems and air vents, etc. Therefore, it has a complex structure, large overall size and height difference, many side holes, BOSS pillars and cross ribs, etc., which are difficult to demold. When automotive instrument panel frame is injected, plastic melt flow distance is long, melt filling and mold cavity venting are difficult, which easily leads to short shot and trapped air phenomenon. We designed a large valve hot runner injection mold with "one mold and one cavity" based on structural characteristics of automotive instrument panel frame. Injection process was simulated by Moldex 3D software to verify rationality of its gating system design and predict its molding defects. A core-pulling method combining hydraulic side core pulling and large spring block side core pulling was adopted to solve demolding difficulty caused by small core-pulling space of hole structure on both sides of plastic part. Spring block side core-pulling mechanism was used to solve demolding difficulties of cross ribs and BOSS pillars on A-side of plastic part caused by small core-pulling space, different structural features, and different demolding angles. After actual mold testing, mold ran smoothly, and plastic part met assembly requirements, providing a reference for design of other large injection molds.

Automotive instrument panel frame is shown in Figure 1a. Plastic part has external dimensions of 1752 mm * 625 mm * 521 mm, a main body wall thickness of 2.5~3 mm, a BOSS pillar and cross rib wall thickness of 1~1.5 mm, and a weight of approximately 4.5 kg, classifying it as a typical large plastic part. Side holes at positions S1, S2, T1, and T2, BOSS pillars and cross ribs at positions S3 and S4 on surface A cannot be demolded normally, requiring a side core-pulling mechanism. BOSS pillars at positions C1 and C2 on surface B require an ejector sleeve mechanism. To ensure proper fit with hot runner, a gate platform is reserved on surface A. To enhance the overall strength and tensile properties of plastic part, polypropylene (PP) + 20% long glass fiber (LGF) is selected as material. 20% LGF content significantly improves tensile strength of PP, providing more reliable support for other parts. Material manufacturer is Kingfa, material type is GFPP-L20.

 

Fig. 1 Automotive instrument panel frame
Mold opening direction is determined based on structure of ribs and holes in plastic part to minimize need for a side core-pulling mechanism and eliminate impact of parting line on appearance. Mold opening direction is shown in Figure 2, with Z-axis as mold opening direction.

 

Fig. 2 Mould opening direction

Car dashboard frame is a functional part, and there are no strict requirements for appearance. Weld lines, gate marks and shrinkage marks are allowed. Car dashboard has a large overall size and many holes and crossbeam structures. Molding material is PP+20%LGF material. Therefore, it is necessary to shorten melt flow length by multiple injection points to ensure its filling balance and subsequent pressure holding effect. After comprehensive consideration, gating system of car dashboard adopts gating method of "13-point valve hot runner + U-shaped cold runner + direct gate/overlap gate". That is, injection starts from middle of plastic part, and melt flows to both sides, controlling flow length to about 360 mm. Due to plastic part structure and to save mold costs, two injection points are used at G4 and G12 valve gates. To prevent molding defects such as premature curing and short shots at gate due to excessively small gate diameter, referring to successful experiences with similar plastic parts and to save costs, a standard hot runner system with a hot runner diameter of 22 mm and a valve gate diameter of 6 mm was selected. U-shaped cold runner is 12 mm * 8 mm, and overlapping gate is 15 mm * 1.5 mm. Gating system is shown in Figure 3a. Opening sequence of valve hot runner is G1~G3/G4~G7/G8~G13. Valve gates G4 and G12 are shown in Figure 3b.

 

Figure 3 Design of automotive dashboard skeleton casting system

To ensure rationality of gating system, mold flow analysis of automotive dashboard skeleton casting system needs to be performed using Moldex 3D software. Plastic part and gating system were imported into Moldex 3D software, then repaired, simplified, and meshed. A 3D solid mesh was used, with 1,856,773 solid meshes for plastic part and 839,690 surface meshes. Mesh was defect-free and met software analysis requirements. Finite element model is shown in Figure 4.

 

Figure 4 Finite element model of automotive dashboard skeleton
Based on work experience and recommended material parameters, molding process parameters were set as follows: melt temperature 241 ℃, mold temperature 41 ℃, holding time 16 s, and holding pressure 41 MPa. Injection molding process was simulated according to opening sequence of valve-type hot runner. Simulation results are shown in Figure 5. As shown in Figure 5a, plastic part is filled evenly and filling process is relatively balanced, without short shots or flow lag, thus proving that gating system design is reasonable. As shown in Figure 5b, fiber orientation is concentrated at 0.727, with some locations at 0.465 due to structural issues. As shown in Figure 5c, encapsulation is mainly distributed at filling end and rib positions, requiring enhanced venting at parting surface and cross ribs. As shown in Figure 5d, indentation displacement of plastic part is small overall, with a maximum value of 0.278 mm, which meets design requirements (<0.3 mm). (mm); As shown in Figure 5e, volume shrinkage rate of plastic part is concentrated at 2.137%, with some locations and filling end experiencing a volume shrinkage rate of 5.006%, and maximum volume shrinkage rate being 13.613%. Volume shrinkage rate is relatively uniform. The total warpage displacement is result of superposition of x, y, and z directions. X and y directions are natural inward shrinkage directions of plastic part material, and z direction is mold opening direction. Since plastic part needs to be assembled in multiple directions, the total warpage displacement is selected for analysis. The total warpage displacement is shown in Figure 5f. Maximum displacement is concentrated at filling end, with a maximum value of 8.087 mm. Since automotive dashboard frame is a large plastic part made of PP+20%LGF composite material, its warpage result is acceptable.

 

Fig. 5 Flow analysis results for automotive dashboard frame mould

A and B sides of automotive dashboard frame have cross rib structures with a wall thickness of 1~1.5 mm. According to mold flow analysis results, there is an encapsulation phenomenon during injection molding. As number of working times increases, cross rib position of mould is easily damaged. Therefore, cavity and core adopt a block structure. This method is convenient for later maintenance and can ensure molding quality of plastic parts. Mold cavity, as shown in Figure 6a, has overall dimensions of 2340 mm * 1080 mm * 686 mm and is made of SW718H mold steel with a hardness of 33~36 HRC. The core, as shown in Figure 6b, has overall dimensions of 2340 mm * 1080 mm * 709 mm and is made of P20 mold steel with a hardness of 32~35 HRC. Insert, as shown in Figure 6c, is made of domestic 718H mold steel with a hardness of 35~36 HRC, is fixed to cavity and core with bolts. Use of high-hardness materials ensures its wear resistance and fracture resistance. To ensure timely venting of gas within mold cavity, a primary venting groove of 0.02 mm * 6 mm and a secondary venting groove of 0.5 mm * 6 mm are provided on both cavity and parting surface.

 

Fig. 6 Automotive dashboard skeleton molding parts design

As shown in Figure 1, 3D model of automotive dashboard skeleton reveals six difficult demolding points in plastic part, necessitating design of a side core pulling mechanism to facilitate smooth demolding. Side core pulling mechanisms are categorized into three different power-driven methods: hydraulic, pneumatic, and mechanical (using guide pillars, sliders, and springs, etc.). Considering both plastic part's structure and side core pulling method, a combination of hydraulic and mechanical side core pulling is adopted. Design of automotive dashboard skeleton side core pulling mechanism is shown in Figure 7. Since plastic parts at T1 and T2 have same structure and position (both have three side holes at the top of plastic part), and neither slider-side core-pulling mechanism nor spring block-side core-pulling mechanism can achieve a good demolding effect, a hydraulic side core-pulling mechanism is used for both. Guide block is 20° and stroke is limited to 17 mm, as shown in Figure 7a. Plastic parts at SJ01 and SJ02 have same structure and position, have many side holes. Since they are located on both sides of plastic part and space in this area is small, slider, hydraulic, and lifter side core-pulling mechanisms cannot be used. Therefore, a large spring block side core-pulling mechanism is used for both. T-shaped guide block is 16° and limit block limits stroke to 144 mm, as shown in Figure 7b. SJ03 is composed of two different structures: BOSS pillars and cross ribs. SJ04 is composed of two BOSS pillars. Since they are both located at the edge of plastic part and on surface A, slider-side core-pulling mechanism cannot be used due to insufficient demolding space. Therefore, spring block core-pulling mechanisms with different strokes and angles are used respectively. A 20° T-shaped guide block is used at S3, with a stroke of approximately 109 mm; an 18° T-shaped guide block is used at SJ04, with a stroke of approximately 64 mm, as shown in Figures 7c and 7d. To prevent spring blocks from jamming and damaging plastic part during mold opening, each spring block is designed with a mechanical hook to provide initial power. Nitrogen springs are used. Side core-pulling mechanism assembly is shown in Figure 7e.

 

Fig. 7 Design of automotive dashboard skeleton side core extraction mechanism
1—Coring core; 2—Guide block; 3, 7—Limit switch; 4—Cylinder; 5—Travel switch; 6—T-guide block; 8—SJ01 pull hook; 9—Boss column pin; 10—Phillips bar inlay; 11, 14—T-guide block; 12—SJ03 pull hook; 13—Boss column pin; 15—SJ04 pull hook

After filling, holding pressure and cooling, plastic shrinks in volume, which will generate a clamping force on core. B surface of automotive dashboard skeleton has many reinforcing ribs, two BOSS columns and cross ribs. In order to demold smoothly, avoid phenomenon of ejection white and ejection bulge, combined with structure and shape of plastic part, ejection mechanism of "ejector pin + ejector sleeve + straight ejector" is selected. Mold is designed with 24 ejector pins of 10 mm, 74 of 12 mm, and 63 of 16 mm, 2 ejector sleeves with an inner diameter of 4 mm and an outer diameter of 8 mm, and 10 straight ejectors. Due to large size of mold and large number of ejector pins and ejector rods, four ejector cylinders are used to provide sufficient ejection power. Ejector pin panel is equipped with a limit block, limiting ejection stroke to 100 mm. Ejection mechanism is shown in Figure 8.

 

Fig. 8 Design of automotive dashboard frame ejector mechanism
1—Straight ejector block; 2—Straight ejector rod; 3—Ejector sleeve; 4—Ejector stop; 5—Ejector retainer plate; 6—Ejector plate; 7—Moving die plate; 8—Ejector pin; 9—Return pin; 10—Ejector cylinder; 11—Ejector plate guide post; 12—Support pillar

A well-designed cooling system can ensure uniform temperature distribution on both sides of mold cavity and core, reducing temperature difference between front and rear molds, product deformation caused by uneven cooling, thereby shortening injection molding cycle, improving production efficiency, reducing production costs. Because automotive dashboard frame is large and has a significant height difference, a straight-through water channel cannot effectively cool certain parts of mold. Therefore, a combination of straight-through and partitioned circulating water channels is used for cavity and core. As shown in Figures 9a and 9b, there are 16 sets of cavity water channels and 14 sets of core water channels. Diameter of straight-through water channel is 16 mm, and diameter of partitioned water channel is 24 mm. Inner surfaces of spring blocks at S1 and S2 have more contact with high-temperature melt. To prevent thermal expansion and deformation due to prolonged operation, two sets of straight-through water channels with a diameter of 16 mm are designed to ensure mold precision and plastic part quality. Spring block water channel is shown in Figure 9c, water channel assembly is shown in Figure 9d.

 

Fig. 9 Design of automotive instrument panel skeleton cooling system

Mold forming parts, side core pulling mechanism, ejection mechanism, cooling system are assembled to obtain the overall assembly drawing and 3D model of automotive instrument panel skeleton mold, as shown in Figures 10 and 11. Mold working principle is as follows.

 

Fig. 10 Auto dashboard skeleton 2D mold assembly drawing
1—Dynamic mold plate; 2—Pusher plate; 3—Ejector pin panel; 4—Pad; 5—Waterline adapter; 6—Dynamic mold; 7—Straight ejector rod; 8—Straight ejector block; 9—Fixed mold; 10—Hot runner fixing plate; 11—Fixed mold plate; 12—Positioning ring; 13—SJ02 bullet block; 14—Square guide pillar; 15—Supporting head; 16—Ejector cylinder; 17—Ejector pin; 18—Specialist cylinder; 19—Hydraulic side extractor; 20—Hot runner; 21—Cold runner; 22—Pulling rod; 23—Push plate guide column; 24—Push plate guide column guide bush; 25—SJO3 bullet block; 26—Reset lever

 

Fig. 11 Auto dashboard skeleton 3D mold assembly drawing
(1) Mold is connected to injection molding machine nozzle via a positioning ring. After fixing fixed and moving mold plates on injection molding machine, water circuit is connected. After plastic granules are heated and extruded into melt by injection molding machine screw, they enter mold cavity in the order of opening valve-type hot runner under high screw pressure. After filling and holding pressure, melt fills cavity; then, cooling system operates, product begins to cool and solidify.
(2) After temperature of plastic part reaches its ejection temperature, under drive of injection molding machine, moving mold plate pulls moving mold part of mold along tie rod to open mold from parting surface. First, core of hydraulic core-pulling mechanism, driven by hydraulic cylinder, begins to move along guide block, stopping when limit block reaches limit switch. Then, as moving mold moves, hooks on moving mold pull hooks on SJ01~SJ04 spring blocks. Driven by hooks and nitrogen springs, each spring block moves in preset direction of T-block, completing core-pulling action of side holes, cross ribs, and BOSS pillars. Plastic part remains on moving mold due to clamping force and continues to move with it.
(3) After mold opens, ejector cylinder drives push plate, which drives ejector pins, ejector sleeves, straight ejectors to eject plastic part and cold runner along direction of push plate guide pillar. After robot removes plastic part, one production cycle is completed. Then, under action of reset rod and push plate guide pillar, ejector cylinder drives ejector mechanism to reset; during mold closing movement, SJ01~SJ04 spring blocks reset, hydraulic core-pulling mechanism resets, mold closes, and next operation begins.

Mold components were actually processed and assembled according to design requirements. After testing by mold clamping machine, all parts of mold could operate normally. Moving mold, fixed mold, ejection state, and actual mold assembly are shown in Figure 12.

 

Fig. 12 Auto instrument panel skeleton mould
According to process parameters and valve-type hot runner opening sequence in Moldex 3D software, on-site actual mold trial verification was conducted. A Tederic injection molding machine, model D3300, with a clamping force of 33,000 kN, was used. After filling, holding pressure, and cooling, an automotive instrument panel skeleton sample was obtained, as shown in Figure 13a. Sample as a whole and at each rib had no defects such as short shots, flash, or obvious deformation. The key positions of automotive instrument panel skeleton were assembled, as shown in Figures 13b and 13c. Assembly effect between parts was good, meeting customer's assembly requirements, and the first trial molding was successful.

 

Fig. 13 Automotive dashboard skeleton samples and assembly

(1) By analyzing structural characteristics of automotive dashboard skeleton, demolding difficulties and mold opening direction were determined. Based on the overall dimensions of plastic part and molding material, a gating method of "13-point valve hot runner + U-shaped cold runner + direct gate/overlap gate" was designed. Injection molding process was simulated using Moldex 3D software to verify rationality of gating system, and subsequent mold design was tailored based on simulation results.
(2) To address encapsulation problem of automotive dashboard skeleton, a block-type cavity and core were designed to allow for smooth discharge of trapped air at cross rib positions. Primary and secondary venting grooves were added to parting surfaces of cavity and core to further enhance venting effect of mold. For side holes in areas T1 and T2 on surface A of plastic part, a hydraulic side core-pulling mechanism was designed; for areas S1 and S2 with a large distribution range and many side holes, a large spring block side core-pulling mechanism was designed; due to different structural characteristics of demolding difficulties in areas S3 and S4, spring block side core-pulling mechanisms with different angles and strokes were specifically designed; to prevent spring blocks from getting stuck and damaging plastic part during mold opening, mechanical hook devices were added to all spring blocks to provide initial power, nitrogen springs were used to provide subsequent power to ensure smooth completion of core-pulling work.
(3) Through actual mold trial verification, all structures of mold met design expectations and could operate stably; plastic part showed no short shots or damage, and assembly effect met customer requirements, allowing for mass production.