In recent years, proportion of plastic parts used in automobile production has become larger and larger. At present, interior and exterior parts of automobiles have basically realized plasticization, which plays an important role in reducing weight, saving cost and reducing fuel consumption.
Plastic parts need to be popularized and quality of technology should be improved. Mold as an industrial matrix must be first on to improve itself. As following takes a car door panel as an example to illustrate manufacturing process of plastic injection moulding and related design points of plastic injection moulding die.
Usually, car has two front and rear doors. According to classification of door holster, it can be divided into with a holster and without holster. According to shape characteristics of door, it can be divided into with a horn net and without horn net. Among them, surface quality requirements of product with a holster are low, and quality of product without a holster is high. For example, surface is not allowed to have a weld line, ejector mart is not allowed, strength requirement is high, and there is no burr.

Door panel is used as an interior part of car, and size varies depending on model. Figure 1 shows left front door trim panel of a certain model. Maximum size is 835mm×520mm×90mm. Shape is more complicated and surface has a grain pattern. There are 21 screw columns in the product and 7 inverted buckles around circumference of product. Dimensions of all inner ribs are 0.8mm for small end and 1.1mm for big end. In order to prevent product from discoloration, speaker mesh is all made on fixed mold with an angle of 8°.

 

According to above technical requirements, material used in actual production of this product is PP-T20, shrinkage rate is 1.2%, average wall thickness is 2.5mm, and it is injection molded by 1600t injection molding machine.

According to above product structure and process requirements analysis, plastic mold design is a one cavity structure. Next, we use MOLDFLOW to simulate manufacturing process of plastic injection moulding. According to shape and manufacturing process of plastic injection moulding, mold adopts a 3-point large nozzle design and is equipped with a lapped side gate. Diameter of flow channel is 14mm, and pouring valve is used to control pouring speed. Detailed location of gate is shown in Figure 2.

 

In terms of cooling system, cooling water circuit (Fig. 3) adopts a straight-through and water tower design. Water path diameter is set to 14mm and water tower is 30mm. Water path is distributed near hot nozzle to enhance mold cooling effect and adjust mold temperature, thus greatly shortening cooling time and production cycle, improving production efficiency.

 

Through software simulation analysis, we got following results:
From final product fusion line distribution map (Fig. 4), it can be seen that junction temperature is higher and junction line is more obvious. Therefore, it is necessary to strengthen exhaust capacity and control temperature.

 

Due to unevenness in cooling and solidification process of injection molded product, product shrinkage is inconsistent (Fig. 5), and it is necessary to move, correct water passage, and control cooling.

 

In summary, following conclusions are drawn:
1). Product has a lower temperature in wavefront, but it is still within temperature allowable range and does not affect quality of product.
2). Product bonding line is not obvious, but it is necessary to pay attention to exhaust at the end.
3). Product is not uniformly solidified, causing inconsistent shrinkage, and it is recommended to cool evenly.

(1) Size of mold base is reasonable, dynamic mold is integral, and material is P20 to reduce material cost.
(2) Fully consider cooling, waterway is arranged as much as possible, and diameter is large.
(3) Ejection is safe and reasonable, and guide bar assists oblique top to eject.
(4) Parting surface is set reasonably, which is convenient for manufacturing process of plastic injection moulding and mold matching, and R angle parting surface is processed into a straight line.

(1) Basic appearance structure of plastic injection moulding die is shown in Fig. 6. Basic appearance structure of auto door molding includes: hot runner protection guide post, positioning ring, lock module, die pit, die foot, ring screw and waterway.

 

Basic requirements of plastic injection moulding die are as follows:
1) Hot runner guide post is 20mm or more higher than hot nozzle and protects during installation.
2) Size of die hole between each two plates is set to 45mm × 45mm and depth is 8mm.
3) Set die on ground side of mold.
4) A and B plates are lifted each at 4 point, and position of rings is set on both sides of mold as much as possible.
Mold positioning system (four-ply structure) is shown in Figure 7. There is a 10° taper fit around outer edge of mold frame to provide overall mold positioning and anti-rolling action. In addition, wear plates need to be added according to actual needs. Four-ply length is 85-90mm, height is 50mm, and bottom is above R5mm, and top corner C6 is finished by formwork company.

 

Mold guide column and runner control valve settings are shown in Figure 8.
1. Because template is too thick, guide posts G and P are fixed by screws, and mating length is 1.5 times or more.
2. Flow path regulating valve is used to control flow rate of pouring and mechanism for closing flow channel, so as to achieve uniform filling of product and control of sintering line. When testing mold, hex wrench is used to rotate on machine, which is simple and convenient to use.

 

1) Dynamic mold horn net is equipped with inserts to play role of deep ribs and fixed-mode horn nets. Stencils are formed by engraving, and internal water circulation cycle reduces coefficient of product deformation.
2) 10 evenly distributed pressure plates are arranged between A and B plates, and material is 40Cr and heat treated.
3) Set mold foot on the lower side of plastic injection moulding die.
4) Screw column is ejected with cylinder, and diameter of cylinder and thimble is usually set to an even size of 6mm, 8mm, 10mm, etc., which is convenient for deep hole drilling and select corresponding tool for processing.

1) Angle of inclined ram should not exceed 15°, in principle greater than 12°, and all guide rods are added to enhance auxiliary function. Diameter of guide rod and inclined rod is usually set to 16mm or more, otherwise it is easy to break.
2) Angle of inclined top is greater than angle of inclined pole 2 ° ~ 3 °. When slanting top is reset, it protects inclined top and compensates for machining angle error.
3) Slanting top material is usually NAK80 or 2738. In principle, it cannot be same material as mold core.
4) When setting guide rod, it is necessary to check oblique top ejection state, inclined top seat can not interfere with guide rod, and it is suitable to ensure distance is more than 5mm.

1) Support columns should be as close as possible to center of mold, quantity be as many as possible and larger in diameter.
2) There are 2 stroke switches, which are set diagonally.
3) Number of forced reset holes is set to 4, which is consistent with top hole of machine.

When plastic injection moulding die is opened, moving and fixed molds are divided, pouring system condensate is pulled by pulling rod, and moves with moving molds along with moving parts. After plastic injection moulding die is opened for a distance, moving mold stops moving, pushing mechanism moves forward under forced resetting force of injection molding machine, plastic parts are pushed out by inclined top, push rod and cylinder, and pushing mechanism is reset under forced resetting when auto door molding is closed.

1. Production cycle of door panel is relatively short, but procurement time of mold base is long, which brings pressure to design order. It must have rich experience to accumulate procurement work ahead of schedule.
2. Such molds have many inclined tops, mold is ejected, reset is unbalanced, product capacity and inclined top are relatively moved. Therefore, it is necessary to add two return rods, at the same time increase thimble plate screws to prevent product deformation, product side should also be improved by adding measures such as adding a block and strengthening slanting top.
3. Elimination of fusion line. After auto door molding was tested, it was found that sintering line was obvious, and it was solved by improving position of gate and strengthening exhaust.
4. Since speaker net can be discharged after mold is closed, electric discharge machining time is long, so pay attention to relationship between assembly and processing of speaker net.
5. Deviation of template inclined top hole is large, and processing technology must be improved to ensure assembly concentricity.

Nowadays, development of automotive industry has put forward higher and higher requirements in terms of technical level and product quality, which also brings great challenges to mold industry. Through continuous experience in manufacture of auto door molding, we have formed a stable working mode between design, processing, assembly and mold frame factory coordination, which is guaranteed in mold manufacturing cycle, processing of mold problems is more targeted and specialized. These have certain reference and guidance for production of similar molds in the future.