Automobile glass window guide rail is shown in Figure 1. Its dimensions are approximately 532mmx108mmx42mm, it is arc-shaped, wall thickness is 3.5mm, and material is ABS+PC. Main function of automobile glass window guide rail is to guide lifting and lowering of automobile glass. Molded plastic part cannot be deformed, otherwise automobile glass cannot be lifted and lowered normally. At the same time, plastic part is also an appearance part, and requirements for outer surface are high. If mold structure design is unreasonable, injection molded plastic part is easy to deform or its outer surface is prone to defects such as gate marks, welding marks, flash, bubbles, etc. After injection is completed, outer surface of plastic part needs to be sprayed with oil. If there are minor defects in plastic part, although it is difficult to see before oil injection, these defects will become obvious after oil injection. In addition, there are 2 buckle positions on plastic part, and there are rib positions inside buckle positions to form a double clamping position. How to make double clamping position demold smoothly is difficulty of mold design.

 

Car glass window guide rails
There are two long strips of buckles on inner surface of plastic part, located on the side of plastic part, one buckle is turned inside plastic part, and the other buckle is turned outside plastic part. A rib is set inside buckle that turns outward to form a double clamping position. Five square punch holes and three round punch holes are set on buckle that turns outward, and several ribs are set on outer surface of buckle; there is a buckle on inner surface of one end of plastic part, there is a round punch hole at the bottom of buckle, and there are three side holes on its side, as shown in Figure 2.

 

Product three-dimensional structure
Buckle position for flanging inside plastic part has a smooth surface and no holes, and an oblique push structure is used to achieve demolding; buckle position for flanging outside plastic part uses a slider mechanism to achieve demolding, but there is a rib inside, and a small slider can be installed on slider to achieve demolding, so this buckle position uses a nested slider structure to achieve demolding. For perforation hole on this buckle position, slider and moving mold core can be used to achieve perforation; buckle position at the other end of plastic part can be demolded by a slider mechanism, perforation holes at the bottom and side of this buckle position can be used to achieve perforation by slider and moving mold core.

Requirements for outer surface of automobile glass window guide rail are high, and no defects are allowed. In order to prevent gate marks from appearing on outer surface of plastic part, gate is set on inner surface of plastic part to be molded; in order to prevent shrinkage marks near gate, try to pour on rib position of plastic part to be molded. If there is no rib position near gate, use a fan-shaped gate for pouring; in order to reduce injection pressure and reduce influence of ordinary runner on flow performance of melt, a hot runner feeding system is selected, and melt enters ordinary runner from hot nozzle. An insert is designed on parting surface of mold to introduce ordinary runner into inner surface of plastic part to be molded, and a gate is opened on inner surface of plastic part to be molded. Gate is fan-shaped. After mold flow analysis of plastic part by Moldflow analysis software, it is found that use of two gates for injection meets molding requirements. Positions of two gates are shown in Figure 3.
For two-point pouring scheme shown in Figure 3, according to performance of ABS+PC, filling time of melt is set to 11s, injection temperature is set to 250~265℃, mold temperature is set to 80℃, maximum injection pressure is set to 125MPa, and holding time is set to 9s. Results of mold flow analysis are shown in Figure 4.

 

Figure 3 2 gate locations

 

From moldability of mold flow analysis results, outer surface color of molded plastic part is uniform and moldability is good, as shown in Figure 4(a); there is a weld mark in the middle of plastic part, as shown in Figure 4(b). Weld mark affects appearance. Weld mark on molded plastic part can be made lighter by increasing mold temperature and setting a push rod near weld mark. Setting push rod also helps to discharge gas in cavity and reduce obstruction of gas to melt flow; pressure distribution at the end of filling is shown in Figure 4(c). Injection pressure at gate position is the highest, which is 29.25MPa, and injection pressure at middle position is the lowest, which is about 2.9MPa. Pressure required for molding is not high, and a medium-sized injection molding machine can meet requirements; melt flow rate is shown in Figure 4(d). 0~2s is high-speed filling stage, and melt flow rate increases rapidly; 2~10s is low-speed filling stage, and melt flow rate is stable; 10~11s is pressure holding stage, which is used to compact melt, and melt flow rate gradually decreases. Mold flow analysis results are consistent with actual mold trial situation, indicating that two-point gating system shown in Figure 3 is reasonable for injection molding of plastic part.

There is a rib on inner surface of buckle position facing outwards of plastic part, which cannot be demolded by force, and a slider structure is required for demolding. Slider nesting structure is implemented in mold. Upper surface of large slider is an inclined surface. A small slider is installed on large slider. A groove is set on the core of small slider. Groove corresponds to rib inside buckle position of plastic part to be molded. When large slider is demolded, spring pushes small slider away, and small slider slides along upper surface of large slider to realize demolding at rib position of molded plastic part. Slider nesting structure is shown in Figure 5.

 

Movement process of nested sliders: After injection is completed, when fixed and movable molds are separated, plastic part remains in movable mold, and T-shaped inclined pressure plate drives large slider to perform lateral demolding movement in slide groove of movable mold. A spring is installed between large slider and small slider. When large slider moves away from molded plastic part laterally, small slider is separated from large slider under action of spring, so that a relative displacement occurs between small slider and large slider. Since upper surface of large slider is an inclined surface, small slider is separated from rib position on plastic part. When small slider is completely separated from rib position, under action of limit screw, large slider drives small slider to move away from plastic part laterally until it is completely separated from plastic part. When fixed and movable molds are closed, movement directions of large slider and small slider are opposite.

There is an inward-bent buckle on inner surface of plastic part, which cannot be demolded normally. Oblique push structure is required for demolding. According to size of molded plastic part, designed oblique push structure size is 346mm*50mm*60mm. In order to smoothly push oblique push structure out of core, two oblique push rods are required. In order to protect oblique push rod from breaking, a T-bar is added next to oblique push rod. When mold is pushed out, T-bar guides oblique push assembly to slide horizontally in slide and bear twisting force. Oblique push structure is shown in Figure 6. Oblique push structure is mainly divided into three parts: oblique push block, oblique push rod and guide slide plate. Oblique push block is processed according to local shape of plastic part; oblique push rod is a standard part, purchased according to different specifications, and directly installed on oblique push block; guide slide plate is customized according to actual situation of mold frame, fixed to push plate with screws, and positioned by push rod fixing plate. Advantages of this oblique push structure are: ① Oblique push block and oblique push rod have good interchangeability, and the two are fixed with screws, which are easy to assemble. When oblique push block or oblique push rod is damaged, only corresponding parts need to be replaced, and there is no need to replace the entire oblique push assembly; ② A T-bar is set next to oblique push rod, which can bear most of twisting force of oblique push rod and protect oblique push rod. Oblique push rod is not easy to break; ③ When disassembling and assembling oblique push assembly, screws on guide slide plate can be loosened from bottom of movable mold base plate, guide slide plate can be removed from push rod fixing plate, then oblique push assembly can be removed from core without disassembling the entire mold; ④) This oblique push assembly can be connected to cooling water. Oblique push rod is a hollow tube. Two oblique push rods are each connected to a water pipe joint, one is connected to water inlet pipe and water outlet pipe to form a separate cooling water circuit, which can control temperature of oblique push block.

 

1. Push block 2. Sleeve 3. Guide slide 4. T-bar 5. Inlet pipe 6. Outlet pipe 7. Push rod
Slant push structure

Because there is a car glass window guide rail on each side of car, shapes are exactly opposite, mold is designed as a 2-cavity structure, 2 plastic parts to be molded are placed opposite each other, 2 oblique push structures are on the inside of mold, and 2 sliders are on the outside of mold, as shown in Figure 7. Advantage of this cavity arrangement is that force on both sides of mold is uniform, and mold parts are not easy to twist.

 

Cavity arrangement
There are 2 large sliders and 2 large oblique thrust components on mold, which are of large weight. In order to reduce impact of weight of these components on opening and closing movements of mold, mold is placed sideways during injection production, and weight of slider is borne by slide, so that weight of components has the least impact on opening and closing movements. Mold has 2 small sliders. In order to prevent 2 small sliders from falling out of slide when mold is opened, one end of 2 small sliders will be facing upward during injection production. weight of 2 sliders will be reasonably utilized, and small sliders can be prevented from falling out of slide after mold is opened under action of gravity. Therefore, during injection production, mold is placed vertically, with 2 small sliders above mold, as shown in Figure 8, so that oblique thrust component and side of large slider can bear weight of oblique thrust block and large slider, and small slider will not fall from slide.

 

Mold clamping direction
Mold adopts an "I"-shaped mold frame. In order to protect various parts on mold, mold feet are installed on fixed and movable mold plates of mold frame. When mold frame is placed on the ground, mold feet bear weight of mold; during injection production, in order to prevent heat from being lost from fixed mold base plate, a heat insulation board (material of heat insulation board is resin material) is installed on upper surface of fixed mold base plate. Mold structure is shown in Figure 9. After injection is completed, movable mold and fixed mold are opened at parting surface, nested slider and slider of end face of molded plastic part (conventional demoulding action) start demoulding at the same time. When movable mold moves to limit position, injection molding machine slider pushes all push rods and oblique push assembly to do push-out movement together to push out molded plastic part. Then injection molding machine slider drives oblique push assembly and push rod to reset. When push rod, oblique push assembly, etc. are completely reset, movable mold moves toward fixed mold, large slider T-shaped oblique pressure plate drives nested slider to reset, oblique guide column and wedge position drive end face slider to reset. When nested slider and end face slider are completely molded, next injection cycle begins.

 

1. Insulation board 2. Fixed mold base plate 3. Hot runner plate 4. Fixed mold plate 5. Small slider 6. Large slider 7. Slider seat 8. Moving mold plate 9. Support column 10. Push rod fixing plate 11. Push plate 12. Pad 13. Moving mold base plate 14. Fixed mold insert 15. Moving mold insert 16. T-type inclined pressure plate 17. Push rod 18. Oblique push rod 19. Oblique push T-type rod 20. Water pipe joint