Rear bumper is one of the most important exterior components of a car. Early rear bumpers were made of metal, but modern bumpers are now made of plastic, injection-molded. Plastic bumpers not only significantly reduce costs but also offer superior cushioning properties, are more resilient and absorb impacts better than metal, can automatically rebound and repair. However, due to rear bumper's large size, complex structure, and high aesthetic requirements, mold design is also quite challenging. Following describes a large, thin-walled injection mold for a rear bumper. We hope that mold designers will understand key design considerations and advanced technologies for large-scale automotive injection molds.

Figure 1 shows a rear bumper for a car. Its shape resembles a bow or saddle. Material is PP+EPDM-T20, with a shrinkage rate of 0.95%. In blend, T20 represents 20% talc, which increases rigidity of bumper. EPDM, a terpolymer of ethylene, propylene, and a non-conjugated diene, is characterized by its superior resistance to oxidation, ozone, and corrosion, as well as improved bumper elasticity.

 

Rear bumper's structural features are as follows: ① Large size, with a flow length ratio of 200, making it a large, thin-walled plastic part; ② Part has a large parting line drop and a complex shape, with 12 undercuts on the inside. Side core-pulling structure is a key and challenging aspect of mold design; ③ High requirements are placed on exterior surface; molding defects such as flash, shrinkage marks, and weld marks are not permitted.

Mold utilizes a hot runner, with feed sequence controlled by a sequence valve, and employs external parting technology. Undercuts on each side utilize core-pulling structures with sliders, inclined lifters, and straight lifters, respectively. Mold's maximum dimensions are 2730 mm * 1360 mm * 1255 mm, making it a typical large, thin-walled injection mold. See Figures 2 and 3 for detailed structure.

 

The first step in molded part design is determining parting surface. Automobile rear bumper injection molds have two types of parting surfaces: external and internal. External parting surface injection molds are simpler in structure than internal parting surface injection molds, but weld marks are visible and affect part's appearance. Internal parting surface bumpers hide weld marks on non-visible surfaces of automobile part after assembly, without affecting appearance. However, internal parting surface molds are more complex and difficult to manufacture, resulting in higher technical risks and mold costs than external parting surface bumpers.
Although this mold utilizes external parting surface technology, it utilizes lateral slides to conceal weld marks on non-visible surface of part. This simplifies mold structure, reduces costs, and ensures part's appearance quality, representing an innovative feature of mold structure.
Molded parts primarily consist of movable mold plate B9 and fixed mold plate A12. Both movable and fixed molds utilize a monolithic structure. Compared to a spliced structure, this offers advantages such as smaller mold size, improved strength and rigidity, a more compact structure.
Molded parts can be made of either 718 or P20 steel. For Japanese bumper molds, mold is often cast using FC250 cast steel, which reduces mold costs.

Rear bumper is a large, thin-walled plastic part, making melt filling difficult. Mold flow analysis and verification are required in the early stages of mold design. Gating system is another technical challenge in mold design. This mold gating system utilizes a hot runner with seven gates, two of which are located between the two light holes in the part, and remaining five below, as shown in Figure 3.

 

Rear bumper is an exterior component. Using a conventional hot runner gating system would ensure that the entire cavity is filled with melt, but weld marks would inevitably remain on the surface. To address this challenge, mold design incorporates sequential valve hot runner gate control technology, or SVG, another innovative and advanced mold technology. Seven hot nozzles in hot runner are all needle valve-type. A hydraulic cylinder and solenoid valve control their opening and closing sequence according to melt filling requirements, achieving ideal result of no weld marks on part surface. SVG technology not only eliminates weld marks on part surface but also reduces residual stress within the part, thereby reducing mold's molding cycle.
In addition, mold utilizes an integrated hot runner system, which offers advantages such as easy assembly and disassembly, low machining precision requirements, no risk of material leakage, reliable assembly accuracy, no need for subsequent disassembly and assembly, resulting in low maintenance and repair costs. Its structure is shown in Figure 4.

 

Rear bumper of car has undercuts at points S1 to S12. Side core pulling mechanism is quite complex and forms core of mold. Figure 5 shows a three-dimensional diagram of lateral core-pulling mechanism for inverted molds S1-S10. All utilize hydraulic core-pulling, consisting of a cylinder, slide, guide block, pressure block, and travel switch. While complex, this lateral core-pulling mechanism is safe and reliable, making it a classic design for lateral core-pulling mechanisms.

 

Lateral core-pulling mechanisms for inverted molds S11 and S12 share same structure, employing a 3° lifter mechanism to achieve lateral core-pulling during ejection process. Lateral core-pulling mechanism for inverted mold S11 consists of an inclined push block 24, an inclined push rod 22, an inclined push rod guide tube 23, an inclined push rod base 25, and an inclined push rod slide 26, as shown in Figure 6. Because upper surface of inclined push block 24 is inclined, forming a 10° angle with mold bottom, guide groove of inclined push rod base 25 must also slide at same angle and direction. Otherwise, lifter could deform or even crack rear bumper part during lateral core-pulling.

 

Temperature control system of rear bumper injection mold adopts a combined cooling method of "vertical water pipe + inclined water pipe + spacer water well", that is, water pipe is used first, followed by water well. Fixed mold adopts 6 sets of water channels, and movable mold adopts 8 sets of water channels, as shown in Figure 7. Mold temperature is fully cooled, effectively ensuring molding cycle and plastic part quality of mold.
Mold belongs to a large automobile part injection mold. Temperature control system is designed in strict accordance with following principles:
(1) 3 m principle. Diameter of straight cooling pipe of a large automobile mold is generally φ15 mm, and the total length of cooling water channel cannot exceed 3 m. If it exceeds 3 meters, deep hole drill will not be able to process and drill bit length is not enough.
(2) Movable mold priority principle. Movable mold structure is more complex and heat is more concentrated, so it needs to be cooled intensively, but cooling water channel must be kept at least 8 mm away from push rod, straight top, lifter and other holes.
(3) Straight hole priority principle. If cooling water channel can be made into a straight hole, do not make an inclined hole. If inclined hole has an inclination of less than 3°, it is better to change it directly into a straight hole. If inclined hole can achieve a good cooling effect, inclined hole should be used first.
(4) Palm effect principle. When arranging water channel of a large automobile injection mold, cooling water should be ensured to flow in same direction and arranged in intervals like a palm. Distance between water channels should be controlled between 50 and 60 mm, distance between water channel and cavity surface should be 25 to 28 mm. Water channel should be arranged along shape of cavity as much as possible to improve cooling effect.
(5) Length similarity principle. Length of different cooling water channels should not differ too much to ensure that mold temperature is roughly same.

 

 

In injection mold of automobile parts, molding cycle of rear bumper mold used to be more than 120 seconds, and some were even as high as 180 seconds. At present, molding cycle of bumper mold is generally between 70 and 80 seconds. Due to advanced and reasonable design of cooling system of this mold, cycle is shortened to 60 seconds. Labor productivity of mold has been greatly improved and has been highly recognized by customers.

Guiding and positioning system design for large automotive part injection molds is crucial. Unlike guiding and positioning systems of other conventional molds, this mold utilizes four 170mm * 60mm * 460mm square guide pins and 1° precision positioning guides. Specific locations are shown in Figures 2 and 8. This method is often used in large automotive part injection molds. Square guide pins not only provide guidance but also provide high positioning accuracy, making subsequent maintenance and adjustment easier, effectively ensuring part precision and mold life.

 

Large plastic parts require high clamping force and long ejection distances, stable and safe ejection is essential. Therefore, mold ejection mechanism utilizes a combined "push block + push rod + lifter" mechanism. Because push rod has a small contact area with part (push rod diameter is φ12mm), localized part deformation is easily tolerated. Therefore, rear bumper injection mold design utilizes as many push blocks as possible. Shape of push block's ejection surface can be determined based on part's shape and size. Given uneven surface of dynamic mold core, all ejector rods and ejector tubes require a rotation stop at their fixed ends.
Due to large number of ejected parts in mold, both demolding force and reset force are significant. Therefore, demolding mechanism utilizes four nitrogen springs as its power source. Nitrogen spring structure is shown in Figure 9. Mold-specific nitrogen springs are a new type of elastic component that uses high-pressure nitrogen as its working medium. Their advantages include long stroke, smooth operation, compact size, high spring force, precision manufacturing, a long service life (up to one million cycles), a flat spring curve, and absence of preload. However, their disadvantages include high cost and inconvenient maintenance. Ejection distance of plastic part is 120mm, controlled by a stopper 19 (see Figure 2). 122mm depth of circular hole in Figure 9 represents a 2mm clearance to prevent interference.

 

Mold working process can be divided into following eight stages:
(1) Melt filling stage: Plastic melt enters hot runner plate 8 from the first-level hot nozzle 11, then enters ordinary runner through second-level hot nozzle 7, and finally enters mold cavity through side gate.
(2) Pressure holding and cooling stage: After melt fills the mold cavity, it is pressure-held, cooled, and solidified. When it solidifies to sufficient rigidity, injection molding machine pulls mold movable mold fixed plate 1 to open mold.
(3) Mold opening stage: Mold opens from parting surface I, and molded plastic part is separated from fixed mold cavity. Mold opening distance is 1200mm, which is controlled by injection molding machine.
(4) Lateral core pulling stage: After completing mold opening stroke, oil cylinders in molding undercut S1~S10 lateral core pulling mechanism (see Figure 6) are started at the same time, and mold begins to lateral core pulling. Lateral core pulling distance is 40mm, which is controlled by adjusting stroke switch.
(5) Demolding stage: After completing lateral core pulling, ejector rod of injection molding machine pushes nitrogen spring 20 on ejector fixed plate 14 through K.O. hole on movable mold fixed plate 1. Nitrogen spring pushes push rod, push block and lifter, while performing inner core pulling, and pushes rear bumper plastic part away from movable mold core.
(6) Part removal stage: Start robot to remove plastic part.
(7) Reset stage: Nitrogen spring pushes ejector fixed plate, then pushes push rod, push block and lifter to reset, and oil cylinder pushes slider in each lateral core pulling mechanism to reset.
(8) Mold closing stage: Injection molding machine pushes movable mold to close mold, and mold starts next injection molding.

(1) Movable and fixed molds greatly reduce mold's external dimensions by adopting an integral structure, while greatly improving mold's rigidity and life.
(2) Pouring system successfully eliminated weld marks and internal residual stress on exterior surface of rear bumper by adopting sequential valve hot runner gate control technology (i.e. SVG technology), greatly improving dimensional accuracy and molding quality of plastic part.
(3) Clever use of lateral sliders successfully solved problem of external parting leaving marks on the surface of plastic part. In addition, although lateral core pulling mechanism S1~S10 uses hydraulic cylinder core pulling, which increases cost of mold, it greatly simplifies mold structure, reduces mold failure rate, and ultimately reduces production cost of rear bumper.
(4) Mold temperature control system successfully reduced molding cycle of rear bumper to 60 seconds by adopting 14 sets of "vertical water pipes + inclined water pipes + spacer water wells" cooling water channels, greatly improving mold labor productivity and economic benefits of enterprise.
(5) Demolding system successfully solved problem of large thin-walled injection molds with large ejection force, long ejection distance, easy deformation of plastic parts during demolding by adopting nitrogen springs for ejection and reset.
During actual production process, mold produced stably and all quality indicators of molded plastic parts met design requirements. A complex, precise, large-scale and long-life injection mold was successfully designed.