Figure 1 shows front bumper of a certain car model. Material is PP-T20 and surface is required to be painted in body color. According to styling characteristics of this car model, front bumper has a front grille that directly overlaps with front hood. When designing front bumper mold, parting line in hood area will be located on R angle of maximum outer contour of plastic part flange, causing parting line to leak out.

 

Figure 1 Plastic part parting line design
External parting line design of front bumper plastic part mainly has following problems.
(1) Parting line in hood area is located on R angle of plastic part appearance. Parting line can be seen after plastic part is installed on vehicle, affecting appearance quality of plastic part.
(2) Before painting front bumper, exposed parting line needs to be polished, which increases manufacturing cost.
(3) Due to mold manufacturing precision and wear during injection production process, parting line position will produce flash and step difference, which reduces qualified rate of plastic parts.
In order to solve above problems, combined with shape characteristics of bumper, it is considered to design parting line of hood area on inner side of plastic part fillet, and design it as an inner parting line, as shown in Figure 2. After plastic part is assembled, parting line can be hidden, which not only solves problem of parting line leakage, but also improves surface perception quality and qualified rate of bumper mold production.

 

Figure 2 Design of inner parting line of plastic part
Design comparison of inner and outer parting lines at flange position of bumper hood area is shown in Figure 3. Inner parting line design scheme makes fillet here form an undercut in main demolding direction. If plastic part is demolded directly, fillet will be broken. Therefore, it is necessary to design a new inner parting deformation mechanism in hood area to realize molding of parting line in this area.

 

Figure 3 Comparison of internal and external parting line design

In order to prevent rounded undercut at flange C in the front bumper hood area from being damaged during mold opening, plastic part needs to be elastically deformed before mold opening. Internal parting mechanism is shown in Figure 4. First, oblique push block makes room for plastic part to deform. Then, pull block pushes plastic part to deform inward under joint action of push rod, T-shaped guide slider, pull rod and deformation guide rail, and pulls undercut at the end C of plastic part inward to achieve normal demolding of plastic part.

 

Figure 4 Parting mechanism in hood area of front bumper
Working principle of internal parting mechanism is as follows.
(1) When mold is opened, under joint action of nitrogen spring and synchronous pull hook, relative position of push plate and fixed mold cavity plate remains unchanged, plastic part is still wrapped in cavity, and oblique push block retreats synchronously to make room for deformation of plastic part.
(2) Taking advantage of deformability of plastic part, when mold is opened and reaches predetermined stroke, plastic part is pushed inward by push rod, T-shaped guide slider, pull rod and pull deformation guide rail to drive pull block to make plastic part inverted and detach from fixed mold cavity.
(3) Mold is opened and plastic part is separated from fixed mold cavity plate.
(4) Mold continues to push out, and then push rod, T-shaped guide slider, pull rod and pull deformation guide rail are used to control pull block to move outward of plastic part to restore plastic part to its original shape.

Internal parting mechanism of front bumper hood area is shown in Figure 5, which mainly includes an inclined push rod, a push block, a pull deformation guide rail and a pull rod. Pull deformation structure is shown in Figure 6, which mainly includes a push rod, a guide sleeve, a T-shaped guide slider, a pull block, a pull rod and a pull deformation guide rail. T-shaped guide slider is made of beryllium bronze and fixed on push rod; pull rod is designed to be L-shaped, bearing rotating block is fixed on pull rod, one end of pull rod is fixed on straight top pull block, and the other end is connected to pull deformation guide rail through bearing rotating block; pull block realizes pull deformation and reset of flange of bumper hood area under joint guidance of T-shaped guide block and pull deformation guide rail.

 

Figure 5 Parting mechanism in the front bumper hood area

 

Figure 6 Pull deformation structure
Installation matching relationship between pull block and oblique push block is shown in Figure 7. Oblique push block and pull block are separated by core. Push rod and oblique push block adopt independent positioning design, in which pull block presses oblique push block to reduce risk of oblique push marks during injection process.

 

Figure 7 Matching relationship between pull block and inclined push block

Internal parting auxiliary mechanism includes 4 synchronous pull hook mechanisms, 6 nitrogen springs and 4 hydraulic cylinders, as shown in Figure 8. In the early stage of mold opening, synchronous pull hook mechanism and nitrogen spring auxiliary mechanism work together to make plastic part wrap mold core and move with core to assist pull deformation mechanism to perform pull deformation action on plastic part.

 

Figure 8 Internal parting auxiliary mechanism

Front bumper mold has two levels of push-out, among which internal parting mechanism of hood area is designed in the first level of push-out, and first level of push-out distance is 160 mm. First level of push-out is divided into two stages. First stage is synchronous push-out and pull deformation stage, with a total push of 50 mm. Push plate and cavity plate are driven to move synchronously by nitrogen spring and synchronous pull hook. At this time, hydraulic cylinder does not move, push plate and hydraulic cylinder are separated by 50 mm, as shown in Figure 9. Second stage push distance is 110 mm, and push plate is driven by hydraulic cylinder to move. During this process, pull deformation mechanism is reset.

 

Figure 9 Hydraulic cylinder and push plate avoid air
In the first stage of first stage of first stage push, first push upward 10 mm, oblique push block retreats to make room for bumper to pull and deform, then push upward 30 mm, pull block pushes plastic part to deform 4 mm inside plastic part, completing pull deformation action; finally push upward 10 mm, pull block only pushes upward, maintaining state when plastic part is pulled and deformed. After the first stage of push is completed, cavity plate is separated from plastic part, and demolding of undercut in bumper hood area is completed. In second stage of push of 110 mm, oblique push block continues to retreat, and pull block moves 4 mm to outside of plastic part to complete reset of plastic part after pull deformation.