1. Analysis of parting line of plastic parts
Figure 1 shows the front bumper of a certain model, the material is PP-T20, and the surface is required to be body color painting. According to the styling characteristics of this model, the front bumper has a front grille, which directly matches the front hood lap. In the design of the front bumper mold, the parting line in the hood area will be located at the R angle of the largest outer contour of the flange of the part, causing the parting line to leak.
Fig.1 Design of parting line for plastic parts
The design of the external parting line of the front bumper plastic parts mainly has the following problems.
(1) The parting line in the hood area is located on the R angle of the appearance of the plastic parts, and the parting line can be seen after the plastic parts are loaded, which affects the appearance quality of the plastic parts.
(2) Before the front bumper is painted, the leaking parting line needs to be polished, which increases the manufacturing cost.
(3) Due to the wear and tear in the mold manufacturing accuracy and injection production process, the parting line position will produce flash and segment difference, which reduces the qualification rate of plastic parts.
In order to solve the above problems, combined with the shape characteristics of the bumper, the parting line of the hood area is considered to be designed on the inside of the fillet of the plastic part, and the inner parting line is designed, as shown in Figure 2. The parting line can be hidden after the plastic parts are assembled, which not only solves the problem of leakage of the parting line, but also improves the surface perception quality and the pass rate of bumper mold production.
Fig.2. Design of parting line in plastic parts
As shown in Figure 3, the design of the inner parting line at the flanging position of the hood area of the bumper shows that the fillet here forms an undercut in the main demoulding direction, and if the plastic part is directly demolded, the fillet will be broken. Therefore, it is necessary to design a new internal parting tensile deformation mechanism in the hood area to realize the forming of the parting line in this area.
Fig.3. Comparison of the design of the inner and outer parting lines
2. Design of parting mechanism in the hood area
2.1 Scheme design of internal parting mechanism
In order to prevent the rounded inverted buckle at the flange C of the hood area of the front bumper from being broken during mold opening, it is necessary to make the plastic part elastic deformation before mold opening. The internal parting mechanism is shown in Figure 4, at first the oblique push block gives up the deformation space for the plastic part, then the pull block promotes the plastic part to its inner side under the joint action of push rod, T-shaped guide slider, pull rod and pull deformation guide rail, and the inverted buckle at the end C of the plastic part is pulled out to the inward side, so as to realize the normal demoulding of the plastic part.
Fig.4. Parting mechanism in the hood area of the front bumper
The working principle of the internal parting mechanism is as follows.
(1) When the mold is opened, under the joint action of nitrogen gas spring and synchronous hook, the relative position of the push plate and the fixed model cavity plate is kept unchanged, the plastic parts are still wrapped in the cavity, and the oblique push block retreats synchronously to make room for the pulling deformation of the plastic parts.
(2) Using the deformability of the plastic part, in the process of opening the mold to reach the predetermined stroke, the plastic part is driven to the inside by the push rod, the T-shaped guide slider, the pull rod and the pull deformation guide rail, so that the plastic part C is inverted and separated from the fixed model cavity.
(3) The mold is opened, and the plastic part is separated from the cavity plate of the fixed model.
(4) The mold continues to push out, and then controls the movement of the pull block to the outside of the plastic part through the joint action of the push rod, the T-shaped guide block, the pull rod and the pull deformation guide rail, so that the plastic part returns to its original state.
2.2 Composition of internal parting mechanism
2.2.1 Tension deformation mechanism
The front bumper hood area flanging inner parting mechanism is shown in Figure 5, mainly comprises oblique push rod, push block, pull deformation guide rail and pull rod. The tensile deformation structure is shown in Figure 6, mainly comprises push rod, guide slide sleeve, T-shaped guide slide block, pull block, pull rod and pull deformation guide rail, wherein T-shaped guide slide block is beryllium bronze material, fixed on push rod; The tie rod is designed to be L-shaped, and the bearing rotating block is fixed on the tie rod, and one end of the tie rod is fixed on the straight top tie block, and the other end is connected with the tension deformation guide rail by the bearing rotating block; Under the joint guiding action of the T-shaped guide block and the tension deformation guide rail, the pull deformation and reset of the flange in the hood area of the bumper are realized.
Fig.5. Parting mechanism in the hood area of the front bumper
Fig.6. Tensile deformation structure
The installation matching relationship between the pull block and the inclined push block is shown in Figure 7, the inclined push block and the pull block are separated by the core, and the push rod and the inclined push block adopt an independent positioning design, wherein the pull block presses the oblique push block, so as to reduce the risk of producing the bias push mark in the injection process.
Fig.7. The matching relationship between the pull block and the inclined push block
2.2.2 Internal parting auxiliary mechanism
The internal parting auxiliary mechanism includes 4 synchronous retractor mechanisms, 6 nitrogen gas springs and 4 hydraulic cylinders, as shown in Figure 8. In the early stage of mold opening, the synchronous hook mechanism and nitrogen gas spring auxiliary mechanism work together to make the plastic parts wrap the mold core and move with the core, so as to assist the pull deformation mechanism to carry out the pull deformation action on the plastic parts.
Fig.8 Internal parting auxiliary mechanism
2.2.3 Launching Agency
The front bumper mould is available in two stages, with the hood area flanging inner parting mechanism designed at a level of 160 mm at a distance of 160 mm. The first stage is divided into two stages, the first stage is the synchronous push out of the tension deformation stage, a total of 50 mm is launched, the push plate and the cavity plate are jointly driven by the nitrogen gas spring and the synchronous pull hook synchronous movement, the hydraulic cylinder does not move at this time, and the push plate and the hydraulic cylinder avoid 50 mm, as shown in Figure 9. In the second stage, the push distance is 110 mm, and the push plate is driven by a hydraulic cylinder, in which the tension deformation mechanism is reset.
Fig.9. The hydraulic cylinder and the push plate avoid space
In the first stage of the first stage of pushing, firstly, 10 mm is pushed upward, the oblique push block retreats to make way for the bumper to pull the deformation space, and then 30 mm is pushed upward, and the pull block pushes the plastic part to deform 4 mm to the inside of the plastic part, and the pull deformation action is completed. Finally, it is pushed upwards by 10 mm, and the pull block is only pushed upwards to maintain the state when the tension deformation of the plastic part is terminated. After the first phase is completed, the cavity plate is separated from the plastic part, and the inverted bumper hood area is demolded. In the process of pushing out 110 mm in the second stage, the oblique push block continues to retreat, and the pull block moves 4 mm to the outside of the plastic part to complete the reset after the plastic part is pulled and deformed.
▍Original author: Shi Bo, Shen Lianghan, Tie Shengwu, He Yechun, Wang Wei
▍Author's Affiliation: SGMW Technology Center