UG is one of mainstream computer-aided design (CAD) software in injection mold industry. Its powerful functional application is conducive to design engineer to complete three-dimensional design of mold, so that entire mold can be displayed and simulated in three-dimensional mode in computer. After verification is correct, then carrying out mold manufacturing, which improves mold manufacturing efficiency and reduces mold manufacturing costs. Main basis of mold design is that customers provide two-dimensional and three-dimensional drawings or samples. Engineers analyze drawings or samples to design a reasonable mold.

 

(A) Gate design

 

(B) Slider design

Figure 1 DFM analysis

According to three-dimensional data of plastic parts provided by customers, through UG software, mold feasibility analysis (DFM analysis for short): gatng system design, parting surface design, slider design, lifter block design, ejection system design, etc. to confirm whether current plastic parts can design a reasonable mold forming. Plastic part shown in Figure 1 is a car horn bracket, external dimensions are about 240mm*215mm*58mm, and material is PC+GF10%.

 

(A) Filling time

 

(B) Injection pressure distribution

 

(C) Overall deformation distribution

Figure 2 MoldFlow simulation results

Through MoldFlow analysis, filling time, injection pressure, melt front temperature, pressure distribution during V/P conversion, curve at pressure injection position, weld mark, warpage and deformation can be obtained. Parameterization of these mold flow analysis can make mold structure design more reasonable, as shown in Figure 2.
(1) Filling time analysis. Filling is basically balanced, and melt can reach end of cavity at the same time.
(2) Injection pressure distribution. Injection pressure is 100MPa, and injection pressure of molded plastic parts is relatively high. Gate design adopts hot runner system and way of increasing gate.
(3) Analysis of warping deformation in the overall direction,  total deformation is 0.24mm.

 

Figure 3 Structural analysis of plastic parts

Slider is a common core-pulling structure of injection molds, which is used to solve demolding of outer or inner side of plastic parts to be molded. There are many types of sliders according to different parts of plastic parts. Generally, a single slider mechanism cannot meet requirements of plastic part reverse core pulling and demoulding,, so secondary core pulling of slider is needed to realize demoulding of plastic parts. Now structure design of slider's secondary core pulling forming plastic part is taken as an example to illustrate its structural design. Plastic part shown in Fig. 3 has two different types of undercuts. Slider cannot be directly core-pulled and formed once, and slider needs a second core-pull to form relevant parts of plastic part.

 

(A) Mold opening sequence

 

(B) Second mold opening distance

Figure 4 Mold opening sequence and second mold opening

1. Fixed mold seat plate 2. Hot runner plate 3. Fixed template 4. Movable template 5. Support plate 6. Pad block 7. Movable mold seat plate 8. Push rod fixing plate 9. Push plate 10. Pull die button 11. Fixed distance rod
Mold opening sequence is shown in Fig. 4(a). Through control of die pull button, fixed mold and movable mold are opened for the first time. When mold is ejected, movable mold plate and supporting plate are opened for the second time, as shown in Figure 4(b).

Movement state of slider is shown in Figure 5.

 

(A) Slider clamping

 

(B) First core pulling of slider

 

(C) State of small slider after first core pulling

 

(D) Second core pulling of slider

Figure 5 Slider motion state

1. Pull hook 2. Moving block 3. Large slider 4. T-shaped block 5. T-shaped block 6. Round push rod 7. Small slider
(1) State of mold closing. Movable block 2 bears against large slider 3, and large slider 3 does not move during first core pulling movement.
(2) First core pulling of slider. Pull hook 1 moves downward by 9mm in synchronization with T-shaped blocks 4, 5, and round push rod 6. Round push rod 6 drives small slider 7 out of inverted position of molded plastic part to complete the first core pulling, and at the same time, pull hook 1 drives movable block 2 out of buckling position of large slider 3.
(3) Second core pulling of slider. T-shaped block 5 continues to move, and at the same time, pulling hook 1 drives movable block 2 out of snap position of large slider 3, T-shaped block 5 drives large slider 3, T-shaped block 4 and round push rod 6 to move. At this time large slider 3 disengages from inverted position of molded plastic part, and second core pulling is completed. The entire core pulling mechanism is completely detached from inverted position of plastic part.

 

(A) Partial structure of mold

 

(B) Hot runner system

Figure 6 Local mold structure and hot runner system

Through computer-aided design, the entire mold structure is displayed and simulated in three-dimensional mode. After verification, mold is manufactured. Final mold partial structure and hot runner system are shown in Figure 6.

Through MoldFlow software analysis, injection pressure of molded plastic parts is large, and a hot runner system is used. DFM analysis in the early stage of mold opening determined that molded plastic parts required 3 sliders, of which 2 sliders can be moved directly in a single direction to achieve core pulling and demoulding, and the other slider needs a second core pulling to make it out of undercut of molded plastic parts. Movement of slider's secondary core pulling structure is complex, and following points should be noted during design process.
(1) Secondary core pulling structure of slider occupies a large space, slider accessories and push rod are easy to interfere. Layout of system should be considered when designing.
(2) Since secondary core pulling structure of slider needs to consider mold opening sequence, a structure to control mold opening sequence should be added, otherwise slider will easily collide with core during mold opening and closing, resulting in scrap of slider.