Tunnel gate is a common gate form in injection mold. Tunnel gate is generally designed on moving mold side of mold and inject from reverse side of plastic part. Advantage is that injection molding of plastic part is achieved without affecting appearance quality of plastic part.

 

(A) Slider type

 

(B) Lifter type

Figure 1 Tunnel runner and gate

Since tunnel gate is fed from reverse side of plastic part to be formed, runner and gate are generally installed in the mold in the form of reverse buckle. There are two types of demoulding for gate: slider type and lifter type, as shown in figure 1. Tunnel gate condensate is usually demolded together with plastic parts, and subsequent manual mechanical trimming of condensate is required, which reduces mold production efficiency and production cost of plastic parts. In response to this situation, an optimized tunnel gate form is adopted, and a combined secondary ejection mechanism is designed to realize push-out of plastic parts. At the same time gate condensate is automatically cut off to meet requirements of automatic mold production.

 

(A) Hot runner system

 

(B) Runner and gate

Figure 2 Automobile front bumper grille gating system

Figure 2 shows gating system of front bumper grille of car. According to improved tunnel point gate, traditional tunnel gate is usually designed in moving mold, gate section is rectangular, and reverse structure is in moving mold. It is necessary to design a slider-type or sloping top structure to demolding, using optimized tunnel point gate, can achieve automatic cut-off gate condensate when plastic parts are pushed out.

 

Figure 3 Launching organization

1. Hydraulic cylinder 2. Push plate I 3. Push plate II 4. Push plate III 5. Pull hook I 6. Pull hook II
Figure 3 shows mold ejection mechanism, which is mainly composed of 3 groups of push plate components, 2 sets of hook components and hydraulic cylinder drive components.

Figure 4 is three groups of push plate components, including push plate I component push mechanism, push plate II component push mechanism, push plate III component push mechanism.

 

(A) Push board Ⅰ

1. Slanted top block 2. Pull rod 3. Plastic parts 4. Push plate Ⅰ

 

(B) Push board II
1. Plastic parts 2. Top block 3. Hydraulic cylinder 4. Push plate Ⅱ

 

(C) Push board Ⅲ
1. Top block 2. Plastic parts 3. Straight top block 4. Push plate Ⅲ

Figure 4 Push plate pushing mechanism

Pushing mechanism of push plate Ⅰ assembly is mainly composed of push plate Ⅰ, pulling rod 2 and lifter block 1 (see Figure 4(a)). When mold is opened, pushing plate Ⅰ drives pulling rod 2 and lifter block 1 to simultaneously push out runner condensate. In addition to pushing out runner condensate, pulling rod 2 also takes care of positioning cold material well to prevent cold material well from sticking to lifter block. Push plate II component is mainly composed of push plate II, top block 2, hydraulic cylinder 3, lifter block, push rod, etc. (see Figure 4(b)). Lifter block realizes demoulding and auxiliary ejection of plastic part reverse button structure, eject block and push rod realize balanced ejection of plastic parts.
Push plate Ⅲ component is mainly composed of push plate Ⅲ, straight lifter block 3, lifter block 1 etc. (see Fig. 4(c)), eject block 1 is adjacent to lifter block 1 (see Fig. 4(a)), eject block and lifter block are pushed out by a hook mechanism to delay tunnel gate condensate automatic cutting.

In order to realize secondary ejection of mold, mold is controlled by two sets of pull hook mechanisms to control push plate Ⅰ, push plate Ⅱ, and push plate Ⅲ components in sequence. Pull hook I connects push plate II and push plate III, pull hook II components connect push plate I and push plate II.

 

(A) Side view of pull hook I

 

(B) Cross section of hook

 

(C) Track section

Figure 5 Pull hook I components

1. Pushing plate I 2. Fixing seat 3. Rotating shaft 4. Pushing plate II 5. Pressing plate 6. Pushing plate III 7. Bottom plate 8. Pull hook I 9. Guide rail 10. Pulling pin 11. Spring cap 12. Spring 13. Stopper 14. Screw
Working principle of pull hook Ⅰ is shown in Figure 5. When hydraulic cylinder piston fixed on moving mold plate drives push plate Ⅱ, fixed seat 2, pull hook Ⅰ and pull pin 10 are driven; when pull pin 10 moves along guide rail 9, pull hook Ⅰ is unhooked from push plate III, push plate III and eject block stop moving, push plate II continues to move under drive of hydraulic cylinder piston.

 

(A) Side view of pull hook Ⅱ

 

(B) Cross section of the hook

 

(C) Track section

Figure 6 Pull hook II components

1. Push plate I 2. Pull pin 3. Guide rail 4. Push plate II 5 Fixed seat 6. Push plate III 7. Base plate 8. Pull hook II 9. Pressure plate 10. Rotating shaft 11. Stopper 12. Spring cap 13. Spring 14 . Screw 15. Pin
Working principle of pull hook Ⅱ is shown in Figure 6. After push plate Ⅲ is unhooked, it stops moving. Hydraulic cylinder piston drives push plate Ⅱ to continue to move, which drives guide rail 3 fixed on push plate Ⅱ, fixing seat 5 on the bottom plate 7 is connected to pull hook Ⅱ, and pull hook II catches stopper 11 on push plate I to keep push plate I at a rest state. When stopper 11 hits pull-out pin 2 on puller hook II, puller hook II is unhooked, pusher board II drives pusher board I to move together until movement of push plate II and push plate I stops after stroke of hydraulic cylinder piston ends.

Lifter block is gradually separated from runner condensate when push plate Ⅰ is pushed out. In order to prevent runner condensate from sticking to lifter block, lifter block comes with a set of ejection system, as shown in Figure 7.

 

Figure 7 Flow channel condensate pushing mechanism

1. Movable template 2. Spring 3. Eject spring pin 4. Wear plate 5. Screw 6. Wedge block 7. Inclined top rod 8. Guide sliding sleeve 9. Lifter block 10. Flow path condensate 11. Top block

 

Figure 8 Pull rod

1. Flow channel condensate 2. Pull rod
Working principle: When lifter block 9 is ejected, ejection pin 3 and lifter block 9 produce relative motion on same level, runner condensate 10 is forced to be ejected. When ejection stroke ends, runner condensate 10 is completely disengages. At this time, spring 2 on ejector pin 3 is compressed first and then popped; when ejector system is reset, ejector pin 3 is reset along guide surface, spring 2 is compressed first and then popped; at the same time, to prevent condensate in the runner from falling out automatically, cone-shaped material rods are positioned at both ends of condensate in the runner to facilitate gripping by robot, as shown in FIG. 8.

After mold injection molding is completed, after moving and fixed molds are opened, ejection mechanism including push plate Ⅰ, push plate Ⅱ, push plate Ⅲ and other components driven by hydraulic cylinder piston to complete a series of actions such as automatic cutting of plastic parts gate condensate, plastic parts demoulding, and robotic parts taking. Working principle is as follows:
Hydraulic cylinder drives push plate Ⅱ and its components, push plate Ⅲ and eject block are pushed out by connecting the hook Ⅰ component. At this time, push plate Ⅱ, its runner and lifter block are still stationary, and there is relative movement between eject block and lifter block, so that gate is separated from plastic part. When movement stroke of piston of hydraulic cylinder is 25mm, pull-out pin on pulling hook Ⅰ is completely uncoupled under action of stopper, pushing plate III and top block are pushed out, and pushing plate Ⅱ continues to push out under action of hydraulic cylinder piston. When stroke moves to 50mm, pull hook Ⅱ is unhooked under action of stopper. At the same time, under push of push plate Ⅱ, movement of push plate Ⅰ drives feed rod and lifter block to push out. As ejection stroke increases, runner and gate condensate gradually disengaged until the end of hydraulic cylinder piston movement, plastic parts and runner condensate are smoothly demolded. Runner condensate is positioned on the draw rod due to tapered design of draw rod, mechanical arm of injection molding machine respectively grips plastic parts and runner condensate to complete automatic pickup action..

 

Figure 9 Plastic parts undercut section

Cross-section of plastic parts is shown in Figure 9. Total height of plastic parts is 130mm, minimum ejection stroke is 85mm, and maximum amount of plastic parts is S=18mm. Considering shrinkage rate of plastic parts and safety of taking parts, horizontal stroke is S+4=22mm, slope of lifter block is 9°, and minimum ejection distance is calculated according to relationship of trigonometric function: 22/tan9°=138.9mm, so total ejection stroke is designed to be 140mm.

 

(A) Stroke Ⅰ

 

(B) Stroke Ⅱ

Figure 10 Second launch schedule

Pusher Ⅱ controls pusher Ⅲ push-out stroke through pull hook Ⅰ assembly. Lifter block is pushed out by a distance of S1=25mm later than eject block to realize automatic cut-off of gate condensate; runner undercut amount is 7mm, safety distance is 2mm, total stroke Is 9mm, slope of lifter block is designed to be 9°, and ejection stroke of push plate III is: 9/tan5°=102.9mm. If ejection stroke of push plate III is designed to be 105mm, delay stroke S2 of push plate III through hook II assembly is: 140-105=35mm, which realizes automatic cutting and demoulding of condensate in the runner, and design formed by second push is shown in Figure 10.

Due to particularity of tunnel gate, following issues should also be noted in mold design:
(1) Due to large size of lifter block and eject block, large area of molded plastic and large number of gates, continuous production is easy to accumulate heat, which affects reliability of ejection mechanism and appearance quality of plastic parts, so a large inclined roof block or roof block needs to be designed with a cooling water channel.
(2) In order to increase reliability of hook mechanism and prolong service life of mold, moving parts such as the hook, stopper, and pin are made of SKD61 steel, heat-treated and tempered to 56~60HRC.
(3) To increase strength of cavity plate, as far as possible without affecting appearance quality of plastic parts, try to avoid sharp corners of mold parts and local thin wall thickness, design round corners of plastic parts and mold cavity plate sharp corners to reduce risk of cracking of mold parts caused by local stress concentration.
(4) Structure of plastic parts grille, buckle, etc. leads to a large number of mold cavity insertion structures, and a small insertion angle (minimum 1.5°), so a balanced insertion protection block needs to be designed around mold, and protection block insertion angle is designed to be 1° to enhance positioning accuracy of mold.
(5) In automated production process, mold uses a mechanical hand to take parts and aggregates. It is necessary to design positioning rib structure of plastic parts and tapered material rod to ensure accurate positioning of plastic parts and runner aggregates for smooth gripping.