[Abstract] A secondary core-pulling mechanism for injection molds is introduced. Mechanism is connected between two sliders by a spring and a tie rod, uses spring force and limit plane to cause movement of two sliders to generate a delay, thereby realizing two core pulling actions.

In design of injection molds, for some plastic parts with special requirements, because of obstacles attached to them, structural design of slider on slider is often used as a solution, which is referred to as secondary core pulling mechanism of slider. Usually this kind of mechanism is composed of two sliders, one large and one small. Small slider is attached to large slider to perform demolding action, which is called mother-and-child slider here.
Secondary core pulling on slider can be summed up as a wide variety of methods, each with its own methods and ingenuity, it is difficult to explain them all in one sentence. But when it comes to its application, generally speaking, there is an obstacle (undercut) in vertical direction of core-pulling mechanism, which cannot be directly pulled out of mold at one time, but core-pulling of obstacle body on core-pulling mechanism needs to be completed in advance. In addition, for some core-pulling mechanisms with relatively large tightening force, because pulling-out force is too large, one-time core-pulling cannot be smoothly demolded, so it is necessary to separate areas to implement successive core-pulling.
So-called slider delay of secondary core-pulling design on slider actually means that after small slider completes first core-pulling in advance, it moves with large slider to complete entire core-pulling process. Commonly used methods can be roughly summarized as follows: Following 4 categories: ①Spring delay; ②Wedge delay; ③Block delay; ④Cylinder delay.
Among several design methods, although spring delay is not a perfect solution, it is widely used because of its clever design, simple structure, and low manufacturing cost. However, spring has a certain fatigue strength. After a certain period of use, performance will be reduced and there is a risk of failure. It is necessary to implement regular inspections of use of spring and replace it when necessary.

Secondary core-pulling on slider is actually a vertical and horizontal core-pulling delay mechanism. Sub-slider first moves vertically and then moves horizontally. The first stage of movement is horizontal movement of female slider to drive vertical movement of sub-slider: Utilizing action of inclined guide rail between mother and sub-slider, when mother-slider moves in horizontal direction, sub-slider maintains vertical movement to pull out horizontal obstacles, vertical distance of sub-slider is reduced from H to H 1, horizontal travel distance of female slider is L 1; second stage of remote movement is synchronous horizontal movement of child and female sliders: child slider stays on female slider, vertical distance H 1 remains unchanged, then it is attached to female slider and continues to move in horizontal direction together. Horizontal walking distance is L 2. At this point, core pull of mother and daughter sliders has all escaped from obstacles in vertical direction. Therefore, position of delayed action of slider involved in two-stage movement must be clearly defined in order to be safe and reliable, otherwise it will strain product or even destroy mold, as shown in Figure 1.

 

Figure 1 Position diagram of moving process of mother and child slider

As shown in Figure 2, there is an obstacle in the middle of long side of this plastic part. Specific situation is cross-sectional view in Figure 2. There are two obstacles in same undercut space: horizontal obstacles and vertical obstacles body.

 

Figure 2 Plastic parts obstacle body
Obviously, undercut structure required by such plastic parts cannot be achieved by a one-time core pulling mechanism to complete demolding of obstacle body. It is necessary to find another way and adopt some feasible measures to solve it. That is to use child and female slider to design a specific core-pulling mechanism.

Component decomposition of mother and child slider is shown in Figure 3. Sub-slider 1, sub-slider pull rod 2 and sub-slider spring 3 are combined into a sub-slider group; female slider 4 and female-slider base 5 are fixed and combined by screws as a whole, as a female slider group. In fact, it is composed of two major assemblies of mother and daughter sliders to complete entire secondary core-pulling process. Sub-slider group is connected to corresponding guide hole of female slider through sub-slider pull rod 2, sliding power is provided by sub-slider spring 3, so that sub-slider group slides obliquely along guide rail of female slider, as shown in FIG. 4.

 

Figure 3 Detailed exploded view of the slider assembly

2. Sub slider 2. Sub slider lever 3. Sub slider spring 4. female slider 5. female slider seat 6. female slider press block 7. female slider stopper 8. female slider lever 9. Female slider spring

 

Figure 4 Sub-slider group and mother-slider group

As shown in Fig. 5, in position where sub-slider mechanism is located in mold clamping state, there are obviously horizontal obstacles and vertical obstacles in undercut area. These two obstacles in different directions cannot be core-pulled at one time.

 

Figure 5 Mother and daughter sliders in clamping state
When cavity is opened after injection molding is completed, female slider group moves outward under drive of inclined guide post, sub-slider group ejects forward under action of spring force on pull rod, but because front end of sub-slider is restricted by mold core and cannot move forward, in fact, only female slider moves horizontally to the right, while sub-slider can only move vertically along front end under action of dovetail rail of female slider, horizontal obstacle Q 1 is disengaged first. When female slider slides outward horizontally to L 1, child slider slides vertically downwards until child slider rod is stopped by limit of female slider (maximum distance between child slider and female slider is M ), sub-slider can no longer slide down vertically, as shown in Figure 6. Note that at this time, vertical downward sliding distance of sub-slider must ensure that A 1> A+ (2~3mm) in order to ensure safety of core pulling of plastic part.

 

Figure 6 State diagram of sliding distance of child and female slider when mold is opened is L 1
At this time, sub-slider can only move horizontally outward along with female slider. At the same time, female slider also completes demolding of vertical obstacle Q 2. Until outward horizontal sliding distance reaches L 2, it is stopped by limit braking of female slider block, demolding process of entire undercut of plastic part is completed. As shown in Figure 7.

 

Figure 7 Position of child and female slider when mold is fully opened (some parts have been hidden)

As shown in Figure 8, clamping sequence of child and female sliders: when mold is closed, it should be reverse movement of mold opening action. Inclined guide column first pushes female slider group and sub-slider group forward together. When limit surface of sub-slider engages with limit surface of mold core, sub-slider group is completely reset and stops moving forward, female slider group continues to move forward, and at the same time drives sub-slider group to slide upward along limit surface to reset, until female slider returns to original position, child and female sliders stop moving at the same time to complete entire mold clamping process.

 

Figure 8 Reset diagram of die-clamping slide block
a — —Core pulling mechanism is fully open b — Sub-slider moves to engagement plane c — —Core pulling mechanism is fully closed

Space distance (H) where undercut obstacles are located should be sufficient, not only to ensure that core-pulling stroke of child and female slider is not interfered, but also to ensure sufficient strength of parts of child and female slider. As shown in Fig. 9, under premise of meeting design strength, D dimension of female slider must ensure that safe sliding distance of sub-slider S exceeds obstacle A, that is, D>A+safe distance must be satisfied. Dimensions in Figure 9 are design parameters selected for this case and are for reference only.

 

Figure 9 Schematic diagram of size of undercut obstacle

When mold is opened, in the first movement of sub-slide group, key point for sub-slider to ensure vertical sliding up and down at fixed position is that there is a limit plane structure between sub-slider and mold core, which is safety guarantee for sub- and mother-slider group during mold opening and closing process, as shown in Figure 10. When mold is opened, sub-slider passes through elastic force of spring on limit rod. When female slider moves back horizontally, sub-slider is kept pressed against limit plane, sub slider can slide vertically down on this plane until limit lever on sub slider is pulled apart by female slider. Figure 11 shows movement sequence of child and female slider on limit plane during mold opening process.

 

Figure 10 Limit plane of engagement between sub-slider and core

 

Figure 11 Movement decomposition of sub-slider and limit surface of mold core when mold is opened
a — —Limiting plane contact position b — —Limiting plane sliding position c — —Limiting plane separation position
In same way, limiting plane plays an extremely important role in movement of female and child sliders during mold clamping process. When mold is closed, sub-slider is kept at limit distance (M) from female slider under pressure of spring. As female slider slides horizontally to the left, limit plane of sub-slider first rubs against meshing plane of mold core and slides vertically upwards (without continuing to move forward and damaging other mold parts). Until female slider slides horizontally and returns to original position, at the same time child slider stops sliding upward and is fixed in original position. It can be seen that this limited meshing plane structure not only ensures that sub-slider does not damage important parts of plastic part during transition from vertical sliding to horizontal movement when mold is opened, but also ensures that sub-slider moves from horizontal when mold is closed. It is an important part that is converted into a vertical sliding and safe return.
This so-called spring-type secondary core pulling mechanism can be regarded as a set of horizontal tilting mechanism to a certain extent. Compared with structure of injection mold, concept of relative motion is used to compare, if female slider is regarded as core of mold, sub slider is regarded as inclined top, there will be equivalent to female slider not moving, but sub slider will push out obliquely on guide rail of female slider to exit obstacle caused by undercut, so that plastic parts can be core-pulled smoothly.

Spring-loaded sub-and-female slider is just one of many secondary core-pulling slider mechanisms. I just tried to analyze core-pulling principle described in communication with simple graphics and text. It is true that feasibility of a core-pulling mechanism must fully consider its reliability in the entire production cycle, that is, to ensure reliability of movement, coordination of various parts and accuracy of stroke positioning during the entire process of mold opening and mold closing. From point of view of components of this set of child and female sliders, it is mainly divided into two parts: child slider group and female slider group. Movement boundaries are clear and number of parts is relatively small.
Characteristics of spring-type secondary core-pulling mechanism: sub-slider is divided into two stages of movement, female sliding block is driven by inclined guide column to slide horizontally, sub-slider is driven to make two vertical and horizontal movements. Through engagement of limit plane, vertical sliding and horizontal movement of sub-slider can be ingeniously converted safely and smoothly. Mold can be opened and closed freely, mechanism is simple and safe, which is convenient for manufacturing, facilitates maintenance.