Common Types of Sliding Mechanisms
Mechanisms with lateral parting and core pulling are collectively called sliding mechanisms. There are many types of sliding mechanisms, and various classification methods exist. Based on the usage characteristics of various sliding structures, common sliding mechanisms can be summarized into the following categories:
(1) Front mold sliding mechanism
(2) Rear mold sliding mechanism
(3) Internal sliding mechanism
(4) Hydraulic mold mechanism
(5) Angled ejector and rocker arm mechanism
(6) Hydraulic (pneumatic) sliding mechanism
7.2 Sliding Mechanism Design Requirements
(1) Each component of the sliding mechanism should have reasonable manufacturability, especially the molding part. General requirements:
a. Avoid sliding clamping lines as much as possible. If unavoidable, the clamping line should be located in an inconspicuous position on the plastic part, and the clamping line length should be as short as possible. At the same time, a combined structure should be used as much as possible so that the sliding clamping line part and the cavity can be processed together. As shown in Figures 7.2.1a and 7.2.1b.
b. To facilitate processing, the molding part and the sliding part should be made into a combined form as much as possible. As shown in Figure 7.2.2. Mold Design Guide - 7. Slide Design
(2) The components and assembly parts of the slide mechanism should ensure sufficient strength and rigidity.
The slide mechanism is generally designed based on experience, but simplified calculations can also be performed (see Section 5.3 of Chapter 5 for calculations). To ensure sufficient strength and rigidity, the following is generally adopted:
A. Maximum structural size. When the space allows, the slide component adopts the maximum structural size.
B. Optimized design structure. For example, the following situations:
1) Positioning the end of a longer slide pin to avoid bending, as shown in Figure 7.2.3
Mold Design Guide - 7. Slide Design
2) Increasing the cross-sectional size of the ejector pin and decreasing the guide slope of the ejector pin to avoid bending of the ejector pin, as shown in Figure 7.2.4. When the space "D" of the plastic part structure allows, increase the cross-sectional size "a" and "b" of the ejector pin, especially size "b". At the same time, while satisfying the side core pulling requirement, decrease the angle "A" to avoid bending of the ejector pin under lateral force. Mold Design Guide - 7. Slide Design
3) Modify the structure of the mold insert to enhance the strength of the assembly part. As shown in Figures 7.2.5a, 7.2.5b, 7.2.6a, and 7.2.6b.
Mold Design Guide - 7. Slide Design
4) Add locking to improve the strength of the mold insert. (See previous analysis)
(3) The movement of the slide mechanism should be reasonable.
To ensure the normal operation of the slide mechanism, it should be ensured that the slide mechanism does not interfere with other structural components during the mold opening and closing process, and the movement sequence is reasonable and reliable. The following points should generally be considered:
A. When using a front mold slide, the mold opening sequence should be ensured. As shown in Figure 7.2.7, during mold opening, the parting should begin from point A-A, and then from point B-B.
Mold Design Guide - 7. Slide Design
B. Use a hydraulic (pneumatic) slide mechanism. During mold construction, the parting and resetting sequence of the slide mechanism must be carefully controlled; otherwise, the slide may be damaged. In Figure 7.2.8, the slide mechanism can only part after locking block 2 has disengaged from the slide. Before mold closing, the slide mechanism must reset, and after mold closing, locking block 2 locks the slide. In Figure 7.2.9, because the slide pin passes through the front mold, the slide pin must be withdrawn before mold opening. After mold closing, the slide mechanism can reset and be locked by hydraulic cylinder pressure.
Mold Design Guide - 7. Slide Design
C. The slide mechanism should prevent interference with the ejector mechanism during mold closing.
When the projections of the sliding mechanism and the ejection mechanism coincide in the mold opening direction, a reset mechanism should be considered to allow the ejection mechanism to reset first. (See Chapter 8, Section 8.6 for the reset mechanism.)
D. When the inclined guide pillar or inclined slide of the driving sliding mechanism is long, the length of the guide pillar should be increased.
Mold Design Guide - 7. Sliding Mechanism Design
Guide pillar length L > D + 15mm, as shown in Figure 7.2.10.
The purpose of lengthening the guide pillar is to ensure that the front and rear molds are fully guided by the guide pillar and guide bushing before the inclined guide pillar or inclined slide enters the driving position of the sliding mechanism, thus preventing damage to the sliding mechanism during mold closing.
(4) Ensure sufficient sliding stroke to facilitate demolding of the plastic parts.
The stroke of the slide is generally taken as the side hole or concave-convex depth plus 0.5~2.0mm. Smaller values are used for slanted ejectors and rocker arms, and larger values for other types. However, when using a composite mold to form plastic parts such as coil frames, the stroke should be greater than the depth of the side concavity, as shown in Figure 7.2.11. The stroke S is calculated using the following formula.
Mold Design Guide - 7. Slide Design
(5) The slide guide should be smooth and reliable, and should have sufficient service life.
The slide mechanism generally uses a T-shaped guide groove for guiding. Figure 7.2.12 shows several commonly used structural forms.
Mold Design Guide - 7. Slide Design
When the slide mechanism completes side parting and core pulling, the length of the slide block remaining in the guide groove should not be less than 2/3 of the total length. When the template size cannot meet the minimum fitting length, an extended guide groove can be used, as shown in Figure 7.2.13.
Mold Design Guide - 7. Slide Design The sliding guide surface (i.e., the moving contact surface and the force-bearing surface) should have sufficient hardness and lubrication. Generally, the sliding component requires heat treatment, and its hardness should reach HRC40 or higher. The hardness of the guide part should reach HRC52~56, and the guide part should be machined with oil grooves.
In sliding mechanisms with inclined ejector pins, the guide surface is the hole wall that mates with the inclined ejector pin. To reduce wear on the guide surface, the actual mating surface should not be too long. At the same time, to increase the hardness of the guide surface, high-hardness inserts should be used locally. See Figure 7.2.14.
(6) Reliable Sliding Positioning
When the sliding mechanism terminates the parting or core-pulling action, the sliding should remain at the position where the movement just ended to ensure successful reset when the mold closes. Therefore, a reliable positioning device must be provided. However, inclined ejector pins and rocker pin sliding mechanisms do not require a positioning device. Below are some commonly used structural forms, as shown in Figures 7.2.15a, 7.2.15b, 7.2.15c, and 7.2.15d. Figure 7.2.15a) is commonly used, but the row spacing is small due to the limitation of the built-in spring.
Figure 7.2.15b) is suitable for slides where the slide block is located on the top or side after mold installation, and for slides with a large row spacing. When the slide block is located on the top, the spring force should be more than 1.5 times the weight of the slide block.
Figure 7.2.15c) is suitable for slides where the slide block is located on the side after mold installation.
Figure 7.2.15d) is suitable for slides where the slide block is located on the bottom after mold installation, using the slide's own weight to stay on the stop block.
Mold Design Guidelines - 7. Slide Design
(7) Slide opening must be guaranteed by a mechanical mechanism, avoiding the use of a spring alone.
Figure 7.2.16a uses a spring alone to provide opening power, which is structurally unreasonable. Figure 7.2.16b mainly uses pull block "3" to provide the opening power, ensuring the slide opening power, and the structure is reasonable.
