【Abstract】By simplifying force condition of lifter structure of injection mold, theoretical analysis is carried out from the two aspects of friction self-locking and deflection deformation, trying to theoretically demonstrate principle of angle limit of lifter ejection, trying to theoretically demonstrate principle of angle limit of lifter and find out main reason for failure of lifter, so as to reduce or avoid risk of failure of lifter function as much as possible during mold design, can effectively reduce cost of mold trial and modification, increase service life of mold, and reduce cost of mold maintenance.

Many plastic products will have an undercut structure. Injection mold of this type of plastic part usually uses slider core pulling or lifter (also called inclined push rod) core pulling mechanism to solve problem of demolding, especially when inner side of plastic part is undercut, lifter mechanism is the best or even the only solution. In addition to function of core pulling, lifter mechanism also has advantages of ejection and small space. Therefore, this mechanism is widely used in injection molds. Figure 1 is a partial structure diagram of a typical lifter structure.

 

Figure 1 Structure diagram of conventional lifter
1. Core 2. lifter 3. Mold base B plate 4. Guide block 5. Slant top sliding seat 6. Thimble panel 7. Thimble bottom plate
Lifter mechanism is usually composed of profile and positioning of head glue position, middle guide inclined hole and pipe position, bottom lifter sliding seat, etc., other functional parts such as cooling water circuit are complicated. Included angle θ of inclined guide hole (angle between direction of ejection force and direction of movement) will cause ejection force to produce a radial component force. This radial component force becomes larger as θ increases, increases risk of failure of lifter function, such as bending deformation, jamming, breaking, etc. Therefore, included angle θ has a certain limit, usually not greater than 15°~18°, so is there any theoretical basis for this empirical value? This article attempts to explore through simplified mechanical analysis.

During operation of lifter, main force is generated during the two actions of ejection and retreat (reset), especially when mold is opened and ejected after injection molding, force is the largest. At this time, ejection resistance of plastic part (plastic part packing force) is the largest, lifter mechanism is also on the eve of transition from static friction to sliding friction. Friction resistance is the largest, cantilever of lifter rod is also in the longest position. Therefore, at this time, lifter should be the time to withstand maximum destructive force. At this time, force of lifter is shown in Figure 1. N is ejection force, F is friction force of base, N1 is supporting force of guide block, F1 is friction force between lifter and guide block, N2 is core supporting force, F2 is friction between lifter and core, N3 is ejection resistance of plastic part, F3 is plastic part packing force and friction, θ is angle of lifter .
For same mold, above N3 and F3 are basically stable and can be understood as a constant value. In order to facilitate research and simplify calculations, influence of these two values is not considered (equivalent to empty top state without glue injection). Supporting forces N1 and N2 of guiding positioning part and core part are simplified to two fulcrum forces, lifter is simplified to a long strip. Such a simplified force diagram is shown in Figure 2.

 

Figure 2 Simplified diagram of force analysis of lifter (at the moment of ejection start)

 

Axial force of lifter is resultant force of ejection component Na and several resistances:

 

Theoretically, coefficient of static friction between steels is 0.1~0.15mm. Considering processing conditions and working environment of mold, it should actually be greater than this theoretical value. Simulate processing technology and materials of lifter and lifter hole, perform a simple verification of friction coefficient:
Take 200*50*50mm rectangular 1.2738H material part A, one side of which is cut out with a slow wire (200*50mm); take 40*40*40mm rectangular material 1.2344 (46~50HRC) part B, ground one surface to Ra0.5μm. Corresponding to bonding of these two surfaces, perform a simple test according to method shown in Figure 4.
Multiple sets of data test, B is critical state of sliding, angle C is between 15° and 19°, with an average of about 17°, find tan17°≈0.31, get friction coefficient of these two materials in this surface state μ≈ 0.31, substituting aforementioned axial force analysis formula to obtain:
Maximum safety angle θ＜30°
Taking into account influence of processing accuracy and assembly accuracy, thermal deformation in injection molding, a safety factor of 1.2 times is adopted for friction coefficient, that is, μ = 0.37. Through above calculation, it can be obtained: θ<21°.
This conclusion is close to empirical value. Under special circumstances, there are examples for oblique apex angle to be 20°.
It can be seen from Figure 3 that reducing friction coefficient will significantly reduce and increase maximum safety angle of lifter, thereby reducing risk of failure of lifter. Therefore, in processing and manufacturing, it is necessary to improve surface finish of mold parts as much as possible, select appropriate materials and surface hardness, improve processing accuracy and assembly accuracy, and improve lubrication conditions of lifter.

 

Figure 3 Relationship between maximum safety angle of lifter and coefficient of friction

 

Figure 4 Verification method of maximum static friction coefficient

From simplified force analysis diagram of lifter in Figure 2, there are only three radial forces, N1, N2 and resultant force Nz. There is no displacement in radial direction, and the overall mechanics is balanced. L1 section has core and tube support, but L2 section is a cantilever, which deforms under force. Radial force is simplified as a cantilever beam structure, as shown in Figure 5.

 

Figure 5 Simplified deformation of lifter deflection

 

 

From comparison of formula and above calculation results, it can be seen that ratio of lifter cantilever L to thickness h (referred to as length-diameter ratio) has a cubic effect on deformation results, and other factors are relatively small. Therefore, in lifter design, pipe position is as low as possible (in direction of ejector plate), and thickness of oblique ejector rod is increased (or oblique ejector rod is thickened) to effectively reduce amount of deformation.
This kind of bending deformation will increase frictional resistance and lead to increase of ejection force, which will produce a series of effects. When deformation reaches a certain level, it will cause plastic deformation (bending) or even fracture of lifter, which will cause functional failure. On the other hand, long-term wear of L1 section may cause deterioration of surface roughness of mating surface, increase fit clearance, etc., which will also increase friction coefficient. With increase of friction coefficient, risk of failure in the above two directions will also increase, thus entering a vicious circle. Therefore, mold must be regularly inspected and maintained during production process, especially for moving parts such as lifters, and failures should be avoided or delayed as much as possible.

In special circumstances, improvement methods, such as large core-pulling stroke and limited ejection stroke, ejection angle of lifter will exceed maximum safety angle. In addition to reducing friction coefficient as much as possible and increasing safety angle, is there a more reliable method? From above analysis of axial and radial forces of lifter, an important parameter can be found, that is, a radial resultant force Nz at the lower end of lifter, which is source of radial force and then generates frictional force. It is also F force that causes lifter to deform and deform in Figure 5. Figure 6 shows that this parallel guide rod structure can directly reduce or eliminate force Nz.

 

Figure 6 Large-angle parallel guide rod lifter structure
1. Core 2. lifter 3. Movable mold plate 4. Auxiliary guide rod 5. Slant top sliding seat 6. Mandrel fixing plate 7. Mandrel pad
Auxiliary guide rod is parallel to lifter, upper and lower ends are respectively fixed on movable mold plate and movable mold seat plate. When ejector force pushes ejector pad up, pressure of auxiliary guide rod on sliding seat will produce a rightward component force. Fg. After this force Fg eliminates frictional resistance of ejector backing plate to sliding seat, resultant force (Fg-F) and ejection force N re-form a resultant force Na to points to axial direction of lifter, pushing lifter movement, which basically eliminates radial force of Nz. Of course, guide rod part of this structure is still restricted by frictional self-locking, and its angle cannot be increased arbitrarily. This mechanical model is similar to aforementioned principle, no further analysis and calculation will be made here.
In actual application, due to machining precision and existence of fit gap, right component force Fg will have risk of lagging, and it cannot reach ideal state of mechanical model, but it can still well protect movement of inclined ejector rod.
Disadvantage of this structure is that there needs to be enough installation space on mold (may interfere with other ejectors, etc.), complexity of structure is also higher, processing and assembly requirements are higher.
On this basis, by calculating and fitting complicated angle of auxiliary guide rod and lifter sliding seat, synchronization problem of lifter and inclined angle core-pulling can be solved. Following two structures are for reference.

1. Parallel guide rods, as shown in Figure 7.

 

Figure 7 Structure diagram of lifter of inclined angle parallel guide rod
1. lifter 2. Core 3. Movable template 4. Guide block 5. Auxiliary guide rod
6. Slant top sliding seat 7. Mandrel fixing plate 8. Mandrel pad
There is a K angle between core-pulling direction of lifter and parting surface. Conventional lifter scheme cannot be demolded. Lifter slide is also designed as a K-angle inclined plane in design, auxiliary guide rods are used to assist in driving and maintaining synchronization of demolding.

2. Cross guide rod double sliding seat, as shown in Figure 8.

 

Figure 8 Structure diagram of double sliding seat of cross guide rod
Cross-assisted guide rod requires use of a compound movement with a double sliding seat, and angle calculation is more complicated. Angle fitting is shown in Figure 9.

 

Figure 9 Angle fitting of cross guide rod

This article is only some personal understanding and analysis, level is limited. In some aspects, I feel that I am not rigorous enough. It is inevitable that there are errors or omissions.