【Abstract】According to requirements of long core-pulling structure in mold structure, a long core-pulling injection mold based on hydraulic power was designed, mold’s problems in core-pulling movement, ejection movement and cooling system were analyzed, relevant mechanisms are respectively innovatively designed. This technology not only realizes automatic production of long core-pulled plastic parts, but also improves quality stability of plastic parts.

When plastic part has side holes, side grooves or side protrusions that hinder demolding, it is usually necessary to design a corresponding core-pulling mechanism in mold. During mold parting process, molded side core is pulled out through core-pulling mechanism, so that separation of movable and fixed molds is no longer hindered, interference of side core on demolding action of plastic part is avoided, and plastic part can be demolded smoothly. Usually power of core-pulling mechanism comes from mold opening force of injection molding machine, and mechanical mechanism is used to convert mold-opening force into core-pulling force. But for some special plastic parts with long core-pulling stroke, mechanical core-pulling mechanism cannot be applied. Special long-stroke oil cylinder must be used to provide core-pulling power.

This design example is a horn-shaped pipe fitting, as shown in Figure 1. Shape of plastic part is relatively long and narrow, all corners of periphery are rounded. Quality requirement of plastic parts is that cracks and deformation defects are not allowed, splicing line must be as shallow as possible, but appearance is not allowed to show any ejection marks; material of plastic parts is ABS; plastic parts are mass-produced, tolerances of plastic parts are converted according to requirements of mold design.

 

Figure 1 Analysis diagram of plastic parts
External dimension of plastic part is 304.5*80.8*80.8mm, and wall thickness is 1.4mm. PP is an unshaped plastic with relatively high mechanical strength and a molding shrinkage rate of 0.5%. Internal structure of plastic part is relatively simple, there is no undercut, as shown in Figure 1. However, because shape of plastic part is relatively long and narrow, internal shape must adopt a long core-pulling structure, appearance does not allow any ejection marks, which brings difficulties to manufacture of mold, especially core-pulling mechanism and ejection mechanism must be carefully designed , otherwise it will affect appearance of plastic parts. Since size of plastic parts is relatively large, output is relatively large, so you can consider using a hot runner system to improve production efficiency.

Since mold has 4 cavities, gate is designed as a side gate with side pouring, as shown in Figure 2.

 

Figure 2 Gating system design
1. Hot runner plate 2. Hot nozzle 3. Manifold 4. Plastic parts

Gating system adopts a mixed design of cold and hot. Cold runner mainly refers to branch runner starting on mold plate, hot runner mainly refers to passage from nozzle of injection molding machine to runner (including hot runner plate and hot nozzle). As shown in Fig. 3, since mold adopts a 4-cavity and hot runner structure, main runner part is injected with a runner hot nozzle 2. Melt enters hot runner plate 1 from nozzle of injection molding machine, is divided into two streams under splitting action of hot runner plate 1, injected into split runner on parting surface through heating nozzle 2. After being split again by runner, passes through latent gate of each cavity and enters 4 cavities.

 

Figure 3 Hot runner gating system design
1. Hot runner plate 2. Hot nozzle 3. Fixed mold plate 4. Fixed mold insert
Structural design of hot runner: Function of hot nozzle in mold is mainly to shorten length of runner and reduce heat loss, so hot nozzle adopts a straight-through large gate. Runner plate is heated by heating belts, and is kept away from surrounding mold plates to prevent heat loss. Only four heat insulation blocks are arranged at two ends of runner plate, and they form supporting points with nozzles.

Cavity part is a molded part on outer surface of a molded plastic part. According to different cavity structure, it can be divided into four types: integral type, integral embedded type, combined type and mosaic type. In this design, an integral cavity is used, as shown in Fig. 4a, and clearance treatment is performed around surface of cavity. If no avoidance treatment is performed, it will affect research and distribution with parting surface of cavity.

Core is a molded part on inner surface of a molded plastic part, which can generally be divided into two types: integral type and combined type. Through structural analysis of plastic part, integral core is used in this design, as shown in Figure 4b.

 

Figure 4 Structure of molded parts
a — —cavity b — —core

As shown in Figure 5, plastic part has two core-pulling mechanisms, one is located at small end of plastic part, namely head core-pulling, the other is located at large end of plastic part, namely tail core-pulling. Head core pulling distance is relatively short, and a conventional inclined guide pillar core pulling mechanism can be used. However, due to long core pulling distance at tail, it is necessary to consider use of hydraulic (cylinder) core pulling, and local mold plate must be lengthened to leave room for core pulling.

 

Figure 5 Analysis of core-pulling structure of plastic parts
Side cores of the other two core-pulling mechanisms are leaning against each other at the ends. Therefore, it is necessary to consider designing a tapered positioning mechanism at leaning position to ensure mutual positioning of two side cores and give side cores a fulcrum. Ensure that core is misaligned under pressure of melt, so as to affect coaxiality of inner wall of plastic part.

Head core-pulling position of plastic part is relatively short (core-pulling distance is 4mm), so core-pulling mechanism is designed as a typical oblique guide-pillar core-pulling mechanism:
As shown in Figure 6, slider part adopts a split type (slider is composed of a side core fixing plate 2 and a slider seat 3), and inclined guide post hole (diameter ϕ 25mm) is machined on slider seat 3; Sliding guide mechanism adopts a sliding groove pressure plate to form an I-shaped guide groove. In addition, in order to improve friction performance of slider, three wear plates are set on contact surface of slider and clamping wedge, and two wear plates with a larger area are also set at the bottom of slider; parting surface of side core adopts a sloped fit, which is beneficial to reduce wear of core plastic part when core is pulled. In addition, a positioning slope is designed on broken surface of short-side core and long-side core to ensure positioning accuracy with long core.
Fixed mold part: Due to relatively large width of slider, two oblique guide pillars are used. Oblique angle of oblique surface is that oblique guide post(diameter ϕ 24mm, inclination of 15°) is fixed on fixed mold plate. Clamping of slider depends on clamping inclined surface (inclination of 17°) on fixed mold, as shown in Figure 7.

 

Figure 6 Inclined guide pillar core pulling mechanism

1. Oblique guide post hole 2. Side core fixing plate 3. Slider seat 4. Side core

 

Figure 7 Hydraulic long core pulling mechanism
1. Cylinder 2. Cylinder fixing bracket 3. Slide groove pressing plate 4. Slide base 5. Side core fixing plate 6. Push plate 7. Side core

Core-pulling position at the tail of plastic part is relatively long (core-pulling distance is 304mm). If oblique guide pillar core pulling mechanism is adopted, distance between oblique guide pillar and mold opening will be too long. Therefore, core-pulling mechanism adopts a hydraulic (cylinder) core-pulling mechanism, as shown in Figure 7.
Characteristics of core pulling mechanism are:

As shown in Figure 7, due to relatively long core pulling distance, length of selected oil cylinder 1 is also increased accordingly. Installation of oil cylinder is fixed on bracket 2 by screws, head of oil cylinder rod is processed into an I-slot, installed in I-slot of sliding seat 4, and integrated with sliding seat. Since oil inlet of oil cylinder can be connected with hydraulic system of injection molding machine, oil cylinder can drive slider for core-pulling movement.

As shown in Figure 7, sliding block part is composed of three templates: a sliding seat 4, a side core fixing plate 5, and a push plate 6.
Sliding seat 4 is connected with oil cylinder 1, connected with side core 7 and side core fixing plate 5 to perform a core-pulling movement. Guiding of core-pulling movement relies on guiding groove formed by pressing plate 3 and movable mold plate for sliding guidance.
Function of push plate 6 is that when sliding seat 4 and side core fixing plate 5 perform core-pulling motion, push plate 6 remains stationary, so that plastic part can be jammed by push plate and does not move with slider. In this way, when core-pulling movement of sliding seat 4 ends, plastic part is also ejected under action of push plate 6.
Since plastic part does not have any locking structure in movable mold part, core pulling mechanism is not only core pulling function, but also plastic part must be pushed out.

Due to relatively high requirements for shape and appearance of plastic part, no ejection marks are allowed on the surface of plastic part, so conventional ejection components such as ejector rods and top plates cannot be used. Structure of ejection mechanism is shown in FIG. 8. Ejection mechanism of mold does not use any elements such as ejector rods and pipes, but uses core pulling mechanism of long core as ejection mechanism. Before pulling core, use long core to push plastic part out of mold cavity, then perform core pulling movement, and use push plate in core pulling mechanism to push out plastic part laterally. Driving mechanism of ejector mechanism is shown in Figure 8. Power for ejection action comes from a hydraulic cylinder. Cylinder 8 is fixed on movable mold backing plate 5, cylinder rod is processed into an I-shape and fixed on sliding seat fixing plate 3. In this way, with expansion and contraction of cylinder rod, movable mold plate and sliding groove pressing plate can be driven to perform ejection and reset movement.

 

Figure 8 Movable mold ejection mechanism
1. Guide sleeve 2. Slide groove pressing plate 3. Slide seat fixing plate 4. Sub guide post 5. Movable mold backing plate 6. Mould foot 7. Movable mold seat plate 8. Oil cylinder
Guidance of ejection mechanism comes from 4 sets of auxiliary guide posts and guide sleeves on sliding seat fixing plate. Auxiliary guide posts 4 are installed on movable mold backing plate 5, guide sleeves are installed on sliding groove pressing plate 2 and sliding seat fixing plate 3, the two form guiding mechanism of ejection mechanism.
Action time of ejection mechanism is that after mold is opened, oil cylinder 8 starts to move. First, the entire long core pulling mechanism is ejected to eject plastic part from movable mold cavity, then long core pulling motion starts to fall off plastic part. After long core-pulling mechanism takes on ejection function, ejection fixing plate and ejection plate of mold lose ejection function, can only play role of supporting mold. In order to increase support strength of movable mold plate, five supporting columns with a diameter of ϕ 50mm are designed in the space of push plate. Purpose is to strengthen strength of movable mold plate, which can resist injection pressure of plastic melt on movable mold plate, and enhance life of mold.

Cooling circuit design of long core pulling mechanism: main cooling part of long core pulling mechanism is inner core of plastic part. Since side core length is very long, if sufficient cooling is not performed, plastic part will be overheated, cooling rate will decrease, plastic part will even be deformed. Therefore, side core must be sufficiently cooled. Since fixed plate of side core is a sliding seat, sliding seat becomes entrance and exit of cooling circuit. In summary, cooling design of long core-pulling mechanism is mainly aimed at side core and sliding seat.
Side core is main cooling part. Because side core is thin and long, beryllium copper parts can be used for cooling. As shown in Figure 9, a water hole is designed inside side core to place beryllium copper; A circular groove for placing a sealing ring is machined at junction of core and sliding seat.
Sliding seat mainly provides inlet and outlet ofexternal cooling water. As shown in Figure 10, there are 4 circuits inside sliding seat, each with 8 entrances and exits. Positions of entrances and exits are all designed on same side of sliding seat to facilitate access to cooling water.

 

Figure 9 Internal structure of side core

 

Figure 10 Internal cooling water circuit of the sliding seat

(1) Core-pulling distance of plastic parts is long, a long-stroke oil cylinder is used for core-pulling. Movement sequence is controlled by injection molding machine. Adjustment is convenient. Oil cylinder moves smoothly and reliably, which is conducive to long-stroke core-pulling.
(2) Shape of plastic part is long and narrow, a series partition type cooling water circuit is designed inside slider core to ensure that inside of plastic part is sufficiently cooled, thereby reducing cooling time and shortening molding cycle.
(3) Gating system adopts mixed use of hot runner and cold runner, which reduces reduction of melt heat and loss of injection pressure, improves molding quality of plastic parts.