This is a mobile phone case, as shown in Figures 2-1 and 2-2.
The mobile phone case is composed of two types of plastics: PC and TPE. The dark part in Figure 2-2 is TPE elastomer, molded by a two-color injection molding machine. From the perspective of mold design, this is an extremely difficult mold with a highly complex structure. Having been engaged in the mold industry for 20 years, we consider this one of the most challenging molds we have ever encountered.
To ensure an aesthetically pleasing and smooth appearance, the parting line must be positioned at the tangent point of the inner arc, as shown in Figure 2-3.
Since the plastic used is PC (commonly known as bulletproof glass), forced demolding is impossible. All outer surfaces must slide open in four directions. The inner side is all inverted, and the core must be pulled out from all directions, which is commonly known as the "explosive core". Moreover, it is a two-color mold, so you can imagine how difficult it is.
Regarding the "explosion core" mold structure, there are already classic mechanisms for ordinary injection molds, which will be introduced in detail below. The problem here is that this is a two-color mold with two sets of moving molds and two sets of fixed molds. All components of the two moving molds are identical and need to rotate 180 degrees on the turntable of the two-color injection molding machine to inject two different plastics into the mold cavity respectively. When injecting hard rubber (PC), the ejection mechanism and core-pulling mechanism of the moving mold do not act. After injecting soft rubber (TPE) and opening the mold, the core-pulling mechanism and ejection mechanism of the moving mold on the side aligned with the soft rubber barrel start to act, ejecting the complete product molded by PC+TPE. After the moving mold rotates, the two sets of moving molds swap positions, and the gate after mold clamping must be at the same position, so handling the gates of soft rubber and hard rubber is challenging, as shown in Figure 2-4.
The mold requires "simultaneous internal and external pulling" around its perimeter. The so-called "simultaneous internal and external pulling" means that at the same position of the plastic product, the inner wall is undercut and requires internal core pulling, while the outer side cannot be directly opened, requiring a slider to pull outward for core pulling. The key questions are: how to arrange the internal and external sliders, and where to set the slider movement track? These are also difficult problems.
Putting aside the complexity of the mold slider mechanism, from the basic principle of two-color molds, the molding of the hard rubber part at the soft-hard rubber joint and the simultaneous internal and external pulling mechanism must be set on the fixed mold side, and the molding mechanism of the soft rubber part must also be set on the fixed mold side. This part consists of a protrusion formed by the simultaneous internal and external pulling mechanism inserted into the groove of the moving mold. After the turntable rotates 180 degrees, this set of protrusions is just inserted into the groove of another moving mold. That is to say, the external shape and size of the protrusions formed by the simultaneous internal and external pulling sliders on the two fixed molds are completely the same; only the mold surfaces for molding soft rubber and hard rubber are different.
The difficulty lies in that the protrusion is divided into two upper and lower layers: one layer moves outward, and the other moves inward (the so-called "simultaneous internal and external pulling"). The side of the combined protrusion is a unified inclined surface. However, traditional sliders must have necessary conditions such as sliding tracks. How to set the track has become the core issue of this case.
Our design for the groove of the moving mold part and the protrusion of the fixed mold part is shown in Figures 2-5, 2-6, 2-7, and 2-8.
The basic mechanism is shown in Figure 2-9.
As can be seen from Figure 2-9, when the A plate and the fixed mold base are separated by 35mm, the shifting block fixed on the fixed mold base toggles the internal slider, and at the same time, the external slider moves outward through gear transmission, as shown in Figure 2-10.
The basic idea is to use the shifting block shown in the figure to toggle the internal slider. While the internal slider slides inward, the external slider slides outward through gear transmission. This enables the separation of the soft rubber part of the product from the mold surface. Similarly, the notch of the hard rubber part of the product can be separated from the mold surface using the same method. All these mechanisms are set on the fixed mold side. Each rotation of the moving mold ensures a precise fit with the fixed mold.
The movement track of the internal slider is stably set on the A plate. However, where is the track of the corresponding external slider located? Would it be like a "castle in the air"?
We added a protrusion on the track of the internal slider, which also acts as a bearing, as shown in Figure 2-11.
The other end of the shaft uses a locking block with a semicircular groove that acts as a bearing, and a small stopper is provided to prevent axial movement of the shaft, as shown in Figure 2-12.
We designed a built-in track (single track), somewhat like an "I-beam", which also serves as the fixed position for the gear shaft—making full use of resources. Since the gear shaft is restricted from moving and can only rotate, the built-in track is equivalent to being fixed on the fixed mold plate, as shown in Figures 2-13 and 2-14.
In this way, the external slider becomes very simple, as shown in Figure 2-15.
The internal slider is more complex, as shown in Figure 2-16.
To realize the action of the shifting block toggling the internal slider, the fixed mold plate and the fixed mold base must first be separated by a certain distance (35mm), so that the shifting block can toggle the internal slider inward while the two plates are separated. The power for this action is achieved by the nylon rubber nail set on the moving mold pulling the fixed mold plate, and the guidance is ensured by the additional small guide pillars set between the base plate and the A plate, as shown in Figure 2-17.
After the two-color injection molding machine completes the injection of hard rubber, the core-pulling part and slider part of the moving mold filled with hard rubber do not act. The main runner and cross runner remain in the moving mold part and rotate 180 degrees with the moving mold on the turntable of the two-color injection molding machine. When clamping again, the fixed mold of the soft rubber part must reserve a position for the main runner of the hard rubber. Unless the main runner of the hard rubber part is removed, which requires an additional action by the robot, taking approximately 3 seconds more—this would greatly reduce production efficiency.
How to set the gate of the soft rubber part? This is actually the most difficult part of this case.
I designed a method called "sharing the same bed but dreaming different dreams" (the metaphor may not be appropriate). The main runner of the soft rubber part is based on the main runner of the hard rubber part, with a tapered semi-elliptical space added as the main runner for soft rubber. When the main runner of the hard rubber rotates with the moving mold to the fixed mold of the soft rubber and clamps, it is directly inserted into the reserved space of the soft rubber sprue bushing. Since the soft rubber sprue bushing reserves a semi-elliptical space, but the main runner of the hard rubber is a cone, this forms a cavity with a crescent-shaped cross-section. Soft rubber can be smoothly injected into the mold cavity of the soft rubber part along this crescent-shaped space, as shown in Figure 2-18.
The sprue bushing for hard rubber is shown in Figure 2-19.
The sprue bushing for soft rubber is shown in Figure 2-20.
The combination of the two sprue bushings is shown in Figure 2-21.
In fact, these two sprue bushings also serve as the clamping blocks for the internal slider and the clamping blocks for the internal slider and all angled lifters on the moving mold side. The two sets of fixed molds, including all fixed mold components, have identical dimensions. However, carefully observing, the depth of the spherical surface where the two sprue bushings contact the injection machine nozzle is different: the sprue bushing for hard rubber is 5mm deeper. The reason is simple: when the molded main runner of hard rubber is inserted into the sprue bushing of soft rubber, a 5mm space is left for the soft rubber to pass through and enter the crescent-shaped cross-section cavity. In this sprue bushing, the molded main runners of soft rubber and hard rubber each occupy half of the space.
Now, let's look at the setting of the cross runner, as shown in Figure 2-22-1.
The cross runner I designed for hard rubber is semicircular, running along the lower semicircle. This allows the hard rubber runner, after being filled, to form an upper semicircular space with the soft rubber runner cavity during clamping, which is exactly the space for the soft rubber cross runner. Figure 2-22-2 shows the mold flow analysis result from Moldex3D.
This runner design facilitates the separation of runners for two different plastics in the future for rational utilization.
In the fixed mold part of this mold, only the runners generate a lot of heat, especially the main runner—its solidification time directly affects the injection cycle. Although the soft rubber molding part is on the fixed mold, it extends into the groove of the moving mold, so the heat is basically concentrated on the moving mold.
Since the simultaneous internal and external pulling sliders are small in volume and cannot accommodate water channels, beryllium copper with high thermal conductivity is used for the internal sliders. I only made independent cooling water circuits on the sprue bushing (also serving as a clamping block) that is in close contact with the internal slider. The main function is to cool the main runner and cross runner, thereby shortening the injection cycle. See Figures 2-19 and 2-20 above. A long water connector is used for direct connection, as shown in Figure 2-23.
The distance between the two sets of fixed molds is determined by two factors: first, the center distance of the two parallel barrels of the two-color injection molding machine; second, whether the various mechanisms of the moving and fixed molds can work normally under the constraint of this center distance. This issue will be introduced in detail in the moving mold design section below. The center distance of the barrels of the two-color injection molding machine I selected is 480mm.
The action principle of the full-perimeter internal core-pulling mechanism for rectangular products: The full-perimeter internal core-pulling mechanism for rectangular products is the most complex structure in mold design. Initially, it was circular, used in plastic pipe joint molds and plastic pipe thermal expansion molds, designed by Italians and commonly known as "explosion core". This case is special: it is rectangular, used in a two-color mold, and requires simultaneous internal and external pulling—there is no precedent at home and abroad. Drawing on our previous experience with circular "explosion cores", I designed this mold step by step，taking more than 20 days to complete.
The internal core-pulling process of the "explosion core" can be summarized in one sentence: "First shrink, then pull, then angle-lift". Specifically, the "explosion core" internal core-pulling mechanism is divided into three parts: first, the middle part shrinks inward to make room for internal core-pulling; second, the internal core-pulling part—after the core shrinks, the internal sliders on the straight sides of the rectangular product can move inward due to the empty space in the middle; third, the corner angle lifters—after the undercuts on the four straight sides are separated from the mold surface of the internal sliders after core-pulling, the corner angle lifters can be lifted obliquely at 45 degrees. At this time, the product moves along the mold opening direction with the angle lifters until the undercuts on the product are completely separated from the mold surface of the angle lifters.
Removing the external slider, the structure is as shown in Figure 2-24.
After internal core-pulling and angle-lifting, the state is as shown in Figure 2-25.
The internal core-pulling only moves inward by 2.5mm, and the core shrinks downward by 30mm. At this time, the full-circle undercut of the product is completely separated from the mold surface.
Internal sliders 1 and 2 are symmetrical. Let's take internal slider 1 as an example, as shown in Figures 2-26 and 2-27.
The internal slider has a set of independent cooling water, a rack, and a built-in track. The built-in track of the concave "T"-shaped groove is difficult to process and is produced by EDM (Electrical Discharge Machining) after heat treatment, as shown in Figures 2-27 and 2-28.
The material used for the internal slider is 8407, with a hardness of HRC48.
Internal slider 1 is pressed outward during mold clamping, with the sprue bushing of the fixed mold serving as the clamping block.
The power for the internal slider to slide inward is transmitted by the external slider through gears and racks.
The external slider is pulled by an external square short-stroke oil cylinder (stroke: 2.5mm), as shown in Figure 2-29.
A detail note: To facilitate the assembly and disassembly of the gear shaft, a 3X3 small groove is made on the internal slider facing the gear shaft. When disassembling the gear shaft, a 2.5mm ejector pin can be used to push it out. The cross-section of this small groove is smaller than the end face of the shaft, which can also effectively prevent axial movement of the shaft, as shown in Figure 2-30.
The design of internal sliders (3 and 4) in the other direction is shown in Figure 2-31.
As can be seen from Figure 2-31, space for the angle lifter is reserved at the lower part of the front end of slider 3. Internal slider 3 itself has a rack, which can be directly processed by wire cutting.
Driven by the external slider, it moves inward (2.5mm) through gear transmission. It is restricted in the groove of the external slider, so no additional track is needed, as shown in Figure 2-32.
The external slider has a complex shape, as shown in Figure 2-33.
The reverse side is as shown in Figure 2-34.
Since the external slider has a friction surface with the internal slider, to avoid galling, a different material and hardness from the internal slider should be selected. I used 738H, with a hardness of HRC34~38.
The cooling water arrangement for external slider 3 is as shown in Figure 2-35.
The moving mold track is an important component of the moving mold part. Although its shape is simple, its design is quite challenging, as shown in Figure 2-36.
We used Cr12MoV, a material with high hardness and low cost, with a hardness of HRC52. It has friction surfaces in two directions and requires high perpendicularity. This reflects my design style: many people prefer to make the moving mold plate very thick, slot it, and then insert hard plates. However, we believe this is not good—deep slots in the moving mold plate can cause deformation, and the deformation amount is difficult to control. Attaching the track directly is simpler, more convenient, and ensures no deformation of the moving mold plate.
The positioning of the moving mold track is achieved by sliding guide sleeves and a set of pins, which is convenient and reliable. It is fixed with 10mm screws for easy assembly and disassembly. The nylon rubber nail is also fixed on the moving mold track.
The reverse view of the moving mold track is as shown in Figure 2-37.
The moving mold track has multiple functions in terms of assembly, as shown in Figure 2-38.
As shown in Figure 2-39.
This inclined surface forms a groove that matches the protrusion of the fixed mold, as shown in Figure 2-40.
Nylon rubber nails, return pins, positioning pins, and limit screws are all arranged on the moving mold track, as shown in Figure 2-41.
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