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There are mainly two kinds of guiding components. The first kind is guide pins and guide bushes, which are used to guide the moving half into the fixed half. A standard injection mould usually requires four guide pins with two pins placed in each cavity. The second kind is tapered interlocks used to ensure precise alignment between the core and the cavity. To ensure that the two halves of cavities for the 87.5° elbow moulding are closely matched, four interlocks are arranged as shown in figure 6. Precise alignment of the two halves can also reduce the visibility of parting lines to improve the aesthetic appearance of the part. Moreover, screws are used to tie the cavity to the support plate. Their locations are also shown in figure 6.
Figure 6. Location of ejector pins, interlocks, screws and guide pins.
2.2.5. Cavity for the fixed half.
After completing the cavity for the moving half, the cavity for the fixed half is almost a direct copy of that of the moving half. The only difference is that there are no ejector pins for the fixed half cavity as the ejector system is located at the moving half. A sprue is assembled to the fixed half to guide the molten plastic into the cavity. The moving half cavity is attached to the support plate while the fixed half cavity is attached to the front clamping plate.
2.2.6. Design of ejector system and support plate.
The main function of the ejector system is to hold ejector pins and to eject the moulded part out of the mould. The ejector system consists of an ejector plate assembly and an ejector plate return system. The ejector plate assembly is located directly behind the support plate, and rests on the back clamping plate. Support pillars and support bars are placed between the support plate and the back clamping plate to provide spacing for the ejector assembly to move (figure 7). Guide pins are attached at the back clamping plate and pass through the ejector plate assembly and the support plate. A spring is placed around each guide pin to allow the ejector plate to return to its original position after ejection.
During ejection, an ejector rod of the machine will hit the ejector plate assembly, causing it to move forward and eject the moulded part. The springs are compressed during the ejection process, and automatically decompress and return the plate to its original position when the ejector rod retracts. The ejector stroke is the distance between the ejector assembly and the bottom of the supporting plate. As a rule of thumb, the ejector stroke should be 5–10 mm more than the height of the moulded part. The ejector assembly and cavities are mounted on different mould plates. Mould plates normally possess recess holes used for mounting purposes.
Figure 7. Ejector assembly.
3. Design of side core
Considering the shape of the part at side A as shown in figure 1(b), we decide to use a side core mechanism. Figure 8 shows the 87.5° elbow together with the side core, which is pided into three parts. Core A will be mounted on a sliding plate that moves in the direction indicated by the arrow. During the moulding process, core A will be inside the cavity as shown in the figure. When the moulding process is over, the sliding plate, where the core is attached, will move out of the mould and thus allow the moulded part to be ejected. The tapered lock is produced as one component of core A and used to guide the side core into the cavity precisely so as to ensure that the matching of the core is accurate.
Figure 9 shows the side core assembly. The core is mounted on the sliding plate. Besides the core, the sliding plate consists of recess holes for mounting the shaft and springs. As the core will indirectly contact with the molten plastic, it should be tolerant to high temperature. Therefore, cooling channels are drilled inside the core as shown in figure 9.
The working principle of the sliding side core is illustrated in figure 9. The sliding plate rests on the shaft that is mounted between the moving half cavity and the stopper. A bush is used to guide the sliding plate along the shaft and to prevent wear of the recess hole. The cam block is used to lock the sliding plate in position during the moulding process while the cam plate is used to activate the sliding plate. When the mould is closed, the cam plate is locked to the sliding plate by the slots. When the mould opens, the fixed half consisting of the cam block and the plate will cause the sliding plate to slide along the shaft, thus moving the side core away from the cavity.