Mastering the Injection Moulding Process Step by Step

The product of the left front cover of the fax machine is shown in Figure 1. The maximum dimensions of the product are 186.80 mm x 58.60 mm x 55.00 mm; the average thickness of the plastic part is 2.50 mm, the material of the plastic part is HIPS-HB, the shrinkage rate is 1.0045, and the weight of the plastic part is 32.40 grams. The technical requirements for plastic parts are that there should be no defects such as peaking, injection dissatisfaction, flow lines, pores, warping deformation, silver streaks, cold material, and jetting lines, and meet ROSH environmental protection requirements.

Figure 1 Product picture of the left front cover of the fax machine

The left front cover of the fax machine is a partial cover on the top surface of a certain Japanese brand fax machine. As can be seen from Figure 1, the shape of the plastic part is composed of a flat plate and an inclined surface, with a deep square hole on the top and multiple bone positions on the back. At first glance, the structure of the plastic parts is very simple, without inclined roofs and sliders. In fact, there are hidden undercuts at the corners of the inclined surface of the plastic part, and it is necessary to design a slider to release the mold. If there is no such design concept and the existence of undercuts cannot be imagined during mold design, big mistakes will occur. In many previous articles, the three elements of mold design have been expounded, namely parting surface, gating system and ejection method. Before the design of the parting surface, the mold opening direction was determined first, and the side of the rectangular deep cylinder was left in the fixed mold, and the side of the multiple bone positions was left in the movable mold. The analysis of the three elements of the mold is shown in Figure 2.

Figure 2 Analysis of the three elements of the mold

Figure 3 3D drawing of mold

The design of the two sliders of the mold adopts the concept of safety design, as shown in Figure 6. After the mold is opened, the slider will retreat a certain distance with the movement of the mold. In order to ensure that the slider can return safely, a limiting device must be installed on the slider to ensure that the slider remains in place after the mold is opened and moves to the required distance. move. Considering the safety factor, the spring is usually used together with the inclined guide post or the bent pin. The spring cannot be used alone as the driving element of the slider. The spring is easy to fatigue and break, which will cause the failure and accident of the mold. Especially the built-in spring inside the mold is prone to failure at high temperature, and the failure of the built-in spring is not easy to detect, and it is easy to damage the mold and even cause accidents. Therefore, for European customers, they generally do not like the built-in spring.

There are generally two safety design methods for the external spring on the slider. For larger sliders, the spring force should not be less than 1.5 times the weight of the slider. Figure 6 shows that the bolt sleeve is used as the limit device of the external spring, and the bolt sleeve is fixed on the slider by bolts. The bolt sleeve is made of mold steel 638 with good toughness and cannot be heat treated. This design is safe and reliable, and avoids the danger of the spring being easily broken by directly fixing the spring with plug bolts and causing the spring to fly out. The other is the design of springs and protection devices outside the mold. The design of adding protection devices to the outside of the slider can effectively play a role in safety protection.

For the selection of the specifications of the spring bolts, choose M8 for the small slider, and M10 or above for the large slider. The unilateral gap between the bolt sleeve and the slider baffle is 0.5, and there is no counterbore at the installation place of the bolt sleeve, just align with the tail of the slider. There is no gap between the bolt and the bolt sleeve, that is, the inner hole of the bolt sleeve corresponding to the M10 bolt is ø10, and the bolt is 8.8 grade bolts, and the limit screw (plug screw) cannot be used.

Figure 4 3D diagram of the rear mold core

Figure 5 Exploded view of the front and rear molds

Figure 6 Mold safety design

Figure 7 The water transport circuit of the mold

The exploded views of the front and rear mold cores are shown in Figure 5. Design the explosion diagram and mark the drawing number, the image is intuitive, and it is convenient for all aspects of mold processing and assembly. For the water transportation of the front and rear molds, a three-dimensional diagram is designed. The function of this diagram is to design and manufacture it on the mold nameplate, so that the injection molding personnel can connect the water transportation circuit. The water transport circuit of the mold is shown in Figure 7.

The gate of the plastic part is a side gate. After cutting, it will be covered during assembly and will not affect the overall appearance of the fax machine. The ejection of plastic parts adopts ejector pin ejection. The thimble brand is MISUMI Japanese brand.

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