This case is a bracket on a sensor of a car. Precision requirement is very high, material is POM, product is very small, the longest dimension is 38mm, a metal insert (copper sheet) should be placed during injection molding, and deformation is required to be small, as shown in Figure 1.

 

Non-concentricity of upper and lower holes of this product is less than 0.02mm. Since POM (polyoxymethylene) products are prone to deformation, in order to minimize internal stress of product, position of glue inlet point is selected. Design of mold should be considered comprehensively, upper and lower holes should be shaped after mold is released, as shown in Figure 2.

 

There is an undercut in gap between upper and lower holes, and core must be pulled in two directions before mold can be released, which brings certain difficulties to design of slider, as shown in Figure 3.

 

Core should also be pulled in this direction, as shown in Figure 4.

 

When injection molding, an insert should be put into moving mold. Insert is a copper sheet with good elasticity, as shown in Figure 5.

 

In order to prevent copper sheet from being biased by plastic during injection molding, two small holes are set on copper sheet, and corresponding cores are set in mold to position them, as shown in Figure 6.

 

After analysis, in order to reduce stress of product and minimize deformation, the best position of glue point is here, as shown in Figure 7.

 

I used form of point gate, see Figure 8.

 

Mold flow analysis is provided by Moldex 3D company, see Figure 9.

 

Due to tight space, gate I designed interferes with fixed mold pins, which is very difficult to deal with. Therefore, I canceled fixed mold pins, and I used original core for forming fixed mold perforation, see Figure 10.

 

This can leave a reasonable position for gate tie rod, see Figure 11.

 

The overall structure of mold adopts a simplified small nozzle structure, and adopts a first reset device, as shown in Figure 12.

 

Lower mold kernel and three slide blocks are arranged like this, see Fig. 13.

 

Hidden and lower mold kernels look like this in reverse, as shown in Figure 14.

 

Front mold core is designed like this, see Fig. 15.

 

This set of models does not seem complicated, but design of slider is still a bit difficult, and all aspects of relationship must be taken into account. First look at slider 1, see Figure 16.

 

Relationship between slider 1 and slider 2 is shown in Figure 17.

 

Since slider 1 and slider 2 and their common boundary is sealing surface, it should be processed into a unified plane here, and there must be a draft slope, which is inserted into fixed mold. Moreover, mating surface should be very precise, so that bonding line on the surface of product should be as small as possible, as shown in Figure 18.

 

All mating surfaces where sliders are inserted into mold should be sloped in the direction of motion to prevent mating surfaces of sliders and mold from being rough due to friction, see Figure 19.

 

Design of slider 3 is shown in Figure 20.

 

End face of slider 3 touches moving mold core to form a sealing position, and mating surface extending into mold core has a 3° slope in the direction of movement to ensure that slider will not be roughened due to friction during long-term work.

Power source of slider is that the three inclined guide pillars push slider apart through force of injection molding machine, and inclined guide pillars are fixed on fixed mold plate by using inclined guide pillar fixing blocks. Side of fixed mold is provided with a plunger with a reset-first structure, as shown in Figure 21.

 

Structure of this set of molds is very compact, and standard 1515 simplified small nozzle mold base is used, as shown in Figure 22.

 

After mold is opened, it looks like this before ejection, as shown in Figure 23.

 

Force to break gate depends on three nylon pull studs in above picture. In order to make reset force more balanced, position of reset lever is also carefully arranged.

In order to reduce internal stress of product and minimize deformation, I used more ejector pins to make ejection force of each part of product relatively balanced. A total of 10 ejector pins are used, which is really rare for such a small product, as shown in Figure 24.

 

Since five ejector pins interfere with slide block, a first-reset structure must be provided, as shown in Figure 25.

 

Now let me introduce one of the most common pre-reset mechanisms, see Figure 26.

 

The first reset mechanism is also called pre-reset mechanism, which is composed of four large parts: inserting rod, swing rod, roller and stopper. When mold is opened, inclined guide post will push slide block apart completely, as shown in Figure 27.

 

Since plunger has been pulled out, pendulum has room to rotate. When top column of injection molding machine pushes push plate, due to action of roller, pendulum rotates along pin axis (15 degrees here), see Figure 28.

 

The first reset mechanism is arranged on both sides of mould, which is completely symmetrical, as shown in Figure 29.

 

Since product is relatively small, and inserts (copper sheets) need to be placed in the gap of injection molding, cycle of injection molding is relatively long, so requirements for cooling water circuit of this mold are not high. I adopted the most simplified design, because mold core is relatively small, water is taken directly from mold base. Fixed mold is 2 straight waterways, see Fig. 30.

 

Same is true for moving mold, as shown in Figure 31.

 

Design points of this set of molds are arrangement of boundaries of slider 1 and slider 2 and selection of position of glue inlet.