This case is a bracket on a sensor of a car. Precision requirements are very high, material is POM, product is very small, the longest dimension is 38mm, metal inserts (copper sheets) must be placed during injection molding, and deformation is required to be very small, see 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, location of glue inlet point is selected. All aspects must be considered when designing mold, upper and lower holes must be shaped after mold is released, as shown in Figure 2.

 

Gap between upper and lower holes is reversed, and core must be pulled in two directions before mold can be released. This brings certain difficulties to design of slider, as shown in Figure 3.

 

Core must also be pulled in this direction, see Figure 4.

 

During injection molding, an insert must be placed into movable mold. Insert is a very elastic copper sheet, as shown in Figure 5.

 

In order to prevent copper sheet from being deflected by plastic during injection molding, two small holes are provided on copper sheet, and a corresponding core is set in mold to position it, as shown in Figure 6.

 

After analysis, in order to reduce stress on product and minimize deformation, the best location for glue entry point is here, see Figure 7.

 

I adopted 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 interfered with fixed mold pins, which was very difficult to deal with. Therefore, I canceled fixed mold pins and used original core for perforations of fixed mold. , 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, see Figure 12.

 

Lower mold core and three sliders are arranged like this, see Figure 13.

 

It looks like this when you drop mold core and look at it from other side, see Figure 14.

 

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

 

This set of molds does not look complicated, but design of slider is still a bit difficult, and all aspects must be taken into consideration. Let’s look at slider 1 first, 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 are sealing surfaces, they must be treated as a unified plane and have a draft angle to form a plug-in fit with fixed mold. Moreover, mating surface must be very precise, so that bonding line on product surface is as small as possible, see Figure 18.

 

Mating surfaces of all sliders inserted into mold core must be sloped in direction of movement to prevent mating surfaces between slider and mold core from being roughened due to friction, see Figure 19.

 

Design of slider 3 is shown in Figure 20.

 

End face of slider 3 collides with movable mold core to form a sealing position. Mating surface extending into mold core has a slope of 3° in direction of movement to ensure that slider will not nap due to friction during long-term operation.

Power source of slider is three inclined guide pillars that push slider apart through mold opening force of injection molding machine. Inclined guide pillars are fixed on fixed template using inclined guide pillar fixing blocks. Fixed mold side is equipped with a plunger with a reset-first structure, as shown in Figure 21.

 

This set of molds has a very compact structure and uses a standard 1515 simplified small nozzle mold base, as shown in Figure 22.

 

This is what mold looks like after opening and before ejection, see Figure 23.

 

Force that pulls off gate relies on three nylon rivets in picture above. In order to make reset force more balanced, position of reset rod 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 were used, which is rare for such a small product, see Figure 24.

 

Since there are five ejector pins that interfere with slider, a reset-first structure must be set up, 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. It consists of four major parts: insertion rod, swing rod, roller and stop. When opening mold, inclined guide pillars push all sliders apart, see Figure 27.

 

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

 

The first reset mechanism is located on both sides of mold and is completely symmetrical, see Figure 29.

 

Since product is relatively small, and insert (copper sheet) needs to be placed in injection molding gap, injection molding cycle is relatively long, so requirements for cooling water path of this set of molds are not high. I adopted the simplest design. Since mold core is relatively small , water goes directly from mold plate. Fixed mold has two straight waterways, see Figure 30.

 

Same is true for movable mold, see Figure 31.

 

Design key points of this set of molds are arrangement of boundaries of slider 1 and slider 2 and selection of location of glue entry point.