In plastic mold production process, there are many fits between molded parts, mating surfaces between moving and fixed molds are usually designed with a certain slope, which can effectively avoid wear between core mating surfaces, as shown in Figure 1. In automotive connector products, due to structural limitations, slope A can only be designed to be 0.05°. However, for plastic parts with square holes, when square hole is formed by movable mold core and side-drawn core, it is difficult to guarantee only inclination of 0.05°~0.1°. There is a gap (0.01~0.02mm) between side-drawn core and movable mold core, which is prone to flashing. When there is a slight interference, it is easy to wear and burn mating surface during long-term production. It is difficult to ensure that molded plastic part will not be flashed, nor will mold parts wear and burn mating surface. Problems and corresponding solutions of this type of mating surface in production are explained through examples.

 

 

 

 

Structure of plastic part is shown in Figure 2. There are 12 square holes and material is PBT GF10, which is formed by side-drawing core and movable mold core. Positional relationship between side-drawing core 2 and moving mold core 3 is shown in Figure 3. There are flashes around square hole of molded plastic part during first trial production. Flash width is 0.05~0.08mm after measurement. As shown in Figure 4 and Figure 5, quality requirements are not met and need to be rectified.

After flash is generated in the first mold trial, measure core size and meet processing requirements of drawing. Clearance of movable mold core of side-drawing type is 0.01~0.02mm. Reduce injection pressure during mold trial, square hole flash of molded plastic part is reduced, but other positions of molded plastic part are insufficiently filled. In order to reduce matching gap, bottom of movable mold core 3 was increased by 0.03mm and flash of molded plastic part was found to be reduced, but flash of molded plastic part became larger after 300 injections, and core mating surface was worn out when mold was opened. After movable mold core 3 is increased by 0.03mm, side-drawn core 2 and movable mold core 3 have an interference fit. Side-drawn core 2 rubs with movable mold core 3 during mold opening and closing process, causing a gap after mating surface is worn, resulting in flash around square hole of molded plastic part.
Through above two mold trials, it is concluded that: ①Matching position of square hole of molded plastic part cannot have a gap after mold is closed; ②Matching core of square hole of molded plastic part should avoid wear during mold opening and closing process.

Contact surface wear is caused by excessive friction during core movement. Friction is calculated as follows:
f=μ×Fn
Among them, f is friction force; μ is dynamic friction factor; Fn is positive pressure.
Friction force f is proportional to dynamic friction factor μ and positive pressure Fn. To reduce friction force, positive pressure Fn and dynamic friction factor μ should be reduced. Dynamic friction factor μ is an inherent property of an object, its size is related to material and roughness of contact surface. Influence factors of dynamic friction factor μ mainly include following three aspects.
(1) Surface roughness. Mating surfaces of side-drawing core 2 and movable mold core 3 are processed by a surface grinder, surface roughness is Ra0.4~0.8μm, and roughness processed by surface grinder is lower than that of electric processing. Although polishing contact surface will reduce roughness value, it will affect dimensional accuracy, core of square hole of molded plastic part is prone to collapse angle defects.
(2) Temperature. Test proves that metal is heated to a certain temperature in a vacuum and kept for a period of time to remove surface dirt. Dynamic friction factor measured after cooling is much greater than dynamic friction factor measured at room temperature. Barrel temperature of PBT material is 230~260℃, and temperature will continue to rise during mold production process. Although there is cooling water to cool down, increase of dynamic friction factor is inevitable.
(3) Molecular structure of material surface. Cohesion between molecules on contact surface of different materials is different, and friction factor is also different, which affects friction between objects. Mold steel grade currently in use is SKD61 with a hardness of 46~52HRC. If a copper-steel combination is used, friction factor can be reduced, but hardness of copper is insufficient and does not meet requirements of mold. You can consider increasing mold core hardness or coating core to increase core wear resistance. However, in mass production, as mold temperature rises, contact surface wear is inevitable, fundamental problem cannot be solved.
Through above analysis, there is no room for optimization of dynamic friction factor μ, and it is necessary to consider reducing positive pressure Fn to reduce friction force f.
Side piece in mold is fixed by a guide rail. Positive pressure Fn acting on side-drawn core 2 comes from side-drawn piece 4. During injection process, positive pressure Fn helps side-drawn core 2 to close to moving mold core 3 to avoid flash. However, during mold opening and closing process, side-drawn core 2 and movable mold core 3 have relative movement, positive pressure Fn will aggravate wear of mating surface. At this time, positive pressure Fn is unfavorable. Therefore, key to problem is to eliminate or reduce positive pressure Fn during mold opening and closing.

 

Based on above analysis, design a separate floating insert at matching position of side drawing core 2 and movable model core 3. Design floating insert 3 in side drawing core 1, floating insert 3 needs to be higher than side drawing after assembly, design a 0.1mm gap between floating insert 3 and hanging table of side drawing core 1, floating insert 4 is designed in the side drawing core 2. And assembly relationship is shown in Figure 6.

 

Mold clamping state is shown in Figure 7. Floating inserts 3 and 4 are tightly attached to movable mold core 6 under action of fixed mold core 1, clamping system of injection molding machine guarantees compaction when mold is closed, and molded plastic part does not produce flash during injection process. After mold is opened, fixed mold core 1 leaves. Since there is a gap of 0.1mm between floating insert 3 and hanging platform of side-drawing core 2, floating inserts 3 and 4 are active in mold opening direction, positive pressure between floating insert 4 and movable mold core 6 that are in contact with each other is Fn≈0, friction force f≈0 during mold opening, no contact surface wear will occur during mold opening and closing process. If space permits, set springs in floating inserts 3 and 4. After mold is opened, there is no function of fixed mold core 1. Floating inserts can also float up under action of spring without contact with movable mold core to avoid friction. Service life of parts is longer, and molding quality of plastic parts is also guaranteed.

 

 

According to above analysis, mold was rectified. After mold test, it was found that there was no flash at square hole of molded plastic part (see Figure 8), and plastic part after mold production for 24h was tested on projector without flash (see Figure 9). Quality is qualified, and there is no wear on mold core.
Through square hole flash problem of molded plastic part in production example, from perspective of principle of mechanical friction, mechanism of molded plastic part flash is analyzed, and key factor to solve problem is to reduce positive pressure. Designed floating insert structure ensures that no flash is produced during injection process, at the same time eliminates positive pressure that causes mold core to wear during mold clamping process, and achieves purpose of mass production of qualified plastic parts. In view of successful case of this mold change, this floating insert structure can be used in similar mold designs in the future to avoid wear between cores, improve quality of molded plastic parts, and extend service life of mold parts.