Protective screen cover product is shown in Figure 1. Maximum external dimension of product is 161.50 mm * 114.40 mm * 62.10 mm, average thickness of plastic part is 3.15 mm, plastic part material is PP-CO, shrinkage rate is 1.018, and plastic part mass is 58.03 grams. Technical requirements for plastic parts are that there shall be no defects such as peaks, underfilling, flow lines, pores, warpage deformation, silver lines, cold materials, and spray lines.

 

Figure 1 Product map of protective screen cover
It can be seen from Figure 1 that plastic part has a Z-shaped structure, with 3 rub-through positions on the top surface and 3 places on the back that need to be designed for slider core pulling. Since plastic parts are twisted in a Z shape and height difference is large, it is primary task of mold design to reasonably arrange the three sliders to prevent plastic parts from being deformed. On the other hand, because parting surface is twisted and inclined, plastic part is made of PP, mold design and manufacture must consider that plastic part cannot produce a sharp edge.
According to shape of plastic parts, mold design and ranking are 2 cavities, mold base is a standard mold base CT 4050 A130 B100 C120, gate is a side gate, and glue is fed from side of plastic part. In order to avoid sharp edge of plastic parts during injection molding, the most effective measure is to design a hard mold, that is, heat treatment of front and rear mold cores. Mold material is 1.2344, heat treatment is 48~52HRC, and mold design diagram is shown in Figure 2. When 3D parting, it is necessary to make runner part flat to facilitate flow of plastic, and parting surface should be smooth. Smooth parting surface is convenient for improving cutting accuracy of CNC tools and for flying molds, so it can effectively improve mold clamping accuracy. Figure 4 and Figure 5 for front and rear die images, respectively.

 

Figure 2 3D drawing of mold

 

Figure 3 Mold parting surface

 

Figure 4 Slider design

 

Figure 5 Front mold core diagram

 

Figure 6 Rear mold core diagram
Soft mold means that hardness of mold core is below 44HRC, and steel used in mold core can meet requirements without heat treatment after buying it. .
Japanese steel NAK80 is one of harder pre-hardened steels, with a hardness of 38-41HRC, its fine and uniform structure is suitable for polishing. Rough machining and finishing of mold cavity mainly rely on CNC machining. Under current technical conditions, the most suitable hardness for steel processing is 38HRC. At this hardness, cutting tool has high durability and the highest processing efficiency.
Machining can both generate stress and eliminate residual stress, which may cause immediate deformation of workpiece, or deformation during subsequent heat treatment. Therefore, for large soft molds, it is recommended to vacuum workpiece after rough machining to relieve stress.
Hard mold means that hardness is above 45HRC, and steel used in mold core needs to be heat treated after buying it, such as quenching and tempering, in order to meet requirements of use. Such an injection mold is called a hard mold. For example, inner module is made of H13 steel, 420 steel, S7 steel and 1.2344 steel.
In addition to large difference in the life of mold, soft mold and hard mold generally have no effect on product.
There are many manufacturing processes for hard molds, and cost of molds will increase by more than 30% compared with same soft molds. Purpose of quenching is to greatly improve strength, hardness, wear resistance, fatigue strength and toughness of steel, so as to meet long life requirements of molds . Since quenching will cause deformation of workpiece, it is necessary to design a reserved amount of deformation.
Typical processing technology of hard mold core is: increase machining allowance by 0.5mm in three directions of length, width and height of mold core. It should be noted that verticality tolerance of blank in each direction cannot be greater than 0.02; Pen marks reference angle and parts to be processed.
Slider design of mold is shown in Figure 4. All sliders are driven by inclined guide columns.
Plastic parts are ejected by thimble, and in design, torso is guide of thimble plate.