The Mold Cavity of Overmolding Handheld Forehead Thermometer Casing

The design of the overmolding mold cavity is central to the design of the complete set of plastic parts. Since the structure between the upper and lower shells in the complete set of plastic parts has a mirror-symmetric relationship, after the design of the overmolding mold cavity for the upper shell plastic part is completed, it is mirrored and copied to obtain the overmolding mold cavity for the lower shell plastic part. When designing the overmolding mold cavity, the basic principles to be followed are: ① Hard materials use the first molding and soft materials use the second molding because soft plastics are easy to deform. ② Transparent materials use the first molding and non-transparent materials use the second molding. ③ Materials with higher injection molding temperatures use the first molding and materials with lower molding temperatures use the second molding. The design of the overmolding mold cavity structure for the upper shell plastic part is shown in Figure 2. In this structure, the base hard shell of the upper shell is molded during the first injection molding, and then the soft shell is injected during the second molding. The base hard shell cavity consists of the base hard shell cavity insert and the base hard shell core insert, while the soft shell cavity consists of the soft shell cavity insert, the soft shell core insert, and the base hard shell plastic part. The shape and structure of the core inserts in both the first and second molding cavities are identical, while those of the cavity inserts differ. The second molding cavity is obtained by rotating the first molding cavity 180° around the mold center and converting the base hard shell cavity insert into the soft shell cavity insert.

 
Figure 2 Mold cavity structure for the upper shell plastic part

When the two cavities of the first and second molding are parted and sealed, the sealant penetrates rather than being inserted. Therefore, the parting surfaces P1 and P2, when the cavity is closed, are planar parting surfaces. When the P1 and P2 parting surfaces are designed in the software, the parting surface P1 of the base hard shell core insert and the parting surface P2' of the coated soft shell core insert must be consistently set, both using the parting surface P2' of the coated soft shell core insert. In contrast, the parting surface P1" of the base hard shell cavity insert and the parting surface P2" of the coated soft shell cavity insert must be set according to the shapes of the base hard shell plastic part and the final two-color plastic part, respectively. The structure of the coated soft shell core insert is set to match that of the base hard shell core insert to ensure that after the movable mold rotates 180° around the centerline of the two-color mold, the coated soft shell core insert and the base hard shell cavity insert can close properly. Simultaneously, the base hard shell core insert and the coated soft shell cavity insert can also close. Additionally, it is crucial to ensure that the base hard shell plastic part located on the base hard shell core insert does not fall off when rotating 180°. The waste materials from the gate GI and runner RI are automatically separated and ejected by the pull rod after the second injection molding. In the second molding cavity design, when the soft plastic completely covers the hard plastic, only the shrinkage rate for the hard plastic needs to be set. However, when the hard plastic and the soft plastic are only connected at the outline, the shrinkage rates for both the hard plastic and the soft plastic must be set. When designing the size of the mold covering the soft shell cavity, only the shrinkage rate of the hard plastic is set, and the size of the covering soft shell cavity is adjusted accordingly (set according to the shrinkage rate of PC + ABS 0.45%). Simultaneously, to ensure the sealing effect of the second molding cavity, the size of the hard shell cavity in the first molding cavity is set larger (according to the shrinkage rate of PC + ABS 0.65%) so that it can be pressed by the covering soft shell cavity insert during the second molding, thus ensuring proper sealing. In the design of the soft shell cavity insert for the second molding cavity, to prevent the soft shell cavity insert from inserting (or scratching) the glue position of the base hard shell plastic part that is better in the first molding, the surrounding area of the local protruding part of the hard shell plastic part in the soft shell cavity insert needs to be locally avoided by 0.2 mm. This avoids deforming the base hard shell plastic part under the injection pressure of the second molding, which could result in flash problems during the second injection molding.
 
Since there is a side groove feature on the upper shell plastic part of the two-color molding, but no soft plastic around it, the side core of the molding side groove feature can be directly withdrawn after the first molding. When the second molding cavity is closed, the side core does not need to be reset, and the reset is completed in the first molding cavity during the next cycle. The cooling water transportation arrangement of the first and second molding cavities is designed to be as efficient, balanced, and consistent as possible, with 0.10 mm pipes used for cooling. During the second molding injection, because the base hard shell plastic part contains multiple 06 mm screw columns, it will not move, and its glue position will not deform. Based on the structural characteristics of the plastic part and the need to simplify the mold structure, the gate GI of the base hard shell plastic part uses a latent gate or a point gate during the first molding injection, which is not conducive to pouring. Therefore, a side gate is used, and its size is shown in the enlarged figure E of Figure 2. The gate for the second molding injection is a latent gate G2, with its size shown in the enlarged figure F of Figure 2. When designing the gate G2, a hole must be reserved at the corresponding position of the base hard shell plastic part to ensure that guide column Z1 can direct the molten material flow from gate G2 into the second molding cavity.
 
Since the overmolding mold for the complete set of plastic parts uses a 180° rotating injection mold structure, the coated soft shell core insert and the base hard shell core insert are identical in structure. When designing the precision of molded parts, the size and accuracy of the coated soft shell core insert (base hard shell core insert), base hard shell cavity insert, and coated soft shell cavity insert must be consistent, ensuring that these components are well-matched. To achieve this matching effect, the slope drop of the insertion and collision surfaces of the coated soft shell core insert (base hard shell core insert) and the base hard shell cavity insert and coated soft shell cavity insert is set to be greater than 0.1 mm. The lower shell overmolding cavity is also designed according to these specifications.