1. Facilitates Machining and Maintenance. Molds are quite complex parts. During machining, we often encounter complex and special shapes that are difficult to process and maintain. For these shapes, inserts can reduce difficulty of machining and maintenance.
2. Facilitates Molding and Demolding. When plastic parts have deep ribs or other difficult-to-mold structures, these structures are prone to defects such as incomplete injection, burning, and joint marks during molding. Inserts can effectively solve this problem. Gaps around inserts not only facilitate venting during molding but also prevent vacuum sticking during demolding.
3. In areas with very strict tolerances and important dimensions of plastic parts, inserts are usually needed to meet requirements for easy adjustment.
4. Replace inserts to increase mold strength. When there are small areas of penetration (or contact) in molding parts such as inner molds or sliders, to enhance mold strength and extend mold life, penetrated (or contacted) part can be removed as an insert and replaced with a better material. Sometimes customers request to change label for same product, requiring interchangeable inserts.
5. Material saving and cost reduction. In molding parts such as inner molds or sliders, when some shapes protrude significantly above other surfaces or are difficult to process, inserts can be removed to save material and reduce processing costs.
6. Convenient mold-saving. For some deep areas, mold-saving is quite inconvenient, so inserts are used.
7. Convenient mold modification. For areas prone to wear or requiring high precision, inserts are used for mold modification considerations.
8. Heat dissipation considerations. This mainly refers to beryllium copper inserts.
9. Processing efficiency. Some large molds are divided into several smaller parts for separate processing, saving time.
Main reason for designing inserts (pins) on molds is to facilitate machining, melt filling of cavity, and replacement of easily damaged parts, thereby reducing machining costs, etc. However, insert (pin) design often reduces strength and rigidity of mold, can cause local stress concentration. Therefore, following points should be followed when designing inserts on molds: First, meet customer's appearance requirements; second, consider the overall service life of mold, avoid local stress concentration as much as possible, and avoid inserting in places where it is not necessary. Figure 4-47 shows router bottom shell and mold inserts. Its top and side surfaces have hundreds of holes with a diameter of 63.5mm. Some of these holes are heat dissipation holes (through holes), while others are blind holes. Center distance between holes is 6.5mm. These holes cannot be inserted into fixed mold and slider; they must be left in mold. If inserts are made, it will reduce mold strength, increase mold cost, and weaken mold cooling effect. Design of leaving fixed mold cavity in place requires attention to fact that small bosses corresponding to these holes need to be machined in one go using high-speed CNC, and attention should be paid to surface roughness to avoid manual polishing.

 

Figure 4-47 Router bottom shell and mold insert

Among mold processing equipment, EDM (Electrical Discharge Machining) is the slowest and has the lowest precision. Therefore, inserts are sometimes chosen to avoid EDM processing.
Figure 4-48 shows insert design for a game console lampshade mold. Plastic part is a dark red transparent plastic part made of PC. Top surface of plastic part is sloping with seven arc-shaped protrusions. Internal top surface and four sides all have arc-shaped wavy protrusions with varying spacing. These wavy protrusions extend to bottom edge of plastic part. Maximum external dimensions of plastic part are 106.95mm x 57.39mm x 47.28mm, and average thickness of plastic part is 2.32mm. Small wavy protrusions on four sides of core can only be pre-machined on insert C2A using a high-speed CNC machine tool, then inserted into core Co1. Inserted part is relatively small, while plastic part is relatively large; therefore, this type of insert is called a mushroom-shaped insert. If C2A and CO1 are manufactured as a single unit, root cannot be CNC machined to required depth. Electrical discharge machining (EDM) results in a rough surface, and polishing will deform wavy protrusions, affecting light transmission and appearance of transparent plastic part.

 

Figure 4-48 Insert design for game console lampshade mold
Figure 4-49 shows insert design scheme for printer cover plastic part. Back of cover has 18 deep ribs. These ribs reach a depth of 36.9mm, and their main function is to guide paper through. Their end faces are rounded, with the deepest point of ribs in the middle. Obviously, these ribs are prone to air trapping during injection molding, necessitating inserts for venting. To solve venting problem, inserts are made horizontally along tangent of ribs, so that insert marks do not affect paper passage. Because these venting inserts are larger at one end of parting line and smaller at bottom fixing part, they need to be assembled from parting line and secured using insert fixing blocks.

 

Figure 4-49 Design of venting insert on paper passage surface of printer cover
Figure 4-50 shows slider insert. Slider has a small protruding part, which requires a large amount of machining work. To simplify machining and facilitate replacement, it is disassembled into an insert.

 

Figure 4-50 Slider insert
Disassembling inserts requires analyzing machining process of mold parts to facilitate mold assembly and improve assembly accuracy. Generally, planar contact between inserts offers higher precision, while curved contact results in lower precision, greater difficulty in mold fitting and assembly, and a higher risk of defects such as burrs. Figure 4-51 shows analysis of plastic insert for paper guide strip at the end of a printer. Figure (a) shows plastic part, made of ABS, 390mm long. One side of plastic part has a long curved rib with a depth of 21mm. This rib is difficult to fill during injection molding, necessitating disassembly of insert. Figure (b) shows mold insert. Long rib is formed in moving mold. Moving mold insert is divided into left and right parts, positioned by a boss in the middle and connected by bolts. Left and right sides of moving mold insert have planar contact, facilitating improved machining accuracy. Curved surface of left side of moving mold insert can be machined by wire cutting, while curved surface of right side can be machined using a CNC machine tool. Figure (c) shows an incorrect insert parting line. Parting lines of left and right parts of moving mold insert extend vertically downwards, contacting with curved surfaces. This requires extremely high precision in machining of curved surfaces, making mold fitting difficult and representing a poor mold design approach.

 

Figure 4-51 Techniques for Inserts
1. Moving mold insert (left); 2. Plate A; 3. Fixed mold insert; 4. Moving mold insert (right); 5. Plate B

Insert involves designing mold forming parts using 3D software after determining mold opening direction and parting surface during mold design process. Its ultimate goal is to facilitate mold processing and manufacturing while ensuring dimensional and appearance quality of plastic part, meeting requirements of mold design specification. Inserts must possess sufficient fixing strength and stability, be easy to process and disassemble, be able to withstand injection pressure impacts, prevent displacement, elastic deformation, bending fracture when clamping force is released.
Advantages of insert are numerous. Analyzing from perspectives of mold machining precision control and mold materials, its advantages are mainly reflected in following aspects:
1. Insert facilitates various processing requirements of parts, greatly improving manufacturing efficiency, precision control, and processability. Especially for molds with complex shapes and large dimensions, a fully inlaid structure can be used, where permissible, to significantly reduce manufacturing and processing time and costs.
2. Different mold materials with varying properties can be selected for inserts, thus meeting requirements for hardness, surface roughness, wear resistance, and corrosion resistance of different structural parts on mold, saving high-quality mold materials while meeting mold requirements. Additionally, adding beryllium copper inserts to specific areas of mold can solve cooling problems.
3. Insert can meet mold forming process requirements. Since molds are prone to producing defective plastic parts due to gas buildup in cavity during injection molding, venting through fit clearance of inserts is typically used to facilitate gas expulsion.
4. Insert facilitates mold repair and replacement. Friction during production can easily cause mold wear or damage. Inserts, where permissible, can be installed in easily damaged areas of mold to facilitate repair and shorten repair time.

1. Assembly issues with inserts. The more inserts there are, the more difficult and time-consuming mold assembly and fitting become. Furthermore, excessive inserts can sometimes affect mold's strength.
2. Machining errors and mold fitting errors. During injection molding, parting lines can easily form on plastic parts at interface between insert and mold core, affecting quality of plastic part.
3. Location of insert can easily affect mold cooling system setup. Because it's difficult to arrange cooling water channels at location of insert , it can easily affect molding cycle of plastic part.