This is one of the key to the injection-molding process, the injection mold forms the melt plastic into the desired shape, provides the surface texture, and determines the dimensions of the finished molded article. In facilitating mold cavity filling and cooling, the mold also influences the molding cycle and efficiency as well as the internal stress levels and end-use performance of the molded part.

Here list some principle of “plastic” mold design, which is a wrap up from C-1 to C-12,A mold design process could be divided into 2 parts as below:

• Before mold design start
• During mold design

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Before mold design start

In this part, mold manufacturer performs following activities:

• Inquiry: gather complete part information and customer’s expectation.
• Customer production ability survey: understand how to fulfill customer’s expectation.
• Generate DFM (design for manufacturing): present how mold manufacturer is going to manufacture the mold.
• Quotation: send cost along with mold specification and DFM to customer.

Before mold design starts, following information shall already become consensus of mold manufacturer and customer.

• Appropriate raw material

• Part design
  • SHALL
    have proper draft angle/dimension tolerance.
    have clear allowed parting line location.
    have clear allowed gate mark/area/location.
    have allowed weld line/sink mark area.
    have reasonable tolerance.
  • SHALL NOT
    lead to weak mold design (thin steel).
    have unsolvable undercut.
    have too high L/T ratio.
    have critical dimension that will cause any unstable measurement result.

• Customer production ability information
  • Injection Molding Machine (IMM)
    brand/type/model
    clamping/Injection unit
    location ring/nozzle head size
    machine plate layout
    other important information
  • Accessories
    robot specification
    hot runner system
    connection to mold

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During mold design

Mold designer starts to add following item into mold design, here are some suggestions for each item respectfully.

1. Location ring
   Diameter shall be slightly less than location ring on IMM plate.
   Radius shall be slightly bigger than nozzle head of IMM.(Prevent raw material leakage)
2. Sprue opening
   Diameter shall be slightly bigger than nozzle head opening of IMM.(Prevent raw material leakage)
3. Back plate
   Clamping hole location shall align with IMM plate design.
4. Runner system
   Proper runner system shall be provided according to component design/raw material/purpose to perform good filling.
   • Transparent component mold shall use semi hot/cold runner with “S” shape.
   • Appearance component is better to use semi hot/cold runner.
5. Gate
   Proper gate shall be provided according to component design/raw material/purpose to perform good filling.
   • Appearance component shall avoid submarine gate since flow mark.
   • Engineering/Function component is better to have direct gate.
6. Insert
   Shall be as many as possible.
   • It could help venting, positioning, reduce wear out…etc.
   • If the component has very long deep rib design, the area shall be made by insert.
7. Ejection
   • The contact area between ejector and component shall be as big as possible to reduce the white mark.
   • Shall equip with limit sensor for safety.
     • Is the part designed to stay on the moving side of the mold?
8. Slider
   • Shall be driven by cylinder if the slider size is too big.
   • The angle is better to keep as less as possible to assure the mold life.
     • Shall be take care more by maintenance.
9. Lifter
   • The angle is better to keep as less as possible to assure the mold life.
     • Shall be take care more by maintenance.
10. Core pulling
    • Shall equip with movement limit switch to prevent mold damage due to wrong operation.
      • Shall be take care more by maintenance.
11. Support pillar
    • Shall equip with enough support pillar to protect mold and extend mold life.
    • Suggest having support pillar at
      Center of mold
      Gate location
12. Cooling
    • The cooling channel distribution in mold shall be as even as possible to reduce component warpage/deformation after ejection.
    • The cooling channel diameter shall be a proper size to provide enough water flow for well heat exchange.
    • Location/accessibility are ease of installation and maintenance.
13. Venting
    Shall start from parting line to atmosphere (outside of mold) to achieve a well/complete venting target.
14. Issues
    Have current mass production issues been improved in new mold design??
15. Free area
    Top of mold has no interference to robot.
16. Sharp edge
    Sharp edge on mold parts shall be avoided to improve mold life and increases part stability.
17. Dowel pin
    Each mold plate shall be equipped with at least one dowel pin to ensure positioning.
18. Fix block
    Cavity insert shall be equipped with enough fix block to ensure positioning.
19. Wear plate
    Shall equip with enough wear plate to
    • Ensure the level of Fixed and moving side.
    • Extend mold life.
20. Zero block
    Shall equip with at least 2 zero block to
    • Ensure the level of Fixed and moving side.
    • Extend mold life.
21. Guide pin
    • Shall equip with 1 set guide pin to protect mold and extend mold life.
    • Guide pins enter at least 25mm before core enters cavity.
22. Ejector plate guide pin
    Shall equip with at least 4 ejector plate guide pins to ensure the level of ejector plate.
23. Return pin
    Shall equip with at least 4 return pins to protect mold and extend mold life.
24. Lifting Block/Ring
    Positioned for safe and balanced lift.

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These suggestions are only based on my experience. Plastic parts and molds design are really diversified and specific, there are too many more principles to be discussed in the industry. Please let me know if you have any question regarding mold design, we can have discussion case by case.