17 Key Considerations for Injection Mold Design

1. Opening Direction and Parting Line

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In designing any injection molded product, the opening direction and parting line must first be determined to minimize core withdrawal mechanisms and eliminate the impact of parting lines on appearance.

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1) After determining the opening direction, structural elements such as ribs, snaps, and protrusions should be aligned with the opening direction to avoid core withdrawal, reduce seam lines, and extend mold life.

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2) Once the opening direction is set, an appropriate parting line can be chosen to avoid undercuts, improving both appearance and performance.

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2. Draft Angle

1) An appropriate draft angle can prevent surface marring. A smooth surface should have a draft angle of at least 0.5 degrees, a fine texture surface greater than 1 degree, and a coarse texture surface greater than 1.5 degrees.

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2) An appropriate draft angle can prevent product damage such as top marks, deformations, and breakage.

3) For deep cavity structures, the outer surface angle should ideally be greater than the inner surface angle to ensure the mold core does not misalign during injection, achieving uniform wall thickness and ensuring material strength at openings.

3. Wall Thickness

1) Each type of plastic has a specific range of wall thickness, generally between 0.5 to 4 mm. Exceeding 4 mm can lead to prolonged cooling times and shrinkage issues, necessitating structural changes.

2) Uneven wall thickness can cause surface shrinkage.

3) Uneven wall thickness can result in gas pockets and weld lines.

4. Ribs

1) Proper use of ribs can increase product rigidity and reduce deformation.

2) Rib thickness must not exceed (0.5 to 0.7) times the product wall thickness to avoid surface shrinkage.

3) The draft angle on ribs should be greater than 1.5° to prevent damage.

5. Fillets

1) Small fillet radii may lead to stress concentration in the product, causing cracks.

2) Small fillet radii may lead to stress concentration in the mold cavity, resulting in cavity cracking.

3) Setting appropriate fillet radii can improve machining processes, allowing direct milling with R cutters instead of less efficient electrical machining.

4) Different fillet radii may shift the parting line; thus, proper selection should be based on practical considerations.

6. Holes

1) Hole shapes should be as simple as possible, typically circular.

2) The axis of the hole should align with the opening direction to avoid core withdrawal.

3) For holes with a length-to-diameter ratio greater than 2, a draft angle should be included. The diameter should be calculated based on the smaller diameter (maximum solid dimension).

4) The length-to-diameter ratio for blind holes generally should not exceed 4 to prevent bending of core pins.

5) The distance from the hole to the product edge should typically be greater than the hole diameter.

7. Core Withdrawal, Slider Mechanisms, and Avoidance

1) If a part cannot be smoothly demolded in the opening direction, a core withdrawal slider mechanism should be designed. This mechanism can form complex product structures but may introduce seam lines, shrinkage, and increased costs, shortening mold life.

2) When designing injection molded products, avoid core withdrawal structures whenever possible unless special requirements necessitate them. Adjust the axial direction of holes and ribs to align with the opening direction.

8. Integrated Hinges

1) Utilizing the toughness of PP material, hinges can be designed as an integral part of the product.

2) The thickness of the hinge film should be less than 0.5 mm and kept uniform.

3) When designing an injection molded integrated hinge, the gate should only be on one side of the hinge.

9. Inserts

1) Incorporating inserts into injection molded products can enhance local strength, hardness, dimensional accuracy, and allow for small threaded holes (shafts), catering to various special requirements while increasing product costs.

2) Inserts are typically made of copper, but other metals or plastics may also be used.

3) The portion of the insert embedded in the plastic should be designed with anti-rotation and anti-pullout features, such as knurling, holes, bends, or shoulders.

4) The plastic surrounding the insert should be appropriately thickened to prevent stress cracks.

5) When designing inserts, consider their positioning within the mold (holes, pins, magnetic).

10. Markings

Product markings are generally placed on a relatively flat inner surface using a raised format, on a surface aligned with the normal and opening direction to avoid marring.

11. Precision of Injection Molded Parts

Due to the uneven and uncertain shrinkage rates during injection molding, the precision of injection molded parts is significantly lower than that of metal parts. Tolerance requirements should not simply adopt mechanical part dimensions but should be based on standards like GB/T14486-93 for engineering plastics. Designers should determine dimensional tolerances based on the plastic material used and the requirements of the finished part, considering factory capabilities and the precision of similar products.

12. Deformation of Injection Molded Parts

Enhancing the rigidity of injection molded structures can reduce deformation. Avoid flat structures where possible, and rationally set up flanges and ribbed designs.

13. Snap Fit Design

1) Design snap fit mechanisms to share multiple snap points, ensuring the overall device remains operational even if individual snap points fail, thus increasing lifespan. Consider adding fillets to enhance strength.

2) Precise tolerance requirements for snap fit dimensions are critical. Excessive undercuts can cause damage, while insufficient undercuts can lead to assembly issues or looseness. A solution is to reserve space for easy modification and adhesive application.

14. Welding (Heat Plate Welding, Ultrasonic Welding, Vibration Welding)

1) Using welding can enhance joint strength.

2) Welding can simplify product design.

15. Rational Consideration of the Conflict Between Process and Product Performance

1) When designing injection molded products, it is essential to consider the conflicts between appearance, performance, and process. Sometimes sacrificing some processability can yield better appearance or performance.

2) If structural design cannot prevent injection defects, aim to position defects in less visible areas of the product.

16. Screw Column Hole Diameter and Self-Tapping Screw Diameter Relationship

17. BOSS Design Principles

1) Support columns should not be used alone but should connect to the outer wall or be used with ribs to enhance strength and improve material flow.

2) The height of the support column should generally not exceed 2.5 times its diameter, as excessively tall support columns can lead to air traps during molding (length can cause gas pockets, burning, insufficient filling, etc.).

3) If the height of the support column exceeds 2.5 times its diameter, especially when far from the outer wall, enhance the strength of the support column using ribs.

4) The BOSS shape should primarily be circular, as other shapes can be difficult to process.

5) The BOSS should be positioned at a sufficient distance from corners or outer walls.

6) Surrounding the BOSS can be thinned (i.e., creating a "volcano" effect) to prevent shrinkage and sinking.

7) The demolding angle for the BOSS: typically 0.5° externally, and 0.5° or 1° internally.