Injection Molding Design Guide: Creating Quality Parts with Efficiency

Introduction

Injection molding is a widely used manufacturing process known for its ability to produce large volumes of high-quality polymer parts with excellent repeatability. To achieve optimal results, it is important to consider various design factors during the injection molding process. In this blog, we will explore essential design guidelines that can help you create successful injection molded parts while balancing cost-effectiveness and functionality.

Material Selection

Carefully choosing the appropriate material for your injection molding design is crucial. Different materials offer varying properties, such as dimensional stability, chemical resistance, and impact resistance. Consider factors like temperature, pressure, and biological or chemical interactions when selecting a material. Thermoplastic resins, for example, can be classified as amorphous or semi-crystalline, each with its own advantages and disadvantages. Understanding the material properties will help you achieve the desired tolerance levels and structural features in your parts.

Tolerance Considerations

Tolerances in injection molding are influenced by the shrinkage that occurs during the cooling process. Understanding the material's shrinkage characteristics is essential for determining the appropriate tolerances for your parts. Amorphous materials, like PLA, generally have tighter tolerances compared to semi-crystalline materials like PEEK. Tight tolerances may be necessary for parts that require proper fit or functionality within an assembly. It is recommended to consult with your supplier during the design stage to discuss tolerance standards and ensure the manufacturability of your parts.

Optimal Wall Thickness

Choosing the right wall thickness is critical for successful injection molding. Thinner walls can reduce cycle time and lower part costs. For many applications, a wall thickness of 1.5-2.5 mm is sufficient, but it is advisable to refer to recommended wall thickness guidelines for different materials. Uniform wall thickness throughout the part is important to avoid issues like sink marks and warping during cooling. Non-uniform thickness should not exceed 15% of the nominal wall thickness and should have a smooth or tapered transition to ensure high-quality parts.

Incorporating Draft Angles

Designing parts with draft angles is essential to facilitate easy ejection from the mold. Without draft angles, parts can get stuck and cause damage to the mold during ejection. A draft angle is a slight slant added to the walls of the part, allowing for smooth release. The amount of draft required depends on the specific surfaces and textures. Near-vertical surfaces typically require a draft angle of 0.5°, while common situations may need 1-2°. Shutoff surfaces and those with textures may require greater draft angles. By incorporating draft angles, you can enhance the functionality and manufacturability of your injection molded parts.

Strengthening with Ribs and Gussets

Certain parts may require additional strength to withstand structural loads. Ribs and gussets can provide the necessary reinforcement, eliminating issues like warping, sink marks, and voids. Instead of increasing the overall thickness of the part, it is preferable to add ribs and gussets strategically. However, it is important to ensure proper rib thickness, typically between 50-60% of the attached wall thickness, to avoid shrinkage-related problems. By incorporating ribs and gussets effectively, you can improve the strength and durability of your injection molded parts.

Smooth Corners with Radii and Fillets

Sharp corners in injection molded parts can lead to weak points and stress concentrations. By applying radii and fillets to corners, you can improve material flow during molding and enhance the structural integrity of the part. Rounded corners are less likely to get stuck during ejection and can help reduce stress points. It is advisable to avoid sharp corners wherever possible, except for parting surfaces or shut-off surfaces. Including internal and external radii of appropriate sizes will contribute to smoother production and better part quality.

Avoid Undercuts and Provide Slots Where Possible

Undercuts, which are features that prevent straightforward ejection from the mold, can complicate the injection molding process. Designing molds with undercuts increases tooling costs and may cause material entrapment during ejection. Instead of using undercuts, consider incorporating slots or pass-through cores to achieve similar functionality. By adjusting the parting line and draft angles appropriately, you can address undercuts on the outer surfaces of the part. Another option is to use stripping undercuts or bump-offs, but these should only be employed when the part can flex and expand during ejection.

Attach Bosses to Side Walls or Ribs

Bosses, cylindrical standoffs used for inserting screws, pins, or other components, should be attached to side walls or ribs rather than being freestanding. Attaching bosses to the structure of the part using ribs or gussets enhances their stability. The outer diameter of the boss should be approximately 2.5 times the diameter of the screw for self-tapping applications. To avoid visible sink marks on the outer surface, the thickness of the ribs or gussets supporting the boss should not exceed 60% of the overall part thickness. Maintaining proper attachment and thickness ratios will ensure the strength and functionality of the bosses within your injection molded parts.

Consider Gating for Visual Appearance

The gate location plays a crucial role in the appearance of an injection molded part. Gates are entry sections where molten material flows into the mold. It is important to strategically position gates to minimize potential quality issues. Gates can leave visible marks or vestiges on the part, even if subtle. To achieve the desired aesthetic and functional requirements, communicate your gating preferences and limitations to your supplier. By highlighting visually important surfaces where no gate marks should appear, you can ensure the overall visual appeal of your injection molded parts.

Conclusion

Designing for injection molding requires careful consideration of various factors to ensure the production of high-quality parts at an efficient cost. By following these injection molding design guide, including material selection, tolerance considerations, optimal wall thickness, draft angles, ribs and gussets, and smooth corners, you can enhance the manufacturability and functionality of your injection molded parts. Collaborating with experienced suppliers like Alpine Mold and leveraging their expertise can further optimize your design for successful injection molding. By integrating these guidelines into your design process, you can create injection molded parts that meet your expectations while maximizing efficiency and quality.