Design of Plastic Injection Molded Part
Designing a plastic injection molded part requires a combination of engineering knowledge, material science, and an understanding of manufacturing processes. A well-designed part not only meets functional and aesthetic requirements but also ensures efficient and cost-effective manufacturing. Below are key considerations and best practices for designing an injection molded part.
Key Considerations in Plastic Part Design
- Material Selection:
Choosing the right thermoplastic material is crucial for achieving the desired performance of the part. Factors such as mechanical strength, thermal resistance, chemical resistance, flexibility, and cost must be considered. Common materials include polypropylene (PP), acrylonitrile butadiene styrene (ABS), polyethylene (PE), and polycarbonate (PC). - Wall Thickness:
Maintaining a consistent wall thickness throughout the part helps prevent defects like warping, sink marks, and voids. Thin walls can reduce material usage and cooling time, but they must be thick enough to provide structural integrity. Typical wall thicknesses for plastic parts range from 1.0 mm to 4.0 mm, depending on the material. - Draft Angles:
Draft angles are necessary to ensure that the part can be ejected from the mold without damage. A draft angle of 1° to 3° is generally recommended, but more may be required for deeper or more complex features. - Ribs and Reinforcements:
To increase the strength and stiffness of a part without increasing wall thickness, ribs can be added. Ribs should be no more than 60% of the adjacent wall thickness to avoid sink marks and other defects. Rounded rib bases also help reduce stress concentrations. - Fillets and Radii:
Sharp edges and corners should be avoided as they create stress points and can cause cracking. Adding fillets (rounded edges) improves the flow of molten plastic during injection and increases the part’s durability. - Undercuts and Complex Geometries:
Undercuts are features that can prevent the part from being ejected from the mold. While undercuts can add functional value, they may increase mold complexity and cost. Features such as internal threads, snap-fits, and recesses should be designed carefully to avoid unnecessary mold modifications.
Design Features for Injection Molding
- Holes and Cutouts:
Holes and cutouts should be aligned with the mold opening direction to simplify manufacturing. Avoid placing holes too close to edges or ribs, as this can weaken the part. - Snap-Fit Connections:
Snap-fits are commonly used in injection-molded parts to eliminate the need for fasteners. They must be designed with flexibility in mind, using materials with good fatigue resistance. - Bosses:
Bosses are cylindrical features used for fasteners or assembly points. To prevent sink marks, the base diameter of a boss should be no more than twice the wall thickness, and fillets should be added at the base for reinforcement. - Living Hinges:
Living hinges are thin, flexible sections of plastic that allow a part to bend without breaking. They are typically used in one-piece molded designs such as containers or caps and require flexible materials like polypropylene (PP).
Gate Placement and Flow Considerations
- Gate Location:
The gate is the point where molten plastic enters the mold cavity. Proper gate placement ensures uniform filling of the part, reduces flow marks, and prevents weld lines. Gates should be positioned in areas where cosmetic defects are less noticeable. - Flow Path and Balance:
The flow of molten plastic should be uniform to prevent uneven cooling and warping. Flow channels within the mold, known as runners, should be designed to minimize flow resistance. - Venting:
Vents allow trapped air to escape from the mold cavity during injection. Poor venting can cause burn marks and incomplete fills.
Tolerance and Shrinkage
Plastic parts shrink as they cool and solidify. Different materials have varying shrinkage rates, which must be accounted for in the mold design. Shrinkage typically ranges from 0.5% to 2.5%, depending on the material and part geometry. Tighter tolerances require precision in mold design and may increase production costs.
Mold Design and Ejection System
- Ejector Pins:
Ejector pins help push the part out of the mold once it has solidified. Proper placement of ejector pins is essential to avoid leaving marks on critical surfaces. - Slides and Lifters:
For parts with complex geometries and undercuts, slides and lifters are used to release the part from the mold. These components add complexity and cost but allow for the inclusion of more intricate features. - Cooling System:
Efficient cooling channels within the mold help maintain consistent part quality and reduce cycle time. Poor cooling can lead to warping, sink marks, and internal stresses.
Best Practices for Designing Injection Molded Parts
- Simplify the Design: Avoid unnecessary complexity to reduce mold costs and improve manufacturability.
- Minimize Sharp Corners: Add fillets to improve plastic flow and reduce stress concentrations.
- Optimize Material Usage: Use ribs and hollow sections to add strength while minimizing material.
- Consider Assembly Requirements: Ensure that parts are designed to fit seamlessly with other components, whether through snap-fits, fasteners, or adhesives.
- Prototype and Test: Use 3D printing or prototype molds to validate the design before full-scale production.
Conclusion
Designing For Manufacturing (DFM) of a plastic injection molded part requires careful consideration of material properties, moldability, and functional requirements. By adhering to design best practices—such as consistent wall thickness, proper draft angles, and well-placed ribs—you can ensure that your part is both durable and cost-effective. Leveraging advanced design tools like SolidWorks can further enhance the design process by providing simulation capabilities that predict potential issues before manufacturing begins. By focusing on design efficiency and manufacturability, you can create high-quality plastic parts that meet performance goals and production demands.