Design rules of plastic parts
Designing plastic parts requires adherence to specific rules and guidelines to ensure the parts are functional, cost-effective, and manufacturable. By considering factors such as demolding, material flow, and structural reinforcements, designers can avoid common pitfalls and produce durable, high-quality components. Below are the key design rules for plastic parts.
1. Demolding Considerations
Plastic parts must be designed with draft angles to facilitate easy ejection from the mold. A typical draft angle ranges from 1° to 3° to prevent the part from sticking to the mold. Parts without proper draft angles may experience defects, damage during ejection, or require costly mold modifications. Additionally, features such as undercuts should be minimized or designed to use side-action cores to allow smooth demolding.
2. Limited Flow Path
The flow of molten plastic during injection molding is limited by material viscosity and the complexity of the mold geometry. Long or narrow flow paths can result in incomplete filling, air pockets, or weld lines. To avoid this, the design should minimize sharp turns and include balanced flow channels. If long flow paths are unavoidable, using multiple gates or flow enhancers can improve filling.
3. Gate Position and Defect Prevention
The gate is the point where molten plastic enters the mold cavity. Proper gate placement ensures even filling and prevents issues such as weak weld lines and air traps. Placing the gate near thicker sections and away from critical surfaces can reduce visible flow marks and strengthen the part. Additionally, the number and size of gates should be optimized to avoid uneven pressure distribution, which can weaken the structure.
4. Avoid Material Accumulation
Material accumulation occurs when thick sections of plastic form due to uneven wall thickness or overlapping features. These thick areas cool more slowly, leading to internal stresses, sink marks, and warping. To avoid this, designers should maintain uniform wall thickness and use features like ribs or hollow sections instead of solid, bulky areas. Gradual transitions between thick and thin sections also help prevent stress concentrations.
5. Uniformly Thin Walls
Uniform wall thickness ensures consistent material flow, prevents internal voids, and reduces cooling time. Thin walls (typically ranging from 1.0 mm to 4.0 mm depending on the material) minimize material usage and increase production efficiency. Avoiding sudden changes in wall thickness also helps maintain the structural integrity of the part and reduces the risk of warping or cracking.
6. Stiffeners (Ribs, Beads, and Crowns)
Ribs, beads, and crowns (raised or recessed features) enhance the strength and stiffness of plastic parts without adding significant weight or material. Ribs should be designed with a thickness no greater than 60% of the adjacent wall to avoid sink marks. Beads and crowns can reinforce large flat surfaces, preventing flexing and distortion. Additionally, adding fillets at the base of ribs and stiffeners improves stress distribution and reduces sharp transitions.
7. Service Temperature and Dimensional Stability
The dimensions of plastic parts can change under varying service temperatures due to the material’s coefficient of thermal expansion. Plastics generally expand and contract more than metals, which can affect tight-fitting parts or assemblies exposed to heat or cold. Designers must account for these changes by adding clearance where needed and selecting materials with lower thermal expansion for high-temperature applications.
Conclusion
By following these design rules, engineers can create plastic parts that are manufacturable, durable, and free from common defects. Proper attention to demolding, flow paths, gate placement, material distribution, and reinforcement features helps ensure consistent quality and performance. Understanding the relationship between service temperature and part dimensions also ensures that the design remains functional in real-world operating conditions. With these best practices in place, plastic parts can be optimized for both performance and production efficiency.