For previous reading, please refer to Small Home Appliance Structural Design – Plastic Part Wall Thickness (with Mind Map and Material Lis.
Design of reinforcing ribs in plastic parts is crucial. It not only affects product's strength and rigidity but also significantly impacts molding quality and cost.
Role of Reinforcing Ribs
Improving Strength and Rigidity: Like I-beams, reinforcing ribs effectively improve strength and rigidity of plastic products, reducing twisting during use and ensuring appliance's structural stability, allowing it to withstand certain external forces without deformation.
Enhancing Molding Flow: Reinforcing ribs help improve molding flow, allowing plastic to flow and fill better in mold, ensuring all parts of product are fully filled and reducing molding defects.
Reducing Material Costs: Properly designed reinforcing ribs can often reduce overall material consumption while maintaining performance. Replacing part of wall thickness increase with reinforcing ribs can save a significant amount of plastic raw materials, reducing material costs.

 

Relationship between Reinforcing Rib Design and Plastic Materials
From a Production Perspective: Material physical properties such as melt viscosity, shrinkage rate, creep characteristics, and ejection characteristics affect reinforcing rib design. For example, height of reinforcing rib is limited by melt flow and ejection characteristics. Increasing ejection angle aids ejection but may affect rigidity. This can be addressed by adding ejector bumps or using flat ejector pins to reduce ejection area.
From a Structural Perspective: Deeper reinforcing ribs can increase product rigidity and strength, but buckling stress must be kept within acceptable limits.
Mold Production: A large number of short and narrow reinforcing ribs are preferable. During mold production, width, depth, and number of reinforcing ribs should allow for adjustments during trial molding.

 

Nine Principles of Reinforcing Rib Design
Thickness of reinforcing rib should be less than wall thickness T of plastic part, generally 0.5-0.7T: This is because if reinforcing rib is too thick, dents are easily formed at connection point with plastic part, affecting product's appearance and quality. Maintaining an appropriate thickness ensures the overall stability and aesthetics of plastic part.
Height of reinforcing rib should not be too high, generally less than 3 times wall thickness of plastic part: Excessively high reinforcing ribs may have opposite effect, reducing their rigidity. Therefore, when designing, if enhanced support is needed, it is better to increase number of reinforcing ribs than to easily increase their height, to ensure effective functioning of reinforcing ribs.
Distance between multiple reinforcing ribs should be no less than 2mm, and at least twice thickness of plastic part: Distance between multiple reinforcing ribs should not be less than 2mm, and at least twice thickness of plastic part. Simultaneously, reinforcing ribs should be properly distributed and staggered. Reasonable spacing and distribution can prevent deformation of plastic part due to uneven shrinkage during molding, ensuring dimensional accuracy and stability of product.
Reinforcing ribs should be 0.5mm lower than supporting surface: This design ensures that when plastic part is placed on a flat surface, supporting surface can make stable contact, avoiding instability caused by excessively high reinforcing ribs, thus improving user experience.
Draft angle of reinforcing ribs is generally 0.5-1.5°, but can be increased to 4°-5° in some special cases: A suitable draft angle can greatly reduce resistance of reinforcing ribs during demolding, ensuring smooth ejection of plastic part from mold and reducing product damage or surface defects caused by demolding difficulties.
Design of reinforcing ribs should adhere to principle of uniform wall thickness: Uniform wall thickness helps plastic flow evenly in mold, reducing shrinkage stress concentration caused by excessive wall thickness differences. This lowers probability of quality problems such as cracks and deformation, improving product reliability.
Root radius of reinforcing rib should be 0.25-0.5 times thickness of plastic part: Appropriate root radius effectively reduces stress concentration, improves melt flow and reduces defects, prevents shrinkage, increases strength of connection between reinforcing rib and plastic part body, preventing breakage during use.
Most reinforcing ribs should align with plastic flow direction (sprue direction): Most reinforcing ribs should align with plastic flow direction, i.e., gate direction. This facilitates smooth filling of mold cavity during injection molding, ensuring reinforcing rib area is fully formed, improving overall quality and molding efficiency of plastic part.
Adding an angled tip to reinforcing rib prevents air trapping: Adding an angled tip to reinforcing rib is a small design change that effectively prevents air trapping. Trapped air can lead to defects such as bubbles and voids inside plastic parts, affecting product performance. Angled tip design guides air to escape smoothly, improving product quality.
Design Considerations for Different Materials
ABS: Thickness of ribs on the surface of main components should not exceed 50% of thickness of intersecting plastic material; for non-deterministic surfaces, this can reach 70%; for thin-film structural foamed plastic parts, this can reach 80%; for thick-film parts, this can reach 100%; rib height should not exceed three times thickness of plastic material; when there are multiple ribs, spacing should not be less than twice thickness of plastic material; draft angle should be set on one side for easy demolding.
PA: Height of a single rib should not exceed three times bottom thickness; small ribs or grooves should be added behind ribs to eliminate back dents.
PBT: Avoid thick ribs to prevent bubbles, shrinkage marks, and stress concentration; for wall thicknesses below 3.2mm, rib thickness should not exceed 60% of wall thickness; for wall thicknesses exceeding 3.2mm, rib thickness should not exceed 40%; rib height should not exceed three times rib thickness; use a 0.5mm radius (R) at rib-to-wall connection.
PC: Rib thickness depends on plastic process and wall thickness. Thinner ribs provide better reinforcement; refer to PS rib design guidelines.
PS: Rib thickness should not exceed 50% of adjacent wall thickness; otherwise, uneven surface gloss will occur.
PSU: Ribs can enhance product impact strength, but poor design can lead to surface shrinkage marks and undesirable impact strength.
These are nine operating principles for reinforcing ribs in plastic parts that I have summarized from my work and studies. When designing reinforcing ribs, you can check whether you have followed these nine principles.
Of course, we still need to operate according to actual situation of product and parts. Principles are rigid, but people are flexible. The most important thing for engineers is to exert their initiative to explore.