Ribs
Ribs can increase the structural strength of a part without increasing its wall thickness, prevent deformation and warping under force, and are also one of the ways to reduce product costs. In some structural designs, ribs connect two different structural units inside the product, which facilitates plastic flow during injection molding. This characteristic is particularly important for large parts.
For ribs in injection molded parts, the following six principles should be followed in specific design:
Thickness Principle (Preventing Sink Marks): The thickness at the root of the rib is the primary design consideration. It must be less than the main wall thickness of the product, typically recommended to be 40% to 60% of the nominal wall thickness. This effectively prevents sink marks on the appearance surface opposite the rib.
Height and Proportion Principle (Preventing Instability): The height of the rib generally should not exceed 3 times its root thickness. If greater height is needed for increased strength, it is recommended to design multiple short ribs instead of one tall rib, or to add supporting structures.
Draft Angle Principle (Preventing Sticking): A sufficient draft angle (typically 0.5°~2°) must be designed to allow for smooth ejection. Too small a draft angle increases ejection resistance and can even scratch the product.
Fillet Principle (Preventing Stress Concentration): A fillet (R angle) must be designed at the connection between the rib and the product wall. This eliminates stress concentration points, avoiding cracking under force or during molding.
Layout and Spacing Principle (Balanced Force Distribution): When multiple ribs are used simultaneously, the spacing between them should be greater than or equal to 2 times the wall thickness. In layout, the direction of the ribs should, as much as possible, align with the flow direction of the melt in the mold cavity to avoid flow obstruction and short shots.
Functional Principle (Goal-Oriented): Ribs should be placed in the areas of the product that bear the most force. They can also serve as channels to improve filling during molding or as support/positioning structures for other components.
Layout Methods for Ribs:
Generally, there are two types of rib layouts:
The first is a square grid distribution, usually in horizontal and vertical directions, or it can be made into a diagonal grid distribution. It can provide uniform structural reinforcement, and this method is also the most common practice.
The second is a circular or fan-shaped distribution. This method is mainly used for circular or fan-shaped parts, providing reinforcement throughout the entire cylindrical direction. For different force requirements, different forms of circular ribs/bosses can be used.
Of course, these two forms can also be combined, for example, combining circular effects with square grids for easy ejector pin placement, comprehensively increasing strength and facilitating injection molding. However, mold machining costs are relatively higher. The specific form of the boss/rib needs to be considered comprehensively based on product structural strength and cost requirements.
Additional Key Design Details:
The height of a rib should generally not exceed 3 times the wall thickness. Excessive height increases mold machining difficulty, affects product ejection, and can cause air trapping during injection molding. Trapped air is sometimes addressed by adding inserts or venting channels on the mold. Depending on product size, wall thickness, and material, rib thickness requirements also vary, but it generally should not exceed half the part's wall thickness. Too thick a rib will cause sink marks on the appearance surface. If the part has high strength requirements, the number of ribs can be increased, or the part's wall thickness can be appropriately increased. When a rib is too thin, causing difficulties in molding and ejection, small support posts can be appropriately added in the middle of the rib to increase strength and facilitate ejector pin placement. The distance between two ribs is generally not less than 3 times the wall thickness. The spacing between ribs should not be designed too small, as this can create thin steel sections on the mold, affecting mold life. Although ribs are a small detail in structural design, sometimes it's the details that determine success. Mastering each detail is a necessary condition for the smooth launch of a product.
