How do you like your ribs? We recommend well done because failure to observe basic design rules on thickness, height and radii could lead to premature failure of an otherwise flawlessly designed part. Poor rib design could also create cosmetic problems in a part or slow molding time.
The primary purpose of ribs in plastic design is to improve the stiffness of the structure. Ribs do this by increasing sectional properties, specifically the moment of inertia. Because stiffness is a function of moment of inertia and Young's modulus, you can also improve stiffness by increasing the modulus of the material. You can do this either by using more glass fiber or by using a material with a higher modulus. However, there are often practical and economic limitations to this approach.
When evaluating the use of ribs, first consider potential causes of stress.
"When we work with customers on parts that failed, we need know if it was due to excessive load or excessive deflection," explains Sebastien Petillon, CAE engineer for Solvay Advanced Polymers of Alpharetta, GA. "These are two different reasons for part failure and cannot be dealt with in the same way."
Failure due to excessive load can be remedied by adding a rib, which increases the moment of inertia and improves stiffness. Failure when deflected, often seen on mating parts of a snap-fit design, requires a change in geometry to decrease the moment of inertia and reduce stress on the part.
Rules for Ribs Use
Once you've determined that a rib is the appropriate solution, you must observe a few basic rules in their design.
Start with rib thickness, which can affect part weight, cosmetics, warpage and moldability. Thick ribs can cause internal voids as well as sink marks on the part surface opposite from where they are attached. The amount of sink is also affected by the type of material, processing conditions, surface texture and relative location to a gate. Materials with high stiffness and low shrinkage rates, such as IXEF® polyarylamide, create less sink. One rule of thumb, as shown in the diagram at the top of this page, is to limit rib thickness to 40% of the thickness of the wall to which it is attached to minimize sink marks. Go up to 60 percent to maximize strength. Consult your materials supplier for a recommendation specific to your application.
The guidelines apply to the thickness at the base of the rib. The rib should be tapered as it rises from the wall to create a draft angle for easier ejection (see discussion of draft angle in Part IV of this series here http://rbi.ims.ca/4398-502).
Keep in mind the surface characteristics of the opposite wall when considering rib thickness. If the appearance is critical or glossy, play it safe with thinner ribs. Another idea: You can disguise slight sink marks with steps, a textured surface, or through the use of product markings on the opposite wall.
Rib thickness can also affect moldability of the part. "It is a common misunderstanding that ribs act as flow leaders and help balance flow in a complex part," comments Kirit C. Desai, CAE manager at Solvay Advanced Polymers. As stated, ribs should be thinner than the intersecting wall. By definition, a flow leader is a local increase in thickness to improve flow in a required direction. "Therefore, most of the time, ribs with thickness less than base wall thickness do not enhance the flow," continues Desai. "In the majority of cases, it winds up acting as a stiffener rather than the intended flow leader."
One thought: If this is a gas-assist application, location of gas channels at the base of thick ribs can avoid problems associated with excessive shrinkage, such as sink marks or warpage.
Additionally, very thin ribs-particularly those located close to a gate-can create filling problems. Melt flow entering a thin rib can slow down and begin to freeze off while thicker wall sections are still filling. Thickness of the rib (or any type of plate) also affects shrinkage, and as a result the tendency to warp.
Consider Rib Height
Tall ribs are a good idea, but height should generally be no more than four to five times the thickness of the adjoining wall. Ribs that are too tall can create mold filling and venting problems. One option is to design multiple, smaller ribs that can provide the same level of stiffness.
It's always easier to add ribs to a design than to remove them. Incorporate a minimum number of ribs in your original design, then add them as testing dictates.
Another design consideration is radii for the internal corners of ribs. Corners with small or no radii are a major cause of failure under load because they concentrate stress. It's important to calculate the stress concentration created by an internal corner. Use handbooks on strengths of materials to obtain formulae that estimate the stress concentration factor for different geometries.
Rule of thumb: Choose an internal corner radius equal to or greater than one-half of the thickness of the part, or at least 0.6 mm. This rule applies to all internal radii, including those for ribs.
To access The Design Engineer's Portal for High-Performance Plastics, please go to http://rbi.ims.ca/4398-502.