Boss and Rib Design - A Key Area for DFM

Boss and Rib design is a crucial aspect of DFM. Maintaining nominal walls is key to reducing sink by ensuring proper rib and boss design. We’ll divide this section into two parts: Ribs and Bosses. Collaboration between ribs and bosses is common in many instances. For example, a rib may be used to support a boss by either tying it into an adjacent wall or used as gussets when there are no other walls to connect to.

Ribs

Ribs are an effective way of adding stiffness to a part or feature without having to use a thicker nominal wall. Ribs are also very effective as alignment features and stand-offs as well as supporting other features like bosses. It is critical to incorporate proper rib design as ribs that are not properly designed can be problematic. Potential problems associated with poorly designed ribs are:

Sink marks

Stress risers

Warpage

Non-fills

Gas Traps

Molded in stress due to high injection pressures

Difficulty ejecting the part from the mold

It’s advisable to strongly consider the following as a general guideline:

Sharp corners should always be avoided to minimize stress risers and notch sensitivity. A minimum .005/.010 fillet radius should be incorporated at all inside corner intersections.

The width of the rib as measured between the outer tangent points of the fillets radii must be a percentage of the nominal wall thickness.

For amorphous materials such as polycarbonate, ABS and PC/ABS, the maximum percentage for avoiding sink marks is 70%.

For semi-crystalline materials such as polypropylene, nylons and polyesters, the maximum percentage for avoiding sink marks is 60%.

The percentages referenced here are general guidelines. For parts that are highly cosmetic, dark in color and/or will have a high gloss finish, these percentages should be reduced by 20%.

The ideal draft for ribs is .5° to 1° per side. Less than .5° per side of draft can become difficult to eject.

Draft greater than 1° per side can be problematic especially on deep ribs. The ribs will become too thick at the top intersection and will create sink marks and they can become too thin at the bottom and become difficult to fill.

X Pattern

There are many things to consider when using ribs. For example, linear, or straight ribs (depending on the direction they are placed) may increase stiffness, but may also increase warp in the part depending on the shrink rate of the material. Higher shrink rate material will have increasingly more warp. One option to help mitigate warp without impacting stiffness is to use an “X” pattern, though this will not completely eliminate it. Above is an example of an “X” pattern.

Deep Ribs

Managing a 2/3 rib intersection to nominal wall in deep ribs presents challenges of balancing thickness at the rib tip and avoiding filling and venting problems. Certain concerns can be resolved through the mold’s construction method. An option is to decrease draft on the ribs and incorporate steel in certain sections to enable venting. When the rib is excessively deep and gases cannot escape, it can lead to material burn or black marks, as well as non-fill or short in the part.

Proper Boss Design

Common design elements of plastic parts include boss features for self-threading screws, brass threaded inserts, and stand-offs. Incorporating appropriate boss design is essential to avoid potential issues with your parts. Potential problems associated with poorly designed boss details are:

Sink marks

Core pin deflection

Molded in stress due to high injection pressures

Difficulty ejecting the part from the mold

There are several common features between good rib design and correct boss design. When designing the boss, the wall thickness should be regarded as a rib attribute and comply with the rib design guidelines. The same guidelines for ribs should also be applied to bosses.

Areas to Avoid

There are multiple considerations when incorporating bosses into your design. To support a free-standing boss, you can either connect it to a nearby wall or rib, or reinforce the rib with gussets. When a boss is directly attached to a wall or rib, it will unavoidably lead to a thick section. There are more considerations to think about when designing bosses for threaded inserts. Examples of these include the type of insert, installation method (heat or ultrasonic), and print tolerances. Additionally, determine if the insert should be flush or sub-flush.

Part design involving ribs and bosses requires careful consideration of multiple factors. Our team at PTA Plastics is here to support you in every step of the design process, guaranteeing a part that is both visually appealing and mechanically sound.