PTA Plastics - Boss and Rib Design
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Rib & Boss Design

By: PTA Plastics June 4, 2021

This is part 3 of our 4-part mini-series on Early Supplier Involvement. One of the key areas of DFM is Rib and Boss design. In many ways, proper rib and boss design is directly related to reducing sink by maintaining nominal walls. We’ll break this section down into 2 parts: Ribs, and Bosses. In many cases, ribs and bosses work together. 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

· Warp

· Non-fills

· Gas traps

· Molded in stress due to high injection pressures

· Difficulty ejecting the part from the mold

Here are some good general practices to follow when incorporating ribs into your part designs:

· 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 noted above 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.

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. Below is an example of an “X” pattern:

Deep Ribs:

Deep ribs pose their own challenges because it becomes a game of give and take when managing a 2/3 rib intersection to nominal wall while not getting too thin at the tip of the rib to the point of causing filling and venting issues. Some of these concerns can be addressed by the way the mold is constructed. For example, less draft on the ribs and inserting steel in localized sections which will allow for venting. If the rib is too deep and gases at the front of the material flow cannot escape to atmosphere, it can result in a burn, or black mark in the material or a non-fill, or short in the part. Figure 2 below shows and example of a deep rib: 

Boss features for self-threading screws, brass threaded inserts, and stand-offs are common design elements of plastic parts. It is important to incorporate proper boss design into your parts as boss features that are not correctly designed can be very problematic. 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

Correct boss design shares many common features of good rib design. The wall thickness of the boss should be considered a rib feature and should follow the guidelines laid out for rib design.

All of the same guidelines for ribs should be applied to bosses. As you can see in the cross section above, the boss has the same characteristics as a rib as it attaches to the adjacent wall.

There are many things to consider when incorporating bosses into your design. It is important to support a free-standing boss by either tying it into an adjacent wall or rib, or by adding gussets to a rib. If a boss is connected directly to a wall or rib it will inherently result in a thick section.

Below are some examples of what to avoid:

When designing bosses for threaded inserts there are additional things to consider. For example, the type of insert, installation method (Heat or Ultrasonic), and print tolerances. Also, whether or not the insert needs to be flush or sub-flush.

There is a lot to consider when dealing with ribs and bosses during part design. Here at PTA Plastics, we are ready to assist you through the design process to ensure that your part is not only ready for tooling, but will make your part both cosmetically and mechanically appealing. Contact us today for your next project.