Plastics Design Tools in Solidworks: Ribs

Solidworks has a collection of features that were intended for plastic parts, although the word “plastic” has been reserved for Solidworks Plastics. The Solidworks Plastics name is essentially reserved for a Moldflow-like analysis module dedicated to plastics flow in the mold, not related to design methods at all. You won’t find the word “plastic” used to refer to these features inside of the software. You’d think it would at least get a CommandManager tab, but no such luck.

This is the first in a series of posts under the heading of plastic design functions in Solidworks to include topics like shell, rib, draft, split lines, indent, snap, lip, mounting boss and evaluation tools. This first post is dedicated to demonstrating the functionality of the Rib feature in Solidworks.

Plastics designers are a little bit of a cult. They are possibly a little secretive. They don’t like to share a lot of information. They don’t tend to show off a lot of cool plastic part designs. I only got to meet a few when I’ve gone into companies to consult on designs or went in on an interview. The only thing more difficult than plastics design to me is mold design. Mold design is plastics design, but inside out and much more conscious of the materials and the process. Not that plastic part designers aren’t conscious of the process, but mold designers are obsessed.

Much of the plastics design work I have done started off in the surfacing track. We’re not going to talk about surfacing today, it’s much too early in the day for that. I’m just going to talk about the Rib feature today.

Ribs

The Rib feature in Solidworks is pretty cool. It has two main modes which I call “Skyline” and “Plan view”. These aren’t names that Solidworks uses, I just had to have something to call the different methods, so I started using these terms. The names kind of describe how they work. The Extrusion Direction option in the PropertyManager shows “parallel to sketch” and “normal to sketch” as tool tips, so I guess these are the SW official names.

The Rib feature uses open contour sketches, and the rib itself will extend up to the next solid or trim itself at the next solid. This means the sketch itself can be a little loose and not necessarily well defined.

When you start the Rib feature, an arrow will show the direction in which it is going to add material. If that direction doesn’t intersect the part, you need to use the Flip Material Side option, and the arrow will go the other direction.

With a Skyline rib, shown here going between two bosses, you sketch along the midplane of the rib, and it allows you to add changes in height of the rib. The sketch can be in the middle of the rib, or on the left or right using the Thickness toggles.

You can’t get the rib to extend to the far side of either boss with a single feature. You’d need a second rib feature to do that. If you make the sketch extend to another area, Solidworks will decide for you which area to put the rib in.

Ribs can add draft right in the rib feature. Both Skyline and Plan view ribs will do this, but it requires some extra thought with a Skyline rib because of the changes in height. You have to decide which level you want the draft to start from using the Next Reference button. This moves the material side arrow from one sketch entity to the next to use as a draft neutral plane. The Next Reference button does not show up until you enable draft for the rib.

And of course with the draft, you can specify the direction in which it should add material with the Draft Outward toggle.

In Plan view, you sketch looking down onto the rib. You can kind of see this in the screen captures shown here. You can see that the Plan view rib can create more disjoint areas, but they are all at the same height as the sketch.

Plan view can use a combination of open and closed sketch elements, as shown in the screen shot. The gray circular boss was created with the rib feature with a closed circle as a part of the sketch with 4 open ended lines. (The blue boss was created with the Indent feature, which will be featured in a later post in the plastics series).

Multi Body Considerations

The Rib feature is maybe singularly sensitive to multibody issues. First, the rib feature only recognizes solids, and doesn’t care at all about surface bodies. Also, if you create a rib in a multibody part, it will not automatically select the body for you. You have to do that manually.

The most odd thing about the rib and multibodies is that if the number of bodies changes at the point in the tree where the rib is, the feature will fail, and you have to reselect the solid body. It’s not hard to do, it’s just hard to understand why it works that way.