Rex Plastics Product Development

Plastic Part Design Fundamentals

Ribs and Bosses

Rib Design

Ribs are a great way to add strength and stiffness to a part while keeping material consumption to a minimum. However, a word of caution:  ribs can cause sink marks to develop where they intersect the main wall. A rule of thumb to follow is the rib thickness should not exceed 60% the nominal wall thickness. Also keep in mind that thin, deep ribs can be very expensive to add to a mold.  Use the table at the bottom of this article as a guideline for rib design. Still, keep in mind that deeper ribs may require an electrical discharge machining (EDM) process.

Rib Width:  50%-60% of nominal thickness

Boss Design

Bosses are one of the most common features seen in plastic parts. However, similar to ribs if bosses get too thick relative to the nominal wall thickness, sink can occur. A boss-rib combination can eliminate sink marks. By using ribs to connect the boss to a side wall, this method of part design will provide the strength necessary to support screws, inserts, etc. Also, adding small radii to break the sharp corners will also greatly reduce stress concentrations.

In the part above, notice the poorly designed boss on the left compared to the well designed boss on the right.

Rib Depth

Avoid thin, deep ribs if possible. Thin, deep ribs are made using an EDM process which will add cost to the mold. It is also worth noting that thin, deep ribs tend to increase the difficulty of hand polishing. Below is a general guideline of rib width to depth ratios.

Pilot Holes

Screws are a common way to fasten two plastic pieces. Self-tapping screws eliminate the need for molded threads. The table below lists common pilot hole sizes for various self-tapping screws.

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Browse additional examples of plastic injection molded parts with draft angles in our gallery. Interested in developing a plastic product?  Learn about our Prototyping and 3D Printing Services here.