(Last Updated On: July 8, 2021)

Unreasonable design of plastic parts often leads to some inherent defects in the product. Problems such as unreasonable structures, plastic material too much used, poor quality may occur. The design of plastic parts is directly related to the quality, cost and level of your product.

This article lists the design basic guidelines of ribs, bosses and fillet, connected with problems that are common in the actual development process. I hope it is helpful for designers and engineers in practical design work.


Ribs are indispensable functional structures on many plastic parts. It can effectively improve the rigidity and strength of plastic parts like the H beam. By using ribs, it is not necessary to greatly increase the wall thickness of the plastic parts, and ribs are not undercut structures like the H beam whose structure will be difficult to make for plastic injection molding. So ribs work well for plastic parts that are subject to pressure, torsion and bending. Ribs can also act as internal flow passages to aid in the plastic melt filling flow, which is of great help to the formation of complex and large plastic parts.

Ribs are generally designed on the non-appearing surface of the plastic part, and its direction is as close as possible to the flow direction of plastic melt filling. Ribs should neither be too long nor too thick, otherwise, it will increase the difficulty of injection molding, causing problems such as voids and sink marks on the back.

Regarding the ribs design, we need to talk about mainly 2 kinds of plastic materials, thermoplastic and thermosetting plastic.

Thermoplastic refers to a plastic that has heat softening and cooling hardening properties. Most of the plastics we use in our daily lives fall into this category. They can be recycled and reused, such as  ABS, Acrylic, Nylon, PE, PP, PS, PC…

Thermosetting plastic refers to a plastic that cures when heated, it is insoluble and does not melt. They can only be formed once, such as Epoxy resin, Polyurethanes, Vulcanized rubber…

Ribs and bosses design guidelines in plastic parts

Generally recommended measurements for ribs in thermoplastics (Figure above)

    • T: Nominal wall thickness
    • A: Base width of a rib: 0.3-0.7T
    • B: Rib height: less than 3T
    • C: Spacing between ribs: more than 4T
    • D: Radius at base of rib: T/8
    • E: Draft of rib: 0.5°-2°

Ribs and bosses design guidelines in plastic parts

Generally recommended measurements for ribs in thermosetting plastics.

    • A: Base width of rib
    • B: Top width of rib: 0.5T
    • C: Rib height: 3T
    • D: Radius at top of rib: T/4
    • E: Radius at base of rib: T/4
    • F: Draft of rib: 5°


Bosses are common structures on plastic parts in home appliances and interior and exterior of automobiles, mainly for positioning and fastening.

Ribs and bosses design guidelines in plastic parts(Size in mm)

Ribs and bosses design guidelines in plastic parts

The base of bosses often cause sink marks on the back surface of a part, this is because of the large wall thickness. The solution is to core out some material at the base, as in the below figure.

Ribs and bosses design guidelines in plastic parts


It is important to create a smooth transition as possible as you can between two intersecting surfaces of a plastic part. There are four advantages to do so:

  1. Disperse the stress, enhance and fully utilize the mechanical strength of the plastic parts.
  2. Improve the fluidity of the plastic melt facilitate filling and demolding, and eliminate defects such as the sink at the corners of the wall.
  3. Conducive to the machining and heat treatment of the injection mold, which is helpful to improve the service life of injection mold.

The chart below shows the relationship between wall thickness and the fillet radius.

F: Stress

R: Fillet radius

T: Nominal wall thickness

Reasonable fillet radius: 0.25≤R/T≤0.6 or R min≥0.4mm

Ribs and bosses design guidelines in plastic parts

Fillet: Poor design VS Good design

Ribs and bosses design guidelines in plastic parts

These are some of the design points and common problems of ribs, bosses and fillets. In future articles, I will continue to share the basic design guidelines for other design elements.

If you have any questions about the and development of plastic parts, please feel free to contact us at www.sidotek.com. Contact Email: info@sidotek.com.

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