If the load-carrying ability or the stiffness of a plastic part needs to be improved, it is necessary to either increase its sectional properties or change the material. Sometimes, it may be enough to change the material grade, for example using a higher glass fiber content. If that doesn’t suffice or isn’t desired for other reasons, increasing the sectional properties is often the solution.
In many cases, the simplest way to improve the sectional properties is increasing the part’s wall thickness. However, as is mentioned here, this has its limitations. Additional reinforcement can be obtained by adding corrugations or placing ribs perpendicular to the parts wall. Adding corrugations typically has a smaller impact on part weight and cooling time, but adding ribs has a bigger potential in terms of increasing the stiffness. Moreover, ribs allow for the facing or mating surface of a part to be smooth, which can be beneficial for aesthetic or functional reasons.
Adding corrugations to the design can stiffen flat surfaces in the direction of the corrugations (see Figure 1). They are very efficient and do not add large amounts of extra material or lengthen the cooling time. The extra stiffness is a result of increasing the average distance of the material from the neutral axis of the part, i.e. increasing the second moment of inertia.
Figure 1 - Corrugations
When adding ribs, the following guidelines for dimensioning should be applied (see Figure 2):
Figure 2 - Recommendations for rib dimensions
When placing ribs, the following guidelines for placement should be followed (see Figure 3):
Figure 3 - Rib orientation.
Figure 4 - Comparison of proﬁles in terms of torsional rigidity and bending.
Figure 5 - Torsional rigidity and resistance to torsional stress as a function of the way in which the ribs are connected to the profile.
Gussets can be used to reinforce corners, side walls and bosses. They can be considered as a subset of ribs, meaning that the guidelines for rib dimensioning and placement are also valid for gussets (see Figures 6 and 7).
Figure 6 - Guidelines for gussets.
Figure 7 - Height of the gusset.