Melet Plastics Inc.

Design Resource Center

To overcome the problems that can arise with thick walls, ribs are an effective means of increasing rigidity while allowing wall thickness to be reduced. Generally, the rigidity of a component can be increased the following ways:

• increasing the wall thickness
• increasing the elastic modulus (e.g. by increasing reinforcing fiber content)
• incorporating ribs into the design

In rib design, a large moment of inertia can most easily be achieved by providing high, thick ribs. However, with engineering thermoplastics, this approach usually creates serious problems such as sink marks, voids and warpage. Furthermore, if rib height is too great, there is a risk that the rib structure will bulge under load. For this reason, it is absolutely necessary to keep rib dimensions within reasonable proportions.

To ensure trouble-free ejection of the ribbed component, it is essential to provide a sufficient draft angles for ejection. For components requiring a very high quality surface finish, rib dimensioning is important. Correct rib design reduces the tendency to form sink marks and thereby increases component quality. Material accumulation at the rib base is defined by the imaginary circle shown below. By adhering to the dimensional proportions recommended there, this circle can be made as small as possible and sink marks can be avoided or reduced. If the imaginary circle is too large in this area of material accumulation, voids can be formed and mechanical properties drastically lowered as a result.