The nature of design analysis obviously depends on having product-performance requirements.
Even though many potential factors can influence a design analysis, each application fortunately usually involves only a few factors. Here we explain some of the factors:
There are different techniques that have been used for over a century to increase the modulus of elasticity of plastics. Orientation or the use of fillers and/or reinforcements such as RPs can modify the plastic. There is also the popular and extensively used approach of using geometrical design shapes that make the best use of materials to improve stiffness even though it has a low modulus. Structural shapes that are applicable to all materials include shells, sandwich structures, and folded plate structures. These widely used shapes employed include other shapes such as dimple sheet surfaces. They improve the flexural stiffness in one or more directions.
In the discussion of uniform wall thickness, the ribbing was one of the suggested remedies. Ribs are also used to increase load-bearing requirements when calculations indicate wall thicknesses are above recommended values. They are provided for spacing purposes, for supporting components, etc. The first step in designing a rib is to determine dimensionally limitations followed by establishing what shape the rib is to have in order to realize a product with good strength and satisfactory the appearance that can be produced economically.
If performance calculations indicate wall thicknesses well above those recommended
for a particular material, one of the solutions to the problem is to find equivalent cross-sectional properties by ribbing. Heavy walls can be responsible for the reduction in properties due to poor heat conductivity during fabrication, thus creating temperature gradients throughout the cross-section, and thereby causing residual stresses. Cycle times are usually longer, thus adding another potential cause for stresses when using too short a cycle time. Also, close tolerance dimensions are more difficult to maintain, the material is wasted, quality is degraded, and material and processing costs are increased.
Example of using ribs and changing the weight part 
Reference: Plastic Design Handbook, DOMINICK V. ROSATO , DONALD V. ROSATO, MARLENE G. ROSATO