Part Details Review
| Strength |
Part thickness directly relates to the amount of deflection it will experience when subjected to an external load. A relatively small increase in thickness can reduce deflection substantially as stiffness is proportional to thickness cubed, e.g. a 25% increase in thickness will nearly double the stiffness.
While increasing wall thickness to improve stiffness seems to be a simple solution, it is not always practical. Due to a large difference in flexural strength of steel versus thermoplastics in general, the plastic part will have to be three to four times thicker than the steel part for equivalent stiffness.
Thick wall sections in injection molding add part weight, face packing difficulties, require increased cycle time, and are associated with shrinkage stresses and sink marks. Therefore, the wall thickness for most thermoplastics is kept under 0.25 inches.
Use of ribs is an efficient engineering solution for increasing rigidity, which minimizes the problems created by simply increasing overall thickness. Figure 1 shows the relative amount of material needed to double the stiffness of a flat part, both by increasing thickness (25% more material) and adding a bi-directional rib (7% more material).
For structures subjected to bending and twisting loads, cross ribbing or diagonal ribbing pattern is used. Use of ribs is discussed in detail under the heading "Ribs", elsewhere in the checklist.
