When it comes to plastic injection molding, often times a plastic engineer is faced with a part that needs to be structurally reinforced. This could be due to the weakness of the part itself or because of something added on the inside of the design for an alternative purpose (I.E. a boss for a screw or circuit board input). A rib design feature is often chosen by a plastics engineer and manufacturer to enhance the strength of a plastic part.
When designing a rib into a plastic part, the engineer has to consider a few things:
- Thickness of the rib
- Daft of the rib
- Radius the base
- Height/Length of the rib
Thickness of the rib
Rib Thickness needs to be determined relative to the thickness of the wall. If the rib thickness is greater than the thickness of the outer wall, at the point of intersection, the plastic will become too thick leading to unequal cooling, and consequently, warp the surface of the part. An engineer will typically choose a thickness or rib width .5 to .75 times the size of the primary wall (W).
Draft is a term used to refer to the degree in which the rib tapers as it comes to an end. This allows for a clean mold release and reduces the risk of problems with ejection once the part is shot.
Radius your edges… Wait, I’ve said that before. Radiusing the corner where the rib and the primary wall intersect increases the structural integrity of the rib and ergo, the strength of the part as a whole. According to plasticsengineering.org, “The size of the radius should be approximately 0.25 times the primary wall thickness with a minimum radius of 0.010 inch.” (Oct 2016).
Height/Length of the Rib
Height/Length of the rib ought to be kept shorter rather than longer. When it comes to the rib height, the engineer will not want to exceed 2.5 times the width of the primary wall. When a rib is too long it can cause difficulties with ejecting the part, as well as create an issue with the integrity of the rib since thinning natural occurs as the rib tapers toward the end.