Designing of Plastic Products for Injection Moulding
When the normal wall thickness is not strong enough to withstand the loading conditions the part is strengthened by adding ribs rather than by making whole wall thicker. Providing running ribs across the part will increase the part rigidity in all directions. These running ribs cross each other at right angle and this create a thick section at the junction which is greater than the normal wall thickness. To maintain the uniform wall thickness at the junction one way is to use a cored out boss at the junction but the
most suggested is to use a normal junction with ribs that are less than 0.75 times the wall thickness. Deep ribs are structurally more efficient than thick ribs because stiffening effect of rib is proportional to cube of its depth.
Design Considerations
- Rib thickness = 50 % to 75 % of wall thickness.
- Fillet Radius = 40 % to 60 % of rib thickness.
- Rib root thickness = 25 % of wall thickness.
- It is suggested to have deeper ribs than thicker ribs.
- Rib depth should be less than 5 times the rib thickness.
- Provide taper to ribs for their easy removal from mold.