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Boosting structural integrity with ribs
Structural integrity: the goal of every design
The major component of designing for structural integrity, in many cases, is to design the
structure to be stiff enough to withstand expected loads. Increasing the thickness to
achieve this is self-defeating, since it will:
l   Increase part weight and cost proportional to the increase in thickness.
l   Increase molding cycle time required to cool the larger mass of material.
l   Increase the probability of sink marks.
Well-designed ribs can overcome these disadvantages with only a marginal increase in part weight.

Typical uses for ribs
Covers, cabinets and body components with long, wide surfaces that must have good
appearance with low weight.
l  
l   Rollers and guides for paper handling, where the surface must be cylindrical.
Gear bodies, where the design calls for wide bearing surfaces on the center shaft and on the
gear teeth.
l  
l   Frames and supports.
Designing ribs
Keep part thickness as thin and uniform as possible. This will shorten the cycle time, improve
dimensional stability, and eliminate surface defects. The use of ribs is an effective way of achieving
rigidity and strength, while avoiding heavy cross-sectional thickness. If greater stiffness is required,
reduce the spacing between ribs, which enables you to add more ribs.

Rib geometry
Rib thickness, height, and draft angle are related: excessive thickness will produce sinks on the
opposite surface whereas small thickness and too great a draft will thin the rib tip too much for
acceptable filling.
Ribs should be tapered (drafted) at one degree per side. Less draft can be used, to one-half degree
per side, if the steel that forms the sides of the rib is carefully polished. The draft will increase the
rib thickness from the tip to the root, by about 0.175 mm per centimeter of rib height, for each
degree of draft angle. The maximum recommended rib thickness, at the root, is 0.8 times the
thickness of the base to which it is attached. The typical root thickness ranges from 0.5 to 0.8 times
the base thickness. See Figure 1 for recommended design parameters.
Location of ribs, bosses, and gussets
Ribs aligned in the direction of the mold opening are the least expensive design option to tool. Asillustrated in Figure 1, a boss should not be placed next to a parallel wall; instead, offset the boss
and use gussets to strengthen it. Gussets can be used to support bosses that are away from the walls.
The same design rules that apply for ribs also apply for gussets.
Alternative configurations
As shown in Figure 2, ribs can take the shape of corrugations. The advantage is that the wall
thickness will be uniform and the draft angle can be placed on the opposite side of the mold, thereby
avoiding the problem of the thinning rib tip.

FIGURE 1. Recommendations for rib cross sections.

FIGURE 2. Corrugations instead of ribs
In terms of rigidity, a hexagonal array of interconnected ribs will be more effective than a square
array, with the same volume of material in the ribs.

FIGURE 3. Honeycomb ribbing attached to a flat surface provides excellent resistance to bending in all
directions.