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Boosting structural integrity with ribs

Posted by admin on July 29th, 2012

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:

Increase part weight and cost proportional to the increase in thickness.

ncrease molding cycle time required to cool the larger mass of material.

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.

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.

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. As illustrated 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.

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