Can this part be saved? Ribs for structure

Editor’s note: In this second installment of design guidelines based on engineering principles, Robert Cramer of Dow’s Materials Engineering Center tackles one of the thorniest design problems for plastics. Structurally speaking, plastics require greater thickness than metals to achieve the same stiffness. Cramer visits the subject of how to improve structural rigidity without adding undue thickness.

For all their advantages, thermoplastics have a major liability compared to metals such as steel or aluminum when used in load-bearing applications. These parts are designed for deflection, not for stress, making modulus of elasticity a key material property. The elastic modulus of the typical thermoplastic is only 1 to 5 percent that of steel, so designers have a challenge getting enough stiffness without excessive wall thickness.

One common approach is to add reinforcing ribs to increase part stiffness while maintaining a reasonable wall thickness. Ribs have the advantage of providing a stiffer part while reducing cycle time and the amount of material used in the part. However, ribs can cause a variety of problems unless they are properly proportioned and located. It is not uncommon to see sink marks on surfaces opposite ribs or ribs that did not completely fill during molding. Designers will often specify extremely tall ribs that make the previously mentioned problems even worse, while making part ejection difficult.

Rib Design

As with many features for plastic parts, rib design guidelines that have been developed through years of experience recognize the constraints imposed by practical processing and appearance considerations. For example, a properly proportioned reinforcing rib will be thinner than the nominal wall to which it is attached. If the rib is too thick, a sink mark will appear on the wall surface opposite the rib. On the other hand, if the rib is too thin, it will be difficult to completely fill during molding. Figure 1 shows the key dimensions for rib design.

The guidelines for basic proportions are based on the nominal wall thickness, W, and depend upon part appearance requirements. Experience has shown the rib height, H, should be limited to two and a half to three times the nominal wall thickness. In designs with amorphous materials, such as ABS, polycarbonate, or polystyrene, the rib thickness T can be as much as 75 percent of the nominal wall thickness without a noticeable sink mark on normal appearance parts.

The draft angle, a, should be 1° per side. If the part is to have a Class A finish, the rib thickness should be limited to 50 percent of the nominal wall thickness and the draft angle reduced to 1¼2°. The use of semicrystalline materials (PP, nylon, PE) with their higher shrinkage usually requires a thickness of approximately 50 percent of the nominal wall to prevent sink marks. Finally, a radius is used at the intersection of the rib and the nominal wall to promote flow into the rib and reduce stress concentration. This radius should be 25 percent of the nominal wall or a minimum of .030 inch (.75 mm), whichever is greater.

Beyond the Guidelines

While guidelines are often easy to apply, they can be restrictive to the designer if their basis is not understood. In the case of ribs, how can the designer estimate how much is "enough?" The answer is found in a series of relatively simple equations that describe the deflection and stress characteristics of a plate and a single rib section. The method for calculating the deflection and stress ratios of ribbed-to-unribbed plates is presented in the sidebar (see p. 50). Using these equations, a series of dimensionless design curves for stress and deflection were developed and are shown in Figures2 and 3. Examining these curves leads to some interesting conclusions.

Figure 2 (p. 48) shows how the maximum flexural stress changes as ribs are added to a flat plate. Each curve represents a particular rib spacing ratio, with Wr = .01 representing very widely spaced ribs and Wr = .33 representing the minimum practical spacing for ribs.

The designer may be surprised to see that adding ribs may actually increase the maximum stress. Why is this? Although a rib increases the overall moment of inertia of the plate, the distance from the neutral axis to the extreme fiber of the cross section can increase more rapidly for short ribs. This effect is most pronounced for widely spaced ribs.

These factors can lead to a puzzling sequence of events. An unribbed part is found to be too flexible, although it doesn’t fail under load. In an attempt to stiffen the part, a few ribs are added. Now the part fails under the same load. The stress curves clearly show the maximum flexural stress can increase by more than a factor of three, depending upon the rib spacing and height. With very widely spaced ribs, that is, Wr = .01, the ribbed plate will have a higher stress than the unribbed plate until the rib height reaches more than six times the wall thickness of the part.

The effect on deflection of adding ribs is shown in Figure 3. As you might expect, deflection decreases as ribs of any proportion and spacing are added. It is interesting to see how these curves support the design guideline that rib height should be limited to two and a half to three times the wall thicknessof the part. Most parts have rib spacing that falls between Wr = .05 and .10. With a rib height of three times the wall thickness, these two curves show that the deflection has been reduced by approximately 90 percent compared to the unribbed plate, and further increases in rib height have minimal impact on deflection. Since very large rib heights, say more than five times the wall thickness, often lead to rib filling and part ejection problems or sink marks, it is better to add a few more ribs of moderate height than to use a few taller ribs.

Applying the Principle

A manufacturer of walk-behind lawnmowers was developing a thermoplastic mower deck (Figure 4, p. 51). The front section provided support for an axle. The designer wanted to ensure this section was very stiff so a diamond-pattern of intersecting ribs was designed into the underside of the section. A prototype mold was built and initial trials revealed a severe filling problem with the ribs (Figure 6). In an attempt to solve the filling problem, the ribs were made thicker. The result was sink marks on the upper surface of the section (Figure 5, p. 51).

The part had a nominal wall thickness of .140 inch (3.5 mm). The ribs were initially 1.25 inch (31 mm) high, .100 inch (2.5 mm) thick, and were spaced on 1-inch (25-mm) centers. While the rib thickness met the guideline of 75 percent of the wall thickness, the excessive height combined with a 1° draft angle resulted in the rib tip being too thin to fill. When the mold was modified to reduce the draft angle to 1¼2°, the rib tip thickness increased and the rib filled, but the part became difficult to eject. Finally, the draft angle was increased to 1° to help ejection, but this change caused the rib base thickness to increase to the point that sink marks appeared on the top surface.

How could the designer have avoided this problem? Let’s look at Figure 3, p. 50. The initial rib spacing falls approximately on the Wr = .10 curve. The chosen rib height (H/W = 8.9) reduces the deflection by approximately 99 percent compared to having no ribs. If H/W were reduced to 3, the deflection would be reduced by 92 percent over the unribbed wall. This more reasonable rib height would significantly reduce the filling problem, while allowing the rib to remain thin enough to prevent sink marks. It would also make the part easier to eject from the mold. If more stiffness were required, the ribs could be spaced slightly closer together.

Another approach often suggested when faced with filling problems is to change to a higher-flow material grade. In general, a higher-flow material will have lower mechanical properties, so part integrity may be compromised. In the case of this particular example, the ribs were so high and thin that even higher-flow grades could not solve the problem.

The manufacturer was unfortunately in a "no-win" situation with this rib configuration. There was no combination of simple mold changes, material substitution, and/or process conditions that would eliminate all the problems. When shown the deflection ratio curves (Figure 3, p. 50), the designer modified the tool with an insert in the rib area. The new ribs had a more reasonable height (H/W = 4) that resulted in sufficient stiffness, were easy to fill, produced no sink marks, and also ejected easily from the tool.

References

Plastic Part Design for Injection Molding, by Robert A. Malloy, Hanser Gardner Publications, 1994, pp. 220-245.

"Designing Ribbed Parts," Plastics Design Forum, March/April 1980, pp. 59-63.

"Calculating Deflection and Stress of a Ribbed Structure," Plastics Design Forum, November/December 1982, pp. 55-57.