By Design: Part design 201 — Projections

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

Many of the functional features that designers incorporate into injection molded parts can be identified as projections. A projection can be defined as anything that adds material onto a part’s nominal wall. Referring to Figure 1, the snapfit latch, the gusset, the reinforcing rib, the flat pad, the hollow boss, and the solid bosses, one with a male thread, are all projections off of that part’s nominal wall. Each of these projections looks different, but the design guidelines are the same for all of these shapes. 

The last nine articles in this series reviewed design guidelines for an injection molded part’s nominal wall, and showed that the most important design consideration is to maintain a uniform nominal wall thickness. 

All of the projections in Figure 1 are attached to the part’s nominal wall. At the junction between the nominal wall and the projection there is an increase in thickness. This increase in thickness violates the single most important rule for designing an injection molded part. All of these projections create problems, but the solid boss and the flat pad are the most troublesome. While all of these types of projections are widely used, reinforcing ribs are the most frequently specified when injection molding parts. 

Reinforcing Ribs
A standing wall, or a rib, can have many functions, but its primary use is to provide the maximum stiffness at the minimum cost. Thick walls provide more stiffness than thin walls, but thick walls require more plastic material and a longer molding cycle. The trend today is to design for efficient manufacturing and the lowest possible cost. The skillful use of reinforcing ribs can maintain or increase a part’s stiffness while reducing the cycle time and the amount of material used. In one example, the use of reinforcing ribs allowed the nominal wall of a part to be reduced from .125 to .090 inch. This change resulted in a 21 percent reduction in part weight and a 25 percent faster molding cycle. 

If the designer’s objective is to reinforce a part’s nominal wall, then more stiffening is better. This line of reasoning could result in the rib shown on the left in Figure 2. This rib is three times thicker than the nominal wall (W). It would provide a lot of additional stiffness, but it is not a suitable approach for an injection molded part. This thick rib will result in an abnormally long molding cycle. 

The thick rib will stay hot longer and shrink more than the thinner nominal wall. This increase in mold shrinkage will result in a sink mark on the top surface of the part. An even more serious problem is that the thicker and therefore stronger rib may overpower the nominal wall, resulting in warpage of the entire part. The variation in mold shrinkage at different locations on the part will also result in an increase in molded-in residual stress. 

A better approach for reinforcing the nominal wall is shown on the right in Figure 2. In this modified design, the one thick rib has been replaced by three thinner ribs. The three thinner ribs use about the same amount of material as the one thick rib. The three thinner ribs have more contact with the mold, and they will cool faster than the one thick rib. 

The thinner ribs cool and shrink at a rate approaching that of the nominal wall. By minimizing the difference in shrinkage between the nominal wall and the ribs, the part will contain less molded-in stress. The resulting part will have improved impact strength and heat deflection temperature, with less of a tendency toward warpage. Best of all, the part shown on the right in Figure 2 can be produced at a lower cost. 

This thin-rib part has a better design, but it is not as good as it could be. In this instance, the designer followed the rule of maintaining a uniform wall thickness without realizing that this would create an increase in thickness in the nominal wall at its junction with the base of the rib. With the minimum radius of 25 percent at the junction of the rib and the nominal wall, there will be a 50 percent increase in thickness in this location. 

This thicker section will be more difficult to pack out. It will take longer to cool and will shrink more than the rest of the part. If the rib is near the gate, it may be packed out sufficiently to avoid excessive shrinkage. On the other hand, if this thicker section is some distance from the gate, an unsightly sink mark will be present in the appearance surface of the nominal wall. 

Material Concerns
If the part is molded in a high mold shrinkage factor material, such as nylon or polypropylene, the sink mark will be large. The same part molded in a low shrinkage material, such as polycarbonate or polystyrene, would result in less of a sink mark. 

The presence of the sink mark, either large or small, also indicates a difference in shrinkage between the rib and the nominal wall. This variation in shrinkage is an indicator that there will be molded-in stress at the junction of the rib and the nominal wall. 

The ideal proportions for reinforcing ribs are shown in Figure 3. If the part is to be molded in a high mold shrinkage material, such as polypropylene or acetal, the thickness at the base of the rib should be limited to 50 percent of the nominal wall thickness. Molding that same part in a low mold shrinkage material, such as ABS or polycarbonate, would allow the rib thickness to be increased to 75 percent of the nominal wall thickness. This thicker rib will provide more stiffness than the thinner rib. The thicker rib will also be easier to fill and pack. 

Adding fillers or reinforcement to a plastic material reduces its mold shrinkage factor. The use of fillers and reinforcements minimizes the differential mold-shrinkage-related problems. This, in turn, allows the use of thicker ribs. 

Avoid Rib Sink Marks
In those cases where aesthetic considerations preclude even a slight sink mark, a designer should consider reducing the recommended rib thicknesses by an additional 10 percent. If trial moldings indicate that these thinner ribs are too hard to fill, the mold can be modified to increase the size of the rib cavities. Having to modify a mold to increase the width of the rib cavities is undesirable. It is, however, easier to increase the size of a rib cavity than to have to weld up and recut the cavity. The old adage of "when in doubt, always design steel safe" is still true. 

In rare instances, the increase in thickness between reinforcing ribs and a part’s nominal wall can be used to an advantage. In these cases, the thicker areas act as internal runners that allow the melt to flow into thin sections. Very thin-walled parts, such as vending machine cups, cellphones, and palm-type computer housings, use this technique. 

Another consideration in designing reinforcing ribs is that many parts wind up being produced in a plastic material that has a different mold shrinkage factor than the material originally specified. Earlier in this series, it was pointed out that the part design had to be changed to accommodate the characteristics of the material to be molded. Marketing considerations can, however, make this desirable objective difficult to achieve. 

For example, when a new product is first introduced, it may be molded in a high-performance material such as polycarbonate to make certain that there are no problems. After a few months of marketplace success, the polycarbonate might be changed to a PPO to reduce costs. With continuing increases in sales and no customer complaints, the PPO is later downgraded to an ABS. In order to squeeze the last few pennies of profit out of the product, the material is finally changed to a calcium carbonate filled polypropylene. 

This common sequence of events ignores the fact that the part was originally designed for a low mold shrinkage factor polycarbonate. The mold that was designed and built to accommodate the unique characteristics of polycarbonate is now being used to mold a filled polypropylene. In these instances, sink marks, molded-in stress, and warpage can become serious problems. The original part design and the mold should be modified to adapt to the final material, but that is rarely done. 

Referring again to Figure 3, the distance that the ribs project off of the nominal wall is specified as 2.5 to 3.0 times the nominal wall thickness. The height of ribs is an important consideration, but that is a big topic that will be covered in a followup article. 

Editor’s note: A one-day seminar covering the part design details being presented in this series of "By Design" articles will be conducted by Glenn Beall during the Appliance Manufacturers Conference & Exposition in Cincinnati, Ohio on September 11, 2000. For registration information, contact Bernice Sharpe, (440) 349-3060, ext. 303; fax (440) 498-9121; or e-mail [email protected].