By Design: Part design 302—Hole configurations

Holes can provide many useful benefits in an injection molded part. Some processes, such as thermoforming and blowmolding, require secondary machining operations in order to produce through holes. Injection molding is the best molding process for producing parts with holes. Every conceivable size and shape of blind and through holes have been molded. 

Holes are useful design features, but they do complicate the molding of a part. If an injection molded component has a hole through its wall, that part will normally contain a weldline. The strength and appearance problems associated with weldlines were reviewed in my April 2001 By Design article. 

The Ideal: Round Holes 
The shape of holes is dictated by their function and the constraints imposed by the rest of the part. In some instances, a hole's only function is to core out a thick section or to reduce a part's weight and to save material. In these cases, the shape of the hole follows the configuration of the rest of the part. Within these constraints, the designer still has a lot of freedom in specifying the shape of holes. 

Round holes are the ideal shape for the injection molding process. The reason for this is that as plastic material cools it shrinks onto the core pin that forms a hole. The core pin prevents the material from shrinking the normal amount. This condition creates stress around the circumference of the hole. With round holes, this stress is uniformly distributed around the hole, as depicted in Figure 1A. This is the ideal situation. 

Square or rectangular holes are probably the second most frequently used shape for injection molded holes. In fact, square holes are so common that they attract no special attention. This is unfortunate, as square holes are significantly weaker than round holes. The way the plastic shrinks against the core pin accounts for this loss in strength. During the cooling portion of the molding cycle, the size of the core pin remains constant. As the plastic between any two corners on a square hole cools and shrinks, it is pulled tightly against the corners on the core pin. The same thing happens on the other three sides of the hole. 

This stretching in two directions causes the long carbon chains that make up most plastic materials to be pulled apart. These molecules tend to orient perpendicular to these sharp corners. This condition is illustrated in Figure 1B. The end result is that square holes contain abnormally high levels of molded-in residual stress. 

Some notch-sensitive materials, such as nylon or polycarbonate, may be so highly stressed that they develop cracks at sharp corners without being subjected to any external load. 

Radiusing the corners on square holes, as shown in Figure 1C, reduces stress in these locations. The advantages to be derived by radiusing corners was reviewed in my December 1999 By Design article. That article pointed out that rounded corners distribute stresses over a larger area, which results in a stronger part. A radius of 75 percent of the part's wall thickness was recommended as the maximum size of radius that would be beneficial. Radiuses that are larger than 75 percent do not provide a significant increase in strength. 

Radiusing Holes 
There are different rules for radiusing holes. With a completely closed structure, such as a hole, the larger the corner radiuses are, the stronger the part becomes. If the radiuses become large enough, a square hole becomes a round hole. Again, this is the ideal situation. 

The big advantage of a round hole is that it is one continuous radius. There are no interruptions in the radius that can become stress concentrators. Independent of where the hole is loaded, the force is transmitted around the radius to be supported by the maximum amount of plastic material. This combination of factors makes round holes strong. 

If the continual radius that forms a round hole is interrupted by something like a keyway, stresses are concentrated around that irregularity. 

The fan shown in Figure 2 is driven by the flat section on the D-shaped mounting hole. High-frequency vibration of the fan results in a crack at one end of the flat section. This crack propagates across the flange and the amplitude of the vibration increases. A second crack then appears and the fan subsequently explodes. These failures were eliminated by radiusing the junction between the flat section and the round part of the hole. 

It is interesting to note that this fan was gated to the left of the D-shaped hole (see Figure 2 close-up). There is a weldline on the right side of the hole. These fans did not fail at the weldlines. In other words, the weldline did not weaken the part as much as the molded-in stresses did. 

In many instances, the function of a part requires a nonround hole. A part might have to accommodate a square drive shaft. This would require a square hole (Figure 3A), but that hole does not have to be designed with sharp corners. The corners of that hole could be radiused (Figure 3D), but that would necessitate the added cost of a corresponding radius on the drive shaft. Another approach would be to leave the drive shaft square and provide outwardly projecting radiuses (Figure 3B, C). Radiuses of this type would reduce the level of molded-in stress caused by the plastic shrinking against the sharp corners (Figure 3A). These outwardly extending radiuses increase the cost of a mold, but they can be the difference between success and failure. 

The glass-fiber-reinforced nylon timing sprockets, depicted in Figure 4, were introduced with sharp corners on the square drive shaft hole. These sprockets failed after several months in the field due to cracks radiating from a corner on the square hole to the root of a nearby gear tooth. These field failures were eliminated by adding the outwardly extending radiuses that can be seen on the modified sprocket. 

The level of molded-in stress can be reduced, and the strength of a molded part increased, by avoiding or minimizing sharp irregularities in the shape of injection molded holes.