Wall Thickness of Injection Molded Parts

The most important guideline to follow when designing injection molded parts is to maintain uniform wall thickness. Non-uniform wall thickness can cause all kinds of molding and part problems, including filling problems (hesitation, air traps), warpage, surface imperfections (sinks), and internal stresses.

The part in Figure 1 suffers from hesitation, and possibly air traps. Because plastic will flow down the path of least resistance, the thick frame will fill first, and the thin section will fill last (if it is not already frozen).

Thicker wall sections take longer to cool. Not only does this increase the cycle time to produce the part, but it affects the part's shrinkage. The amount part shrinkage is dependent on cooling rate, especially for semi-crystalline materials like polypropylene, polyethylene, and nylon. Because thick sections slow down cooling rate, more shrinkage results. This can lead to warped parts, and internal stresses. Figure 2 shows an example of a warped part due to non-uniform walls. A solution to the problem is also shown.

One of the few times that non-uniform walls are recommended are for designing ribs that are directly across from cosmetic surfaces. The rib should be thinner than the wall to minimize the amount of sink on the cosmetic surface (see section on rib design for details).

Flow Problems from Non-Uniform Wall Thickness

Effect of a Flow Leader

The above pictures illustrate a possible effect of non-uniform wall thickness in the filling of a part. As shown in the cross-section, the part has a thick outer frame and a thin inner wall section. The part is gated on the end of the thick frame. Because plastic flow will always take the path of least of resistance, the flow tends to favor the outside frame and hesitates to flow into the thin center region. As can be seen from the flow analysis pictures, this ultimately causes an air trap and part cannot be filled.

Hesitation Effect

The pictures above illustrate a flow related problem called hesitation. The sample part has one leg that is 0.150” thick, and another leg that is 0.060” thick. It is gated in the thick section, as shown in the diagram. Again, the plastic will always flow towards the path of least resistance. As shown in the fill pattern from a filling analysis, flow is favored towards the thick section. When the plastic reaches the thin section, it hesitates and slows down. As seen in the final fill pattern picture, the thick section, even though it is longer, completely filled before the shorter thin section was even close to filling out. Hesitation such as this can cause major quality problems. The flow front becomes colder in the hesitating section, possibly resulting in a short shot. The colder flowing plastic will lead to greater molded-in stresses and could weaken the part.

Conclusion

This is just one illustration of the negative effects of non-uniform wall thicknesses in an injection molded part. Maintaining a uniform wall is the most important guideline to follow when designing a robust injection molded part. In addition to the flow problems explained in this section, variation in shrinkage also occurs (click here for more information).