It may seem funny to read, but it is true. When it comes to injection molded plastic products, roundness, flatness, and wall straightness are more an ideal than a reality. The problem with such features in injection molded plastic involves gate location, part geometry, and molecular and fiber orientation. There are several things that can be done to maximize the roundness, flatness and straightness potential of your product, but the perfection of such features that we might take for granted in machined metal products are nearly impossible in plastics.
For more in-depth information on this tip and the others, you can download our free PDF of the Top 10 Design Tips for Plastic Injection Molded Parts.