Plastic Design Guidelines

Plastic design guidelines to follow when designing plastic parts.

By following a few simple design guidelines listed below one can achieve successful results during the moulding process and also produce plastic parts that are robust.



Sometimes there is a need to increase the stiffness of a plastic part for various reasons mostly to withstand a certain load applied to it.
Adding ribs to a part is an easy way to accomplish this, thus increasing the structural strength of a part.
Below are some design guidelines to follow when adding ribs to a part model without causing sink marks or surface irregularities.

  •  Rib thickness should be less than wall thickness. A rib thickness of 60% to 80% of nominal wall thickness is recommended.
  • To increase stiffness increase the number of ribs or “gusset plates”, another feature designed to strengthen the plastic part.
  • For a given stiffness, it is better to increase the number of ribs, not the height.
  • For thick ribs ” core out ” the rib from the back. This creates a hollow space underneath the part and maintains a uniform wall thickness.


Bosses are used for locating, mounting, and assembly purposes.
There are boss design guidelines that must be followed to insure the highest quality in molded parts.
One of the main points to consider is nominal wall thickness. Too many times bosses are designed with thick wall sections that can affect the appearance of the plastic part and the final product.

Rule of thumb: The outside diameter of bosses should remain within 2.0 to 2.4 times the outside diameter of the screw or insert

Boss diameter, wall thickness, and height design parameters.
While boss heights vary by design, the following guidelines will help avoid surface imperfections like sink marks and voids: the height of the boss should be no more than 2.5 times the diameter of the hole in the boss.
Please observe the “60/40” rule (see above) for the wall thickness at the bottom of the boss.


In the design of injection molded parts, sharp corners should be avoided.
Sharp corners act as stress risers or concentrators, reducing part strength and causing premature failures.
Sharp corners may also effect plastic flow, producing parts with objectionable surface flow patterns.

For Bosses & Ribs the nominal radius where the boss/rib meets the part surface should be 1/4 of the nominal part thickness, with a minimum radius of 0.38mm
Corners: The inside radius should be at least half the part wall thickness. The outside radius should equal the inside radius plus the part thickness.