Determination of the Third Digit for Injection Molded and Die Cast Parts

(Used under permission of Dixon and Poli,
Engineering Design and Design for Manufacturing)

To determine the cavity detail, and thus the value of the third digit, enter the appropriate values into the following form. Remember to include only those features which are parallel to the direction of mold closure.

For details concerning the types of features mentioned, read the text following the form.

Number of holes or depressions: Circular Rectangular Irregular

Number of bosses(1): Solid Hollow

Number of non-peripheral ribs and/or walls and/or rib clusters(2):

Number of side shutoffs(3): Simple Complex

Number of letterings:


  1. A boss is an isolated projection with a length of projection which is less than about three times its overall width, the latter measured parallel to the surface from which it projects.
  2. A rib is a narrow elongated projection with a length generally greater then about three times its width (thickness), both measured parallel to the surface from which the feature projects, and a height less than six times its width. A cluster of two closely spaced logitudinal ribs or lateral ribs (spacing less than three times the rib width) are counted as one rib.Bosses and ribs are sometimes supported by a cluster of ribs of variable height called gusset plates. This cluster of ribs is counted as one rib or one cluster of ribs. Examples of these three features can be found in the figure below:

  3. Holes in a part that are not considered undercuts are classified as side shutoffs. Isolated grooves or cutouts on the external surface of a part are also considered side shutoffs. It is considered isolated if the dimension of the cutout normal to the direction of mold closure is less than 0.33 times the envelope dimension in the same direction. NB: For parts with a first digit of 2, 4 or 6 side shutoffs are not to be taken into consideration. The following figure shows examples of side shutoffs: