A shutoff angle has an entirely different reason for being than does a draft angle. A draft angle exists for purposes of part release, while a shutoff exists to prevent the two halves of the mold from crashing into one another if there is any slight misalignment upon the mold closing, (and there will be). Shutoffs also prevent galling that would occur if vertical metal faces were rubbing against each other.
The #1 rule for shutoffs is "no less than 3 degrees or 0.010" misalignment on closing". For example if the shutoff were only 0.010" tall the angle would have to be 45 degrees. It is a good idea to avoid creating shutoffs under 0.020" tall however, as they are difficult to make and tend to get deformed over time.
There are four basic forms of shutoffs, Flat, Wipe, Saddle, and Hidden. Hidden shutoffs are very rare and will not be discussed here. A wiping shutoff is usually coplanar with the outside of the plastic wall and a saddle is roughly perpendicular to the plastic wall. These shutoff types may take on many forms however and may not be easily recognized. Different types may even be combined under some situations. Variants include conical, radiused saddle, stepped parting line, split draft and freeform.
FLAT- A flat shutoff is just that, a flat surface perpendicular to the line of draw that shuts off on another flat surface.
WIPES- The simple wipe is the easiest shutoff to construct. However it forces the plastic wall to be no less than 3 degrees, and the sides of the opening only need draft angles. Short of using a side action this will get you the squarest opening. Obviously one can not texture the face of a shutoff so the metal must be masked in the area of the shutoff prior to etching. Texturing around wiping shutoffs is tricky and rarely results in perfect texture on the part edges. The conical shutoff is a variant on the wipe.
SADDLES- The simple saddle shutoff is probably the most common shutoff type. It requires at least 3 degrees on the sides of the opening and allows minimum draft on the face. The radiused saddle is more difficult (expensive) to make but it allows for a fillet in the corners of the opening for reduced stress in the plastic part. In non-cosmetic applications such as chassis or internal parts an alternative to the radiused saddle can be accomplished by combining a radiused saddle with a split draft shutoff.
Split draft is used for creating steps in the part that deviate from parting line. They are frequently used in non cosmetic applications for purposes of mold simplification. By splitting draft a stepped parting line can many times be eliminated and be replaced by a flat shutoff. A half saddle is used for creating a stepped parting line. Split draft frequently is necessary when the parting line must be on one level to create a feature but then jump to a different level to create another feature.
FREEFORM- Freeform parting surfaces frequently incorporate all of the above shutoff types in one free-flowing shape. All of the considerations of all the other shutoff types still remain important to maintain tool longevity. Freeform parting however introduces another factor, that of knife edge metal conditions. As the parting surface traverses along the side of a part it can freely transition from one orientation to another. As these transitions occur, it is imperative that the parting surface remain very close to normal to the part face to prevent the occurrence of knife edges.