Ever wonder why the little plastic buckets kids use at the beach have tapered sides? Hint: it isn't for hauling water. It's for that other popular beach pastime of the pre-preteen set: making sand castles. Moist sand is pretty fragile stuff and the surface tension created by the water between the grains is just barely strong enough to turn a bunch of particles into a temporary solid. A good shaking would pull them apart, and since the same surface tension that sticks sand grains to one another also draws them to the sides of the bucket, a good shaking is what it would take to get damp sand out a straight-sided bucket (see Figure 1). The result would be a pile of loose damp sand and a powerful lesson on the importance of draft for the young castle-builder.
Figure 1: Because the surfaces that are in contact with the sand are undrafted, surface tension will cause the sand castle to fall apart upon lifting the mould. Similarly to prevent damage or drag marks on the part, surfaces that are parallel to the line of a mould opening may have to be drafted-angled away from the line along which the part will be ejected.
In the more typical tapered bucket, however, as the mass of sand begins to move out of the bucket it also pulls directly away from the bucket wall. Less contact means less surface tension between the sand and the bucket (essentially, less friction), and less friction means no need for shaking; a simple tap is all it takes, and gravity does the rest. This mode of ejection may leave a few grains stuck to the plastic, but it also leaves the turret of your sand castle intact (see Figure 2).
Figure 2: Draft facilitates the removal of the part from the mould and is particularly important where the moulds are straight pull only, similar to this sand castle mould. Using at least 0.5 degrees of draft on all "vertical" sides eases release of your part, or, in this example, sand castle*.
The same principle applies to the design of plastic parts for injection moulding. Moulded plastic may be more solid than damp beach sand, but if the plastic gets dragged during ejection it's no match for a metal mould wall, and while the resulting part may hold its shape it won't hold its finish. So as long as your parts have surfaces that are more-or-less parallel to the direction of mould opening you'll want to draft those surfaces no matter how complex or simple they might be.
One obvious example of such a part is the same plastic beach bucket we've been discussing. The bucket that acts as a mould for sand is itself a moulded part. Its outside is formed by the A-side mould half, and its inside is formed by the mould's B-side. Shrinkage causes the cooling plastic to pull away from the outside mould half making that part of mould opening easy, unless, of course, the surface of the bucket is textured. Texture can act like tiny undercuts. To prevent damage to the surface, increase the draft. Light texture (PM-T1) requires three degrees of draft; medium texture (PM-T2) requires at least five.
At the same time, shrinkage makes the bucket cling to the B-side mould half from which it needs to be separated by the action of ejection pins. It is here that significant draft (and avoidance of texture) helps ejection and allows the injection moulder more flexibility to improve the quality of the parts due to reducing difficulty of ejection.
The bucket we've described so far is essentially a cup. If we want to attach a handle we may want to add features that protrude outward from the surface or add holes in the sides of the bucket. To do so we might want to use side actions, which must be drafted parallel to the direction of cam opening (read more about side actions).
*NOTE: Sand may need 5+ degrees of draft where plastic may only need ½-2 degrees.