Injection molding is the technique where molten plastic is injected into a metal mold. The mold is composed of two halves, the “A” side and “B” side. The halves are separated and allow the plastic component to be removed once it has solidified, thus creating plastic parts.
Draft angles allow for removal of the plastic from the mold. Without draft angles, the part would offer significant resistance due to friction during removal. Draft angles should be present on the inside and the outside of the part.
The deeper the part, the larger the draft angle. A simple rule of thumb is to have a 1 degree draft angle per inch. Not having enough draft angle may result in scrapes along the sides of the part and/or large ejector pin marks(more on this later).
Having radiused edges and corners (both inside and outside) of a part is a multipurpose feature. This allows for better removal during part ejection, — in conjunction with drafted sides — and better material flow (more on this later). Most importantly, however, is that it will not only prevent excessive expenses when creating the mold, but it will prevent cracks from forming due to stress concentration.
Keep in mind that radiused corners should maintain same wall thickness, which means that if inner r=½ thickness then outer R=3*½ thickness.
Undercuts are items that interfere with the removal of either half of the mold. Undercuts can appear just about anywhere in the design. These are just as unacceptable, if not worse than a lack of a draft angle on the part. However, some undercuts are necessary and/or unavoidable. In those instances, necessary undercuts are produced by sliding/moving parts in the mold.
Keep in mind that creating undercuts is more costly when producing the mold and should be kept to a minimum.
As the plastic cools in the mold, it also shrinks, which is a common characteristic for most materials.
Making a component a shell versus a solid helps reduce the amount of shrinkage or warpage that happens during the cooling process. It also helps lowers the cost of material needed to be used in that part. There are numerous ways to shell a design, one example is shown on the sphere below.
Sink marks are as they sound, a spot or segment of the plastic surface that appears and/or feels as if it has sank into the part.
These marks are caused by a number of items: