Part Details Review
| Draft |
Parts with surfaces perpendicular to the parting line (opening surfaces of the mold) will experience sticking and drag when ejected from an injection mold. Also, a greater force will have to be used to effect ejection. During the ejection process, the parts made of rigid materials may suffer damage. Parts ejected before they are sufficiently cooled may deform.
Designers allow the perpendicular surfaces to be slightly tapered in the direction of mold opening to facilitate easy breakaway of the surfaces and facilitate air movement to compensate for the vacuum effect between the part and the mold walls. Because of the taper, the ejection force is only needed for the initial breakaway, after which the part becomes free from the mold surface.
The term "draft" is used for this taper, which is used in almost all injection molded parts. When functional requirements dictate that no draft angle be used, special techniques such as collapsible cores have to be used, which add to the cost of the mold and the molding process.
Part removal from the mold is a two-step process. The first step is to separate the part from the cavity. Draft on the cavity sidewalls makes this step easier. This step is relatively easy due to the shrinkage through the part thickness, which tends to pull the part away from the cavity walls. Next, it has to be ejected off the core which necessitates the use of proper draft on the core. Generally, the same draft angle is used for both the core and the cavity to maintain a uniform wall thickness.
The effect of draft on the sides of blind bosses contributes to thinning at the top of the boss. Both the inside and the outside diameters of blind bosses are formed by the same side of the mold (see figure). Because of this, as the draft angles are increased, the thinning effect also increases. Draft on the sides of the ribs also has a similar effect on them.
