Holes are easy to produce in molded parts. Core pins that protrude into the mold cavity make the holes when the part is molded. Through holes in the molded parts are easier to produce than blind holes, which do not go all the way through a part. Blind holes are made by core pins supported on one end only. The pins can be deflected and pushed off center by the pressures of the molten plastic material during the molding process. A good rule of thumb: the depth of the blind hole should be about twice the diameter of the core pins up to 3/16" and up to four times the diameter of core pins over 3/16". The guidelines for blind and through holes are provided below. Blind hole (shown with draft) L = 2D for diameters less than 3/16" dia. core pins L = 4D for diameters more than 3/16" dia core pins Through Hole (shown with draft) L = 4D for diameters less than 3/16" dia. core pins L = 6D for diameters more than 3/16" dia. core pins There are definite rules for the placement of cored holes in a molded part. If these minimum distances shown below are not followed, the holes will be egg shaped or the part will deform in the areas around the holes. T = wall thickness of part D = diameter of hole As depicted above, the distance from the edge of a hole to a vertical surface (i.e., rib) or the edge of a part should be twice the part thickness (or more), or at least one diameter of the hole. This same rule applies between holes—at least two part thicknesses or one hole diameter should be specified. As easy as it is to make holes in molded parts, it does not come without some concerns for the strength of the part. For every cored or molded hold, there will be a weld line. The weld lines are caused by the flow of the melted plastic around the core pins. These weld lines are not as strong as the surrounding plastic material and may also detract from the overall appearance of the molded part. The part designer should consider these points when designing holes in a molded part.
The coring of holes is easy when the axis is parallel to the parting line. But when holes and other features run perpendicular to the parting line, then retractable cores (or cams) are required. Split pins and cores (called passing steel shutoffs) can be used to create some of the features. The designer needs to be aware of the problems of side action cores and the added expenses associated with these types of molds. With a little understanding of how the mold opens and where the parting line will exist, these costly features can be modified. Rule of thumb: whenever possible, all design features should be incorporated in the same direction of the mold opening so that cam action can be avoided. |