Deep holes may be molded by building the holes up in steps as shown in Fig. 2. 24a. It shows the solution of that the pins used in molding butt holes should be in different radius by at least 0.38mm to compensate for the misalignment of cavity and plunger sections. Through holes, molded with a pin supported at both ends (fig. 2. 24b), are not always as practical from a molding standpoint as holes are made by butting the pins.
There is another opposite effect to the workpiece that the flow of the compound around a pin making a hole and the welding of that compound on the other side of the pin are causes for stress concentration because both plastic materials flow and knit inside the mold, stress is set up within the workpiece. Deep holes can be molded for a short length or spotted, and then drilled a deep hole.
As we have discussed previously, if a hole is molded, knit line or weld line will occur on the opposite side of the holes from the gate or occur between the hole and the side wall at the other side. If maximum mechanical strength is required, the material between the wall of a hole and exterior wall of a workpiece should be at least the thickness of the hole diameter, and it should never be less than 0.25 of the wall thickness and the side wall should be at least 3mm. The interior wall between holes should be at least one thickness of the diameter of hole, and never less than 3mm. Fig. 2. 26 illustrates the recommends of the distances between holes and the distance of a hole from side wall. The recommended distances between holes and the distance of the hole from the side wall are presented in Tab. 2.6 It is very important that wall thickness should be kept fairly uniform. Not only non-uniform wall thickness will cause inequalities in curing time during mold process, but the workpiece will be more susceptible to warping at thin wall sections especial when it is joined much thicker wall sections.
