Mould: Hole Design (six points)

 Mould: Hole Design

1, There are holes or opening in plastic workpieces which used for a variety of purpose, such as assembly with other parts, decorate the workpiece or give it more eye appeal, or as ventilators and so om. Holes or openings can be defind as round, squre, rectangular, eliptical, etc, they may be classified as blind holes, through holes, step holes, recess step holes, and intersecting holes (FIG 2.18).

Design of holes or opening of workpieces affect the construction of mould which leads to change the cycle time if holes or openings are deisgn incorrectly, the following machining may need for workpiece, and that also may cause the overall cycle to slow down and the high cost of mold manufactory. Holes can also affect workpiece integrity and strength due to the formation of knit lines or weld lines.

2, A hole in the workpiece, there is a core pin'in the mold correspondingly. Core pin Is commonly referred to as core pin shift. A general rule, holes design is that a wall located near a hole should be a minimum of one hole diameter away to obtain maximum workpiece strength and facilitate melt flow.

In a plastic workpiece, a through hole is preferred to a"blind hole". A through hole, core pins that are used to form the holes are supported by both mold halves and can withstand the high injection pressures of the plastic melt. Blind holes, means to use an unsupported core pin, which may bend or deflect when the ratio of length to radius is over 2, because it expose to the high injection pressures of the plastic melt.

Holes in plastic workpieces may be molded perpendicular to movement of the mold, parallel movement of the mold, and at oblique angles. The orientation of hole will affect the structure of mold.A hole in workpiece or the forming core pin parallel to the draw as it opens and closes, the draw will simplify the mold. Holes may also be molded at right angles and at oblique angles tha need an watercut insert system to form. Fig. 2. 19 shows different orientation of holes. If it is possible that the oblique-angle holes should be avoided, because the mold cost will increase greatly compare with the mold with the core pin that parallel to the draw. Right angles are not recommended in general design, because they require split molds. and retractable core pins.

                                

3, Be attention please, the hole in workpiece means a pin insert or core in the mold, the pin will be subject to pressure which may cause pin breakage and wear. If the depth is no more than 2 times diameter,the pin can be supported only at one end, as shows in Fig. 2. 20. If the hole diameter is less than 0.062in, the depth of hole should less than ID If the diameter is more than 0.062in, the depth of hole should no more than 2D. If the length is greater than 4 times diameter, the hole should be through Fig 2.21 to Fig 2.23 list holes molded by slender pins with the various types of molding methods.

If the depth to diameter ratio is over 2, the following is what we can do. Holes may extend entirely through the workpiece, in which case the core or pin may be supported at both ends. Fig. 2. 21 shows a slender hole molded by one pin which is supported at one end and withstand in other end. The height of hole is less than 2D for compression molds, 6D for transfer molds or injection molds.

Fig.2.22 illustrates a slender hole molded by two pins. The height of hole is less than 6D for compression molds, 1SD for transfer molds or injection molds. If a slender hole molded by two pins as Fig 2.22, that may have a misalignment between the two pins. It may be ditticult to assembly the workpiece with other part, namely a bolt or shatt Provision should be made for the misalignment of cavity and plunger and the possiblity of the pin's bending. If either misalignment or bending occurs, the companion part may not go through the hole.

4, Fig 2.23 illustrates the recess pin which make through holes is supported at both ends, but the recess may usually contain a small particle of flash prior to a new molding cycle. If the pin entering the recess butts against this hard flash, the pin may break. This method is most frequently used in molding holes. The strength of the pin making the hole is affected by the ratio of its depth to its diameter which called"slenderness ratio"or depth to-diameter ratio.

5, 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 section.

6, Distances between holes and distances of hole from side wallFor blind hole, if the wall at the bottom of the holeis less than 1/6 the diameter of the hole, the bottom will tend to bulge after molding as shown in Fig.2.27b and can not to obtain a desired designed workpiece as Fig2.27a. A better design is shown in Fig2.27c, and to keep hole proportions uniform throughout, and there are no sharp corners for stress concentrations to occur.