Opening holes on plastic parts is a common method to connect with other components or add functional assemblies. The size and location of the holes should minimize the impact on the product's strength or increase production complexity. The following are several factors to consider when designing holes.
The distance between connected holes, or the distance between a hole and the adjacent straight edge of the product, should not be less than the diameter of the hole, as illustrated in the diagram for key points on distance from edges or inner walls. At the same time, the wall thickness around the hole should be as large as possible; otherwise, the pierced location is prone to cracking. If the hole contains threads, the design requirements become more complex because the thread area is prone to stress concentration. Based on experience, to reduce the stress concentration factor at the edge of a threaded hole to a safe level, the distance from the edge of the threaded hole to the product edge must be greater than three times the diameter of the threaded hole.
1 Through Holes
From an assembly perspective, through holes are used much more frequently than blind holes and are easier to produce. From a mold design perspective, the design of through holes is also structurally superior because the core pins used to form the holes can be supported at both ends. Through holes can be formed either by a single core pin fixed at both ends in the mold or by two core pins joined together, each fixed at one end in the mold. Generally, the first method is considered better. When applying the second method, the diameters of the two core pins should be slightly different to avoid undercuts in the product caused by slight misalignment of their axes, and the contacting end faces must be ground flat.
2 Blind Holes
Blind holes are formed by ejector pins (also called core pins) in the mold, which can only be supported on one side in the mold. Therefore, they are easily bent and deformed by the molten plastic, causing the blind hole to become elliptical. Thus, the length of the ejector pin cannot be too long. Generally, the depth of a blind hole is limited to twice its diameter. If the diameter of the blind hole is 1.5mm or less, its depth should not be greater than its diameter.
3 Drilling
In most cases, additional drilling processes should be avoided as much as possible. Consider designing holes that can be formed in a single molding operation to reduce production costs. However, when the hole to be formed is long and narrow (i.e., the length of the hole is greater than its depth), the additional cost of replacing broken or bent core pins may be higher than the cost of a secondary drilling process. In this case, adding a secondary drilling process should be considered. The drilling process should utilize drilling jigs to speed up production and improve quality, and also to reduce the additional cost and time associated with broken drill bits or frequent resharpening. Another method is to add small, shallow locating holes on the plastic part instead of using a drilling jig.
4 Side Holes (Side Cores)
Side holes often increase the difficulty of mold design, especially when the direction of the side hole is perpendicular to the mold opening direction, as side holes easily create undercuts on the plastic part. Common methods include using angle pins and split molds (side-actions) or hydraulic core pulls. Pay attention to whether the core pin will deform or break under the pressure of plastic filling; this situation is common with long, small-diameter core pins. Because the mold structure becomes more complex, the manufacturing cost of the mold is higher. Additionally, production time increases accordingly because the core pin must be retracted before the part can be ejected.
5 Other Design Considerations
Other considerations regarding hole design in product design include the following:
5.1Stepped Holes (holes with multiple connected diameters of different sizes) allow for greater depth than holes with a single uniform diameter. Furthermore, hollowing out part of the mold in the hole area can also shorten the required depth of the hole. The diagram below illustrates the application of these two methods.
5.2Using angle pins, split molds, or hydraulic core pulls for side holes will inevitably complicate the mold structure and increase costs. This problem can be eliminated by increasing the draft angle of the side hole wall or by replacing the original side hole with a stepped hole design, thereby removing the undercut caused by the side hole. The diagram for methods to eliminate side hole undercuts illustrates the application of these two methods.
5.3A straight land (vertical section) of at least 0.4mm should be reserved at the edge of the hole. Designing a complete chamfer or fillet at the hole edge is often impractical economically or in practice. Please refer to the diagram for hole edge design.
