Meaning
Design technique that removes material from the centre of a cylindrical protrusion to create a hollow structure with uniform wall thickness across a part. Coring out thick bosses prevents the accumulation of excess plastic in areas where fasteners or assembly pins are located. This modification ensures that the cooling rate of the boss is consistent with the rest of the part, which reduces the risk of sink marks and internal voids.
The process also saves material and lowers the overall weight of the moulded component. It is a fundamental rule of plastic part design that helps maintain dimensional accuracy and structural integrity. The technique stops being effective if the remaining wall thickness is insufficient to support the mechanical loads applied to the boss.
Material Distribution
Creating a hollow centre in a boss allows the polymer to flow more evenly during the injection phase of the moulding cycle. When a boss is left solid, the thick section of material takes much longer to cool than the surrounding walls. This difference in cooling rates causes the plastic to shrink towards the centre, pulling the surface inward and creating a visible depression known as a sink mark.
By coring out the centre, the designer creates a structure where the walls of the boss have the same thickness as the main body of the part. This uniformity leads to more predictable shrinkage and a much better surface finish. The hollow space can also be used to house a metal insert or a self tapping screw.
Proper material distribution is the key to producing high quality parts with a professional appearance.
Cooling Efficiency
Hollow bosses allow the mould to extract heat from both the internal and external surfaces of the plastic. This dual surface cooling noticeably reduces the time required for the part to solidify before it can be ejected from the tool. In many cases, coring out thick bosses can reduce the cooling phase of the cycle by thirty percent or more.
This improvement in efficiency allows for faster cycle times and higher production rates. The reduction in heat buildup also prevents the plastic from overheating, which can lead to material degradation or gas traps. Moulders prefer parts that are designed with cored bosses because they are easier to process and less likely to cause production delays.
The faster heat removal also results in a more stable crystalline structure in resins like polypropylene or nylon.
Structural Integrity
Careful material removal can enhance the strength of the part by eliminating internal stresses. The uniform wall thickness reduces the internal tensions that are created during the cooling process. These stresses can lead to premature failure or cracking when the part is put under load.
To maintain the necessary strength, designers often add small ribs to the outside of the boss to provide additional support. These ribs help to distribute the forces more evenly and prevent the boss from bending or breaking. The final design must balance the need for material savings and cooling speed with the mechanical requirements of the application.
A correctly cored boss provides a reliable mounting point that will not fail during assembly or use.
