The closed volume of the mold cavity is created using (at minimum) two mold halves. In two-part tool molds, these halves are either cores or cavities.
The core is the side of the tool mold that contains the interior or non-cosmetic side of the part. The core is also where (in most configurations) the ejector system is located. In the above visual, the inside of the cup is the non-cosmetic side, and so the core forms the inside of the cup.
The cavity is the side of the tool mold that forms the exterior or cosmetic side of the part. It is often polished to a finish that represents the desired final surface finish of the part.
When placed together, the core and cavity will leave a thin-walled cavity that will eventually be filled with melted plastic and allowed to cool.
What Affects Core and Cavity Placement?
It is vital to determine the best arrangement of mold components; otherwise, the injection molded part will fail. The above image intentionally removes many parts of a mold to highlight the core and cavity but note that there are several components and concepts that will affect the core/cavity placement (and the success of the project).
Below is a brief list of what affects core and cavity placement:
- Part shape: the above image shows a simple, cylindrical shape, but many injection molded parts are highly complex. The part shape will not only determine where to place the core and cavity in the mold but also where ejector pins, cooling channels, and other necessary components go (which in turn affect core and cavity placement, etc.). Also, uniform wall thickness is a priority in part shape, as variable wall thicknesses will cause issues in both cooling and ejection.
- Ejector System: When the mold is opened after the cooling stage, the injection molded part must ALWAYS be stuck to the side of the mold with the ejector pins. The ejector system and its specific shapes will affect where the core and cavity are placed, such that consistent, repeatable ejection is achieved.
- Material: The shrink rate, flow characteristics, and injection speed of the material will partially dictate how the core and cavity are arranged. If a material has a low shrink rate, it will be more challenging to get the part to stick to the ejector pin side, therefore calling for design considerations. Luckily most plastics do shrink considerably, but it is important to understand how the material could potentially affect where the core and cavity should go.
- Gates, Runners, and Sprues: The sprue is the primary channel from the injection molding nozzle that feeds plastic into the mold cavity. It is typically perpendicular to the mold cavity’s axis but can sometimes be directly piped into the cavity (in that instance, the sprue is known as a direct sprue gate). Runners are the channels that feed multiple mold cavities with molten plastic from the sprue. Most injection molds use runners as it is more economical to mold multiple parts in one cycle (especially in the case of small parts such as model kit parts, for example). Gates are the entry points from the runner into the core and cavity and are typically much narrower than the runner.
Each of these components affects core and cavity placement and must be smartly designed. The sprue and runner locations must be placed in such a way that allows for sufficient filling of all features of a part. Proper gate location affects core and cavity placement, as they influence how the core and cavity fill and how the part will look aesthetically, as it is the point at which the part is cut off from the runner system (assuming a cold runner design).
The image below illustrates the plastic mold feed system typically employed with plastic injection molding.