The Influence of Tonnage, Shot Capacity, and Part Geometry
Press selection begins with tonnage – the clamping force required to keep the mold closed during injection. Filling a mold cavity requires significant injection pressure, and that pressure, acting across the projected surface area of the part, generates tremendous force that attempts to push the mold open. The clamp unit must provide enough force to keep the mold closed throughout the molding cycle. Too little clamp force allows the mold to separate, causing plastic to escape along the parting line as flash. Too much clamp force can restrict proper mold venting and, over time, place unnecessary stress on the mold. The required clamp tonnage is influenced by the material being molded, part geometry, and gate location. We begin every project with injection molding simulation software to accurately predict the clamp force required.
Shot capacity is a separate consideration. It defines the maximum volume of plastic a press can inject during a single molding cycle. A press may have sufficient clamp tonnage but insufficient shot capacity – or vice versa – so both must be evaluated before quoting a project. As a general guideline, the ideal shot size falls between 20% and 80% of a press’s rated shot capacity to promote consistent processing and part quality.
Part geometry also plays an important role in press selection. Features such as undercuts, holes, threads, and other complex details often require mechanical side actions or hydraulic cylinders within the mold. While the molded part itself may be relatively small, these mechanisms can significantly increase the overall size of the mold. As a result, a larger press may be required – not because of the part size, but due to the mold dimensions and the space needed for these tooling features. Evaluating the anticipated mold size during the quoting process is essential to selecting the right press and avoiding manufacturing constraints later in the project.
How early should I involve my molder in press selection?
Press selection begins during the quoting stage, not after the project has been awarded.
Understanding the required clamp tonnage, shot capacity, and anticipated mold size provides critical inputs for the mold designer from the very beginning of the project. As mold development progresses, decisions regarding part orientation, runner layout, gate locations, cooling, and other design details are made. Each of these decisions can influence mold size, processing requirements, and ultimately the molding press best suited for the job. Close collaboration between the molder, mold designer, and customer throughout this process helps ensure the mold is designed around the production equipment that will manufacture the parts – not the other way around.
Taking this approach reduces risk, avoids costly design changes, and helps ensure a smooth transition from mold design to production.
If you’re working through a new component or evaluating whether your current molding setup is the right fit, our engineering team is ready to review your requirements and help you move forward with confidence.
