Injection molding machines, also known as presses, consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. The molds are clamped to the platen of the molding machine, where plastic is injected through the sprue orifice to create injection molds.
Presses are rated by tonnage, which is the calculation of the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection molding process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations. The total clamp force needed is determined by the projected area of the custom part being molded. This projected area is multiplied by a clamp force of from 2 to 8 tons for each square inch of the projected areas. As a rule of thumb, 4 or 5 tons/in can be used for most injection molded products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage is needed to hold the mold closed. The required force can also be determined by the material used and the size of the part, larger plastic parts require higher clamping force.
Today, electric presses are taking over the typical hydraulic injection molding machines. Companies who produce injection molds prefer them as they offer 80% less energy consumption and nearly 100% repeatability, by utilizing electric servo motors. While the cost of an electric molding machine is typically 30% higher than a hydraulic press, higher demand for injection molds is closing the gap on cost. It is estimated that in the next 20 years hydraulic molding machines will be a thing of the past, as more molding companies are making the switch to stay competitive.