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Injection Molds A. Basic mold design An injection mold consists of at least two halves that are fastened to the two platens of the injection molding machine so that they can be opened and closed. In the closed position, the product-forming surfaces of the two mold halves define the mold cavity into which the plastic melt is injected via the runner system and the gate. Cooling provisions in the mold provide for cooling and solidification of the molded product so that it can be subsequently ejected. B. Types of Ejection For product ejection to occur ,the mold must open. The shapes of the molded product determines whether it can be ejected simply by opening the two mold halves or whether undercuts must be present. The design of a mold is dictated primarily by the shape of the product to be molded and the provisions necessary for product ejection. Injection-molded products can be classified as: ○ 1 Products without undercuts(e.g., plaques, strips, half-shells, cups). ○ 2 Products with external undercuts or lateral openings(e.g., spools and bobbins, beverage crates, threaded bolts). ○ 3 Products with internal undercuts (e.g., threaded closures, housings). ○ 4 Products with external and internal undercuts(e.g., bumper fascias, electrical and automotive instrument housings, cameras, etc.). C. Design Rules There are many rules for designing molds. These rules and standard practices are based on logic, past experience, convenience, and economy. For designing, mold making, and molding, it is usually of advantage to follow the rules. But occasionally, it may work out better if a rule is ignored and an alternative way is selected. In this text, the most common rules are noted, but the designer will learn only from experience which way to go. The designer must ever be open to mew ideas and methods, to new molding and mold materials that may affect these rules. D. The Basic Mold 1. Mold Cavity Space The mold cavity space is a shape inside the mold, ”excavated”(by machining the mold material) in such a manner that when the molding material(in our case, the plastic)is forced into this space it will take on the shape of the cavity space and, therefore, the desired product The principle of a mold is almost as old as human civilization. Molds have been used to make tools, weapons, bells, statues, and household articles, by pouring liquid metals (iron, bronze) into sand forms. such molds, which are still used today in foundries ,can be used only once because the mold is destroyed to release the product after it has solidified. Today, we are looking for permanent molds that can be used over and over .Now molds are made from strong, durable materials, such as steel, or from softer aluminum or metal alloys and even from certain plastics where a long mold life is not required because the planned production is small. In injection molding the (hot) plastic is injected into the cavity space with high pressure, so the mold must be strong enough to resist the injection pressure without deforming. 2 . Number of Cavities Many molds, particularly molds for larger products, are built for only 1 cavity space(a single-cavity mold),but many molds, especially large production molds, are built with or more cavities. The reason for this is purely economical. It takes only little more time to inject several