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Shaping liquid plastic resins into solid, usable objects and parts is a modern science that is commonly performed using an injection molding machine. The machines come in various sizes, with the size of the machine determined by the desired dimensions of the finished product. Invented in 1872, the injection molding process revolutionized the plastics industry. It is currently a multi-billion dollar business that produces about 32% by weight of all the world's plastics. Its popularity is credited to the injection molding process, and the machines that make millions of consumer products and manufacturing parts and components that are affordable, yet strong and long-lasting.
The injection molding machine construction and operation are fairly simple. The machine consists of six basic parts. The major components of the machine include a hopper where the raw materials are inserted, a barrel to carry the materials to the heating unit, a heater to break down the materials into liquid, a nozzle to pump the liquid into the mold, a clamping unit to solidify the shape, and an ejector to expel the finished product.
To produce an injection molded product, liquid resin is poured into the injection molding machine hopper, followed by dyes or tints. Gravity draws the resin into the barrel, and the heating process melts the resin into a smooth liquid. An injection mechanism, usually a reciprocating screw or ram injector, forces the liquid into the mold. If small amounts of the resin are desired in the mold, the reciprocating screw is used as it can inject as little as 5% of the entire amount in the hopper. The ram injector is used when at least 20% of the total amount in the hopper needs to be forced into the mold.
The mold determines the shape for the finished product, and cools the liquid into a solid. While the cooling is in process, the plates of the mold are held together by either mechanical or hydraulic force. This clamping procedure determines the final shape of the finished product. Since different resins have different shrinking values, molds are designed with specific resins in mind.
Problems with injection molding machine performance are normally uncomplicated and easily solved. Burned or singed parts can be avoided by lowering the hopper temperature or reducing processing time. Warpage is normally solved by adjusting the mold's surface temperature or adjusting the mold's thickness. Imperfections in the product's surface may be corrected through adjusting the mold's temperature, moisture levels, or pressure.
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