Injection molded products are familiar to all of us. Ball point pen bodies, computer key boards, drinking glasses, eyeglass frames, automotive parts, etc. etc. It seems that we live in a world of plastic molded products. Molded plastics can be crystal clear, opaque, colored and textured. They may be as disposable as a toothpick or as durable as a plumbing fitting. They can be as hard as a kitchen cutting board or as soft as a pencil eraser. They can be found in products such as a cereal box give-away or as expensive as a fine watch. They can be in items as mundane as a puppy’s chew toy or as critical as a heart patient’s pacemaker.
All injection molded plastics are made in the same way. First, a mold (often called a ‘tool’) is made. The mold is a reverse, or mirror image of the desired part. It is made in such a manner that can be opened and the finished part removed. Movable round or rectangular pins are placed in the mold to push the finished part out. The mold is held in the closed position by a molding machine which applies pressure to the mold to hold it closed, then molten plastic is pressed into the mold under high pressure. The plastic is allowed to cool, often aided by a water jacket built into the mold, until it is solid enough to be ejected. This cycle is then repeated to produce another part.
The most expensive and time consuming part of the entire process is the making of the mold, for once the mold is made, parts may be produced quite rapidly. It is this speed of production, and the fact that the cost of the mold can be amortized over a large number of parts, that makes injection molding an inexpensive process for producing various products. An injection molding machine is capable of producing 100 or more parts per hour, compared to conventional machining which may produce a half dozen equivalent parts per hour.
Besides the price per part advantage enjoyed by injection molded items, the wide selection of properties available in molded plastics cannot be duplicated by any other practical process; for example, soft rubber, used for tool handgrips, toothbrush handles, etc. is virtually impossible to machine, but is easily molded; clear materials can be molded with a smooth finish to produce lenses and sight glasses without the expensive and time consuming polishing necessesary with machined parts; delicate and flexible parts may be easily produced that are difficult to impossible to hold for conventional machining processes.
In cases where a solid plastic part may not be suitable, such as threaded parts intended for screws and bolts, metal inserts may be molded into the plastic during the molding process. In cases where multiple properties are required in one part, such as in a rigid tool with a comfortable soft grip, plastics with varying properties may be co-molded together. Many injection moldable plastics can be modified with fillers to give special properties, such as glass fibers to impart additional rigidity and strength, or lubricants to produce long-lasting bearings. In addition to these mechanical advantages, plastics may be colored, textured and coated in numerous decorative ways to produce attractive consumer products.