21-06-2013, 12:51 PM
Injection Molding
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ABSTRACT
This is the most common method of producing parts made of plastic. The process includes the injection or forcing of heated molten plastic into a mold which is in the form of the part to be made. Upon cooling and solidification, the part is ejected and the process continues. The injection molding process is capable of producing an infinite variety of parts designs containing an equally infinite variety of details such as threads, springs, and hinges, and all in a single molding operation. A plastic is defined as any natural or synthetic polymer that has a high Molecular weight. There are two types of plastics, thermoplastics and Thermo sets. Thermo sets will undergo a chemical reaction when heated and once Formed cannot be resoftened. The thermoplastics, once cooled, can be ground up and reheated repeatedly. Thus, the thermoplastics are used primarily in Injection molding.
There are four major elements that influence the process. They are:
• The molder
• The material
• The injection machine
• The mold
Of these four, the injection machine and the mold are the most varied and mechanically diverse. Most injection machines have three platens. Newer Models use just two platens and may be electrically operated as opposed to the traditional hydraulic models. They can range in size from table top Models to some the size of a small house. Most function horizontally, but
There are vertical models in use. All injection machines are built around an Injection system and a clamping system.
The injection system mechanism may be of the reciprocating screw type or, less frequently, the two-stage screw type. Also included is a hopper, a heated injection barrel encasing the screw, a hydraulic motor, and an Injection cylinder. The system’s function is to heat the thermoplastic to the Proper viscosity and inject it into the mold. As the resin enters the Injection barrel, it is moved forward by the rotation of the screw. As this Movement occurs; the resin is melted by frictional heat and supplementary Heating of the barrel encasing the screw. The screw has three distinct zones.
Plastic Injection Molding
Injection is accomplished through an arrangement of valves and a nozzle, all acted upon by the screw and the hydraulic pump that pushes the resin into the mold. This so-called “packing action” occurs at pressures from 20,000 to 30,000 psi and higher. The temperature of the resin at this time is between 320o and 600o F. The clamping system’s function is to keep the plastic from leaking out or “flashing” at the mold’s parting line.
Machinery & Equipment:
Injection molding machines consist of a material hopper, an injection ram or screw type plunger, and heating unit. They are also known as presses, they hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations.