Injection molding machine is not a sole hero for the many plastic products we have today. Who does the real job? It’s its every part and piece that does the magic! And, these are the small yet great things you must discover!
Injection molding machine or the injection press is the equipment used for the sort-of-complicated process of injection molding. Through it, a polymer is heated to a plastic state and pushed to flow under intense pressure inside a mold cavity. The molded section, also called the molding, is then removed from the cavity. The machine is mainly composed of an injection unit and a clamping unit- named based on what it does during the process.
The injection unit is consist of a hopper, a barrel assembly, a reciprocating screw and an injection nozzle. It basically acts as an extruder, to confine and carry the plastic as it goes through the feeding, compressing, degassing, melting, and packing phases. The hopper is a thermoplastic material which contains the plastic pellets which then progress into the second injection unit section, the barrel. Heated by the electric heater bands, the barrel supports the reciprocating screw. The latter, the most important part, acts to compress, melt and convey the material. It has three zones, namely the feeding zone, the compressing or transition zone and the metering zone. The screw turns to mix and heat up the polymer and acts as a ram to rapidly move and inject the molten plastic forward into the mold. A non-return valve at the tip of the screw avoids the melt from flowing backwards. Because of this dual action, it is called the reciprocating screw. In general, the injection unit functions to heat up, melt and homogenize the polymer and then transport it right into the mold cavity.
The clamping unit completes the other part of the story. It is focused on the mold operation. Its vital functions include holding the two halves of the mold for proper alignment, keeping it closed during injection by projecting as a clamping force against the injection force, and opening and closing the mold at an exact time during the molding cycle. The entire unit is composed of two platens- a fixed platen and a movable platen. A particular mechanism operates the latter, though a hydraulic piston or a mechanical toggle device. The clamping or the molding unit shapes the plastic inside the cavity and ejects the molded part.
Upon ejection, the machine’s delivery system takes over the process. It provides smooth passage for the molten plastic from the nozzle to the part cavity. It generally involves a sprue, slug wells, a main runner, branch runner and gates. The delivery system design is critical for the filling pattern and mold’s quality. It can either be through cold runners or hot runners. The first is normally used to consume material while maintaining a desirable pattern for the molten plastic. Hot runners, on the other hand, keep the plastic in a molten state at all instance to save every material and eliminate repetitive trimming procedure.
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How does polymer turn into our mostly used plastic products? injection molding machine and its parts provide the answer!