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Plastic injection molding is a popular and economical industrial process for manufacturing a wide range of plastic consumer products. These include knobs, washers, valves, pulleys, gears, power tools, bottle caps, toys, car panels, computer monitors, keyboards, furniture and so on. These items are mass-produced using molding machines of various sizes and various specific requirements.
The process of injection molding was invented by John Wesley Hyatt. In 1868, he produced billiard balls by injecting celluloid into a mold. Later he built a plunger type injection molding machine to facilitate production. Another inventor, James Hendry, improved on Hyatt's invention and built a screw injection molding machine in 1946.
Both plunger type and screw injection type molding machines are used in the modern plastics industry. The difference between the two is in the way the plastic is transferred to the mold. Screw injection types are more convenient and are more widely used.
In the plastic molding process, plastic resins are used in pellet or granular form. Choice of plastic type depends on the kind of product being made, its requirements and the overall budget. Of the hundreds of available plastics, only a few are safe for consumer use. Some of the ones used in the plastic injection molding process include polystyrene, polycarbonate, polypropylene, polyethylene, polyamide, polyvinyl chloride, Teflon®, Delrin®, and acrylic.
Plastics are non-degradable and therefore not environment friendly. The plastic injection molding process, however, plays a positive role in reducing waste. Plastics used in the process can be reused as often as required.
The plastic pellets or granules are poured into the feed hopper of an injection molding machine. The feed hopper is a large container that opens into a heating cylinder. In the heating cylinder, the pellets are subjected to intense heat until the plastic melts.
A plunger or a screw then pushes the molten plastic forward through a nozzle into a split-die mold. The mold is the reverse of the part to be made and can have one cavity or several to make more parts at the same time. Molds can be made of durable and expensive steel, or the more affordable, less-durable aluminum or beryllium-alloy metal.
As with selecting the right plastic type, selecting the mold metal depends upon product, cost and durability factors. Manufacturers often use a less expensive metal mold for prototype molding, and get the more long-wearing, precision-machined molds when the production demand increases.
The molten plastic cools in the mold, hardens and takes on the shape of the mold. The mold then opens and the part is ejected out. It is then either ready for packing or is prepared for any other secondary operations. The whole plastic injection molding process is completed within a few minutes and can be carried out automatically.
@mrwormy- I also worked for a few plastic injection molding companies over the years, and I can't say I enjoyed it much. I mostly made the bottoms of cigarette lighters and bottle caps. Every so often, the mold would jam and the hot plastic would start piling up behind it. That's when I had to call maintenance people to clear out the mold and remove the hot plastic. When the system worked right, though, it was an easy job.
I worked at a plastic injection molding company for a few years after I graduated from high school. Each plastic injection molding machine ran nonstop, because the pellets weren't supposed to cool down once the process started. Someone had to stand by the machines and pull off parts as they came out of the molds. Sometimes there would be scrap parts that needed to be tossed into a grinder for recycling.
It wasn't the most interesting work I've ever done, but it was steady. I could have stayed at that job for my entire adult life if I had wanted.