Injection molding is the most prevalent plastic manufacturing process in India. The process is applied to thermoplastics and thermoset materials that produce plastic products ranging from toys, furniture, electronic casings to Legos and disposable cutleries. Both thermoset injection molding and thermoplastic molding have similar working principles but differ extensively on the attributes these material exhibits when exposed to heat. In this article, you will know about thermoset injection molding in detail and their different manufacturing stages from the leading plastic mould manufacturer in India.
Injection molding manufacturing began in the early nineteenth century when a need arose to replace the ivory billiard ball with a plastic ball. John Wesley Hyatt developed an injection molding apparatus to produce plastic balls out of celluloid (considered the first plastics) and subsequently patented it.
An injection molding apparatus has a simple workflow: melting plastics, injecting them into a mould, letting it cool and ejecting out the final product, albeit the process is a tad complex in real-time. The thermoset materials like phenolic are passed through the molding machine's material hopper, which is then fed to the barrel and hot screw at a temperature of 170F to 240F. The screw injects this molten plastic into the mould system and fills it inside the mould cavities. The cured plastics are then ejected out at the end of the process.
Here is a detailed look into the production phases of thermoset injection molding.
The molding machine has three primary parts; clamp, mould and the injection unit. Here is how each part of a molding machine contributes to the entire production processes
The injection unit consists of a hopper feed system, barrel, heater bands and a screw. The hopper feeds the plastic pellets into the barrel. Pellets are small plastic granules that usually are a few millimetres in diameter. These are mixed with pigments called colourants and a small number of recycled materials before being fed into the hopper feed system.
The reciprocating screw inside the injection unit transports the plastic pellets through the entire barrel, with the pellets being exposed to an even heat throughout the injection unit. The rotating screws have flights that transport the pellet through the barrel. Flights also facilitate the mixing of plastic pellets. The movement of the screw in the barrel causes friction, and the action heats the plastic along its traverse in the barrel. Another interesting thing to note is the diameter of the screw, which increases towards the end. The reduced space between the shaft and the barrel wall squeezes and shears the pellets, ultimately heating and melting the plastic. The shearing process contributes to the maximum heat inside the barrel. The rest of the heat is provided by the heater bands wrapped around the barrel. The plastic melts entirely by the time it reaches the tip of the barrel in the injection unit.
Once the molten plastic fills up adequately in the barrel's front, the screw rams the molten pellet out of the thin nozzle of the barrel. The system works like the plunger of a syringe. Injected plastic is filled inside the hollow part of the mould, also known as a cavity.
Inside the mould
As the injection unit injects the molten plastic into the moulds, the air-filled inside the cavity escapes through minute vents. Vents are tiny channels within the size of five to forty microns. The viscous molten plastics have less chances of escaping through the vents. The plastic inside the mould cavity is solidified by the coolants or water channels that surround the moulds. Once the plastic solidifies, the mould opens slowly.
Ejecting the plastic
Ejecting the solidified plastic can be difficult as cooled plastic shrinks during the curing and sticks tightly to the mould. The moulds have ejector pins, which pushes the stuck plastic off from the moulds.
Once the plastics eject out, the pins slushes into the other half of the mould and the injection & curing processes are repeated. Since the ejector pins do not fit properly inside the other half of the moulds, they often leave witness marks on the final products.
The process is vital for the production of innumerable plastic products in the industry currently. And thermoset injection molding is undoubtedly a continually evolving manufacturing process that is a boon to the plastic manufacturing industry.