Injection Molding Process

Injection molding is a manufacturing techniques used to manufacture parts from thermoplastic and thermosetting plastic materials. Plastics are synthetic non metallic compounds which are easily molded and hardened. The injection molding process is used to manufacture tiny parts as well as the entire components of a car. It is often thought to be a hard process which is a misconceived notion.

Thermoplastic and thermosetting plastics are both polymers of carbon. Thermoplastics commonly used are polyethylene, polystyrene, polypropylene and nylon. Whereas vulcanized rubber is a commonly used thermosetting plastic. Elastomers are also commonly used. These are a form of plastics with very high elasticity.

The machines used for this process are complex, sensitive and expensive.  They are called injection moulding machines. They are mostly heavy weighing tons of kilograms. The injection moulding machines have two components, the injection unit, and the mold and clamping unit.

The machines need to be able to produce high temperatures and withstand them.  This is a reason why such a complex system is required. The process is not as complicated as the machinery. It is a simple and systematic process.

The injection molding process has four basic steps:

  • Plasticizing

  • Injection

  • Chilling/ Dwelling

  • Ejection

Plasticizing is the conversion of the normal granular polymer into a melted liquid. This requires very high temperatures. The resins are heated till melting point and are poured into the mold which is of the desired shape. This is it the process of injection.

The next step is chilling; this is the process of allowing the plastic to cool down into the mold. Before the chilling is a process called dwelling.  It is the application of hydraulic pressure or mechanical pressure. This helps the trapped air in the mold to escape. The trapped air in the mold if not removed can cause changes in the final plastic product, thus it is necessary to get rid of this trapped air before the melted plastic is allowed to cool. This causes the air cavities to move to one corner of the mold. Finally the mold cools and the plastic solidifies back to its original rigid state.

Ejection is the removal of the solidified plastic from the mold. This is carried out either automatically or manually. While ejecting manually care is taken not to damage the mold.

The automatic injection molding process is carried out using ejecting pins. These pins remove the plastic efficiently without causing any damage to the molds. The process of mold manufacturing plays a very important part in providing a good quality finished plastic product. The mold used has to be of uniform thickness. This helps in keeping the properties of the plastic intact. The cooling process can alter the chemical and physical properties of the plastic part, as the heating and subsequent cooling cause a change in the polymer bonding. This change in bonding causes a change in the physical and chemical properties of the plastic. A mold of uniform thickness helps in keeping the chemical properties of the plastic intact.