Injection moulding is a way of making plastic parts by pushing molten plastic into a mould. The mould is expensive to make. After that, each part is cheap, so the method suits large numbers of the same part. A bucket, a bottle cap, a tiffin box lid and the plastic body of a pen are made this way.
The machine has four main parts. Plastic granules, small grains of plastic about the size of rice, fall from a hopper into a heated barrel, where a screw melts them and pushes them forward. Industrial heaters and temperature controllers keep the barrel at the right temperature. The screw pushes the molten plastic into the mould, which is made of two steel halves held tightly shut by a press. The plastic cools and hardens. The mould opens, and pins push the part out. The whole cycle can take under a minute.
Each step has a name and can go wrong. The hollow in the mould that gives the part its shape is called the cavity. The plastic enters through a small opening called the gate. Once the cavity is full, the machine keeps pressing for a few seconds, to make up for the shrinking of the plastic as it cools. If the plastic is too cold or the pressure too low, the cavity is not filled and the part comes out incomplete. If the mould is not shut tightly enough, plastic leaks out at the edges as a thin fin called flash. If one part of a thick wall cools more slowly than the rest, it shrinks and leaves a dent called a sink mark.
The mould is the costly part. It is cut from hardened steel, usually with CNC machines, and it is designed from the part's CAD model. A good steel mould can make hundreds of thousands of parts, and the best can make more than a million. Some moulds have many cavities, so that one cycle makes four, eight or more parts together. A mould also needs design rules. Walls should have an even thickness. The sides of a part should slope slightly, a feature called draft, so the part can slip out. These rules are checked during design.
Take, for example, a company in Indore that needs a plastic cap for a bottle. A mould costs ₹8 lakh. Each moulded cap costs ₹1. The same cap printed on a 3D printer costs ₹180, with no mould. So the mould costs ₹8,00,000 more at the start, and each moulded cap saves ₹179. The mould pays for itself after about 4,500 caps. For 500 caps, printing is cheaper. For 50,000 caps, moulding is far cheaper. CNC cutting is a third choice. Like printing, it needs no mould, so it suits small numbers of caps.
Moulding is used for medical items such as syringes and the housings of devices, and for car dashboards, toys and pipe fittings. Many Indian tool rooms specialise in making moulds, and a manufacturer often orders the mould from one company and the parts from another. The plastic itself is chosen for the job: polypropylene for buckets and caps, ABS for the casings of appliances, nylon for gears, and polycarbonate for clear, tough parts. On-demand manufacturing platforms also offer moulding, but their quote must include the mould. Ceramics are also formed in moulds, with a similar idea, as in ceramic machinery.