Plastic injection moulding is a manufacturing process widely used in the manufacture of products from plastic trinkets and toys to cell phone cases, automotive body panels and water bottles. A liquid plastic is forced into a mold and hardens - it sounds simple, but is a complex process. The fluids used vary from hot glass to many different plastics - thermosetting plastics and thermoplastics.
The plastic used in the process of plastic injection moulding are either thermoplastic or thermosetting. Thermosetting plastic is set by the catalytic reaction or application of heat. Once they are cured, they can't be reused or remelted. Curing is a chemically irreversible process. Thermoplastic however, can be melted and reused.
Thermosetting plastics include epoxies, phenolic resins and polyesters, while thermoplastics include polyethylene and nylon. There are almost 20,000 plastic compounds that can be used for injection moulding, meaning that it is an ideal solution for nearly all casting requirements.
Glass isn't a polymer, and then it generally does not fit the accepted definition of thermoplastics - although it can also be melted and reused.
Making moulds has historically been done by highly skilled men. A mould is formed when two different assemblies are squeezed together in a press. Creation of a mold often needs various machine operation, complex designs, and a high degree of skill. The tool generally used is beryllium copper or steel, which is often used in mold making. This tool needs heat treatment to harden the metal. Aluminum is cheaper and easier to manage, and can be utilized for immediate or fast production. Today, computer aided milling and spark erosion ("EDM") techniques have allowed a high degree of automation of the entire process of tool manufacture.
There are three main parts that make up an injection moulding unit-heater barrel, feed hopper and the ram. The plastic used in the hooper is in powdered or granular form, but materials like silicone rubber may already be liquid and may not need further heating.
Once in the hot liquid, the plunger ('screw') forces the liquid into the tightly stretched mould. More viscous and molten plastic requires a higher pressure (and higher press loads) to force the plastic in every crack and corner. The plastic cools down as the metal mould conducts the heat away. The press is cycled at a later stage to remove the casting. However, with thermosetting plastics the mould is heated to set the plastic material inside.
Plastic injection molding allows complex shapes to be produced, some of which might be almost impossible to produce economically by other means. In volume, this can be an inexpensive process, no doubt with minimal environmental impact.
A top plastic injection moulding manufacturer can always swing out hundreds and thousands of impressions. The plastic itself is pretty cheap, and despite the energy needed to heat the plastic and the cycling press, the process can be economical for even some of the most basic items like bottle caps.
With hundreds of plastic compounds researched and developed every year with ultra modern techniques, plastic injection moulding is sure to continue increasing in its use. Though, thermosetting plastics can't be reused, their use, in particular for high-precision components, is also set to grow.
Shzen Hoong Plastic Industries has been manufacturing plastic injection moulds since 1975. They have been known for their quality since the onset of the company.