Injection moulding

Injection moulding is a high-speed manufacturing process used to create parts by injecting molten material into a mould.

A mould is a hollow container that gives the material its final shape. This process is essential because it allows for the mass production of identical objects very quickly. It is the most common modern method for manufacturing plastic parts. Engineers and industrial designers use it to create everything from tiny mechanical components to large automotive body panels. The versatility of this method comes from the wide range of materials and designs that can be used.

The process begins when raw material, often in the form of small pellets, is fed through a hopper into a heated barrel.

Inside the barrel, a reciprocating screw rotates to move the material forward. This rotation mixes the material and creates frictional heating, which helps melt the polymer. As the temperature rises, the Van der Waals forces between the molecular chains weaken. This reduces the viscosity, or thickness, of the material so it can flow easily.

The screw then pushes a specific volume of this molten material, called a shot, toward the front of the barrel. A small portion of the shot, about 10%, remains in the barrel to act as a cushion. This cushion helps transfer pressure from the screw to the mould cavity.

Once enough material has gathered, the machine forces the molten polymer into the mould cavity at high pressure and velocity. To prevent sudden pressure spikes, the machine often shifts from constant velocity to constant pressure control when the cavity is 95% to 98% full. This stage is known as packing pressure. Packing pressure is applied to compensate for thermal shrinkage, which is the tendency of the material to shrink as it cools.

This pressure is maintained until the gate, which is the entrance to the cavity, solidifies. Once the gate is solid, no more material can enter. The screw then moves back to collect material for the next cycle while the part cools inside the mould. To speed up this cooling, many machines use cooling lines that circulate water or oil.

Different types of polymers require different handling during the moulding process. Thermoplastics are the most common because they can be melted and reshaped, making them easy to recycle. In contrast, thermosetting polymers involve a chemical reaction. When two chemical components are injected into the barrel, they undergo irreversible chemical crosslinking. This process transforms the liquid into a permanent, solid network of molecules.

Because thermosets can harden inside the machine and damage the screw, manufacturers must carefully manage their residence time and temperature. Some machines use a cold injection unit to keep the chemicals stable before they enter the hot mould.

Moulds are also categorized by their material and how many parts they can make. Most industrial moulds are made from tool steels, which are very durable. A steel mould can be designed to produce over one million parts during its lifetime, though these moulds can cost hundreds of thousands of dollars to fabricate.

Aluminium moulds are a cheaper and faster alternative for low-volume production. However, aluminium is less suited for high-volume work because it has inferior mechanical properties. It is more prone to wear, damage, and deformation during the high-pressure clamping cycles.

The history of injection moulding shows how much the technology has evolved. In 1834, Joseph Morgan filed a British patent for moulding candles. In 1846, Charles Hancock patented an injection moulding machine. By 1872, American inventors John Wesley and Isaiah Hyatt patented a machine that functioned much like a large hypodermic needle.

This early machine used a plunger to inject material through a heated cylinder. While those early machines were simple, they laid the groundwork for the complex, high-precision systems used in modern factories today.

Modern advancements have introduced even more complex techniques like two-shot moulding and insert moulding. Two-shot moulding allows two different materials to be combined into a single part, such as adding a soft grip to a hard plastic knob.

Insert moulding involves placing pre-made components, like metal screws, into the mould cavity before the plastic is injected. This allows the plastic to form around the object, creating a single part with multiple materials. Furthermore, new hybrid approaches use woven fabric reinforcements, such as carbon or glass fibers. These composites can increase the tensile strength of the base polymer by as much as 4.18 times, providing a lightweight and strong alternative to metal in the automotive industry.