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A comprehensive guide to the different types of injection moulding processes and their applications.
Introduction
Injection moulding is a manufacturing process that allows for parts to be produced in large volumes by injecting molten material into a mould. Injection moulding can be performed with a host of materials, including metals, glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers.
Injection moulding is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of cars. It is a versatile and efficient process that can produce complex and intricate shapes with high accuracy and repeatability.
However, not all injection moulding processes are the same. There are different types of injection moulding processes that have different characteristics, advantages, disadvantages, and applications. In this article, we will explore some of the most common types of injection moulding processes and how they work.
Table of Content
- Introduction
- Table of Content
- Injection Moulding Equipment
- Injection Moulding Cycle
- Types of Injection Moulding Processes
- Die Casting
- Gas-Assisted Injection Moulding
- Liquid Silicone Rubber Injection Moulding
- Metal Injection Moulding
- Thin-Wall Injection Moulding
- Two-Shot Injection Moulding
Injection Moulding Equipment
The basic equipment for injection moulding consists of three parts: the injection unit, the mould, and the clamp.
The injection unit is responsible for heating, mixing, and injecting the molten material into the mould cavity. The injection unit consists of a hopper, a barrel, a screw, a nozzle, and a heater.
The hopper is where the raw material is fed into the injection unit. The barrel is a cylindrical chamber where the material is heated and melted by the heater. The screw is a helical device that rotates and pushes the molten material forward. The nozzle is the opening at the end of the barrel where the material exits and enters the mould cavity.
The mould is the part that shapes the injected material into the desired form. The mould consists of two halves: the core and the cavity. The core is the part that forms the inner surface of the part, while the cavity is the part that forms the outer surface of the part. The mould also has channels for cooling fluids to circulate and regulate the temperature of the moulded part.
The clamp is the part that holds the mould together and applies pressure to keep it closed during injection. The clamp also opens and closes the mould to allow for ejection of the finished part. The clamp can be either hydraulic or mechanical.
Injection Moulding Cycle
The injection moulding cycle is the sequence of steps that takes place during injection moulding. The injection moulding cycle can be divided into four stages: clamping, injection, cooling, and ejection.
Clamping: In this stage, the clamp closes and secures the two halves of the mould together. The clamp applies sufficient force to prevent the mould from opening due to the high pressure of the injected material.
Injection: In this stage, the screw moves forward and injects the molten material into the mould cavity through the nozzle. The amount and speed of injection are controlled by a timer or a pressure sensor. The injection pressure can range from 30 to 200 MPa depending on the material and part geometry.
Cooling: In this stage, the molten material inside the mould cavity begins to cool and solidify. The cooling time depends on several factors, such as the material properties, part thickness, mould temperature, and cooling system efficiency. The cooling time can range from a few seconds to several minutes.
Ejection: In this stage, after sufficient cooling time has elapsed, the clamp opens and separates
the two halves of the mould. The finished part is then ejected from
the mould cavity by ejector pins or other mechanisms. The part may require further trimming or finishing operations before it is ready for use.
Types of Injection Moulding Processes
There are many different types of injection moulding processes that have different characteristics, advantages, disadvantages, and applications. Here are some of the most common types of injection moulding processes:
Die Casting
Die casting is a type of injection moulding process that uses metals as the material. Die casting is similar to injection moulding, except that the material is injected into a metal mould under high pressure and speed. The metal mould is usually made of steel or aluminium and has a higher temperature than the melting point of the metal. The metal solidifies rapidly in the mould and forms a dense and strong part.
Die casting is suitable for producing parts with complex shapes, thin walls, and fine details. Die casting can also produce parts with smooth surfaces and accurate dimensions. Die casting is commonly used for producing parts such as engine blocks, pistons, gears, housings, and other automotive components.
However, die casting also has some disadvantages, such as high initial cost, high energy consumption, limited material choices, and potential defects such as porosity, cracking, and shrinkage.
Gas-Assisted Injection Moulding
Gas-assisted injection moulding is a type of injection moulding process that uses both plastic and gas. Normally, it begins with injecting molten plastic into a mould but only about 70-80% full. Then gas (nitrogen) is introduced via a different channel into the mould under pressure. The gas pushes the plastic to fill the remaining space and creates a hollow core inside the part. The gas also helps to reduce the pressure and temperature of the plastic and prevent warping and sinking.
Gas-assisted injection moulding is suitable for producing parts with thick sections, large projections, or complex shapes. Gas-assisted injection moulding can also reduce the weight, material usage, cycle time, and clamping force of the parts. Gas-assisted injection moulding is commonly used for producing parts such as handles, levers, knobs, furniture components, and medical devices.
However, gas-assisted injection moulding also has some disadvantages, such as high equipment cost, high gas consumption, difficulty in controlling gas flow and distribution, and potential defects such as gas leakage, incomplete filling, and surface blemishes.
Liquid Silicone Rubber Injection Moulding
Liquid silicone rubber injection moulding is a type of injection moulding process that uses liquid silicone rubber as the material. Liquid silicone rubber is a type of thermosetting polymer that has high elasticity, durability, and biocompatibility. Liquid silicone rubber injection moulding is similar to injection moulding,
except that the material is injected into a heated mould and cured by heat or radiation. The cured part can be easily removed from the mould without sticking or deformation.
Liquid silicone rubber injection moulding is suitable for producing parts with soft touch, flexibility, resistance to heat and chemicals, and medical grade quality. Liquid silicone rubber injection moulding is commonly used for producing parts such as seals, gaskets, valves, keypads, medical devices,
and baby products.
However, liquid silicone rubber injection moulding also has some disadvantages, such as high material cost, high viscosity, difficulty in colouring, and potential defects such as air bubbles, flash, and curing problems.
Metal Injection Moulding
Metal Injection Moulding (MIM) is a process that combines the advantages of powder metallurgy and plastic injection moulding. MIM can produce complex metal parts with high precision, strength and durability. MIM involves mixing metal powders with a thermoplastic binder, injecting the mixture into a mould, and then removing the binder and sintering the metal part.
Thin-Wall Injection Moulding
Thin-Wall Injection Moulding (TWIM) is a process that produces plastic parts with very thin walls, typically less than 1 mm. TWIM can reduce material consumption, cycle time and weight of the parts. TWIM requires high injection pressure, speed and cooling to achieve uniform wall thickness and avoid defects.
Two-Shot Injection Moulding
Two-Shot Injection Moulding (2K) is a process that produces plastic parts with two different materials or colours in one moulding cycle. 2K can create parts with enhanced functionality, aesthetics and quality. 2K involves injecting the first material into a mould cavity, rotating or transferring the mould to another injection unit, and then injecting the second material over or around the first one.
