How does a plastic injection moulding machine work?
But before we answer that question, let’s take a broader look at the industrial uses of plastic injection moulding. How it has become such an important tool in the manufacture of an extraordinarily wide range of components, many of which we are likely to encounter in our everyday lives.
Plastic injection moulding is a universal manufacturing technique that can produce many different types of parts or components. Indeed, the vast majority of manufactured products rely to some extent on the use of plastic injection moulded parts. These can be produced economically in large volumes and at high speed, sometimes at rates that are expressed in terms of tens of thousands of parts per hour. Typical applications range from extremely small high precision components, such as gears or spindles used in desktop printers, to large automotive parts, such as bumpers and interior panels.
Plastic injection moulding is capable of producing parts from a variety of thermosetting and thermoplastic resins. These are generally specified according to end-use requirements for strength, impact resistance, durability, flexibility, finish and so on.
There are many reasons for companies to choose injection moulding; you can read more about those in our recent blog post Choosing to use Injection Moulding.
Plastic Injection Moulding Process
The essential process of plastic injection moulding is to convert the polymer resin, introduced in the form of solid pellet or bead, into an easily flowing, viscous liquid state. This is done by the application of direct heat and the shear action of the resin feeding system. Then injecting the hot molten resin under pressure into the cavities of a mould.
A combination of good mould design and sustained high pressure ensures that the molten resin reaches every part of the mould cavity without voids or inclusions.
On completion of the injection phase, the molten resin is cooled under carefully controlled conditions to form the final shape of the component, as defined by the contours of the mould. The component is ejected after an appropriate length of time (in as little as a few seconds to several minutes), depending upon the size of the component and the rate at which the resin is allowed to cool.
Plastic Injection Moulding Machine
Converting resin pellets or beads into a hot viscous liquid, injecting it at considerable pressure into a mould and finally releasing the cooled, moulded component requires a machine with three distinct elements: the feeding and injection stage, the mould and its retaining system and a hydraulically powered clamping mechanism.
- Injection – Resin pellets are transferred to a motor-driven, reciprocating screw feeder via a hopper. The screw feeder chamber (called the barrel) is enclosed by a heating element and it is this, in combination with the shear action of the screw that heats the resin and causes it to become molten.
This molten polymer resin is conveyed forward by the screw to the nozzle of the barrel (called the sprue orifice) and is prevented from flowing backwards by means of a check ring. This arrangement ensures that the resin arrives at the point of injection at the right pressure and in a homogeneous state.
- Mould – Molten resin enters the split mould assembly (comprising a platen-mounted moving core and a stationary cavity) via runners that direct the flow away from the sprue orifice towards the part cavity. Runners end in gates at the cavity boundary, with the type, size and location of these gates having a significant influence over the quality of the final moulded part. Moulds incorporate internal channels through which liquids are pumped following the injection phase to aid part cooling prior to ejection.
- Clamp – The clamp, which in most cases is hydraulically actuated, must keep the two parts of the mould tightly closed together to sustain the extremely high pressures generated during the injection stage. Most injection moulding machines are defined, in terms of size, by the achieved clamping force (usually expressed in tonnes). The clamp is released when the part is properly cooled, allowing it to be ejected before the machine is readied for the next cycle.
That concludes our guide on how an injection moulding machine works. If you work in an industry that operates in fast-moving and time sensitive markets, why not speak with one of our industry experts today to see how our plastic injection moulding service can help you.