Process, Design And Applications
Injection moulding is a process used to manufacture plastic parts from synthetic polymer materials such as thermoset plastics and thermoplastics. The products thus produced are identical in dimensions, shape and color. High impact polystyrene (HIPS) is one of the most common thermoplastics used in this process.
Injection moulding is big business in the US. According to the North American Injection Molders Association (NAIMA), the demand for moulded plastic stood at $10 billion as of 2010. The same association valued the US plastic molding industry at $8.69 billion as of the year 2010.
The process
The process has several stages as explained below:
Stage One
Powdered or granulated plastic is fed to a heated barrel from the hopper through gravitational force. Both the hopper and barrel are parts of the plastic moulding machine. The barrel is hollow and has a screw that carries the granulated plastic to the mould through screw action.
Stage Two
As the plastic passes the barrel, it encounters heaters which melt it while the screw pushes the melted plastic into the mould.
Stage Three
Once enough plastic has collected at the barrel end, the screw is withdrawn and the plastic is rammed into the mould cavity via a sprue by the hydraulic ram. This stage of the process happens quite quickly to avoid the setting of the plastic. The mould is usually warm which also prevents the hardening of the plastic before it is completely full.
Stage Four
Once the melted plastic has filled the mould, the pressure that injected the plastic into the mould is maintained at original levels to prevent a flow back of the material during the hardening process. This also prevents the formation of hollows and shrinkage which ensures a high quality product. The time that the pressure levels are maintained at original values is known as dwell time.
Stage Five
The moulding is then left in the mould cavity where it cools down before being ejected. The resulting shape of the moulding is similar to that of the mould.
Design
The low cost associated with mouldings is as a result of the ability of the mould to produce thousands of similar mouldings. Modern moulds and die come with interchangeable parts which help in the alteration of the mould.
Stainless steel alloys are used to make the moulds and dies. A typical injection moulding machine is made up of the mould, the screw which is encased by the heater, the hopper for granular plastic input, the ram which contains the hydraulic fluid and a motor.
Applications
Of all the manufacturing processes that use plastics, injection moulding is arguably the most important since it produces products such as disposable razors, model kits and children’s toys. Other products include one piece chairs, camera parts, sink plugs and mechanical parts such as gears. Big products like dustbins, body shell car parts and dingy hulls are also made using the same process of injection moulding.
|
No related posts.