This process is very important for producing many different products for a large number of different services including medical. The products produced are extremely diverse ranging from combs to musical instruments to one piece chairs.
Plastic Injection Moulding tends to have the same manufacturing as the other types of Injection moulding. The process that actually happens is that the plastic granules are entered into a hopper that is part of the moulding machine. The granules are moved along by a screw type plunger It is at this stage that any extras are added, for example colourants. After this, the mix enters the heated area, this is where the granules become molten. After this the mix is injected with a very high force through a nozzle into a mould tool and this is different for every different part that is produced. Here, the mix is cooled and when it is totally cooled, the mould opens itself and the product can be removed. To increase the speed of this final process, the mould is kept very cold so that the plastic cools instantly upon reaching the mould.
Like another industrial processes there are a number of factors that should be controlled to ensure that output is optimum, with as little contamination as possible. The injection speed can be improved by varying the viscosity of the material. The higher the viscosity, the higher the pressure loss and thus the lower the speed. The higher the speed, the higher the shear rate and viscosity. Although increasing mould and melt temperatures will reduce the viscosity, thus increasing pressure, it will also reduce shear rate. Also, the cooling time can be manipulated, the hotter the melted plastics, the longer for the plastics to cool and thus the thicker the item produced. The Gate Speed and Metering Phase can also be adapted to produce optimum results.
The cost of moulds depends on a large number of factors including number of cavities how complex the mould itself is, how long these moulds are expected to last. There are a number of difference companies offering a large range of services for these machines each offering different ideas.
Advantages of this process include repeatable results with high production rates, low labour costs, little scrap and the ability to use a very large range of materials. However, the set up of this process is expensive as many of the parts are very expensive. Also, there can be large running costs and the fact that each mould is different for each part can be time-consuming and expensive.