Technology of plastics processing:

Injection moulding

Injection moulding is a way of processing plastics, in particular the thermoplastics, where the material in molten state is processed by injection to a shaped cavity of the mould (the temperature of which is by far lower than the temperature of the processed thermoplastics). The molten material in the cavity is then cooled and hardened under pressure. The procedure is carried on in injection machine.

The injection machine is the equipment, in which the melting and homogenization of plastics, and, subsequently, the injection of the melting under pressure in a closed mould secured against opening, is carried on.

Injection machines consist of the following main parts:

• injection unit - the parts of injection unit are shuttle, dosing device, plasticizing and injection chamber (cylinder) with piston or worm, nozzle, heating and regulation.
  
  
• the injection unit has two main tasks:
  a) to change pellets of the plastic dosed from the shuttle to a homogeneous melt of a particular viscosity
  b) to transport (inject) the mould with high speed and under high pressure into the closed mould
  
  
• closing unit - this unit consists of a closing assembly /joint or piston/, fixing assembly and a mould
  
  
• the task of the closing unit:
  a) to open and close the mould
  b) to fix the closed mould with such a force that the force acting on the walls of the shaped cavity of the mould, created by the pressure of the melt, does not open the mould neither at the injection, nor at the pressurizing of the mould
  
  
• accessories of the injection machine , consisting of the source of energy (pressure needed for the generation of the injection, closing and fixing forces), mould tempering device, and regulation and control elements.

According to the mutual position of the injection and closing units, there are two basic possibilities for the arrangement of the injection machine: horizontal and vertical. We posses the horizontal machines only, i.e. the machines at which the axis of the injection unit is in the horizontal position, perpendicular to the dividing plane of the mould.

Pressing tool - mould:

The growing use of plastics causes rise of requirements relating to their processing. One of the plastics processing technologies is injection moulding of thermoplastics, which bases in the injection of melted material in a shaped cavity of a mould, i.e. of a pressing tool.

All pressing tools used for injection moulding of thermoplastics, beginning with the simplest one, and ending with the most complicated one, can be properly operated only if they fulfill several basic functions. Their shaped cavities have to correspond tightly to the demanded shapes of the pressed pieces, depending on the types of the used processing and of the processed plastics, it has to be possible to temper the pressing piece at the highest speed to the required temperature, and last but not least, it has to be possible to remove the pressed piece from the mould in a proper and fast way.

The moulds usually consist of two main parts, the first is fixed on the firm side of the injection machine, and the second on the movable side of the machine. The part fixed on the firm side of the injection machine is usually named moulded block, and its main function is to secure the supply of the melt into the cavity of the mould via the entrance system. Main function of the other part of the mould, which is fixed on the movable side of the injection machine, is the execution of removal of the pressed piece from the mould cavity by the throw-out system of the mould. In the same time, the both parts of the pressing tool provide for the proper tempering of the pressed piece before its removing from the mould. For this purpose, the both parts of the mould are provided with a distribution system of the tempering fluid.

Every mould consists from ten up to several hundred of parts. To get the proper shape of the pressed product, the designer often uses one or more movable slide-rules or jaws, the moving of which at the opening of the mould is controlled by the press or by the accessory systems, e.g. accessory hydraulic systems. The removal of the pressed piece from the cavity of the mould is subsequently performed by a system of throw-out tools fixed on the throw-out plate, which is - after opening of the mould - controlled by the mechanism of the injection moulding press. Other parts of the mould are frames or fixing and supporting plates, including different accessories. Besides the proper functioning of the mould, provide these parts for fixing of the mould to the press, and for connecting the mould to different media.