What is a mould?

Mould is necessary in any mass production of plastic articles

Mould - complicated technological device which is intended for production of large quantity of plastic workpieces (articles) exerting to them pressure that is created in injection or blowing machines.

From the viewpoint of construction, mould is a device which consists of huge amount of details and mechanisms. All details are classified into three groups by their purpose:
1. Moulding parts. They include core, cavity, inserts, ejectors, etc.
2. Parts which ensure supply of molten material (plastic) to moulding cavity of mould. Such details include injectors, runner inserts, hot runner or manifold, injector nozzle, injector nozzle tips, etc. They form injection system of mould.
3. Parts with constructive role – plates (shifters, supports, pads), columns and guides bushes, spacers, etc. They ensure reliable and precise assembly of moulding cavity details in relation to each other, removal of article out of mould, its connection with working machine, etc.

Mould may have one or several moulding cavities which are reverse (negative) imprints of article’s external surface. Number of cavities determines number of articles to be produced per one moulding or blowing cycle, for example, a mould with one cavity can produce 1 article per one production cycle, but a mould with four cavities – 4 articles per one cycle, etc.

Injection moulds:
Depending on configuration of article, surface requirements of article, as well as productivity, injection moulds may have two types of injection systems: moulds with hot runner system and moulds with cold runner system. 

Hot runner injection system – it is a system which ensures supply of molten plastic from injection moulding machine maintaining constant temperature, its distribution inside a mould (in case of moulds with several cavities are used) and supply of molten plastic directly in moulding cavity.

Advantages of hot runner injection systems:
- increased working productivity of mould,
- waste-free production,
- extended technological opportunities of article production.

Coldrunner injection system – it is a system which delivers molten flow without additional heating. On completion of process, runner becomes cold and is removed out of mould.

Advantages of cold runner injection systems:
- lower price,
- simple maintenance,
- high reliability.

Principle of injection moulding of plastic articles:
Thermoplastic (plastic, polymer) articles are produced with injection machines using the method of injection moulding. Plastic (in form of pellets) is loaded in hopper of injection moulding machine.The dosage amount of granules required for one injection, screw is captured and transferred to the material cylinder, externally heated by electrical elements. Moreover, plastic, at first, moves in continuous flow, but after that overflows torpedo and passes through annular space between cylinder and torpedo in thin layer. Because of this, it gets mixed very well and its temperature rises up to the temperature specified by technological mode.
Injection cycle begins in the moment when a mould closing. First of all, moveable part of the mouldmove and touch stationary part of the mould. In this moment, screw under high pressure injects plastic in a mould, which is cooled with water. Since the temperature of mould is lower by 100—160 °С than temperature of molten material, it becomes cold quickly and hardens. In order to avoid shrinking of plastic as a result of quick cooling, screw hold pressure in a mould forsome time after injection. After that, it moves back and the next portion of plastic is passed to cylinder from hopper in the same order.
Plastic is cooled in a mould for some period of time.Waiting period is determined guiding by viscosity of molten material and wall thickness plastic product. After that, a mould is opened. After opening of mould, ejectors start working with the help of hydraulic cylinders of injection machine, and they push an article out of mould.