Manufacturing methods
The moulding processes can be categorised into three main groups as mentioned earlier:
1. Compression moulding
2. Transfer moulding
3. Injection moulding
The first method mentioned above is the oldest and simplest method. Development has since been moving towards transfer moulding and above all towards injection moulding with the thermoplastic industry being the primary leader in this field.
Compression moulding
In compression moulding, the rubber compound is formed into a blank, which is then placed in the mould cavity. In order to ensure that the mould is filled properly, a certain excess charge of between 5 - 10 % is required.
A simple punching or cutting procedure directly from the rubber slabs produces blanks or alternatively the rubber compound is squeezed into shape with the aid of a plunger or screw through an extruder die. The strip obtained is then cut into suitable lengths.
The supply of heat depends on the rate of heat conduction from the mould to the rubber. Since the heat conductivity of rubber is poor, heating is slow and the curing time is therefore relatively long.
Typical curing times are from 3 minutes for thin goods to several hours for products with thick walls. In the latter case, it is more efficient to pre-heat the rubber. This can take place in a hot air oven or in a microwave oven up to temperatures of approximately 100 °C. When heated over 100 °C the risk of scorching becomes excessive.
The advantages of compression moulding are primarily the following:
1. The method is simple and only requires relatively simple presses and moulds. It is an appropriate method for short runs.
2. Allows manufacturing of composite products which contain non-rubber reinforcing material (laminates).
3. Suitable for products with large surface or large spreading.
4. Can be used for rubber compounds with high viscosity and poor flow properties.
The disadvantages of compression moulding are the following:
1. The preparation of blanks and the insertion of blanks into the mould are time consuming.
2. Complicated cavities can be difficult to fill out completely.
3. The formation of flash is extensive and the variations of the hot mould cavity dependant dimensions are relatively extensive.
4. The production rate is relatively low.
Transfer moulding
Transfer moulding is a natural progression in the development when attempting to limit the disadvantages of compression moulding. The blank is loaded into a loading chamber and is then distributed into several cavities. The rubber is squeezed out of the loading chamber by means of the closing mechanism of the press itself, or with separate pistons, into each respective cavity.
Since the rubber is forced to flow through channels and gates, preheating takes place in the rubber. This preheating significantly reduces the curing time. Provided that the injection pressure does not exceed the closing pressure of the mould, a well balanced dosage of rubber into each cavity will be obtained and the formation of flash will be reduced to a minimum.
The advantages of transfer moulding are primarily the following:
1. The preparation of blanks and their handling is made considerably easier.
2. The preheating of the rubber reduces the curing time.
3. Since the rubber is preheated it flows easier and fills the mould cavities more efficiently.
4. The mould is closed when the rubber is injected into the cavities, which means that less flash is formed and smaller dimensional variations of the product are obtained.
5. For rubber-metal bonding, the rubber makes better contact with the metal in the form of a new and “fresh”, clean surface.
The disadvantages of transfer moulding are primarily the following:
1. The moulds are more complicated and more expensive.
2. Parts with textile inserts cannot be produced.
3. The method requires materials that are relatively simple to process.
Injection moulding
During injection moulding, the press unit and the injection unit are two separate units with separate operating controls. The press unit can have the mould placed horizontally or vertically. The injection unit can have a combined plasticizing and feeding screw or alternatively the screw is only intended for the purpose of plasticizing and the injection itself is carried out using an attached plunger unit. The positioning of the injection device varies between different makes of machines.
It is an advantage if the feed opening for the rubber strip is placed at comfortable working height. One extruder unit can sometimes serve several presses by moving in a pre-programmed pattern. By means of a high injection moulding pressure, a short injection time and a considerable amount of preheating of the rubber is obtained. The preheating can achieve the same or higher temperatures than those of the mould and the curing time then becomes extremely short.
A special variety of injection moulding is the use of gear pumps as an injection unit. The injection process then becomes slower but the quantity of rubber per shot is almost unlimited.
The advantages of injection moulding are primarily the following:
1. The handling of blanks is completely eliminated.
2. The mould is filled in a closed position and the formation of flash can in certain cases be completely avoided.
3. Complex cavities and flow channels can be filled easily.
4. The curing time is very short
5. The process can be easily automated.
The disadvantages of injection moulding are the following:
1. The moulds and machines are considerably more expensive.
2. If an injection unit serves several moulds, the material in the common injection unit is limited to one particular type of compound.
3. The process requires greater technical know-how.
4. There are strong requirements for the rubber compound to have low viscosity and to be homogenous.
5. Gates and injection runners contain vulcanised rubber, which must be discarded.
6. Laminated parts cannot be produced.
