Elastomer Injection Moulding Compounds
This section will discuss issues relating to the injection moulding of rubbers, specifically to the moulding of liquid silicone rubber (LSR) which is of large commercial interest. Firstly however, a quick introduction to rubber materials is given.
There are two types of rubber, natural rubber and synthetic rubber.
Natural rubber was for a long time the only raw material of the rubber industry. It is obtained from the sap of the rubber plant of the rubber plantations in Asia (rubber milk or latex). The thickened material is rolled, dried and pressed.
Synthetic rubber is a product of organic chemistry. The basic material is oil, especially heavy petrol that is produced when cracking oil in the refinery. It is polymerised by means of various chemical combinations and additions. This results in synthetic latex milk. As with natural rubber a solid material can be extracted. The raw material for liquid silicone rubber is quartzite or quartz powder. All rubber types can be processed on injection moulding machines.
7.2.1 Processing Procedures for Elastomeric Materials
7.2.1.1 Compression Moulding
With this procedure a blank, prepared in shape and weight, is put into the opened mould and pressed under high pressure. This is shown in Figure 7.1.
The disadvantages of this production method are:
• Long cycle times
• High material consumption
• Considerable additional treatment is necessary
• No accurate parts are obtained
• Only semi-automatic operation is possible. Practical Guide to Injection Moulding 116
7.2.1.2 Injection Moulding
In this method the injection moulding screw plasticises prepared cords whereby the elastomeric material is homogeneously heated up.
Advantages of heating the rubber:
• Shorter vulcanisation times, 12-16 s/mm wall thickness
• A higher accuracy of the parts
• A better quality of the parts
• Any additional treatment is not necessary
• 10-20% material is saved
• Flash-free parts or parts with minimum flash are obtained
• Less wear on the mould
• Automatic production.
7.2.1.3 Injection Moulding
Machines for Rubber Compounds Standard machines can be prepared for the processing of elastomeric materials. The machines must, however, have the following equipment:
• Cylinder for elastomeric material
• Mould blow unit
• Dosage delay
• Special control for cleaning and brushing devices in the mould (to remove bloom).
NB: Special 10 D cylinders with compressionless screws and a big feed bag for specially sensitive rubber materials are available for many injection units.
7.2.1.4 Peripheral Device
A temperature control unit is needed for heating the plasticising cylinder. Depending on the rubber material a temperature between 50 and max. 170 °C is needed inside the plasticising cylinder. In most cases such a temperature control unit works with oil.
7.2.1.5 Mould
Mould heating is achieved by means of electrical heating. It is recommended that both mould halves are controlled separately. The heating power for each mould half should be between 1,200 and 1,600 Watt. The mould temperatures vary according to the elastomeric material. Standard values range from 110 °C to 210 °C. In view of the high mould temperatures insulating platens are required with a thickness of 10 mm for the mould.
