General comments on compound selection
This information can be used as a guide for selecting rubber compounds. Most products can be made in most of the base polymers and in many colours. The chemists at Macam Rubber can develop a compound to meet specific operating criteria. We have our own testing facility to test for hardness, tensile strength, compression set, cycle testing and heat aging and use outside testing services where necessary.
This table provides general information to assist in the choice of a type of rubber compound. For material to meet specific operating criteria the advice of our chemist must be sought. A rubber compound may need to be developed and appropriate testing carried out before the component parts are used in the application. For moredetailed operating specifications see the attached table.
Rubber base polymer selection
NATURAL RUBBER (NR)
Good dynamic, cut growth and abrasion properties, high mechanical strength and elasticity. Continuous operating temperature application range from 80°- 90° C
SBR (STYRENE-BUTADIENE RUBBER)
Good dynamic and abrasion properties, good resistance to swelling in alkalis and acids. Continuous temperature application range 80° - 90° C
NEOPRENE (CR - POLYCHLOROPRENE RUBBER)
Excellent oil resistance, self extinguishing, non flammable, good weathering resistance. Continuous temperature application range 80° - 90° C
EPDM (ETHYLENE-PROPYLENE-DIENE TERPOLYMER)
Good weathering, ozone and acid resistance. Continuous operating temperature range 110° - 120° C, can be increased to approximately 150° C using a peroxide cure system
NITRILE (NBR - ACRYLONITRILE-BUTADIENE RUBBER)
Resistance to petroleum products, acid and solvents. Good swelling resistance especially in aliphatic hydrocarbons, oils and grease. Continuous temperature application range 80° - 90° C
PVC/NITRILE (POLYVINYL CHLORIDE /ACRYLONITRILE-BUTADIENE RUBBER)
Resistance to petroleum products, good acid and solvent resistance, good ozone and abrasion resistance, continuous temperature application range 80° - 90° C
VITON (FLUOROELASTOMERS - FPM)
Good oil, acid and solvent resistance at high temperatures. Continuous temperature application range of200° - 210° C
CHLORO SULPHONATED POLYETHYLENE (CSM)
Good weathering, ozone, acid and heat resistance. Continuous operating temperature range of 120° - 140° C
Good abrasion, rebound resilience. Low gas permeability, high damping behaviour. Continuous temperature application range 80° - 90° C
Good ozone resistance, good thermal resistance, especially dry heat, good dielectric properties. Oil resistance can be compounded into silicone rubber. Continuous temperature application range 230° - 250° C
LIQUID SILICONE (LSR)
As for silicone, with a minimum hardness at 10 Shore A. It is popular for medical applications.
TYPICAL EXTRUSION MOULD: Most extrusion moulds are simply one round piece of steel with the profile of the intended extrusion wire cut into them. Allowances are made for the shrinkage / expansion of the intended compound. Extrusion dies are by far the least complex of the moulds.
MOULDING PROCEDURE: Although extrusion moulds are quite simple the extrusion moulding process requires great care in the set up and manufacture and final processing to ensure consistency of product during the run and over a period of time.
The beginning of a continuous strip of rubber compound is fed into the extruder. The material is heated and under pressure is forced through the die plate that has the correct profile cut into it. Variations in feed rate, temperature and pressure need to be controlled.
Unlike compression or injection moulding the rubber is not cured when it comes out of the mould. The raw rubber is laid out on circular or long trays (depending on the profile) and loaded into an autoclave for curing under heat and pressure.
For long continuous lengths a salt bath or microwave curing system may be used and for silicone extrusion a continuous heating line is used. The curing process used is dependent upon the quantity and profile of the extrusion required.
After curing the strip is further processed - cut to lengths etc - and rolled or packaged for dispatch.
GENERAL COMMENTS: These moulds are relatively cheap to build but because of the processing involved minimum run quantities will vary. Cut extrusion is often used for packing pieces rather than moulded parts as the unit cost is not as high, however, as cutting is not entirely automatic process, tolerances can't be controlled as easily as they can be with moulded parts.
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TYPICAL INJECTION MOULD
Injection moulding machines operate on a continuous process once the initial set-up is complete.The mould remains in the machine and the press closes, a large ram or screw forces preheated uncured rubber through the injection nozzle, through the runner system, down the sprues and into the cavities. The uncured rubber is then forced into the cavities in the mould. Sometimes a slight excess of material (flash) flows out of the cavities, along the grooves and vents.
The mould remains closed until the rubber is cured. The mould then opens, the parts are ejected and brushes wipe away the excess flash. When required the machine operator demoulds parts or rubber pieces and restarts the cycle.
A high level of automation in an injection mould adds significantly to the cost, however, for high volumes this can mean a significantly lower part price.
Injection moulds are ideal for high volume production runs because once they are set up the cycle time is much faster than that of compression moulding . This is because the rubber is pre-heated in the injection cylinder, there is a rapid transfer of heat to the rubber while it is being forced through the runners and sprues and the mould opens, usually ejects the product and closes automatically.
Injection moulds are well suited to moulding delicately shaped or precision parts of high consistency. The time, temperature and pressure is computer controlled and an accurate shot of rubber compound is metered into the mould.
Although the design and manufacture of an injection mould is more complex this is often the most suitable alternative for production volumes.