- Rubber ACCELERATORS
- Chemicals which are added to rubbers to accelerate the rate of vulcanisation. Rubber without accelerators takes twenty or thirty times longer to cure.
- Rubber ACID ACCEPTORS
- Mostly used in Neoprenes to absorb the acid produced by the chemical reactions which take place during curing. Acid acceptors are usually metal oxides.
- Rubber ACTIVATORS
- Chemicals which are added to rubbers to activate curing.
- Processing Rubber Accelerators
- Chemicals which vary the speed and timing of the curing reaction.
- Processing Rubber Anti-Oxidants / Anti-Ozonants
- Chemicals which are added to help the compound resist surface attack, especially by ozone.
- Rubber BACKRINDING
- A "breaking up" of the rubber normally observed around tool split lines. It results from the rubber shrinking during cure and tearing away at points where the moulding is held.
- Rubber BANBURY MIXER
- An internal mixer commonly used in the rubber industry. It consists of two shaped rotors which turn inside a sealed chamber.
- Rubber BLEEDING
- The exudation of droplets of excess plasticiser onto the surface of the rubber. This happens when rubbers contain unsuitable levels of plasticisers which are not fully compatible.
- Rubber BLOOM
- A white or light coloured crystalline deposit on the surface of the rubber. Many chemicals added to rubber have a limited solubility. If they are present in a cured rubber at levels higher than their solubility, they crystallise out onto the surface.
- Rubber BUILDING TACK
- Term used to describe the ability of a rubber to stick to itself in the uncured state, so allowing pre-building of the material before curing.
- Processing Rubber Buffing
- Parts can be smoothed using a variety of abrasive belts, wheels and mops.
- Rubber CALENDERING
- Producing rubber in continuous sheets by processing it through a series of rollers. Harboro does not specialise in this sector of the industry.
- Rubber CLAY
- Commonly used as an "extender" in rubber. Treated clays reinforce the rubber and are termed "hard" clays. Those which do not provide reinforcement are called "soft" clays.
- Rubber COMPOUND
- A general word used to describe a particular formulation or "recipe", or as a descriptive term for unvulcanised rubber in general. It is used in such applications as "Compounding ingredients", "Compound no. xyz", etc.
- Rubber COMPRESSION SET
- The degree to which a rubber does not recover fully to its original state after it has been compressed for a long period of time. (See also "Measurement of Rubber Properties").
- Rubber CRYSTALLISATION
- Rubber materials tend to crystallise on standing, resulting in an increase in hardness. This increase is usually small, but can be more significant in the case of some Neoprenes (polychloroprenes).
- Rubber CURATIVES
- The chemicals involved in curing the rubber, e.g. accelerators, vulcanising agents, activators etc.
- Rubber CURING
- Also known as vulcanisation, this is the permanent change which the rubber undergoes during moulding. When mixed, rubber compounds are thermoplastic and will melt and deform easily. After curing, the rubber is heat stable.
- Processing Rubber Curatives
- Active chemicals which bring about the cross-linking of the long chain rubber polymer. Sulphur was the first to be discovered and is still commonly used.
- Processing Rubber Compression
- A piece of uncured rubber of the correct size is placed between two halves of a heated mould. The mould is closed in a press under a pressure of around one ton/sq in and the rubber is forced into the exact shape of the cavity. The rubber gains heat by conduction from the mould surfaces and "cures". When the rubber has had sufficient time to cure, the mould can be opened and the part removed. Compression moulding is a relatively simple process and is often used for components required in fairly low quantities. It is also the most economic method for parts with simple shapes. Parts moulded by this method will always have some flash because the mould surfaces are held apart by the necessary excess rubber in the "blank".
- Processing Rubber Cutting
- Parts are die cut by machine using a range of tools from precision steel knives to wood-formes, or in extreme cases are trimmed with scissors.
- Processing Rubber Direct Injection
- A screw injection system delivers a metered quantity of rubber into the closed mould. The injection unit is fed from a continuous strip or a reservoir of uncured rubber and is cooled to avoid premature curing. This process is generally used for multi-cavity moulds and can produce hundreds of components per press cycle. Because of the amount of rubber in the system, it is inadvisable to change materials frequently Large moulds require complex feed systems to balance the pressures in each cavity. Generally these are in the heated top half of the mould and cure at the same time as the components. Unlike thermoplastics, cured thermoset rubber cannot be reground or reused and the additional waste has to be included in the material usage per piece. Where very large volumes of mouldings are required, cold runner systems should be considered. These are justified by material savings over ￡10,000 pa. This process lends itself to relatively large quantities, a large number of cavities and infrequent changes of materials or moulds. Parts are repeatable and can be made to a high level of precision.
- Rubber ELASTOMER
- General term used to describe all natural and synthetic polymeric materials which have rubbery or "elastic" properties.
- Rubber ELECTRIC STRENGTH
- Describes the ability of a rubber to withstand high voltages. It is usually quoted in KV per mm of rubber thickness.
- Rubber EXTRUSION
- Producing continuous lengths of rubber with a constant profile by forcing it through a die under pressure and vulcanising it. Harboro does not specialise in this sector of the industry.
- Processing Rubber Extrusion
- Extruders force warmed compound through a shaped die. Any reasonable length of shaped material can be produced. Once cooled this is fed into the direct injection presses.
- Rubber FILLERS
- General term describing the bulk materials which are added to the rubber (usually as powders).
- Rubber FLASH
- Excess rubber found on mouldings as a result of the moulding process, but which does not form a part of the moulding itself. This does not usually include feed gates but refers particularly to thin films of rubber formed at tool split lines etc.
- Rubber FLEX CRACKING
- The tendency of some materials to crack as a result of repeated bending or stressing at the same point.
- Rubber FORMULATION
- The term used to describe a rubber "recipe".
- Processing Rubber Fillers
- Relatively inert chemicals, such as clays, which increase the bulk of the compound.
- Processing Rubber FINISHING
- Compression moulded and some injection moulded parts require deflashing. This is done in various ways depending upon the shape and size of the component and the type of rubber used.
- Rubber HEAT
- (see "Lift")
- Rubber HEAT AGEING
- Also known as "accelerated ageing". Rubber materials are often tested for shorter times at higher temperatures to predict what will happen over longer times and at lower temperatures.
- Rubber HYSTERESIS
- The difference between the amount of energy absorbed when a rubber is stretched and the amount of energy released when the rubber is relaxed. High hysteresis indicates a high loss of energy (and so is good for energy absorbing applications). Low hysteresis rubbers are more resilient.
- Rubber LIFT
- The set of rubbers resulting from a single mould cycle or pressing. Also called a "heat".
- Rubber MODULUS
- The relationship between stress (force) and strain (extended length). Modulus is usually given as the stress (or force) required to extend a rubber (e.g. 100%, 200% extended).
- Processing Rubber MIXING
- The constituents are weighed out and combined by a mixing process which must blend the ingredients thoroughly in a repeatable way. This is achieved either by an internal mixer, where the compound is mixed by two meshing rotors in an enclosed case; or by open mill mixing, adding the ingredients carefully into the "nip" between two steel rollers, typically of 30" diameter.
- Rubber PLASTICISERS
- Liquids which are incorporated into rubber, generally described as plasticisers because of their softening effect.
- Rubber POLYMER
- General term used to describe all rubbers and plastics. In fact, it is the chemical term used to describe all organic materials which are formed from chains of repeated chemical units.
- Processing Rubber Pigments
- Added to produce specified colours. They can only be used with compounds which do not contain carbon black.
- Processing Rubber Plasticisers
- Added to aid processability or to produce specified properties.
- Processing Rubber Process Aids
- Resins, soaps, low-weight polyethylene.
- Processing Rubber PRE-FORMING
- Each moulding process has its own requirements for uncured material. Compression moulding, for example, requires a "blank" of material in a size which will fill the cavity exactly. Direct injection moulding needs relatively large quantities of compound in a continuous strip. Due to the nature of the injection process, material properties must be precisely measured and controlled to achieve the planned flow and cure behaviour, as well as the desired final characteristics of the rubber.
- Processing Rubber Pre-Forming
- Extrusions as above are cut to required lengths as they emerge from the die. This process can be accurately controlled to produce blanks of precise volume for compression moulding.
- Rubber RESILIENCE
- The technical term used to describe "bounce" or "snap". Increasing resilience means an increasing "bounciness".
- Processing Rubber Reinforcing Fillers
- Materials which increase the strength of the material. Carbon black and silicas are the most commonly used.
- Rubber SCORCH
- The onset of cure, when chemical reactions begin to take place in the rubber as it is being heated. A "scorched" rubber is no longer processable.
- Rubber SET
- The degree to which a rubber does not fully recover to its original shape after it has been deformed for a long period of time. Also called "permanent set".
- Rubber STRAIN
- The change in length of a rubber under tension or compression. A rubber stretched to double its original length would be at 100% strain.
- Rubber STRESS
- An applied force or load. A load of 1kg applied to a test piece would be a stress of 1kgf.
- Processing Rubber Sheeting
- Uncured material is produced in sheets of the desired thickness. Sometimes "blanks" are cut from the sheet, like pastry cutting.
- Processing Rubber Sub-Zero Finishing
- The most modern and efficient method of finishing uses cryogenics. Parts are frozen to temperatures as low as -120°C and then tumbled and/or bead-blasted while cold to remove the brittle flash. The machines are individually programmed with the optimum temperature and running times for each particular type and number of components, which are tested and proven during the development stages.
- Rubber THERMOPLASTIC
- Any material which melts on heating and resets on cooling. This melting and refreezing can be repeated indefinitely.
- Rubber THERMOSET
- Any material which melts on heating but then undergoes a permanent chemical change after which it is heat stable (i.e. it will degrade on further heating rather than "melt").
- Processing Rubber Transfer Injection
- The heated mould is closed in a press and the rubber injected by a hydraulic cylinder through a feed hole in the cavity. The cylinder can either be incorporated in the press or sometimes in the mould. Provision must be made for air to escape from the cavity as the rubber enters, and the feeding method chosen to suit the operational requirements of the part. This method of moulding can produce high-precision parts in moderate quantities without high tooling costs. In the simplest case, the mould can be the same as a compression mould with the addition of a feed hole. Maximum weights and number of cavities are governed by the capacity of the transfer cylinder and the clamp pressure.
- Processing Rubber Tear Finishing
- The tool is designed to produce a very thin section of flash around the part which is torn off at the press during de-moulding.
- Rubber UNDERCURE
- Removing rubber from the mould before the vulcanisation process has been completed. Often results in porosity in the rubber or unusually poor compression set.
- Rubber VULCANISATION
- The permanent chemical change that a rubber undergoes on heating together with chemicals. The chemicals which accomplish this change are called "vulcanising agents".
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