Rubber

There are several processes used in the manufacture of rubber products. Some of the most common techniques used for natural rubber processing are extrusion, immersion in latex, moulding and calendering. We offer you a wide range of temperature control systems that meet the manifold requirements of temperature ranges, volume flows and system integration and are adapted to the various process methods.

The reliability of raw material quality, exact weight proportions of the various ingredients, a controlled mixing procedure and optimised forming processes are essential requirements for the production of components made of elastomer materials. Within this complex environment, we meet all your temperature control requirements – either in the form of standalone solutions, or fully integrated into your system.

Elastomers are created from crosslinked macromolecular polymers that form a three-dimensional wide mesh structure. The links between the individual polymer chains (vulcanisation) create the elastic properties of this material. In general linguistic usage, elastomers are therefore commonly referred to as rubber. Up to their decomposition temperature, elastomers are vulcanised polymer materials that are as hard as glass at low temperatures and do not exhibit viscous flow even at high temperatures. Instead, between room temperature and their decomposition temperature, they exhibit elastic properties. At room temperature, elastomers can withstand small forces that cause significant, temporary deformation. Once the force is no longer applied, the macromolecules return to almost their original position.

The following briefly describes the materials that can be optimally processed with help from our temperature control units.

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Elastomer compounds
Natural and synthetic rubbers are not materials in the normal sense. They are raw materials which require a range of other substances to be added prior to vulcanisation. Besides the base material natural rubber, all elastomers contain numerous additives such as filler materials, plasticizers, processing agent, antioxidants, vulcanising agents, vulcanising catalysts, activators, vulcanisation retarders, pigments, etc. On average, an elastomer compound is made up of 10 to 20 components. The sum total of these components is known as the elastomer compound. The type of natural rubber used determines the fundamental properties of the vulcanised product, in particular the ageing stability and cold flexibility as well as its behaviour when exposed to mediums such as oils, fuels, water and solvents. The mechanical properties such as elasticity and hardness also depends on the polymer base as well as the use of filling materials such as carbon black or light filling agents. The use of various additives allows certain properties to be modified (e.g. the hardness) and improved (e.g. cold flexibility, impact resilience, compression set, heat and swelling resistance). The composition of elastomers is highly complex and is altered depending on the application.

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Elastomer foams
Foamed elastomers are created by adding an expanding agent to the non-vulcanised natural rubber compound. During vulcanisation, gas separates out and builds pores which expand the compound. This process is used to create products such as: Filters, cushioning, packing, mattresses and technical items.

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Composites: Rubber-metal, rubber-plastic
Moulded rubber parts allow a huge amount of design freedom due to the excellent expansion properties of the elastomer compounds which allow even more difficult contours to be removed from moulds. Rubbers can be used to make a variety of composites. Composites using moulded rubber parts and metallic elements, or rubber and plastic, allow for significant costs savings to be made. A key factor affecting the quality of rubber-metal composite parts is the selection of a suitable composite technique. Even under the heaviest of loads, the elastomer layer must not separate from the metallic or plastic material.

Composite parts such as these offer many advantages:

• Reduction in the total number of individual parts and thus easier handling of just a single part
• Assembly work is no longer required, eliminating errors arising from assembly processes
• Reliability and operational safety thanks to a firmly bound composite
• Parts can withstand higher loads

Mixing / compounding
The compound is created in two stages. A pre-mixture of the raw materials is made before the final compound is created by adding the cross-linking agents and catalysts and ensuring homogeneity. Both pre-mixing and creating the final compound is done using kneading machines or rolling mills. The end results of this process are non-vulcanised natural rubber compounds in the form of granules that can be used for shaping.

Shaping
Shaping can be caried out using a variety of different methods:

• Compression moulding
• Calendering
• Hand shaping
• Injection moulding
• Extrusion

This processing step generates both non-vulcanised slugs and vulcanised finished or semi-finished parts. Temperature control for both variants can be optimally supported by our temperature control units.

Vulcanisation
Our temperature control units provide optimum support for vulcanisation. Vulcanisation is the chemical and physical transformation of ductile natural rubber into a rubber-elastic state. This process, also known as cross-linking, creates links between macromolecules at reactive points. Vulcanisation requires a vulcanising agent. The oldest and most commonly used vulcanising agent is sulphur. Compared to the raw rubber material, the rubber produced by this process has permanently elastic properties, returns to its original shape after being subjected to a mechanical load, has a higher tensile strength, elasticity and resistance to the effects of ageing and weather.

The elasticity of the rubber depends on the number of sulphur bridges. The more sulphur bridges there are, the harder and less elastic the rubber. The number of sulphur bridges itself is dependent on the amount of sulphur added and the duration of the vulcanisation. As the rubber ages, these sulphur bridges are replaced by oxygen bridges; the rubber thus becomes more brittle and porous.

Vulcanisation is often carried out in the same process step as shaping. Vulcanisation can therefore be carried out either in a heated mould or in a special vulcanisation oven.

• Compression shaping
  Compression moulding is the traditional way to produce shaped rubber parts. A quantity of non-vulcanised natural rubber compound sufficient for the quantity of parts to be produced is usually placed manually into a vulcanisation mould. The mould is then closed and pressure applied. The pressure and temperature softens the compound which flows to fill the mould cavity. Temperatures between 140 and 200 °C vulcanise the rubber compound. The vulcanised (cross-linked) part now acts more like a thermoplastic and is removed hot from the mould without any deformation. The mould must be designed so that no shrinkage can occur in the finished part after cooling. Compared to injection moulding, the compression moulding process generally requires much longer heating times as the cold rubber compound is heated to the vulcanising temperature exclusively by the walls of the mould. Compression transfer moulding is a further development of this. Here the moulding compound is additionally heated using friction during injection. This shortens the time needed for crosslinks to form. The disadvantage of this process is that the mould costs are much higher as a result of higher wear and there is a larger drop in cross-linked elastomers. Compression moulding is more cost effective when producing simple parts in limited quantities.
• Injection transfer molding (ITM)
  Injection transfer molding (ITM) is a variant of the compression transfer molding process in which the transfer unit in the upper part of the mould is filled with the plasticized natural rubber compound via the injection unit of an injection moulding machine. Transferring an already plasticized natural rubber compound makes it possible to more evenly fill the vulcanisation mould. Cold transfer moulding channels are used in compression moulding and injection transfer moulding (ITM) to produce shaped rubber parts. A cooling medium cools the transfer unit and is thermally separated from the heated vulcanisation zone in the mould by an insulation plate. Water is usually used as the cooling medium. The transition from the cooled to the heated zone is carried out via nozzle elements. Unlike nozzle cold runners, the nozzle elements used with transfer moulding cold channels are not cooled individually, but only across the entire cooling block. The elastomer material in the transfer unit is not vulcanised in each product cycle and can be used in the next cycle for producing shaped parts. Whilst nozzle cold channels are most often used with small calibres and large volume parts, transfer moulding cold runs are better for high calibres and smaller volume rubber parts.
• Calendering
  Calenders operate similar to a rolling mill. Parallel rollers arranged on top of each other can be temperature controlled. Depending on the size of the gap between the rollers, different thicknesses of film can be produced. Vulcanisation is carried out either intermittently in an autoclave in the press, or continuously in a vulcanisation machine.
• Injection moulding
  When using injection moulding to produce shaped rubber parts, the natural rubber compound is first pre-heated in a screw unit and plasticized (80 – 100 °C) and then injected into the heated mould via sprue channels. Two different types of machine are used: horizontal and vertical injection moulding machines. The injection process takes place either by directly pushing the screw in the cylinder or via a separate cylinder (injection ram). Compared to compression moulding, injection moulding is a more modern process and offers efficiency benefits due to it being fully automated. Plasticization in the injection unit results in significantly shorter heating times. The use of nozzle cold channels helps reduce wastage to a minimum.
• Extrusion
  In the extrusion process, plasticization of the natural rubber compound is handled by the rotating screw. The screw compresses and heats the material, pressing it through a nozzle into the open. After passing through a corresponding calibration unit and vulcanisation unit, the extrudate is cut to the required lengths. Vulcanisation can be carried out using an unpressured saline bath, a UHF hot air channel or in a separate autoclave.