Calenders

5.3.2 Frictioning ……… 163 5.3.3 Coating ………. 164 5.3.4 Profiling ………. 164 5.3.5 Embossing ………... 164 5.4 Compound Drying/Cooling ……….………….. 164 5.5 Compression Moulding Presses ………. 165

5.5.1 Presses with Vacuum Chamber Extraction ……… 165

5.6 Continuous Vulcanisation ………. 165

5.6.1 Hot Air Systems ……….. 165 5.6.2 Infrared Systems ………. 166 5.6.3 Liquid Cure Medium (Salt Bath) ………...……… 166 5.6.4 Microwave (Ultra High Frequency, UHF) Systems ……… 166 5.6.5 Fluid Bed System ……… 166 5.6.6 Gamma Irradiation Systems ………. 167

5.7 Conveyors, Material/Haul-Off ……… 167 5.7.1 Flat Belt ……… 167 5.7.2 Screw Conveyor ………. 167 5.8 Cutting Equipment ……….. 167 5.8.1 Bale Cutters/Guillotines ………. 167 5.8.2 Clicker Presses ………... 168 5.8.3 Gasket Cutting Lathes ……… 168 5.8.4 Laser and Water Jet Cutters ………. 168 5.8.5 Automatic Cutting Machines ………. 168

5.9 Deflashing ………. 168

5.9.1 Buffing Machines ……… 168 5.9.2 Cryogenic Deflashing Units ……….. 168

5.10 Dipping ……… 168

5.10.1 Latex Dipping, Principal Methods ……… 169 5.10.2 Latex Dipping, Ancillary Processes ……….……… 169 5.10.3 Solution Dipping ………. 170 5.10.4 Dip Coaters ………. 170 5.10.5 Cord Resorcinol Formaldehyde Latex (RFL) Dipping/Drying Lines ……… 170

5.11 Dusting Devices ………... 170 5.12 Extruders, Cavity Transfer Mixer ……… 170

5.13 Extruders, Cold-Feed ………. 171

5.14 Extruders, Dump/Pelletiser/Strainer/Roller Die ………... 172

5.14.1 Dump Extruders ……….. 172 5.14.2 Pelletisers ……… 172 5.14.3 Strainers ……….. 172 5.14.4 Roller Die Systems ………. 172

5.15 Extruders, Gear Pump ……… 173

5.15.1 Combined Extruder and Gear Pump ……….……….. 173

5.16 Extruders, Heads and Dies ……… 174

5.17 Extruders, Hot-Feed ……… 174

5.18 Extruders, Mixers ……….……… 175

5.18.1 Powder Feed Extruder-Mixers ……….. 176

5.19 Extruders, Piggy Back Systems ……….. 176 5.20 Extruders, Pin Barrel ……….. 176

5.21 Extruders, Ram ………. 177 5.22 Extruders, Vacuum ……….. 177 5.23 Granulators/Shredders/Grinders ……….……… 178 5.24 Hose Machinery ……… 179 5.24.1 Braiding Machines ……….. 179 5.24.2 Knitting Machines ……… 179 5.24.3 Hose Builders ……….. 180 5.25 Hydraulic Drives ……….. 180 5.26 Injection Moulding Machines ……… 180

5.26.1 Reciprocating Screw (Screw-Ram) Machines ……… 181 5.26.2 Screw Machines With a Separate Ram ……….. 182 5.26.3 Multi-Station Machines ……….. 182 5.26.4 Shuttle Machines ……… 182 5.26.5 Machines and Moulds for Special Purposes ……….. 183 5.26.6 Injection-Compression Moulding ………..……… 183 5.26.7 Injection-Transfer Moulding ……….. 183

5.27 Internal Mixers ……….. 183

5.27.1 Intermeshing Rotor Machines ……….. 184

5.28 Laboratory Processing Equipment ……….……… 185 5.29 Marking Devices ……….……….. 185

5.29.1 Printing Machines ………... 185

5.30 Metal Preparation for Bonding ………. 186

5.30.1 Degreasing Systems ……….. 186 5.30.2 Blasting Systems ……… 186

5.31 Microwave Heating ……….. 187 5.32 Mills, Cracker ……… 187

5.33 Mills, Mixing ……….. 187

5.33.1 Influence of Mill Variables ………. 188 5.33.2 Laboratory Mills ……….. 188

5.34 Mills, Stock Blenders ……….. 188 5.35 Mixers for Rubber Cement/Dough and Solution ………. 189

5.35.1 Z-Blade (Sigma-Blade) Dough Mixers ……… 189 5.35.2 Paddle Stirrers ……… 189 5.35.3 Planetary Mixers ………. 189 5.35.4 Rotational Mixers ……… 189

5.36 Mould Cleaning ……….……… 190

5.37 Moulds ……… 190

5.38 Ovens for Cure, Post-Cure and Ageing Applications ……… 191

5.38.1 Cure/Post-Cure Ovens ……….. 191 5.38.2 Ageing Ovens ………. 191

5.39 Preheating Bales ………. 191

5.39.1 Hot Room Equipment ……….……… 191

5.40 Preheaters Other Than Microwave ………. 191 5.41 Presses, Lead ……… 192 5.42 Presses, Rotary Curing ……….. 192 5.43 Spreading/Coating Machines ……… 192 5.44 Temperature Control Units ……… 193 5.45 Thickness Gauges ……….. 193 5.46 Transfer Moulding ……… 194

5.46.1 Presses with Vacuum Chamber Extraction ……… 194

5.47 Tyre Building Equipment ……….. 195

5.47.1 Tyre Reinforcement ……… 195 5.47.2 Bead Forming Devices ……….. 195 5.47.3 Extruder Dies for Tyre Profiles ……….……… 196 5.47.4 Tyre Building Machines ………. 196 5.47.5 Tyre Moulds and Mould Containers ……….… 196 5.47.6 Tyre Vulcanising Presses ……….. 196 5.47.7 Tyre Press Loading Equipment …………...……… 196 5.47.8 Tyre Testing Equipment ………. 196

5.48.1 Buffing Machines ………..………..……… 196 5.48.2 Retreading Using Unvulcanised Extrusions ………... 197 5.48.3 Retreading by Direct Extrusion ………..……….. 197 5.48.4 Retreading by Smooth Tread Extrusion and Pattern Cutting ……….. 197 5.48.5 Prevulcanised Tread Replacement ………..……… 197 5.48.6 Vulcanisation Autoclaves ………..……… 197 5.48.7 Retread Testing ………..……… 197

5.49 Weighing Equipment ……….. 197

5.49.1 Weighing Units For Liquid Dispensing ………..………. 197 5.49.2 Weighing Rubber Chemicals ………..……….. 197

5.1 Autoclaves

Autoclaves for rubber product vulcanisation are available in a wide range of sizes to suit the product type. The main use for autoclaves is in the production of extrusions, sheeting and components which are of unsuitable size or construction for conventional mould vulcanisation. Examples of this type of product can range from roller coverings to hand-built products such as footwear.

There are two main types of autoclave available. Steam curing is still the more widely used method and involves the use of a gas or electric boiler supplying steam to a single-cased autoclave. Increasing in popularity is the use of hot atmosphere curing in autoclaves, where the internal pressurising medium (either air or nitrogen) is heated by electric heater elements. This method has the advantage of providing independent control of pressure and temperature, unlike with steam, and eliminating the detrimental effects of condensation. Use of nitrogen also eliminates surface deterioration of the product from oxidation.

An advantage of the autoclave for smaller components, is that a large volume of the same or different products can be vulcanised at the same time, provided that the curative systems in use do not interact between the different compounds. It is usual to support extrusions and low hardness products in talc. Products formed on mandrels and sheeting, for example, are cloth wrapped to prevent distortion.

The temperature is raised using a number of steps to achieve uniformity of temperature throughout the product mass, before the final vulcanisation stage is reached.

It may be necessary to use a gas or steam circulatory system to ensure uniformity of temperature throughout the whole of the autoclave and its contents.

5.2 Cable Manufacturing

Cables with elastomeric covers are manufactured by extrusion of the compound around a metallic conductor. The compound must be prepared in a manner to ensure that there are no residual conglomerations of ingredients, which could act as a source of electrical failure. In smaller diameter cables extrusion of the compound to cover the conductor is usually by a crosshead die system, which can apply one or two compound layers to give the required insulation properties. It is usual to use duplex extruders applying insulation and sheath compound at one pass, directly before the covered cable enters the curing tube system. The construction of the die head is designed to apply the compound co-axially to ensure correct dimensional concentricity. The metal conductor/cable is controlled both before and after the extruder/vulcanising system by capstans which are co-ordinated so that the correct tension can be maintained on the cable throughout the process.

Cables which are of larger dimensions can be manufactured by the older batch process. The conductor covering portion of the process is similar to that for small cables, but the covered cable does not enter a vulcanising tube. Large diameter cables, with multi-conductor construction, can be covered by using a compound tape lapping system. The individual conductors are covered with insulation by passing the conductor with two flat tapes applied through grooved rollers to nip the tapes together to form a seam. The combination of conductors bunched together are then lapped using a continuous compound tape. The lapping method can also be used to apply the initial insulation layer. Cables manufactured by this method require a cotton support tape application prior to being wound on to large drums and vulcanised in an autoclave. An alternative method is to apply a lead sheath, for cable consolidation, using a lead press. After vulcanisation the cotton tape or lead sheath is stripped from the cable.

5.2.1 Continuous Vulcanisation Tubes

Steam tube vulcanisation of cables is a common method used by the cable industry. The cable after covering is passed through a steam tube connected to the extruder crosshead. The end of the tube connected to the extruder is fitted with a seal system. After traverse of the steam tube the cable with its now vulcanised sheath emerges through a sealing system, is cooled and then coiled. There are three tube systems used, horizontal, catenary and vertical. The catenary is shaped to follow the natural drape of the covered cable as it passes through the steam tube, thus eliminating contact with the walls of the curing vessel, especially in the case of large diameter cables. The length of the steam tube depends on the size of cable to be vulcanised, as it is necessary for full cure to be achieved before the cable emerges from the end distant from the extruder. Gas tubes are also available for dry curing.

5.2.2 Fluid Bed Systems

Although to some extent fluid beds have diminished in popularity for vulcanisation, this method can be used in cable vulcanisation. The heating medium is a bed of tiny glass spheres fluidised by steam or hot air.

5.2.3 Hot Air/Infrared Tunnels

Hot air tunnels are often used for vulcanisation of cable covered with rubbers such as silicone. These systems may also incorporate infrared radiation as a means of boosting heat transfer to the product.

5.2.4 Lead Presses

Large diameter cables can also be vulcanised in ‘short’ lengths using a lead cover. This method can be a problem for coloured cables coated with rubbers with sulphur content, which can form a lead sulphide stain on the surface of the rubber in contact with the lead sheath. It is necessary to use very large drums to support the cable during vulcanisation in an autoclave, and it may also be necessary to rotate the drum to offset any eccentricity of the cable cover which may occur, due to gravity causing compound movement, owing to the reduction of compound viscosity with temperature rise.

5.2.5 Salt Bath Systems

Systems are available which use molten salt as the curing medium. These systems can be either unpressurised or pressurised, and can have a salt recirculation system fitted. The advantage of this method of vulcanisation is the high temperatures which can be achieved with consequent greater product throughput rates.

5.3 Calenders

A rubber calender in basic form consists of two or more hardened and accurately machined metal rollers known as ‘bowls’ rotating in bearing journal boxes which are set in rugged iron frames. One bowl in each pair is equipped with nip adjustment to control the thickness of processed material. Adjacent pairs of bowls rotating in the same or opposing directions form a ‘nip’ where the material being processed is squeezed into sheets or is laminated to form the desired product. The drives for the bowls include constant or variable speed motors and reduction gearing, to achieve the bowl surface speeds specified by the processing requirements of the materials. Modern calenders are usually fitted with individual DC motors via double universal joints, or directly fitted hydraulic

drives to each bowl. A calender, depending on the number and the design of its bowls, is capable of sheeting, frictioning, coating, profiling and embossing.

Calenders for processing rubbers are made in a number of bowl configurations, horizontal and vertical, and in sizes ranging from laboratory units to giants weighing many tons. The three-bowl vertical calender with 24 in ((60.96 cm) diameter) x 68 in ((172.72 cm) face length) bowls and the four-bowl ‘Z’ and ‘L’ with 28 in (71.12 cm) x 78 in (198.12 cm) bowls are typical of the machines used for mass production of tyres, belting, sheeting and the like. Three-bowl calenders have been widely used for processing of mechanical rubber goods. The four-bowl calenders are popular in tyre plants. The four bowls permit simultaneous application of rubber compound to both sides of tyre cord fabrics and steel cords. Two-bowl calenders are used to produce strips and profiles, often in combination with extruder feeding, in which case they are commonly referred to as ‘Roller Dies’.

5.3.1 Sheeting

This basic operation utilises a two-bowl calender in horizontal or vertical configuration. The feed material, either in strip or ‘pig’ form, is fed into one side of the nip and is squeezed by the bowls, thereby emerging as a sheet which is pulled from the bowl by some manual or mechanical means or supported by a ‘liner’ cloth. Because of their versatility, three-bowl calenders are now more widely used for sheeting as well as other basic calendering operations. Thickness control is accomplished by use of the adjustable nips and may be further refined by automatic control systems using thickness sensors. It should be noted here that the force required in the nip to flatten the feed material causes deflection of the bowls, however slight. If some corrective steps are not taken, the product thickness will vary across the sheet, resulting in excessive variations of the product and possibly excessive use of expensive materials. In order to overcome these problems, three basic techniques are used to achieve uniform product thickness:

1. Grinding or ‘crowning’ of the bowl faces to compensate for deflection. The bowls are mechanically ground so that the diameter at the centre is larger than that at the ends of the bowl face. Under load the ‘crowned’ bowls deflect so that the gap between them is more uniform. Grinding and regrinding after service wear must be carried out at normal running temperature if the ‘crowning’ is to be correct.

2. Bowl ‘crossing’ whereby the bowl axis can be mechanically deflected from a planar relationship. This effect will create a progressively greater gap between the bowls from centre to ends. Because bowl crossing is a deliberate misalignment of the horizontal axis of the bowls, it is also known as ‘axis skewing’.

3. Bowl ‘bending’ whereby the bowl is physically bent by the application of force to the outside of each bowl journal bearing. Positive bending causes an increase in bowl convexity while negative bending causes an increase in concavity.

In order to minimise air entrapment and blistering, the thickness of each sheet is generally limited. To build up the required thickness of the final sheet, two or more plies of calendered sheet are usually laminated on the bottom bowl of a three-bowl calender.

5.3.2 Frictioning

Frictioning involves rubbing or wiping an elastomeric compound into a substrate of textile or metallic cords, which may or may not be held together by ‘pick’ threads or fill yarns, or the substrate may consist of a ‘square woven’ fabric like ‘hose ducks’ or ‘belt ducks’.

Usually a three-bowl calender is employed wherein the rubber sheet is formed between the upper and middle bowls while the resulting sheet is simultaneously being frictioned into the substrate

between the middle and bottom bowls. In this operation the upper and middle bowls may be moving with either ‘friction’ or ‘even’ surface speed, but the middle and bottom bowls will only be run with ‘friction’ or unequal surface speeds so that the rubber is effectively wiped into the substrate being carried on the bottom bowl.

5.3.3 Coating

A coating or skim-coating operation is similar to that described for frictioning except that the middle and bottom bowls will run at ‘even’ surface speed so that the rubber sheet is merely laid and pressed against the substrate. This is particularly true of a multi-pass operation where the substrate will have previously been frictioned. The coating operation may produce a heavy deposit or merely a thin ‘skim’ coat depending upon the product requirement. Generally, multi-purpose calenders such as a three-bowl unit, are equipped with ‘even’ and ‘friction’ gearing arrangements so that a number of combinations of bowl speed ratios are possible.

A more complex form of coating calender is the four-bowl ‘Z’ or ‘L’ arrangement. A four-bowl calender can simultaneously apply a rubber coating onto both sides of a fabric. In effect, the No. 1 and 2 bowls and the No. 3 and 4 bowls form pairs where two rubber sheets are produced. The sheets are then laminated to a substrate between the No. 2 and 3 bowls. Very sophisticated devices are usually incorporated into the calender design to control thickness and width of the individual sheets and the resulting laminate.

5.3.4 Profiling

Many rubber products require uncured components which are not rectangular in cross-section. In such cases, at least one calender bowl may have a peripheral design cut into its surface to produce the desired cross section. This method is particularly useful for long production runs, but becomes expensive in terms of bowl change and bowl inventory necessary when many different sections are required. For shorter run working, the calender bowl may consist of a heavy basic mandrel onto which may be clamped solid cylindrical or split cylindrical steel ‘shells’ into which the appropriate profile design has been cut. Such calenders often have the bowls outside the frames to improve access and facilitate bowl changing. Consequently the bowls are comparatively short.

5.3.5 Embossing

Some rubber products are made from uncured components which must have a surface design that cannot be economically formed by subsequent moulding. One such example is the cover strip around the sole of canvas shoes. The method of producing such strips is similar to ‘Profiling’ in that the required design is engraved into the calender bowl or ‘shell’ as a mirror image of the design itself.