Developments in the extrusion of tire components.

It is well known that tires comprise many pre-fabricated extruded and calendered products, plasticized as economically and homogeneously as possible and, above all, shaped with as much precision and under as little stress as possible, cooled, temporarily stored and re-processed without further deformation. Low maintenance machines with a high level of automation and minimum re-tooling and down-time are required for this. Additionally, the entire process and all line areas relevant to product quality must be controlled with maximum precision.

Pin extruders

The pin extruder represents the only well-known system which is equally suitable for practically all machine sizes and screw flight depths. This machine has therefore prevailed throughout the tire industry (except for some NR truck compounds where hot feed machines are also used).

The operating principle of the pin extruder is to split and reverse the lamina flow. At low shear rates, the melt flow in the screw flight is split into an increasing number of new partial streams. Having been split, reversed and slowed down locally, the streams merge again (figure 1).

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The picture explains the effect the immersed pins have on the material flow. The layers lying against the direction of flow are reversed by the pins into the direction of flow and slowed down in comparison with the undisturbed layers in neighboring zones, and stretched in this way and by the passage of the pins through the slots in the screw flights.

The length of the pins and therefore the depth of the screw channel clearly has no effect on the function described.

The surface area is expanded by reducing the thickness of the layer, and excellent plastification and homogenization are achieved if this process is repeated a sufficient number of times.

Homogenization

Efforts must be made, however, to ensure that, whenever possible, only compounds with a similar degree of plastification reach the row of pins. Where viscosity differs widely, the more highly viscous pans can evade the flow splitting elements. This means that plastification errors must already be eliminated in the feed zone. This consideration led to the development of the mixing zone. Here, several superimposed flight geometries with differing pitches and depths ensure that the compound is thoroughly kneaded, broken down and mixed in the feed area itself.

Screw mixing zone (feed area)

This type of extruder is essential if high output rates under conditions of perfect plastification, acceptable extrudate temperatures and good product tolerances are to be achieved.

With extruders, as with all technology, the general aim is to achieve optimum output by the simplest means. The typical pin extruder, therefore, has a screw with a mixing area in the feed zone, and is otherwise of a relatively simple design. The pins are designed to ensure optimum strength and flow, and remain immersed at a constant depth. A spiral undercut is pan of the hopper section liner, while the barrel is totally smooth except for the holes for the pins.

As already stated at the beginning, the standard pin extruder is able to plasticize and extrude the most widely diverse compounds with great precision and in a homogeneous way, almost always without the need for any mechanical modifications (e.g., pin adjustments or the introduction of flow restrictors).

Some special designs have not become widely used because they are unnecessarily complex or because they have several disadvantages. They include:

* Pin extruders with adjustable pin depths;

* multi cut Transfermix (MCT);

* a Transfermix zone combined with flow restricting adjustable pins (convert-extruder or MCTD).

Radially adjustable cylindrical pins are required for the first type, and these are prone to fatigue failure due to the notch effect in the area where they protrude through the barrel. This design is therefore less suitable for problem-free production. In addition, bent pins (for instance caused by contact with foreign bodies) are extremely difficult to remove (figure 2).

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The MCT is a modification of the Transfermix extruder, introduced several decades ago, i.e., a special extruder with flights in the screw as well as the barrel. The flight depths in both screw and barrel change alternating from a maximum to a minimum value in a way that, for instance, the screw is at its maximum depth while the barrel is shallow. The compound is thus forced between screw and barrel, and the compound is carried in layers from flights that reduce in size to the opposite flights, which increase in size.

To increase the dispersion effect, the number of flights was steadily increased over the years, and finally produced the MCT.

Transfermix extruders are more complex than other extruders because they also have flights in the barrel. They also have the disadvantage of poor self-cleaning and the problem of removing the screw after longer (unplanned) stoppages due to scorch of the compound.

The "convert extruder" combines an MC Transfermix zone with radially adjustable pins functioning mainly as variable flow restrictors. The MCTD achieves its goal with the aid of flat-topped, rotatable pins.

The L/D ratio is normally lower than for other extruders, so the machine is of a more compact construction, but also has the disadvantages common to all shorter extruders: A high head pressure-output dependency and, with respect to product tolerances, a strong reaction to feed irregularities.

Compounds that are easy to plasticize require processing with as little flow restriction as possible. If the level of plastification difficulty increases, the resistance also has to be increased, and with it the compound dwell time. This also means that output is often substantially reduced and melt temperature increased.

As a general rule, an extruder should be designed to be as simple as possible, i.e., it should achieve high output rates at acceptable extrudate temperatures and good tolerances by the most simple method possible.

When a high degree of plastification is required, it is therefore better to design the extruder with a sufficient number of pin planes and a number and geometry of mixing zones to meet requirements early on, avoiding the need for additional devices.

Simple, but very effective, is the combination of a mixing zone on the screw with a pin area. The different flight pitches and depths and the varied flow speeds in the mixing zone assist the function of the pins (figure 3).

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Feed unit

The substantial influence this device has on a consistently even extruder output was not given sufficient recognition in the past. Until 15-20 years ago, it simply comprised a hopper with guide rolls enabling one or two slabs to be fed automatically to the extruder.

Today, these have, in some cases, become very complex pieces of equipment which allow several sheets to be drawn automatically and simultaneously from pallets, to be checked for metal inclusions, and fed to the extruder over dancer rollers or via loop control systems. Whereas earlier developments were directed solely towards making better use of capacity by feeding more than one slab to the extruder, the present aim is to balance out any differences in the compound in the extruder by additional blending, i.e., to some degree the final mixing of the compound has been transferred from the mixing room to the extruder.

Metal detecting devices detect metallic contamination in the slabs, and trigger an alarm. The contaminated area is then marked by additional devices, for instance chalk markers, so that the contaminated section can be removed. If this section is not removed promptly, an emergency stop is triggered automatically.

Multi-component piggy-back units for profiles

In modern tire manufacturing, the various components for treads and sidewalls are formed simultaneously, either as single or twin profiles in multi-component heads with a high level of precision, without any displacement of the individual components, and with no air pockets between the individual parts.

While the tread cap, base and wings are generally combined in one unit, there are different opinions about the production of the cushion. Both four-component piggy-back units and downstream calenders, used to apply the cushion, have advantages and disadvantages.

Use of a four-component piggy-back extrusion unit has the advantage of saving the space needed for a downstream machine and eliminating any handling problems, but it also has the disadvantage of being an even more complex extruder unit with poor on-line control of the cushion thickness.

Most sidewall units comprise two extruders, if no additional whitewall component is needed. Both tread and sidewalls are extruded as single and dual profiles.

The widest range of designs has been developed for clamping the head sections together, all with the purpose of producing short compound changeover times and the minimum amount of scrap. All movements of the piggy-back head, including clamping, are hydraulic in the more common designs. The most usual versions are the clamping head and the hammer head (figure 4).

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The clamping head is held together by two integral, hydraulically activated clamps. These very powerful wedge-shaped clamping devices press the head sections together. The forces produced in the flow channels by melt pressure are thus self-contained. The advantage of this design lies in the powerful locking force produced with low hydraulic pressure.

However, this design does not allow a single channel to be opened separately. For example: If only the cap compound is to be changed, the complete head has to be opened and the compounds removed from all flow channels.

The hammer head design allows the upper and lower head section to be closed and opened separately. Instead of clamps, pairs of hydraulic cylinders apply pressure to lock the head sections together. The cylinders are flanged onto the fixed middle section and have hammer-like extensions at the end of the piston rods, which are hinge-mounted in fork-like recesses above the hydraulic rotary cylinder.

The clamping mechanism is purely hydraulic, without the assistance of any extra devices, so that considerably higher hydraulic pressures are necessary. All hinge and bolt elements are situated on the head to give excellent front and side access. As already stated, with the hammer head design, the cap compound can be changed without the need to open the lower head section. However, if the wings are to be applied from above - as is usually the case - the wing extruder must be placed underneath and the compound fed through openings in the base flow channel (figure 5).

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The tool design of this extruder arrangement is also substantially more complicated. For optimum material flow in the head, the material flow channels must be easily interchangeable so that, depending on the product, the best conditions for conveying the material to the die cassette are established, and impact surfaces (dead spots) in front of the cassette are kept to a minimum. To facilitate changeover, the flow channels are always positioned in the lower parting sections.

If only the final die is to be changed, this can be done quickly by partially releasing the die clamping mechanism. If the complete cassette has to be changed, it is hinged so that it swings outward when the clamping mechanism has been released, and a new one can be swung in from the other side. There are also two automatic options for this operation:

* The changing device allows the cassette to be exchanged by moving it hydraulically across the extrusion direction. This device requires a lot of space on either side of the head because the cassette take-up unit is mounted on both sides. Also, the exchanged cassette cannot be removed on the operator's side.

* The better alternative is a multi-level changing device that removes the cassette, then transfers the new cassette vertically (hydraulically) to the level of the head-die-mounting. The cassette can then be swung into position.

The advantage of this device is that it requires little space, and operates on one side only. It can also accept up to four cassettes, which can be pre-heated by internal rod heaters while in the stand-by position.

It is also possible to pre-heat the final dies and/or complete cassettes in a pre-heating oven.

Machines for forming innerliners

Today, two types of machines are in common use, including the roller head system and the single roll roller die unit. Both designs are used in lines singly or in combination so that, in one line, two component innerliners can be produced by doubling, even with additional gum strips if required. Features of modern roller head units include:

* Cold feed pin-type extruder;

* roller head;

* head locking frame (independent of the calender frame);

* two roll calibrating calender (for contoured innerliners with quick-change profile sleeves).

When the head clamp has been released, the head and extruder, mounted on a base plate, can be moved backwards away from the head frame and the calender area. To be able to remove the extruder screws, the base frame must either be moved backwards a sufficient distance, or if space is limited, swung to the side out of the way.

The head distributes the compound as evenly as possible to the required width, and conveys it to the calender nip as e.g., 5 - 20 mm thick sheet (for innerliners: approx. 7.5 mm).

The flow channels are designed basically like those in a tread head, and are chrome plated to prevent the compound from sticking to the surface. So that the compound can be easily removed, both halves of the die head should be hydraulically hinged. After cleaning, the head is closed again and re-positioned with the extruder in the locking frame.

At the front of the head, a pair of dies spans the entire width. By changing one or both die plates, the thickness of the extruded sheet can be altered to the specified range.

By inserting adapters, the width of the material leaving the head can be changed so that small product widths can be produced with reduced compound dwell times and/or edge trimming.

A pressure transducer is fitted to the extruder head to automatically adjust the screw speed, thus ensuring a constant head pressure, which is one of the conditions for accurate tolerances. The measuring point is usually positioned directly following the screw tip.

The frame of the locking mechanism comprises a top and bottom cross-beam and lateral connecting columns, which ensure that the head locking forces are contained, i.e., the calender frame only has to take the weight of the equipment. While the lower section of the head rests on the lower crossbeam, hydraulic cylinders (mounted on the upper crossbeam) press the upper section downward.

Other designs have the disadvantage, particularly with large widths and/or high pressures, that the head bends open in the middle, having a negative effect on the product thickness.

For the manufacture of contoured innerliners, the top roller of the calender comprises a core and a profile sleeve which can be changed in just five minutes using a sleeve changing device. The sleeve change, i.e., replacement by a pre-heated new sleeve, takes place through the front calender frame.

Single roll roller dies

The single roll roller die comprises an upper section with material flow channels and a die retained by a wedge shaped gib, and a lower section with a driven cylindrical roller. Hydraulic cylinders operate the locking mechanism of both main head sections, the hinge mechanism of the upper section and the movement of the wedge shaped gib. For precision adjustment of product thickness, the gap between the roller and the die can be changed. This is done by moving the roller vertically by means of motor driven spindles. The main advantage of this head is that, owing to the drag-effect of the roller, the pressure required to overcome the die resistance is supplied only partially from the extruder, and so pressure and melt temperature can be substantially reduced. Due to adhesion to the roller, even products with large differences in thickness leave the head without tension in a straight line, which simplifies the design of the die.

However, for thin sheets or those with thin areas, compounds free from contaminations must be ensured, or a screen changing device must be used to prevent the sheet from tearing or the roller from becoming damaged as a result of impurities trapped between the die and the roller.

Although single roll roller dies were originally developed for much narrower widths, machines are in use today for a trimmed product width of approximately 1,200 mm (approximately 47-1/4").

Trimming devices are required for both roller head and single roll roller dies. The knives can be operated either manually or automatically. The complete cutter bar can be swung in and out of position pneumatically, ensuring the smooth passage of the product sheet. The edge trims are returned to the extruder feed hopper via conveyer belts.

Conclusions

To meet the expectations of fire manufacturers requiring solutions to processing problems, reduction of manpower, improvements to personnel-dependent processes and increased efficiency, machine manufacturers have been compelled to make continuous developments and detailed improvements to what are, by and large, already familiar machines and equipment.

Today's goal is a "perfect" low maintenance production line, which is easy to operate and control.