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Tandem mixing.

The processes used at present for producing rubber compounds are as follows:

* Two or multi-stage processes. The two-stage or multistage processes are at present the most common methods of mixing. However, they have disadvantages with regard to multihandling methods and the storage space and storage time for masterbatch.

* Addition of reactive materials (sulfur and accelerators) on a two-roll mill. Although this process is often used for industrial rubber goods, it also has disadvantages, e.g. different cycle times and capacities between mixer and two-roll mill, dependence on personnel and handling problems.

* Single-stage mixing process. For many years, the single-stage mixing process in the mixer was not technically possible. This process only gained strength with the acceptance of the superiority of intermeshing rotors and the increased cooling available on an interlocking mixer, along with the discovery of other improved technical processes to compliment the mixer.

The problem in single stage mixing is that the metal of the mixer which has absorbed work heat in the primary mixing operation must be cooled before the mix is reduced in temperature sufficiently to add curatives.

Despite all advances, the single-stage process is still limited to fairly soft and slow vulcanizing compounds processed in small to medium sized mixers and is normally restricted to industrial rubber goods applications. It cannot yet be used for tires.

To make significant progress here, a completely new process is required and one such process is termed tandem mixing because of its internal mixer configuration.

Tandem mixing

The process described here uses a combination of a ram type mixer and, below this, a ramless mixer, where the masterbatch produced in the ram mixer is transferred without intermediate storage to the ramless mixer, cooled and final mixed, while at the same time a new masterbatch is prepared in ram mixer. The upper mixer is preferably but not necessarily interlocking. The mixer below must be interlocking to achieve the ramless feed.

The tandem mixing process was invented and patented by Dr. J. Peter (a retired director of the Continental Rubber Co.) and the process has been promoted with the technical assistance of G. Weckerle, who is the technical manager of the Continental technical rubber factory in Northeim, Germany. Francis Shaw & Co. and licensees have sole world rights for supply of the interlocking tandem mixer.

The tandem process works with a combination unit consisting of a ram mixer and a ramless mixer below it. The masterbatch passes from the primary mixer into the tandem mixer below, where it is cooled and finish-mixed while the next masterbatch is being prepared in the upper mixer. It is then emptied into a two-roll mill or a dump extruder and processed in the normal way.

Technical description of the process

The masterbatch function is carried out in the upper mixer, at relatively higher speed and ram pressures. Discharge temperatures may be in the region of 150 [degrees] to 160 [degrees] C and mix times may be typically 3-1/2 - 4 minutes.

The upper machine may preferably be an interlocking mixer but this is not essential. As the masterbatch function does not involve the addition of curatives or accelerators and is essentially a heating operation, the mixing cycle may be carried out as rapidly as possible because there is no need to await cooling of the mixer between mixes.

The function of the lower tandem mixer is to accept the hot batch and to cool it prior to addition and incorporation and dispersion of the curatives. The lower machine must be an interlocking Intermix in order to enable self-feed without the pressure and work heat effect of a top ram.

The final mixing function is normally a shorter process than the masterbatch stage and assuming a final mix time of, say, two minutes then the tandem machine may run idle typically for a minute or so following discharge and before receiving the next hot masterbatch. This idle period with discharge door open allows the tandem mixer to cool considerably and the cool machine forms a huge metal heat sink ready immediately to absorb heat from the hot masterbatch as it is received.

As stated previously, the interlocking rotors are self-feeding, but the self feed operation is much assisted by sizing the tandem mixer larger than the upper machine so that the tandem mixer receives a relatively small batch. The optimum fill factor for the tandem mixer is 0.45 to 0.47. The suitable combinations of sizes of intermixes would be: K2A/K4 Intermix K4/K5 K6/K6A K7/K8

Between the two mixers is a discharge flap and chute which would be closed at all times except when the tandem mixer receives the masterbatch. It is desirable to fit air extraction to the discharge chute to encourage a cooling draft of air, but with suitable sealing, the chute could be under vacuum if required. As the tandem mixer receives only hot plasticized batches, the tandem rotor speeds are low (up to 25 rpm) and motor torque less than half the masterbatch requirement.

As the tandem mixer system is entirely dependent upon excellent cooling of the tandem machine, the following graphs of cooling of full machines and empty machines are relevant and interesting.

Figure 1 shows the cooling effect of a K4 Intermix running at slow speed with a batch size giving 0.5 fill factor both with and without a ram. The cooling rate is relatively slow.

Figure 2 shows the cooling rate of the hot masterbatch having been discharged into an empty cool K4 or K1. Here the cooling rate is much faster and the batch is cooled to a safe temperature after 30 seconds or so.

Figure 3 shows the rapid cooling rate of an empty mixer following discharge. This indicates the importance of the idle cooling period while the tandem machine awaits dump of the masterbatch.

Possible applications and layouts for the tandem mixer

* Applications: a) replacement for single and two-stage systems; b) reduction in the number of mixer passes in a multi-pass system; c) reduction in the number of mills in a multi mill final mix system. (This system appears to be gaining favor in the tire industry.)

* Layouts - various layouts available offer these obvious advantages: a) no masterbatch storage space or store time; b) much reduced transport of compound; c) reduced labor (no final mix line); d) weighing of masterbatch eliminated; e) reduced energy requirements; f) less anti-tack in use and on product; g) simple logistics.

Initial testing

This was carried out in 1989/90 in the Continental Northeim plant which uses K2A, K4, K5 and K7 mixers. The hot masterbatches were transferred between mixers using insulated boxes. In all cases the finished compound was subjected to normal factory quality control checks and finished product was successfully made.

Further testing was also carried out in the factory of Kraiburg GmbH near Munich. These people are in high repute and are the largest custom compounders in Europe.

The masterbatch mixer was a peripheral type and the final mixer was an interlocking machine of K7 size. Again, the compounds were subjected to factory quality control tests and product was successfully produced.

First pilot plant - observations

Following successful early tests in the firms of Kraiburg and Continental, a pilot plant was installed in Kraiburg which has been working since February 1991.

The plant is arranged so that the tandem mixer can be by-passed when required but it has been in successful use as a tandem system for 85% of its working time since installation

The layout of the plant comprises the following equipment:

* An existing W&P GK.110E interlocking rotor mixer for masterbatch. 85-110 kg batch, 6-60 rpm, 610 kW DC drive.

* A connecting chute with hinged feed flap and inspection door.

* K5 Intermix tandem mixer (ramless) 2-25 rpm, 173 kW DC drive, 85-1 10 kg batch.

* Skip hoist

* Two-roll mill with stock blender. Note - the plant is employed on colored compounds only.

Results obtained from previous tests have been confirmed more ideal situations and in nearly all cases the output of the system is determined by the optimum masterbatch time.

In the months since the plant was installed, the quality of the compounds made has been at least as good as that made by the two-stage process.

The tandem mixer as installed uses the mixing chamber, etc., of a standard K5 mixer and this has proved adequate for normal mixing work but could be improved using the knowledge already gained.

The performance of the tandem mixer depends very much on good cooling facilities for the mixer and on the feed behavior of the rotors for a given compound. The batch tends to self-feed immediately and then disgorge before being taken in again on a repeating cycle until the batch remains within the rotors. The reduction in batch temperature following a pass through the rotors is remarkable. The curatives are normally well distributed after 100-120 secs., but due to the feed behavior described above, the temperature across the batch is erratic initially. The initial instability of batch cooling rate does not seem to have an adverse effect or scorching. This is probably because of the fact that as a particular piece of compound (dusted with curative) is drawn into the rotors, the temperature of that piece is immediately and drastically reduced below danger levels as the curative is mixed in. Consequently, the heat history of the total compound is good.

It has been found that the best feed behavior occurs at higher rotor speeds (15 - 25) but a lower final temperature is achieved using lower rotor speeds.

At the time of writing, a program was being arranged to modify the machine in order to speed up rate of intake into the rotors.

The cooling water flow rate has also proved more important than low water temperatures. It is desirable that tandem mixers have recirculating cooling water systems to ensure good water flow and chemical dosing to ensure clean cooling passages.

Prospects

A tandem mixer may be inserted into a new mixing line for about an additional 15% of the basic machinery cost, taking into account mixer, speed equipment and downstream equipment.

Potentially, the output of the line can be increased by over 50% by adding the tandem mixer and eliminating a second mixing pass.

This system will obviously not accommodate all types of compounds and consequently is most interesting in plants where other mixing facilities exist, so that streaming of suitable compounds can occur.

Certainly, if a technical products plant was able to increase its single stage output from 25% to 50% of the total which we believe is possible, then this would constitute an enormous saving.

A machine designed specifically as a tandem mixer need not be so robust as the standard machine because bending loads and motor powers are much reduced. Consequently cooling capability can be vastly improved. The machine handles only premixed hot compound and therefore need not be hard weld protected. The dedicated tandem mixer would, therefore, be a relatively inexpensive investment.
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Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Johnson, F.
Publication:Rubber World
Date:Feb 1, 1993
Words:1832
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