Twin roll mixing revolution.

Two roll mills offer users the opportunity to meet the criteria long applied to such machines. They are said to be easily cleanable, versatile, compact, quiet and energy efficient, and offer automatic mixing and safety.

Two roll mills are used and have the potential to be used in a wide variety of industries, and were traditionally used in the manufacture of rubber compounds. Prior to the advent of high speed extrusion systems, two roll mills were used for plastics, where they were mainly used to accept a charge from a batch mixer. Two roll mills in different formats have been used in the medical field, as well.

The use of the two roll mill in its traditional role in the rubber industry is well documented, with many companies diversifying into silicone manufacture, for example in the medical field After a specific request from one such company to J.R. Dare Limited, manufacturer of the Twin Roll Mill, to supply a purpose built mixing mill for use in a clean room environment, the scene was set for an extremely versatile mill design range which could be used in a wide variety of applications.

The two roll mills used in the manufacture of silicone compounds prior to the advent of the Twin Roll Mill were an adaptation of older traditional designs of rubber mills (figure 1). When milling rubber compounds, due to the rubber's mainly higher viscosity (compared to many silicones), it was necessary for the rubber to remain in place between the material guides. Cross contamination, especially on the carbon black rubbers where long runs were the norm, was not too much of a problem. However, with lower viscosity and colored compounds, the traditional design of the two roll mill was not the ideal.

[Figure 1 ILLUSTRATION OMITTED]

In a lot of cases, these mills used bronze sleeve bearings which involved a constant loss grease system, i.e., the bearings were unsealed, and subsequently any low (or even higher) viscosity material touching these areas resulted in scrap batches. Material guides had running clearances with the rolls - again a contamination trap. The inside of the side frames were a combination of corners and ledges which were a haven for solids, powders and oils/greases to lodge. Any material inadvertently coming into contact with these areas was contaminated. Being ex-rubber mills, the drive powers were invariably too high, so energy usage was not optimized. On the older mills, unless they incorporated motorized nip adjustment (very rare), the batches had to run at a constant thickness, and for these thicknesses to be changed, the material had to be dumped, the nip gap adjusted and the material re-introduced - again, not the ideal machine for the job.

The new mill design (figure 2) was approached mainly as a scaled up version of laboratory mills, which incorporate very smooth stainless steel side frames, sealed roller bearings, lightweight spring loaded material guides in contact with the rolls, thermally efficient rolls, a color touch screen PLC access unit and hydraulic nip control. They are also very compact in design. A patented roll reversal system for automating the mix cycle (Mix Assist) was incorporated, plus a unique automatic safety test feature. The machines were also extremely quiet.

[Figure 2 ILLUSTRATION OMITTED]

Twin electrical variable speed drives are fitted to provide a variable friction ratio facility, which is of great benefit when mixing rubber and silicones.

The mill range enables a wide variety of types of materials to be mixed, due to each mill following a standard but flexible design that is known to be relevant to virtually all mixing parameters.

Nip adjustment is hydraulic, and capable of adjusting the roll nip gap, even when fully loaded with compound. The hydraulic cylinders are integral within the mill frames to ensure smooth lines.

The back roll is the adjusted roll, ensuring a fully accessible front roll free from large manual or electrical nip adjustment equipment,

Spiral flow designs for high thermal efficiency and extremely rapid roll working face temperature adjustments are used for thermally sensitive compounds. Other roll designs are available.

Mills can be cleaned in minutes. The material guides are lightweight and spring loaded in contact with roll surfaces. By operating the lever, they are removable for cleaning (figure 3).

[Figure 3 ILLUSTRATION OMITTED]

All internal surfaces of the side frames are stainless steel, without any projections to act as contamination traps. Roll edges give adequate room for cleaning. Roll bearings are sealed for life.

The mill operator has a color touch screen PLC access unit to control all of the mill functions (figure 4). Recipe repeatability of roll speeds, roll reversal procedure, timing, etc., thermoregulation fluid temperatures and nip gap settings is possible.

[Figure 4 ILLUSTRATION OMITTED]

Graphs available on the color touch screen include time (mixing cycle), roll speeds profile (RPM), thermoregulation fluid temperatures profile ([degrees] C), nip settings profile, torque profile (Nm or lbs. ft.) and energy profile.

Also offered are front and rear roll individual speed/ reversal controls; front and rear roll speed indication; friction ratio indication; individual drive current indication; torque indication (if required); automatic nip settings (adjustable at any time during the mix cycle); an automatic roll stopping distance/brake check; and an emergency stop push button.

Regarding data retrieval, a control system can be supplied with a network card or port to allow direct connection to a printer, existing office PC network or a supervisory PC. Also, a full data retrieval system can be supplied, either as a stand alone or integrated system, incorporating a PC, monitor, network card, Windows, Office 97 and printer.

A roll stopping distance/brake check diagnostic system allows the individual drive running times to be pre-set. At the end of the pre-set running time, the system allows any milling operations to be completed, and when the mill is next stopped, requests the mill operator to initiate the auto brake test sequence, where the rolls are run up to full speed and braked. The result is then displayed.

If a fault is found, the rolls are allowed to run only under manual control, for minimal time periods, to enable the maintenance personnel to establish why a failure oc-curred. When the fault is cured, the system acknowledges the same, and operation can continue.

All emergency stop and guard switches are monitored by separate safety relays.

Normal mill mixing occurs with a rolling bank between the two opposite rotating rolls. Reversal of one roll has the effect of shearing the sheet at the nip and promoting the direction of the stock around the periphery of the reversed roll.

Maintaining the rotation of the reversed roll for a set period of time ensures the stock is supported on top of the two rolls and above the nip. After the selected time period, the rolls are put into normal mixing mode and the rolled bank and sheet are moved back together in a different position to assist dispersion.

To enable the stock to be removed from the mill on completion of a mix cycle or for the re-introduction to the nip for further milling, the roll reversal is pre-set for a longer period, thus removing the material to the mill tray. The tray is roller bearing mounted to enable it to slide to the operator for collection, or one side of the rolled material is lifted and pushed to the roll face for remilling.

Stripping stock from the rolls now only requires the press of a button, or inclusion into the mix program. The stock rolls up into the bottom nip and drops into the tray.

The very compact design ensures a minimum of production space is used. The individual roll drives are mounted on the roll ends. Electrical panels and hydraulic systems are both housed under the mill with pull-out access for the electrical system. The back roll is the adjusted roll, ensuring a fully accessible front roll.

Drives are optimized to suit the materials being milled. The mills are said to be extremely quiet. The loudest noise is invariably from the drive motor cooling fans.

Recent innovations include compact mill/calender production units primarily developed for ceramic sheet and slab production, which can also be used for rubber, plastics and silicone manufacture. These units are a combination of a mill and calender where the mixing is done on the two roll mill and, on completion, product is automatically fed directly to the calender for accurate sizing.

Recently, inclined mill designs have been used in medical applications where wide and very thin medical adhesive compounds were initially processed on an extruder and fed directly to an inclined mill to laminate the compound between backing material and release paper. The system incorporates a tension and web guiding system, all of which is controlled by a program via a color touch screen PLC access unit.