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Advances in mixing line equipment.

The typical rubber mixing system consists of an internal, intensive batch mixer followed by varying downstream equipment. One common line involves discharging the mixed batch onto a two-roll mill usually referred to as a drop mill. Often the drop mill is followed by a secondary mill for continuously sheeting the compound. Once in sheet form, the compound is immediately coated with an anti-stick agent, and then cooled in some form of batch-off machine.

Alternately, discharge from the intensive mixer may drop directly into a large single screw extruder or transfer mix which has a large stuffing ram (see figure 1). Extrudate from the single screw extruder is either sheeted directly through a roller die, a tube die or it is fed into another two-roll mill for sheeting. These two conventional down-line systems have many problems which are summarized in table 1.

TSR development and description

It was the aim of Kobelco to develop an under mixer, sheeting device which improved or eliminated the problems. Kobelco introduced the TSR (twin screw roller head extruder) in 1986. Simply stated, the TSR consists of a two-roll sheeting calender, close coupled to a short twin screw extruder. The intermeshing twin screws pull in material without assistance from a pusher and feed the sheeting calender which, in turn, produces a precise sheet automatically.

Hopper section

As can be seen in figure 2, the hopper opening is very large, having approximately the same dimensions as the discharge opening of a correspondingly sized intensive mixer. By positioning the TSR dump chute directly under an intensive mixer, the rubber batch can fall freely onto the twin screws without bridging or lodging. Alternatively, the TSR can be set freely, away from the mixer. Stock can be transported from mixer to TSR hopper via a conveyor.

Smooth material flow

During the design and development stage, considerable investigation was made concerning screw design. It was found that with proper screw geometry design, screw length could be significantly shortened without adversely affecting outlet pressure uniformity and, subsequently, sheet quality. Screw diameter in the hopper area is very large and the twin screws are deeply intermeshed. With the screws counter-rotating, material is drawn into the screw flights without the assistance of a pusher.

Material exits the extruder screw through full circle openings slightly larger than the screw diameter and uniformly feeds across the entire calender roll width. Because of the large open area and smooth, even material flow, extrudate pressure is very low. This smooth, even feeding of the calender rolls results in a consistent sheet, absent of voids and rough edges.

In order to maintain a uniform sheet, it is necessary to precisely control the rubber bank in the calender roll bite. This control is accomplished automatically through a floating sensing bar. This bar contacts the top surface of the rubber bank and is attached to a load cell. As the bank enlarges, pressure on the load cell increases. A signal is fed from the load cell to the screw or calender motor to speed up or slow down, thus, holding the bank in an equilibrium position.

TSR operation and control There are two basic modes of operation for the TSR. These can be simply described as continuous and start/stop. In either case, two sensors, one in the TSR hopper and one in the drop chute, automatically adjust screw and roll speeds, depending on the volume of material in the chute.

Continuous

In continuous operation, the TSR is run at such a speed as to have a small portion of the previous batch remaining in the TSR hopper when the next batch is dropped. Operation in this mode yields a continuous sheet without batch separation and without the need to rethread the batch-off line with the leading edge of each batch.

Staff/stop

With start/stop operation, the TSR is started automatically when a batch is dropped and runs at a higher speed than it does with continuous operation. Once all material is purged from the TSR, it stops until the next batch is dropped.

Start/stop operation generally requires higher motor power, but offers discrete batch to batch separation while minimizing heat history. Each batch enters the cooling line very shortly after discharge from the mixer.

Design features

Effective cooling

In order to maximize the cooling efficiency of the TSR, all surfaces which contact rubber are cooled via water circulation. This includes the hopper, chamber sides and bottom, screws and calender rolls. (Generally it is desirable to maintain accurate calender roll temperatures, Therefore, a two-zone temperature control unit is recommended.)

Cooling begins the instant the batch is dropped and, in many cases, as much as 15-18 degree C can be removed from the compound as it moves through the TSR

( see table 2).

Fixed clearance screw

Conventional single screw extruders have a very long screw which must float at the free end. Because of contact between the screw and barrel at the free end during operation, screw and barrel life is very short. In addition, costly hardfacing must be applied in the contact areas.

By reducing screw length in the TSR, it is possible to fix the screws in a pair of radial bearings adequately spaced at the shaft end. Once fixed, a constant clearance can be maintained between the free screw end and the chamber. Since there is never any screw to barrel contact, there is no need for expensive hardfacing and the screw life is very long - up to ten years.

Low power and heat generation

Since the screws are very short and smooth rubber flow is obtained at very low pressure, power input requirements are very low. Motor power may be reduced by at least 50% when compared to similar capacity conventional extruders, thus reducing initial costs and operating costs. In addition, energy input (and, therefore, heat generation) to the compound is minimized.

No pusher required

Since the twin screws are self-feeding, a mechanical stuff ing device is not necessary. This results in three benefits when compared to conventional drop extruders:

* Since no compressed air is required to actuate a stuffer, significant energy savings can be realized.

* Maintenance requirements are substantially reduced. Many wearing and control components are eliminated which previously required a great deal of attention.

* Possibility for contamination is reduced since remnants cannot become lodged within the stuffer clearances and later fail out as cured lumps or non-compatible material.

Reduced labor/compact design

Labor requirements are much lower especially when compared to conventional mill lines. The TSR can be started automatically when the batch is discharged and no attendants are needed (once leading edge is threaded into the batch-off) during normal operation.

Because of the TSR's short screw design and close-coupled calender, the TSR consumes considerably less floor space than any other conventional systems.

Summary With recent advances and improvements in intensive mixer design, it is now possible to complete a high quality mix within the mixer. Thus, the need to improve mix quality on mills or other downstream equipment has been eliminated. Present mixing philosophy is to do all mixing inside the mixer, then discharge the batch and cool it down as rapidly and consistently as possible while minimizing further energy input. Such a mixing process is possible by placing a Kobelco TSR underneath a modern intensive mixer. The TSR provides a long, maintenance tree service life. reduces conventional mill line labor requirements (and the associated human inconsistencies imparted on the compound), and is much more energy efficient than conventional single screw extruders.

Multiple program controller - MP-100/A Internal, intensive rubber mixers have historically been controlled manually by the mixer operator. The operator would set up, load and run the mixer to parameters assigned each different recipe by a plant chemist or other technical representative. By watching a batch timer, mix temperature meter, power meter and/or other variables, the operator would attempt to match the assigned mixing procedure as closely as possible.

However, operator's reaction time, physical state, attitude, attention, etc., vary considerably over time, and different operators have their own unique and differing approach to the operating task. As a result. considerable batch to batch variation in actual mixing conditions is inevitable, even though the prescribed mix sequence is fixed and well defined. Consequently, it is not possible to obtain highly consistent mixes under the historical manual control.

Moreover, due to the large number of variables in the mixing process and the wide range of recipes (and mix sequences), it is a difficult task to automate mixer operation. Such automatic control equipment must be highly flexible (to allow for many possible control schemes), simple to program and operate (minimizing operator interface) and inexpensive to purchase and install.

MP100 controll scheme

Alter considerable development work, it was concluded that all mixes have a characteristic torque/time mixing pattern such that the peaks and troughs of the mixing curve are equivalent for particular states of mix in the mixing process.

Like torque and time, temperature, instantaneous motor power and integrated motor power accurately reflect the state of mix. By monitoring the above characteristics, the mix state can be closely followed and mix steps can be set according to an optimum combination of time, temperature, instantaneous power and integrated power. Through the MP100, a series of and/or conditions are set to drive each mix precisely in accordance with these optimum conditions.

Such a control scheme can be used to control any type and size of internal mixer. A summary of advanced functions is in table 3.

Benefit to users

For operators:

* Improve safety by sounding an alarm before actuation of ram or charging door.

* Prevent misoperation during mix since operator only has to push one proceed button after charging material.

* Inform operator of the recipe which is being charged now and will be charged next by a flashing display. No recipe cards are required.

* Easily change mixing conditions or production schedule by communication with screen.

For production and quality control staff:

* Maintain consistent quality of mixing by controlling power, temperature, time and/or power consumption.

* Provide easy to read screen to display mixing process; able to see abnormal conditions instantly.

* Establish complicated mixing procedures by easily setting format of screen.

* Store mixing data on hard disk.

* Able to recall necessary data promptly.

* Easily change production schedule if required for emergency.

* Enable total factory automation by on-line connection with host computer (as option).

* Enable fully automated mixing-line control by connecting with upstream and downstream equipment, like material weighing and handling devices (as option).

MP100 function

The MP100 serves to control all aspects of the mixing operation. In this way, there is no human influence induced into the mix once initial ingredients are charged.

All functions described below are maintained by or through the MP100.

Setting of mix procedure sequences

Through selection of up to 16 different mixing steps, it is possible to set various mixing sequences and to operate the ram, charging door and discharge door at every step. For example, the following steps are available:
1 - Rubber 1; 2 - Rubber 2; 3 - Rubber 3;
4 - Carbon 1; 5 - Carbon 2; 6 - Carbon 3;
7 - Oil 1; 8 - Oil 2;
9 - Chemical 1; 10 - Chemical 2; 11 - Chemical 3;
12 - Curative; 13 - Rotor speed set; 14 - Ram pressure set;
15 - Ram cleaning; 16 - Drop door.


It is possible to set up to 12 of the above steps in any order including divided or simultaneous charge.

Set timing of material charge and discharge

And/or conditions for time, temperature, instantaneous power and integrated power are set for the termination point of each mix step. Time and integrated power can be set individually for each step or as an accumulated value from start.

For example: Mix discharge conditions:

A = Time 60 seconds.

B = Temperature 300 degrees F.

C = Integrated power 2.3 KWH.

D = Instantaneous power 150 HP [+ or -] 5%.

Possible combinations are shown in table 4.

Instantaneous power can be considered as equivalent to torque which, in turn, shows viscosity as a mixing property. Passing from step to step is normally handled automatically once the set conditions for the previous step are met. For safety reasons, an alarm will sound briefly prior to the automatic step proceed. The operator can manually delay a step proceed by holding in a "pause" button.

Alternately, step proceeds can require operator acknowledgement by pressing a step proceed button.

Setting of various alarms Upper and lower limits can be set for total time, integrated power and temperature. An alarm is output if upper limits are exceeded within each step, or if the lower limits are not reached prior to discharge. Optionally, an alarm can be set to sound should actual ingredient weighing error exceed the set tolerance. For example the upper limit could be five minutes, 350 degrees F and 30 KWH integrated power with a lower limit of one minute, 300 degrees F and 20KWH integrated power.

An alarm will sound if actual mix data do not fail within the set limits. Any combination of limits or no limits at all can be set.

Announce finish of mix step

A buzzer can sound a certain time before each step is finished helping the operator to work efficiently and safely. For example - the alarm is set for 90%. If step time is set for one minute, the alarm will sound 54 seconds after start of step and remain on until step is finished.

Set mixing data

Mixing data can be selected in any one of three ways:

* New compound data can be input.

* Previous test mixing results can be used.

* Existing stored mixing data can be modified or reused.

Set daily production schedule

The production schedule is made up of stored compound numbers (mix sequences) followed by the number of batches to be produced with that sequence. The mixer then automatically follows this schedule. The operator can:

* Create or modify mixing schedule and store data.

* Display schedule and change pages.

* Set the number of batches, operator I.D. and start number.

* Break into the schedule at a selected point with a new compound number.

For example: Run 1 - compound C, five batches; Run 2 compound A, three batches; Run 3 - compound D, 13 batches and Run 4 - compound K, three batches. While mixing run no. 1 as per above schedule, two batches of compound X are needed as an emergency. Without stopping the mixer, the operator can change the screen from a running mode to a schedule mode and add two batches of compound X. The mixer will then run as follows: five batches of compound C; two batches of compound X; three batches of compound A and 13 batches of compound D.

Actual mix data record

Following completion of each batch, a hard copy printout of compound number, date, time and discharge time, power and integrated power at batch end is generated. The operator can select the report in either digital values or analog chart.

Optionally, all mixing data can be transferred to a plant host computer.

Actual mix data storage

Following completion of each batch, mixing data can be transferred to hard disk memory. Approximately 3,000 batches can be stored in this manner. This memory data can also be transferred to floppy disk and reviewed on any other compatible personal computer for instant comparison with past results. Data can be located by order of operation date or alphabetical order of compound name.

Display of real time operating data-running screen

Time, temperature, instantaneous power and integrated power are displayed graphically on the operator's screen with major parameters digitally displayed. The operator may select an all numeric screen simply by pressing the function key. Features of the graphic screen include:

* Power curve is colored in for easy viewing.

* Bar graphs are displayed along horizontal and vertical axis.

* Vertical scale can be changed depending on magnitude of data.

* Horizontal lines show conditions at end of each step change.

* If mixing time exceeds five minutes, vertical scale automatically changes.

* Present values and step number are displayed with large characters.

* Each step end and/or current condition and set values are displayed.

* Next step is always displayed.

Numerical data screen displays present step values and values at start and completion of each step. Current step is lighted, while the next step is flashing.

Test mixing

This function is used to generate new mixing data. The mixer is run manually and the operator must press proceed button at the end of every step. Mixing data, time, temperature, power and integrated power are stored. It is also possible to modify these collected data. No separate data logger is necessary.

Mixing results report

Results from previous mixes can be reviewed and compared with other results by graph or numerical values. Vertical axis can be scaled to fit in both mixes in order to compare in detail.

Compound list

All mix sequences currently in memory can be displayed. Individual mix sequences can be copied or deleted from the list. Specific sequences can be located by order of date or by alphabetical order of compound name. Entire compound list can be placed on a 3.5" floppy disk for back-up.

Utility

Through this function, the following items can be set or selected:

* Graphic displays: color, content, scales, prints.

* Analog input scale. Analog output scale.

* Sampling time (frequency of update).

* Spare output setting. Prediction signal output timing.

* Time setting: accumulated or section.

* Running screen: graphic or numerical mode.

* Data memory.

* Data storage pass code. Step name and ram, charge door condition.

* Spare digital output signal name set.

* Spare analog output signal name set.

Battery back-up

In order to protect mixing data in event of a power failure, a battery back-up is provided. Work on the MPIO0 can continue even if the mixer stops due to power failure.

Input/output of analog signals

In addition to the standard power and temperature transducers provided, other transducers for indicating motor speed, ram presses, etc., can be provided as an option.

Control for weighing and conveying equipment

It is possible to expand the MP100 System to control upstream and downstream equipment (option). For example, weight values with tolerances can be set on screen and then downloaded to weigher and conveyor.

The alarm can be set if out of tolerance weighing results are obtained. Specified and actual values are stored and printed.

Data input by separate personal computer

As an option, it is possible to develop mixing data and dally schedule from a separate personal computer in the office. Data are stored and transferred by floppy disk system.

Communication with host computer

As an option, any data mixing, daily schedule, weighing data and mixing results, etc., can be transmitted mutually to a host computer. This enables large scale quality and production control of multiple mixer lines. Mutual communication is made via a serial communication link. Hardware needed is shown in table 5.

Summary

After years of studying the mixing process, Kobelco has developed a high powered, highly flexible, easy to use and inexpensive control system which can be used on any new or existing internal batch mixer. Because of its flexibility, it is readily adapted to any application, even small quantity production of many different recipes. By totally removing the operator from the mixing process, it is possible to produce highly repeatable, uniform mixes to pre-determined characteristics while improving overall productivity.

Table 1 - common problems associated with conventional mill line equipment

Mill line

* Labor intensive as each mill requires an operator.

* Operator induced variation form batch to batch and operator to operator causes inconsistent product.

* Considerable time lag form dischange of batch to entering cooling line results in unwanted heat history.

* Equipment consumes a great deal of floor space.

* Mixer productivity can be limited by the mill line capacity.

* Mill operation is inherently unsafe and requires considerable skill.

Single screw extruder

* Operator induced variations on downstream sheeting mill is used.

* Considerable maintenance required on pneumatic stuffing ram.

* Pneumatic stuffer consumers much energy.

* Contact between screw end and barrel causes rapid wear.

* High pressure at end of extruder increases energy requirement and raises compound temperature.

* Contamination resulting from remnants lodging in stuffer clearances and fall ing out in subsequent batches.

Table 3 - summary of advanced functions

* Set any sequence of mixing.

* Set any timing of material charge and discharge for the above set mix sequen ces.

* Set various alarms for abnormal mix conditions.

* Set mixing data very easily.

* Set daily production schedule in advance.

* Printer provided to record mixing results automatically.

* Store compound recipe and mixing data - up to 3,000 compounds.

* Provide easy to read screen.

* Record and utilize data form any mixing trial.

* Review mixing data instantly.

* Provide compound list table.

* Provide full utility screen with variable parameters.

* Provide back-up battery.

* Receive amy input signal from up and down stream equipment as option.

* Input data through separate personal computers as option.

* Communicate with host computer for factory wide production control as option .
 Table 4 - possible combinations
Only A A and B A or B A and B and C A or B or D
Only B A and C A or C A and B and D A or B or D
Only C A and D A or D A and C and D A or C or D
Only D B and C B or C A and C and D B or C or D
 B and D B or D A and B and C and D
 C and D C or D A or B or C or D


Table 5 - hardware requirements

CPU
 * Computer: IBM 32byte
 * Drive: 3.5" floppy disk
 * Hard disk: 20 Mbyte
 * CRT: 14" color
 * Keyboard: Flat touch sheet


* A/D and D/A Board

* Printer: 80 Digits

* Expansion slot for host and personal computer (option).

PLC

* Maker: A/B (or equivalent)

* Transducers: Temperature and power

* Power supply
COPYRIGHT 1993 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Asada, Mitch
Publication:Rubber World
Date:Jul 1, 1993
Words:3634
Previous Article:Flow visualization studies of internal mixing using rotors of different design.
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