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Mixing guidelines for proper control and tracking--part 2.

(Part 1 appeared in the November, 2006 issue.)

Gauge control

A very important part of running any mixing operation is the calibration of the various gauges used in the system to provide measured feedback and/or control of functionality. If a sensor or sender is incorrectly calibrated or becomes damaged or malfunctions, it will be important to detect and correct this as quickly as possible. Performing simple daily checks and having the appropriate system monitoring is vital to maintain not only the quality of the product, but to also reduce downtime. An auditable gauge control system should therefore be instituted to keep track of every gauge considered important to the integrity of the process and which could adversely affect the performance and quality of the product being produced. An audited gauge control system should be maintained to cover inspection of all such devices in the system. Where visual gauges are in use, it is a good idea to mark the gauge with colored bands indicating to the operators/maintenance staff the desired control areas and out of control areas.

Controls

To ensure the best possible repeatability from any process related equipment, scales, mixer, mills, etc., computer and/or PLC controls are essential. Such systems are more commonplace these days and are designed so that they can be configured/tailored to an individual site's process and configuration needs. Many of the latest control systems prompt operators through the process, thus negating the need for wordy work instructions and process sheets. These new systems offer greater advantages to the users because they always present the operators with the current revision of process related data and they never get tired. Electronic process sheets and work instructions further support the customer's need for verification of control plans and PFMEAs. They also ensure that the process is less, or in most cases not, operator dependent, because each is given the exact same instruction and he or she must respond to the control system's needs as required. Today's systems now data-log not only the parameters associated with the proportioning of ingredients and the mixing cycle, but can also monitor and record operator response times--reaction to prompts. This can be a very useful tool in determining if operators require additional training.

Materials proportioning

There are three methods for material preparation:

* Hand weighing onto platform scales equipped to handle containers or bags;

* Automatically feeding materials from storage locations into containers or bags; and

* Automatically feeding materials from storage locations to weighing vessels that discharge the pre-weighed batch directly into the mixer.

With any system it is important to ensure that the feeding and weighing equipment is compatible with the raw material being handled. It is also important to ensure that any aspiration or dust collection needs are fully satisfied to ensure not only the cleanliness of the plant, but the safety of its operators. At hand weighing stations, it is recommended that a computer driven prompting display be used to guide and direct the operator through the desired procedure. Displays will typically show recipe number, number of batches to be produced, material code number and an associated target weight for each ingredient required from the preparation center. Each station should have mechanisms in place to capture the raw material's personality data via keyboard entry, barcode label or RFID. At a minimum, this information should comprise the material code number, expiration date, lot number and quantity. The purpose here is to have the computer take the legwork out of the mundane task of performing the data collection and doing all the verification to ensure that the appropriate material is being dispensed and that the material is within its expiration date. Where materials have expired, the system should prevent the proportioning and advise the operator and any other staff of the material's condition.

Fillers proportioning systems should be designed so as to minimize any material degradation and should, if possible, and equipment permitting, introduce materials directly into the mixing chamber via one of the side or rear entries of the hopper section of the mixer. This will help minimize any fly Joss at the hopper door and aid plant cleanliness, since the hopper door can remain closed during the mixer charging sequence. Using these techniques should also improve the mixing cycle time, since the loading sequence will be more repeatable. Be wary of systems generating high levels of fines levels in the carbon blacks, as these can cause material handling and mixing difficulties and potentially high levels of fly loss. One additional problem associated with high fines is that the volume required for an equal amount of the filler can be increased markedly, thus forcing the introduction of these fillers to be broken into two charges, thus adding time to the mix sequence. Automatic weighing and charging of fillers are very clean and reliable with today's pneumatic systems. Such systems are available with bag tip stations, supersack (flexible IBC) stations and silo stations to accommodate most of the suppliers' packaging and delivery formats.

Liquids proportioning systems should preferably be designed to pre-heat any oils having higher viscosity levels. Reducing the viscosity and warming others will aid the automatic handling and introduction into the mixing chamber. Preheating also has the added benefit of ensuring that the mixer batch is not cooled when these liquids are introduced. Why put all that energy into a batch only to quench it with cold liquids? If possible, liquids should be introduced via positive displacement pumps and pneumatically actuated liquids injectors, or a series of injectors mounted directly to the mixing chamber where larger quantities of oils are required in a batch. Introducing liquids in this manner will, in most cases, allow you to keep the ram in float or down position such that there is no loss of temperature or extension of the mix time. Where there are few oils or plasticizers in use. one might consider using a metering system in place of the proportioning system. Metering systems are generally less complex than weighing systems and are thus less expensive.

Polymer proportioning systems come in all shapes and sizes. One of the best recommendations I can make for mixers having chamber sizes of less than 270 liters would be to cut full bales of polymer into smaller pieces to improve mastication times and prevent loading hang-ups in the smaller hopper throats. Manufacturers of bale cutters offer various levels of automation to their machines to facilitate such needs. If equipment is available, it is also recommended that masterbatch rubber be cut into smaller pieces to aid mastication and mixer loading. These devices also allow the loading of multiple sheets of rubber at a time, permitting a high degree of batch blending prior to mixing the final stage, again allowing a reduction in possible batch-to-batch variation caused by the masterbatch material. A warming room can also be a very useful addition, not only for natural rubbers, since this can also aid with mastication, mix time, consumed energy and batch consistency, because the viscosity of the polymer can be reduced and the temperature of other materials raised.

Small chemicals are best prepared and then sealed in bags to prevent ingress of moisture and foreign materials. Each bag should be clearly identified so as to ensure that each is used in the correct batch and within an appropriate time frame. Where possible, bags should be identified with bar code labels such that computer verification can be used. Always be cognizant of reactivity of materials when grouped together, and the duration that these may be grouped; bags are best produced one day ahead of mixing. One may also wish to consider pre-weighs from one of the many suppliers now offering these services. All have the capability to identify bags with bar code labels, if required. If the process and compounds permit, one might also consider batch inclusion bags and tickets; these will allow you to pre-weigh the bags, seal them, identify them and then introduce an unopened bag directly into the mixer. Both bags and labels are available with melt points as low as 75[degrees]C. Many mixing plants will also employ secondary check scales to verify the gross weight of such bags to ensure no losses have occurred, and will scan them prior to loading into the mixer to ensure that the correct components are being added at the appropriate point in the cycle.

Mixer

The "fill factor" of the mixer is a very important parameter with very obvious results. So sensitive can some mixers be that when sizing a batch it can mean the difference between a homogeneous mix and a rework. Real time ram position sensors can be a very useful addition to a mixer because one can see how well the batch is being incorporated (coming together) during the mix cycle. A ram position sensor will also give one a good indication of the overall size of the batch and the tilting factor. A ram that bottoms out too soon indicates an under-filled chamber, and a ram failing to bottom or near bottom out can indicate over filling. Since the volume of the mixing chamber never changes, it is very important to ensure that the sum of the ingredients is maintained each and every batch. Some control systems can make minor adjustments to less active filler ingredients, within specification limits, in an attempt to maintain the overall volume of the batch, thus ensuring optimum mixing. Temperature control of the various zones of the mixer body is also very important, as is having a responsive and reliable chiller system. Mixers come with various zone configurations, the most popular being three, but with more people choosing four these days. Zone tempering systems make life much easier when mixing a wide variety of compounds, since one has a better opportunity to fine tune and control the temperature of the batch.

Mixer controls

When it comes to controls, there are four options:

* Basic manual systems comprise a control enclosure having switches and/or buttons to manipulate the various features of the mixer, along with feedback gauges dispaying time, temperature (with chart recording in some cases), and in some instances, amps, speed and energy. If the mixer is equipped with a variable speed drive, there will also be rotor speed controls and indication. While these designs were adequate fifty plus years ago, they are less than suitable for today's complex formulations and productivity goals. Many mixing plants utilize hand-operated mixers where an operator reads from a process sheet and mixes to the values indicated by the various instruments. The downside of these systems is twofold; they do not record the actions of the operator or any feedback during the mixing cycle, and they rely too heavily on the repeatability of the operators. Performing the same functions at exactly the right moment based on feedback from the machine, time after time, batch-to-batch and shift-to-shift, takes more than skill alone.

* Basic PLC controls add much to the functionality of the mixer and can in addition provide both storage of mixing procedures and record the data from the mixing process for reporting purposes. These systems manipulate the mixer in accordance with pre-entered programs comprising steps and actions, and perform these repeatably time after time. The complexity of these systems can vary dramatically based upon cost and the mixer's basic features. Control of items like temperature controllers for zone cooling, ram position, ram pressure and mixer rotor speed, based upon values of absorbed power, motor amps, time or temperature can be incorporated into the mixing algorythms, providing users with a level of control previously unavailable or considered.

The big downside to the basic manual and PLC systems is that they seldom integrate the mixer with the other important upstream and downstream elements of the mixing process, all of which are vital to the overall consistency and repeatability of the compounds. It is also difficult to provide extensive storage and reporting functionality without the addition of a computer which adds cost and complexity for little gain.

Regarding integrated controls packages, many PC based control systems available today offer materials management, recipe handling, mixer and downstream procedure handling, scaling of batches to suit mixers of differing sizes, lot traceability, auditable reporting systems and improved batch-to-batch consistency. These systems typically comprise a combination of PC based software and PLCs so as to permit distribution of control and minimize plant wiring. There are also underlying reasons for having the two sets of equipment; many users have less faith in the PC than they do in the PLC for some reason, and therefore do not want to have their operations relying solely on a PC that their maintenance department has less of an opportunity to fix. After all, who wants to stop the mixing process because a PC went down? However, what has been realized over the years is that the PC is equally as reliable as the PLC.

With today's demands for higher quality and documentation, the need to monitor downstream processes has also become very important. There is little point in applying all this technology to the mixer and its upstream if the operations following are not under control. Today's control systems can help in this respect too, since downstream mills, extruders and batch-off units can be incorporated into the package, thus providing the end user with a totally integrated package. Parameters, set points, instructions and actuals for these machines can be preset in the recipe, and batch data captured real time.

The major reason for using a combination PC/PLC package is that the PC alone is just not ideally suited to manage all the realtime activities and continue with operator requests and reporting activities because there is a lack of suitable operating systems and applications. PCs are great for manipulating data, and PLCs are great for real time control. Today's suppliers of these integrated systems offer the end user packages tailored for their needs; there is no "one fits all" solution. A user can start with mixing control only and can expand as their needs grow. Systems can be configured such that additional software packages and features can easily be incorporated with little disruption. What can one of these systems offer? Total traceability of raw materials and mixed product in both directions, automatic and manual weighing controls, mixer controls, mill controls and prompting stations, extruder controls, batch-off controls and reports. Should one be prepared to make a more sizable investment, a mixing control system is currently available with "adaptive" features. This system is unique because it has the ability to analyze a compound's properties and can optimize the mixing process while the compound is being mixed. No other system has this feature. Careful design can ensure that the individual components of the process can be incorporated to achieve an efficient, reliable and accredited control system.

The above is but an overview of some, certainly not all, of the areas associated with the compounding of elastomeric compounds which can in some way have an influence on the overall quality and repeatability of compounds mixed in an intensive mixer. In addition to the management of raw materials and the downstream handling of mixed product, there is that all important factor, chemistry. The timing and introduction of the various materials into the mixer based upon such characteristics as viscosity, melt temperature and activity are also vital to the final properties of any compound.

Summary of suggestions

Raw materials receiving

* All material suppliers shall be on an "approved suppliers" list;

* each raw material shall have a "specification sheet" detailing key material characteristics, product form, packaging form and any other specific requirements;

* each material shall be detailed on a "periodic testing" list; and

* each item shall be clearly identified at the receiving point to show that it is "approved for use," "on hold," or "reject:

Production floor

* Department operating procedures;

* standard operator procedures (operator instructions) for each plant element;

* process data sheets, where applicable (paper and/or electronic); and

* control charts, where applicable.

Typical control system reports

* Raw materials directory:

* raw material stocks and usage;

* weighing batch reports;

* mixing batch reports;

* finished materials stock report; and

* alarms log.

Raw materials management

* Approved vendor/supplier control;

* material parameter database;

* inventory control;

* acceptance, quarantine and rejection procedures;

* storage and warehousing control;

* shelf life observation;

* assignment of unique lot number; and

* verification and checking of materials at the point of usage.

Formulae management

* Fully auditable revision control;

* total process application--weighing, mixing, mills/extruders, batch-off and storage;

* integrated off-line applications, e.g., small chemicals; and

* material parameter database.

Production

* Tracking of materials and product through the plant;

* verification and checking of materials at the point of usage;

* inventory and usage reports;

* construction of standard operating procedures (operator instructions);

* integration and automation of all process specific plant equipment;

* downloading of machine-specific set points;

* monitoring actual values against set points;

* data logging the machine-specific actual values; and

* electronic process flow sheets.

Laboratory

* Identification of individually mixed batches with lot number;

* full weighing and mixing batch reports;

* production stock reports; and

* transfer of batch data between controls package and lab package providing a single comprehensive report.

Quality

* Identification of all materials used with bar code;

* tracking of materials and the product through the plant;

* verification and checking of materials at the point of usage;

* reports by plant location or by material type;

* traceability of "all" materials;

* inventory and usage reports;

* finished goods stock reports;

* electronic process sheets; and

* integration and automation of plant equipment, from material proportioning to product packaging.
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No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2006, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

 
Article Details
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Title Annotation:Tech Service
Author:Kent, Christopher J.
Publication:Rubber World
Date:Dec 1, 2006
Words:2914
Previous Article:Process and apparatus for comminuting particle rubber.
Next Article:DoE use in FKM custom compounding.


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